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Nitrogen Release from Slow-Release Fertilizers in Soils with Different Microbial Activities

  • CREA - Council for Agricultural Research and Agricultural Economy Analysis
  • CREA - Council for Agricultural Research and Economics
  • Council for Agricultural Research and Economics

Abstract and Figures

Soil microbial activity is recognized as an important factor affecting nitrogen release from slow-release fertilizers. However, studies on the effect of size and activity of soil microflora on fertilizers degradation provided contrasting results. To date, no clear relationships exist between soil microbial activity and the release of nitrogen from slow-release fertilizers. Hence, the aim of this study was to better understand such relationships by determining the release of nitrogen from three slow-release fertilizers in soils with different microbial activity. Soils were amended with Urea-formaldehyde, Isobutylidene diurea, Crotonylidene diurea. Urea, a soluble fertilizer, was used as positive control. Fertilized soil samples were placed in a leaching system and the release of nitrogen was determined by measuring ammonium-N and nitrate-N concentrations in the leachates during 90 days of incubation. Non-linear regression was used to fit nitrogen leaching rate to a first-order model by non-linear regression. In all the treated soils, nitrogen was released according to the order Urea (89-100%) > IBDU (59-94%)> UF (46-73%) > CDU (44-56%). At the end of incubation, nitrogen released from CDU did not differ (p>0.05) among soils. On the contrary, Urea-formaldehyde and Isobutylidene diurea released lower (p<0.05) amounts of nitrogen in the soil with the higher microbial activity and lower pH. The rate constant (K0) for UF was lower (p<0.05) in the soil with the lower pH. Taken together, our results indicate . that the size and the microbial activity of the soils used had a marginal effect on fertilizers mineralization.
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Pedosphere 28(2): 332–340, 2018
ISSN 1002-0160/CN 32-1315/P
2018 Soil Science Society of China
Published by Elsevier B.V. and Science Press
Nitrogen Release from Slow-Release Fertilizers in Soils with
Different Microbial Activities
Pierfrancesco NARDI1,, Ulderico NERI1, Giovanni DI MATTEO1, Alessandra TRINCHERA1, Rosario NAPOLI1,
Roberta FARINA1, Guntur V. SUBBARAO2and Anna BENEDETTI1
CREA-Council for Agricultural Research and Economics, Research Center for Agriculture and Environment, Via della Navicella 2,
Rome 00184 (Italy)
Japan International Research Center for Agricultural Sciences (JIRCAS), Tsukuba 305-8686 (Japan)
(Received January 13, 2017; revised March 14, 2018)
Soil microbial activity is recognized as an important factor affecting nitrogen (N) release from slow-release fertilizers. However,
studies on the effect of size and activity of soil microflora on fertilizer degradation have provided contrasting results. To date, no clear
relationships exist between soil microbial activity and the release of N from slow-release fertilizers. Hence, the aim of this study was to
better understand such relationships by determining the release of N from three slow-release fertilizers in soils with different microbial
activities. Soils were amended with urea-formaldehyde (UF), isobutylidene diurea (IBDU), and crotonylidene diurea (CDU). Urea, a
soluble fertilizer, was used as the control. Fertilized soil samples were placed in a leaching system, and the release of N was determined
by measuring ammonium-N and nitrate-N concentrations in leachates during 90 d of incubation. Non-linear regression was used to fit
N leaching rate to a first-order model. In all the treated soils, N was released in the order: urea (89%–100%) >IBDU (59%–94%) >
UF (46%–73%) >CDU (44%–56%). At the end of incubation, N released from CDU did not differ (P > 0.05) among soils. On the
contrary, UF and IBDU released significantly lower (P < 0.05) amounts of N in the soil with higher microbial activity and lower pH.
The rate constant (K0) for UF was lower (P < 0.05) in the soil with lower pH. Taken together, our results indicated that soil microbial
size and microbial activity had a marginal effect on fertilizer mineralization.
Key Words: ammonium-N, crotonylidene diurea, isobutylidene diurea, nitrate-N, non-linear regression, urea-formaldehyde
Citation: Nardi P, Neri U, Di Matteo G, Trinchera A, Napoli R, Farina R, Subbarao G V, Benedetti A. 2018. Nitrogen release from
slow-release fertilizers in soils with different microbial activities. Pedosphere.28(2): 332–340.
Slow-release fertilizers (SRFs) are condensation
products obtained by reacting urea, the most common
mineral fertilizer characterized by a high nitrogen (N)
content and relatively low cost, with several aldehy-
des. These SRFs release N at slower rates compared
to conventional N-fertilizers, such as urea releasing N
rapidly by hydrolysis; thus, in theory SRFs facilitate
better N uptake and utilization by crop plants. Poten-
tial benefits from SRFs include improved N use effi-
ciency (NUE), reduced volatilization loss and nitrate
leaching, increased N availability during plant growing
season, and reduced costs of application (i.e., multi-
ple applications of conventional N-fertilizers vs. a sin-
gle application of SRFs) (Allen, 1984). Among SRFs,
urea-formaldehyde (UF), isobutylidene diurea (IBDU),
and crotonylidene diurea (CDU) have gained popu-
larity (Trenkel, 1997). Urea-formaldehyde is obtained
by reacting urea with formaldehyde and consists of
a mixture of chain polymers with different lengths.
The degradation and subsequent N release from UF
is driven by the size and activity of soil microflo-
ra (Alexander and Helm, 1990) and factors that in-
fluence microbial activity, such as soil moisture and
temperature. Crotonylidene diurea, a ring-structured
compound, is produced by condensation of urea with
acetic aldehyde. Both microbial activity and hydro-
lysis drive the degradation of CDU. Thus, soil mois-
ture, temperature, and pH influence the CDU-N re-
lease (Trenkel, 1997). Isobutylidene diurea is a single
oligomer compound, formed by the reaction of urea
and isobutyraldehyde. The IDU-N release occurs by
chemical hydrolysis, and its decomposition in soil is
affected by soil temperature, moisture, and pH (Guer-
tal, 2009).
Slow-release fertilizers have been largely studied
under different temperature and soil moisture regimes
Corresponding author. E-mail:
(Engelsjord et al., 1997; Agehara and Warncke, 2005;
Fan and Li, 2010; Fan et al., 2011), whereas the effect
of size and activity of soil microflora on SRF degra-
dation has received less attention with contrasting re-
sults observed (Benedetti, 1983; Koivunen and Hor-
wath, 2004). For example, in the case of UF, a signi-
ficant positive relationship between microbial activity
and N release was found under laboratory conditions
(Trinchera et al., 2010). However, Koivunen and Hor-
wath (2004) observed that both size and activity of soil
microflora have no significant effects on N release. In
addition, due to the role played by the pH in regulating
hydrolysis reactions, the evaluation of N release from
SRFs under different soil pH has implications on their
performance in different soil types. To date, the ques-
tion of whether N release from SRFs proceeds faster in
soils with higher microbial activity is still open. The
aim of this study was therefore to better understand
such relationships by determining N release from UF,
IBDU, and CDU in soils with different microbial acti-
vities. Given that N release from UF should be mainly
driven by microbial activity, we hypothesized a faster
N release from UF in soils with higher microbial acti-
Soils and fertilizers
Four soils with different physicochemical and bio-
logical characteristics were selected for this research.
Soils were all classified according to the World Refe-
rence Base for Soil Resources (FAO, 2015), as refe-
rence European classification. The first soil was a Gleyc
Cambisol with sandy loam topsoil (431831.28′′ N,
114915.02′′ E), developed on recent alluvial deposits
with seasonal herbaceous crop land use inside the Ce-
sa Research Centre for Agricultural Technologies and
Extension Services (CATES) in the Province of Arez-
zo, Italy. The second soil was a Calcaric Fluvisol with
loam topsoil (434059.05′′ N, 102032.06′′ E), deve-
loped on the reclaimed alluvial floodplain of the Arno
River with seasonal herbaceous crop land use inside
the Interdepartmental Centre for Agro-Ecological Re-
search Enrico Avanzi (CIRAA), located near the city
of Pisa, Italy. The third soil was a Cambic Umbrisol
with clay loam topsoil (42226.83′′ N, 121944.03′′
E), developed on a volcanic tuff plateau of the Vico
complex with permanent crop (hazelnut orchards) land
use, near Viterbo, Italy. Finally, the fourth soil was a
Relictigleyc Calcaric Cambisol with a loamy sand top-
soil (492450.27′′ N, 82356.76′′ E), developed on re-
cent sandy alluvial deposits of the Rhine River with
seasonal herbaceous crop land use, near Limburgerhof,
Germany. Topsoil samples were collected from the 0–
15 cm depth (Ap horizon), dried at room temperature
(approximately 25 C) for 2 weeks, and then passed
through a 2-mm sieve. Main soil physicochemical pa-
rameters are presented in Table I. The N contents in
the SRFs were 380, 320, and 320 g kg1for UF, IBDU,
and CDU, respectively, and the urea with 460 g kg1
N was used as a control.
Selected physicochemical properties of four soils used in this stu-
dy, including a Gleyc Cambisol in the Province of Arezzo, Italy,
a Calcaric Fluvisol near the city of Pisa, Italy, a Cambic Um-
brisol near Viterbo, Italy, and a Relictigleyc Calcaric Cambisol
near Limburgerhof, Germany
Propertya) Arezzo Pisa Viterbo Limburgerhof
Sand (%) 66 40 39 84
Silt (%) 16 38 32 10
Clay (%) 18 22 29 6
Soil textureb) Sandy Loam Clay Loamy
loam loam sand
pH 7.8 7.8 5.2 7.4
EC (dS m1) 0.19 0.24 0.10 0.22
OM (g kg1) 10.30 20.20 34.40 15.10
CEC (cmol kg1) 15.43 17.74 16.47 9.22
Organic C (µg g1) 6.0 11.7 19.9 8.7
N (µg g1) 0.6 1.1 1.1 0.9
C/N ratio 9.9 10.5 18.3 9.7
a)EC = electrical conductivity; OM = organic matter; CEC =
cation exchange capacity.
b)According to USDA Soil Taxonomy.
Microbial biomass carbon (MBC) and basal soil respi-
Soil MBC was measured by the chloroform
fumigation-extraction method (Vance et al., 1987) u-
sing air-dried soil samples conditioned by a 21-d in-
cubation in open glass jars at 33 kPa water tension
and 30 C to restore soil microbial activity. Two por-
tions of 20 g (oven-dry basis) moist soil sample were
weighed: the first one (non-fumigated) was extracted
with 80 mL of 0.5 mol L1K2SO4for 30 min by oscil-
lating shaking at 200 r min1and then filtered using
Whatman No. 42, and the second one was fumigated
for 24 h at 25 C with ethanol-free CHCl3. Following
fumigant removal, the soil was extracted similarly as
the non-fumigated soil sample. Organic carbon (C) in
the extracts was determined after oxidation with 0.4
mol L1K2Cr2O7at 100 C for 30 min, and MBC
was calculated as follows:
MBC = EC/kEC (1)
where EC is the difference between organic C extrac-
334 P. NARDI et al.
ted from fumigated and non-fumigated soils and kEC,
the conversion factor, is 0.38 (or 1/2.64) (Vance et al.,
To measure soil basal respiration, 25 g (oven-dry
basis) moist soil sample was weighed in a vessel and
then placed inside a 1-L stoppered glass jar. The CO2
evolved was trapped, and 2 mL of 1 mol L1NaOH
was placed in another vessel inside the jar. After 1, 2,
4, 7, 10, 14, 17, and 21 d, CO2was determined by titra-
tion of the excess NaOH with 0.1 mol L1HCl (Alef,
1995). The average of the 21-d measurements was used
as the basal respiration value for each soil.
It should be noted that soil MBC and basal respira-
tion were only measured once, just prior to the soil in-
cubation. We recognized that this may represent a limit
of our approach as both MBC and microbial activity
can change during the incubation. This excludes the
possibility to determine causal links between microbial
activity and N release from SRFs. On the other hand,
our approach allows for the determination of the rela-
tionships between pre-existing microbial activity and
fertilizer degradation. Therefore, it provided valuable
information on the use of soil microbial activity as a
putative indicator of fertilizer degradation.
Soil incubation procedure
Modified leaching method of Stanford and Smith
(1972) (Benedetti et al., 1994) was used to measure N
release from N sources. Briefly, 50-g air-dried soils were
mixed with quartz sand at a 1:1 ratio (weight:weight),
followed by the addition of N-fertilizer at the ratio
of 250 mg N kg1. The mixtures (soil, quartz sand,
and N-fertilizer) were then transferred to the B¨uchner
funnels and incubated at 60% water-holding capaci-
ty (WHC) (pF = 2.5) at 30 C. After 7, 14, 28, 45,
60, 75, and 90 d, released inorganic N (Ning ) was
leached as NO
3-N and NH+
4-N with 500 mL of 0.01
mol L1CaSO4solution. To avoid nutrient limiting
effects, after each leaching, a nutrient solution without
N containing 0.002 mol L1CaSO4, 0.002 mol L1
MgSO4, 0.005 mol L1Ca(H2PO4)2, and 0.002 5 mol
L1K2SO4was added to the mixtures. During the in-
cubation, B¨uchner funnels were covered with punctu-
ated aluminium foil and weighed every 3 d to monitor
soil moisture. Distilled water was added to maintain
soil at 60% WHC. Non-fertilized soil was considered
as the control. Leachates were filtered by Whatman
No. 42 filter paper and analyzed for NH+
4-N and NO
N with a Technicon II autoanalyser (Bran & Luebbe,
Sydney, Australia). Net NH+
4-N and NO
3-N on time
from fertilizer release in each soil were calculated after
the subtraction of those of the control soil. Nitrogen
concentrations were reported on a dry matter basis.
Each treatment was replicated three times.
Statistical analysis
Soil MBC and basal respiration were subjected to
one-way analysis of variance (ANOVA). A two-way
ANOVA was performed to test differences in net cu-
mulative Ning (NH+
4-N + NO
3-N) at the end of in-
cubation (90 d). We performed a 7 ×4×4 three-
way mixed between-within subject ANOVA to test the
effect of soils from Arezzo, Limburgerhof, Pisa, and
Viterbo, fertilizers (CDU, IBDU, Urea, and UF), and
incubation time on soil Ning concentrations during 90-
d incubation. In the mixed ANOVA model, time was
the within-subject factor, whereas soil and fertilizers
were considered as the between-subject factors. The
Mauchly’s test of sphericity indicated a violation of the
sphericity assumption for N concentrations; therefore,
the Greenhouse-Geiser adjusted degrees of freedom of
F-distribution was used (von Ende, 1993). In the two-
way ANOVA, as well as in the three-way ANOVA,
main effects were superseded by significant interacti-
ons. To break down these interactions, we performed
a simple effect analysis with Bonferroni-adjusted sim-
ple effect tests, holding the alpha level at 0.05. We
used non-linear regression to fit N leaching rate to a
first-order model (Agehara and Warncke, 2005) as fol-
Nrel =N0[(1 exp(Kt
0)] (2)
where Nrel (mg N kg1soil) is the cumulative N re-
leased from each fertilizer at time t(d), N0(mg N
kg1soil) is the potentially mineralizable N in the ferti-
lizer, and K0(d1) is the mineralization rate constant.
All data points were used in the regression analysis, al-
though data were reported as mean values. The N0and
K0were considered statistically different at P < 0.05,
if the 95% confidence intervals did not overlap. Data
were analyzed using SPSS version 22.0 software.
Soil MBC and basal respiration
Among the four studied soils, MBC was significan-
tly the lowest (P < 0.05) in Arezzo soil (52.04 µg C
g1soil) and significantly higher in Pisa (115.57 µg
C g1soil) and Viterbo (144.31 µgCg1soil) soils
(Fig. 1). All soils showed significant differences in basal
respiration in the range from 1.78 µg CO2-C g1soil
in Limburgerhof soil to 12.39 µg CO2-C g1soil in
Viterbo soil.
Soil effects
At the end of the incubation period, CDU released
approximately 51% of the applied N with Ning concen-
trations being 110.3 and 138.9 mg N kg1soil for Arez-
zo and Viterbo soils, respectively (Table II). However,
the difference was not statistically significant (P >
0.05). Isobutylidene diurea released more than 80% of
the applied N with a significantly lower amount of N
detected from the Viterbo soil (147.9 mg N kg1soil;
P < 0.05). The recovered urea-N ranged from 89% to
100% of the applied N, with non-significant differences
among soils. Nitrogen released from UF was signifi-
cantly lower in Viterbo soil (116.4 mg N kg1soil;
P < 0.05) than the other soils.
Fertilizer effects
In all soils, fertilizers released a decreasing amount
of N in the order: urea >IBDU >UF >CDU (Table
II). Net cumulative concentration of Ning was signifi-
cantly different (P < 0.05) among fertilizer treatments
for the Arezzo soil. In Limburgerhof and Pisa soils, net
cumulative Ning concentration was significantly higher
(P < 0.05) for urea and IBDU treatments than UF
and CDU treatments, with no differences between the
former two fertilizer treatments. In Viterbo soil, the
net cumulative Ning released from CDU and UF was
not statistically different (P > 0.05).
Dynamics of N concentration as affected by soil type,
fertilizer, and incubation time
A two-way ANOVA was performed to evaluate the
effects of soil and fertilizer on net Ning at the end
of incubation (90 d) (Table III). However, the main
effects of soil (F= 29.40, P < 0.001) and fertilizer
(F= 177.05, P < 0.001) were both significant and su-
perseded by significant soil and fertilizer interactions
(F= 6.54, P < 0.001). The three-way mixed between-
within subject ANOVA showed that incubation time
was the within-subject factor of Ning concentration,
whereas fertilizer and soil type, factorially combined
with incubation time, were the between-subject fac-
tors. For NH+
4-N and NO
3-N, as well as for Ning, there
were significant main effects for both within- and bet-
Fig. 1 Microbial biomass C and basal respiration in four soils prior to the incubation. Values are means with standard deviations
shown by vertical bars (n= 3). Different letters indicate statistically different means between soils at P < 0.05.
Net cumulative concentrations of inorganic N (NH+
4-N + NO
3-N) in four soils amended with slow-releas fertilizers (crotonylidene
diurea (CDU), isobutylidene diurea (IBDU), urea, and urea-formaldehyde (UF)) at the end of the incubation period (90 d)
Soil Net cumulative concentration of inorganic N
mg N kg1soil
Arezzo 127.8 ±3.4a)aAb) 217.7 ±5.8aB 249.8 ±17.7aC 168.1 ±5.8aD
Limburgerhof 132.6 ±10.4aA 220.8 ±8.9aB 222.6 ±8.6aB 181.8 ±8.7aD
Pisa 138.9 ±5.5aA 235.8 ±3.2aB 249.5 ±5.6aB 175.8 ±4.8aD
Viterbo 110.3 ±3.8aA 147.9 ±3.2bB 241.2 ±2.8aC 116.4 ±6.3bA
a)Means ±standard errors (n= 3).
b)Means followed by different lowercase letters within a column and uppercase letters within a row are statistically different for a given
soil and fertilizer, respectively (P < 0.05).
336 P. NARDI et al.
Three-way mixed analysis of variance results for the effects of slow-release fertilizers (crotonylidene diurea, isobutylidene diurea, urea,
and urea-formaldehyde), soil type (Arezzo, Limburgerhof, Pisa, and Viterbo), and incubation time (7, 14, 28, 45, 60, and 90 d) on soil
net N concentrations
Factor(s) Inorganic N NH+
4-N NO
dfa) Fvalue Pvalue df Fvalue Pvalue df Fvalue Pvalue
Between subjects
Soil type (ST) 3 29.40 <0.000 1 3 592.22 <0.000 1 3 488.74 <0.000 1
Fertilizer 3 177.05 <0.000 1 3 43.51 <0.000 1 3 174.45 <0.000 1
ST ×fertilizer 9 6.54 <0.000 1 9 134.76 <0.000 1 9 39.33 <0.000 1
Error 32 32 32
Within subjects
Incubation time (IT) 3.21 1 172.3 <0.000 1 3.85 651.22 <0.000 1 2.78 633.12 <0.000 1
IT ×ST 9.64 22.7 <0.000 1 11.55 203.19 <0.000 1 8.35 100.08 <0.000 1
IT ×fertilizer 9.64 951.5 <0.000 1 11.55 369.75 <0.000 1 8.35 618.48 <0.000 1
IT ×ST ×fertilizer 28.90 16.3 <0.000 1 34.65 197.47 <0.000 1 25.04 77.67 <0.000 1
Error 103 32 32
a)Degree of freedom.
ween-subject factors. However, these effects were obta-
ined in the context of significant three-way interacti-
ons: F= 197.47, P < 0.001 for NH+
4-N, F= 76.67,
P < 0.001 for NO3N, and F= 16.27, P < 0.001 for
Crotonylidene diurea-amended soils
Net Ning concentrations were quite low during the
first 14 d in all soils amended with CDU with the
exception of Pisa soil, and then increased significan-
tly from days 15 to 28 in all the soils, but general-
ly decreased afterwards (Fig. 2). In particular, on day
7, net Ning concentration was in the range of 14%,
6%, 5%, and 3% of the applied N in Pisa, Arezzo,
Limburgerhof, and Viterbo soils, respectively, with si-
gnificant differences between Pisa and Limburgerhof
soils (P < 0.05) and between Pisa and Viterbo soils
(P < 0.01). The net Ning concentration significantly
decreased (P < 0.01) in Pisa soil by day 14. From days
15 to 28, net Ning concentrations increased 3.5-fold to
27.5 mg N kg1soil, 11-fold to 43.3 mg N kg1soil,
15-fold to 53 mg kg1soil than the former period (from
days 8 to 14) in Pisa, Limburgerhof, and Viterbo soils,
respectively (P < 0.001). On day 28, Arezzo soil ex-
hibited the significantly lowest net Ning concentration
(P < 0.001) compared to the other three soils. From
days 29 to 45, net Ning concentration significantly de-
creased in Viterbo soil (P < 0.05) and remained quite
constant in Limburgerhof and Pisa soils. Nitrogen dy-
namics differed among soils. Pisa soil, with the excep-
tion of the first week, was clearly dominated by the
prevalence of NO
3-N during the incubation period.
Yet, NO
3-N was prevalent in Arezzo soil for at least
the first three-incubation periods and in Limburgerhof
soil from days 29 to 45 and 46 to 60. In Viterbo soil,
3-N was almost absent during the first two weeks,
but increased afterwards.
Isobutylidene diurea-amended soils
Isobutylidene diurea released N at very low rate
during the first week of incubation, with net Ning con-
centration consistently less than 17 mg N kg1soil,
with non-significant (P > 0.05) differences among soils
(Fig. 2). Conversely, approximately 68%, 76%, and
79% of the applied N was recovered in Arezzo, Pisa,
and Limburgerhof soils, respectively, during the next
two incubation periods, i.e., days 7–14 and 15–28. Dis-
tinct peaks in net Ning concentration were also found
for the Viterbo soil during the same incubation periods,
but the amount of N released was significantly lower
(P < 0.05) compared to the other three soils. During
days 8–14, net Ning concentration increased 10-fold to
83 mg N kg1soil in Arezzo soil and 64 mg N kg1
soil in Limburgerhof soil, 7-fold to 16 mg N kg1soil
in Pisa soil, and approximately 4-fold to 32 mg kg1
soil in Viterbo soil (P < 0.001) compared to those du-
ring days 0–7. From days 15 to 28, net Ning concentra-
tion increased 2-fold in Limburgerhof and Viterbo soils
(P < 0.001), but it remained almost constant in Arez-
zo soil (P= 1.000), and significantly decreased in Pisa
soil (P < 0.001) compared to the former duration. Af-
ter 28 d, net Ning concentration significantly decreased
in all soils and became negligible for the remaining pe-
riod. Nitrogen was nitrified faster in Arezzo, Pisa, and
Limburgerhof soils, but persisted in NH+
4-N in Viterbo
Urea-amended soils
As expected, urea released N very fast, and almost
all N added was recovered during the first 2 weeks of
Fig. 2 Net concentrations of inorganic N (Ning ) containing NH+
4-N and NO
3-N in four soils (Arezzo, Limburgerhof, Pisa, and Viterbo)
amended with slow-release fertilizers (crotonylidene diurea (CDU), isobutylidene diurea (IBDU), urea, and urea-formaldehyde (UF))
during the 90-d incubation.
incubation (Fig. 2). The remaining N was recovered on
day 14, with no difference (P > 0.05) among soils.
Urea-formaldehyde-amended soils
In soils amended with UF, net Ning concentration
decreased with incubation time in Pisa, Arezzo, and
Limburgerhof soils (Fig. 2). The UF-N recovered on
day 7 was 26%, 24%, and 19% of the applied N in Pisa,
Arezzo, and Limburgerhof soils, respectively, with no
significant difference. During the same incubation peri-
od, approximately 10% of the applied N was recovered
in Viterbo soil with significant differences (P < 0.01)
compared to Pisa and Arezzo soils. During days 15–
28, a significantly higher net Ning concentration (52.5
mg kg1soil) was released in Viterbo soil than Arez-
zo and Limburgerhof (P < 0.001) and Pisa (P < 0.05)
soils. The released N was nitrified very fast in Pisa soil,
as NO
3-N was the prevalent N form during the incu-
bation period. Arezzo soil followed similar dynamics,
with NO
3-N concentration always higher than that of
4-N. The balance between the two N forms was dif-
ferent in Limburgerhof soil because of the prevalence
of NH+
4-N during days 0–7 and the presence of both
4-N and NO
3-N during days 8–14, whereas NO
338 P. NARDI et al.
N dominated the remaining incubation period. As ob-
served for the other treatments, Viterbo soil showed
different N dynamics, with NH+
4-N as the dominant
form during the first 28 d of incubation but higher
3-N concentration during days 29–45 and 46–60.
Nonlinear regression
The first-order model generally fit the obtained da-
ta (Fig. 3, Table IV), with the only exception of Arez-
zo soil fertilized with CDU, where the model predic-
ted an unrealistic N0. The analysis of residuals (data
not shown) revealed systematic patterns in their dis-
tribution; thus indicating that the observed data for
CDU-amended Arezzo soil were not well described by
the model. Therefore, both N0and K0predicted for
Arezzo soil amended with CDU were excluded and not
compared with the other regression coefficients. Non-
significant differences (P > 0.05) in both N0and K0
were observed in the other soils treated with CDU (Ta-
ble IV). For IBDU-amended soils, N0was significantly
lower (P < 0.05) in Viterbo soil than the other three
soils, whereas no statistical differences were detected
for K0. In soils amended with urea, Limburgerhof soil
showed the lowest N0, and K0values were not sta-
tistically different among the four soils. Interestingly,
for UF-amended soils, both N0and K0were signifi-
cantly lower in Viterbo soil (P < 0.05) than the other
soils, and the highest K0was observed in Arezzo soil,
followed by Pisa and Limburgerhof soils, with statisti-
cal differences between Arezzo and Limburgerhof soils
(Table IV).
It should be noted that in this study, we were pri-
marily interested in the elucidation of the relationships
that exist between soil microbial activity and N release
from slow-release fertilizers. Regarding the effects of
the size and activity of soil microflora on UF degra-
dation, the following can be discussed. Soil microbial
activity was in the following order: Viterbo >Pisa >
Fig. 3 Nonlinear regressions of net cumulative inorganic N (Ning) in four soils (Arezzo, Limburgerhof, Pisa, and Viterbo) amended with
slow-release fertilizers (crotonylidene diurea (CDU), isobutylidene diurea (IBDU), and urea-formal dehyde (UF)) against incubation
time according to the first-order model.
Potentially mineralizable N in the fertilizer (N0) and mineralization rate constant (K0) in four soils amended with slow-release fertilizers
(crotonylidene diurea (CDU), isobutylidene diurea (IBDU), urea, and urea-formaldehyde (UF)) estimated by nonlinear regressions,
assuming first-order kinetics
mg N kg1soil d1mg N kg1soil d1mg N kg1soil d1mg N kg1soil d1
Arezzo 1 025.7a) 0.002a) 235.7a 0.038a 250.4a 0.482a 158.4a 0.062a
Limburgerhof 192.6ab) 0.014a 242.0a 0.038a 222.8c 0.368a 179.0a 0.037bc
Pisa 155.4a 0.024a 246.2a 0.049a 249.6ab 0.432a 163.5a 0.053ab
Viterbo 147.3a 0.018a 170.3b 0.030a 241.3b 0.407a 131.1b 0.031c
a)Statistical significance was not reported, because data did not fit the model used.
b)Means followed by the same letter(s) within a column are not significantly different.
Arezzo >Limburgerhof. If microbial activity was the
main driving factor of UF decomposition, N release
would be expected to decrease from Viterbo to Lim-
burgerhof soils, according to the scheme above. Inte-
restingly, net cumulative Ning was the highest in Lim-
burgerhof soil, followed by Pisa and Arezzo soils. Most
importantly, it was significantly lower in Viterbo soil,
which was characterized by the largest and more ac-
tive soil microflora. These findings were additionally
supported by K0predicted for Arezzo (K0= 0.062)
and Viterbo (K0= 0.031) soils. Although the influ-
ence of microbial activity on UF degradation has been
well-documented (Benedetti, 1983; Nicolardot et al.,
1994), our results generally agree to those of Koivunen
and Horwath (2004), who did not observe a direct
link between N release from methylene urea, a slow-
release fertilizer, and soil microbial activity. This lack
of consistency could be explained as follows. A mixture
of methylene urea with different long-chain polymers
forms UF, which could be broken-down by both soil
bacteria and fungi (Alexander and Helm, 1990). To
date, different bacterial methylene urea-degrading en-
zymes have been purified (Jahns et al., 1997; Jahns and
Kaltwasser, 2000; Koivunen et al., 2004), and these
were structurally different, depending on the specif-
ic microorganisms and geographic locations. A limit of
our approach was that we neither performed enzymatic
assays, nor identified UF-degrading microorganisms.
However, it is reasonable to conclude that the degra-
dation of this fertilizer in soils was driven by the pre-
sence of specific UF-degrading microorganisms provi-
ding methylenediurea deaminase activity, and thus be
higher in soils harboring such microorganisms. There-
fore, specific enzymatic activities rather than total soil
microbial activity may better represent a predictor
of UF degradation in soils. Another important aspect
characterizing UF was that a relatively fast N release
occurred during the first 2 weeks of incubation, du-
ring which the applied N was recovered from 30% to
40%. Although this could be explained by the prese-
nce of unreacted urea in the UF, it is noteworthy that
in Pisa soil, and to some extent also in Arezzo soil,
the N released was rapidly nitrified, creating the po-
tential for N losses through NO
3leaching or denitri-
fication. Therefore, the use of this fertilizer does not
necessarily avoid environmental problems.
Nitrogen release from IBDU occurred mostly du-
ring the first 28 d of incubation with a lag phase
in the first week, followed by a quick release during
days 14–28. Among parameters influencing IBDU-N
release, granule particle size, temperature, soil water
content, and pH are considered the most important.
In addition, as hydrolysis is faster in acidic condition,
N release should increase with lowering pH (Trenkel,
1997). Our results showed that N release did differ a-
mong Arezzo, Limburgerhof, and Pisa soils, which were
characterized by similar pH. However, the expected
higher N release in Viterbo soil due to its lower pH was
not observed. Conversely, IBDU degradation was found
to be higher in soils with the higher pH, where hydroly-
sis was presumed to be slower. However, the discovery
of the soil bacteria Rhodococcus erythropolis, a strain
able to degrade IBDU through the activity of a speci-
fic enzyme designated as an IBDU-amidinohydrolase,
challenged the view that IBDU degradation occurs on-
ly abiotically (Jahns and Scheep, 2001). In addition,
the optimum pH for the IBDU-amidinohydrolase acti-
vity has been shown to be in the range of 8.0 to 8.5,
which is closer to those of Arezzo, Pisa, and Limburger-
hof soils. Therefore, the biotic pattern of IBDU degra-
dation and, most importantly, the optimum pH for en-
zymes could explain our finding of higher IBDU degra-
dation in the soils with a higher pH. Finally, regarding
CDU degradation, it involves chemical hydrolysis and
microbial attack. However, throughout the incubation
period, neither the amount of the released N nor the
340 P. NARDI et al.
predicted regression coefficients differed significantly a-
mong the studied soils. Yet, our findings stand in con-
trast to previous studies reporting a faster N release
in acidic than alkaline soils (Varadachari and Goertz,
2010). Moreover, as for UF and IBDU, CDU-degrading
microorganisms have been isolated from soils (Jahns et
al., 2003). Our results indicate that both total micro-
bial activity and pH are weak indicators of CDU be-
havior in soil, which led us to speculate that the pre-
sence and activity of specific microorganisms capable
of degrading CDU may represent the key to predicting
the N release rate.
We did not observe significant relationships be-
tween soil microbial activity and N release from UF-
amended soils. It is possible that the latter occurs
mainly due to the activity of microorganisms able to
produce UF-degrading enzymes. Therefore, soil micro-
bial activity appears to be a poor indicator of the UF
behavior in soils. The effect of pH on IBDU degrada-
tion was of secondary importance because higher N
release in acidic conditions was not observed. Yet, N
release from CDU-amended soils was neither directly
affected by microbial activity nor by soil pH. There-
fore, although we recognize the importance of micro-
bial activity for N release from SRFs, the results of the
present study suggest that the size and the total acti-
vity of soil microflora had marginal effects on fertilizer
We thank the reviewers for the insightful comments
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... Microbial activity in the soil has been identified as a key component influencing nitrogen (N) release from slow-release fertilizers. The N released from S-coated urea is mainly governed by the activity of soil microbes (Nardi et al., 2018). In addition, nitrogen release can be delayed by 6-8 weeks by reducing soil microbial populations (Nardi et al., 2018). ...
... The N released from S-coated urea is mainly governed by the activity of soil microbes (Nardi et al., 2018). In addition, nitrogen release can be delayed by 6-8 weeks by reducing soil microbial populations (Nardi et al., 2018). Sulfur-coated urea is hydrolyzed by microbial ureases into ammonia and carbon dioxide (Rutherford, 2014). ...
Full-text available
Nitrogen (N) is an important macro-nutrient required for crop production and is considered an important commodity for agricultural systems. Urea is a vital source of N that is used widely across the globe to meet crop N requirements. However, N applied in the form of urea is mostly lost in soil, posing serious economic and environmental issues. Therefore, different approaches such as the application of urea coated with different substances are used worldwide to reduce N losses. Urea coating is considered an imperative approach to enhance crop production and reduce the corresponding nitrogen losses along with its impact on the environment. In addition, given the serious food security challenges in meeting the current and future demands for food, the best agricultural management strategy to enhance food production have led to methods that involve coating urea with different nutrients such as sulfur (S) and zinc (Zn). Coated urea has a slow-release mechanism and remains in the soil for a longer period to meet the demand of crop plants and increases nitrogen use efficiency, growth, yield, and grain quality. These nutrient-coated urea reduce nitrogen losses (volatilization, leaching, and N 2 O) and save the environment from degradation. Sulfur and zinc-coated urea also reduce nutrient deficiencies and have synergetic effects with other macro and micronutrients in the crop. This study discusses the dynamics of sulfur and zinc-coated urea in soil, their impact on crop production, nitrogen use efficiency (NUE), the residual and toxic effects of coated urea, and the constraints of adopting coated fertilizers. Additionally, we also shed light on agronomic and molecular approaches to enhance NUE for better crop productivity to meet food security challenges.
... This suggests that the use of a PSB bio-inoculant in combination with encapsulated nano-rock phosphate adds value, because it can provide energy support for efficient root colonization and consequently higher P solubilization. In agreement with previous studies, the addition of biobased polymer coated fertilizers can compensate for the need of C substrate for microbial energy metabolism and up-regulate the functional diversity of root associated beneficial bacteria (Nardi et al., 2018;Zhang et al., 2020). An efficient plant root system is fundamental requirement for water and nutrient uptake, and a crucial determinant of plant adaptability under changing environmental conditions. ...
... nano-fertilizer to microbial biomass build up due to the stimulatory effects of bio-based coating materials (Nardi et al., 2018;Zhang et al., 2020). Our study shows that PSB inoculation had the most favorable contribution to the labile P (e.g., resin-P) compared to uninoculated conditions. ...
Full-text available
In semi-arid regions, post-restoration vegetation recovery on abandoned agricultural lands often fails due to inherently low organic matter content and poor soil fertility conditions, including phosphorus (P). As such, amending these soils with controlled release P fertilizer, especially with suitable P solubilizing bacteria (PSB) may promote plant growth and productivity by stimulating biological P fertility. To this aim, a pot study was performed to evaluate the agronomic potential of maize and soil biological P pools, using encapsulated (ENRP) and non-encapsulated (NRP) nano-rock phosphate as the P fertilizer source, on reclaimed agricultural soil in the presence and absence of PSB inoculant. The experiment was setup following a 3 × 2 factorial arrangement with four replicates. Without PSB, NRP treatment showed marginal positive effects on plant growth, P nutrition and P use efficiency (PUE) compared to control treatment. Although larger gains with NRP treatment were more noticeable under PSB inoculation, ENRP was the most convenient slow-release P fertilizer, increasing plant growth, P nutrition and grain yield compared to all treatments. Importantly, PSB inoculation with ENRP resulted in significantly higher increase in soil CaCl2-P (8.91 mg P kg soil⁻¹), citrate-P (26.98 mg P kg soil⁻¹), enzyme-P (18.98 mg P kg soil⁻¹), resin-P (11.41 mg P kg soil⁻¹), and microbial-P (18.94 mg P kg soil⁻¹), when compared to all treatment combinations. Although a decrease in soil HCl-P content was observed with both types of P fertilizer, significant differences were found only with PSB inoculation. A significant increase in soil biological P pools could be due to the higher specific area and crystalline structure of nano materials, providing increased number of active sites for PSB activity in the presence of biobased encapsulated shell. Furthermore, the increase in PSB abundance, higher root carboxylate secretions, and decreased rhizosphere pH in response to nano-structured P fertilizer, implies greater extension of rhizosphere promoting greater P mobilization and/or solubilization, particularly under PSB inoculated conditions. We conclude that cropping potential of abandoned agricultural lands can be enhanced by the use of nano-rock phosphate in combination with PSB inoculant, establishing a favorable micro-environment for higher plant growth and biochemical P fertility
... Using slow-release or slow-release fertilizers with control in rice cultivation is considered one of the optimal solutions to reduce losses, increase fertilizer use efficiency for plants and save 20-30% of fertilizer. Fertilizers are compared to conventional fertilizers (4). Besides, the study also recorded an increase in rice yield, improved root structure, more extensive leaf area index and higher photosynthetic capacity when used (5). ...
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The study aims to assess the effect of mixed fertilizers, including controlled slow-release NPK (NPK-CRF) and urea, potassium humate fertilizers, on soil fertility and rice yield. The on-Farm Trials experiment was carried out on alluvial soil, with two models corresponding to two farming techniques: (i) Traditional fertilization, applying conventional fertilizers with the formula 92.2 N–82.8 P2O5–22.8 K2O kg/ha; (ii) New generation fertilizers (NPK-CRF, urea humate, and potassium humate) with the formula 50.1 N–39.9 P2O5–30.0 K2O. Each pattern was repeated three times, corresponding to 3 farmers. Each household's area is 1,000m2, cultivating continuously through three seasons of Winter-Spring (WS), Summer-Autumn (SA), and Autumn-Winter (AW) in Chau Thanh A district, Hau Giang province. The results showed that the new generation fertilizer application significantly improved rice yield and yield composition of the Winter-Spring cropping season (6.92 tons/ha), Summer-Autumn (5.94 tons/ha), and Autumn-Winter (6.15 tons/ha), which are different from farmers' fields. Furthermore, the combined application of NPK-CRF, urea-humate, and K-humate fertilizers for rice in SA and AW crops significantly reduced the total acid content, Al3+ exchange in the soil, and improved soil fertility of pH, N, and available P, organic matter (%C). However, there was no difference in soil's physical properties over the three farming seasons. Finally, adding humic acid to controlled-release fertilizer can improve soil fertility, and increase yield and yield components, nitrogen uptake, enhance nitrogen usage efficiency, all of which have positive yield and soil consequences.
... This steady release encourages the improved distribution of nutrients to the plants which promotes early germination and high nutritional content (Lateef et al. 2016). CRFs have been mostly considered under altered temperature and soil moisture systems (Nardi et al. 2018). ...
Full-text available
The use of fertilizer in the agricultural field is essential for plant growth but an excess amount of pure chemical contents in fertilizers becomes harmful to every living being. To reduce this chemical exposure, the use of materials coated with Controlled Release Fertilizers (CRFs) are being used. The coating of materials outside the fertilizer does not allow the chemicals to spread completely within one application of fertilizer but its spread can be extended as will be done in 2-3 applications of fertilizer. The features of the undercoating material are thus vital to attain this delayed or slow release of the nutrients present in the fertilizer. The longevity of CRFs depends upon the width of the material coating surrounding the fertilizer, temperature, and moisture. The review focuses on the consequences of conventional fertilizers, the need to control the release of fertilizers and types of coatings used, and their application in sustainable agriculture.
... A large number of studies has shown that slow release or controlled release fertilizers, coated fertilizers and fertilizer synergists can significantly improve FUE. [40][41][42][43][44]. The methods of increasing the depth of fertilization have also been proved to be effective [45,46]. ...
Full-text available
In the old course of the Yellow River area, most orchards are over-applied with nitrogen (N) fertilizers. To improve N management in this area, a 15N tracing experiment was conducted to investigate the absorption, distribution and loss of spring-topdressing urea in pear orchards from March to August 2019. The 7-year-old Sucui 1 pear was used as the test material, and 277.5 g of ordinary urea and 15 g of 15N-urea N were evenly applied to each plant. The N absorption, distribution and utilization efficiency of different organs from the flowering stage to the post-harvest stage were analyzed, and the residual and loss of N in the soil were also discussed. The N fertilizer utilization rate increased with the advancement of the phenological period. The N fertilizer utilization rate in the full bloom period is 10.39%, which is the fastest growing period, and reached a maximum of 23.62% in the post-harvest stage. In the young fruit stage, the amount of N derived from labeled fertilizer (%Ndff) of the fruit was only 1.02%, and most of the new vegetative organs were above 1%. Residual amount in the 20–40 cm soil layer was significantly higher than that in other soil layers. Direction of N fertilizer is N fertilizer loss>soil residue>tree absorption. N loss in the fruit expansion stage and the harvest stage is higher, which are 3.76 g and 3.74 g, respectively. N utilization rate in this area is low throughout the year. There is nutrient competition between reproductive growth and vegetative growth, which can be effectively alleviated by spring top-dressing. The N loss during fruit expansion and harvesting is serious. Attention should be paid to split fertilizer application and the timely supplementation of an appropriate amount of N fertilizer to improve N use efficiency.
... Some studies have shown that CRFs decreased N losses by reducing nitrous oxide emissions, ammonia volatilization, and runoff (Anitha and Bindu, 2016;Mohanraj et al., 2019). Moreover, there are some NFs that can increase the water absorbance, soil microflora, microbial activity and can maintain the soil p H which plays a crucial role in crop production (Table 2) (Nardi et al., 2018;Chen et al., 2020). Cairo et al. (2017) has observed that zeolite NFs had a potential role in improving physical characteristics in terms of hydraulic conductivity, infiltration, ventilation, etc. ...
The extreme use of commercial fertilizers (CF) for maximum agricultural crop production leads to severe environmental pollution, reduces land fertility as well as directly or indirectly affects human health. Besides, it lowers down the nutrient use efficiency (NUE) by decreasing the rate of crop production with the increased rate of fertilization which expedites the cost of production. In this review, we have explored the recent advancements of slow and/or controlled released fertilizers and their impacts on crop production in terms of the growth, development, and yield of agricultural crop plants. However, we have found that slow-release fertilizers (SRFs) cost much because their pattern of nutrients release is not in a controlled manner that leads to nutrient loss into the ecosystems. Contrarily, the controlled released fertilizers (CRFs) releases their nutrients with a controlled pattern, however, the size of both SRFs and CRFs are not always specific for the particular crop plants, therefore, there would have a great chance to be aggregated of SFs and/or CRFs into the plant rhizosphere due to lower uptake by the plants causing the plant toxicity and polluting the groundwater and environments. Thus, from this review, an innovative nano-fertilizers that could be named as controlled released nano-fertilizers (CRNFs) has been proposed to be developed that will regulate the nutrient release with the plant’s demand with time, increase the NUE and reduce the environmental pollution suggesting the sustainable crop production. Moreover, we believe that the industrialization of CRNFs could replace CF which will greatly contribute to the cleaner agricultural production process.
Both inorganic and organic fertilizers are widely used to increase rice yield. However, these fertilizers are also found to aggravate mercury methylation and methylmercury (MeHg) accumulation in paddy fields. The aim of this study was to reveal the mechanisms of inorganic and organic fertilizers on MeHg accumulation in rice grains, which are not yet well understood. Potting cultures were conducted in which different fertilizers were applied to a paddy soil. The results showed that both inorganic and organic fertilizers increased MeHg concentrations rather than biological accumulation factors (BAFs) of MeHg in mature rice grains. Inorganic fertilizers, especially nitrogen fertilizer, enhanced the bioavailability of mercury and the relative amount Hg-methylating microbes and therefore intensified mercury methylation in paddy soil and MeHg accumulation in rice grains. Unlike inorganic fertilizers, organic matter (OM) in organic fertilizers was the main reason for the increase of MeHg concentrations in rice grains, and it also could immobilize Hg in soil when it was deeply degraded. The enhancement of MeHg concentrations in rice grains induced by inorganic fertilizers (5.18–41.69%) was significantly (p < 0.05) lower than that induced by organic fertilizers (80.49–106.86%). Inorganic fertilizers led to a larger increase (50.39–99.28%) in thousand-kernel weight than MeHg concentrations (5.18–41.69%), resulting in a dilution of MeHg concentrations in mature rice grains. Given the improvement of soil properties by organic fertilizer, increasing the proportion of inorganic fertilizer application may be a better option to alleviate MeHg accumulation in rice grains and guarantee the rice yield in the agricultural production.
Slow-release nitrogen fertilizers have received little attention as alternatives to conventional fast-release fertilizers in N-limited boreal forests. Slow-release fertilizers better match the rate at which trees can uptake nutrients from the soil solution and are less likely to be vulnerable to losses to the environment. Ureaformaldehyde is a polymer that is synthesized from formaldehyde and urea and mainly used as a nitrogen fertilizer in lawns and greenhouse cultivations. Our goal was to assess ureaformaldehyde (UF) as a forest fertilizer. We aimed to determine its long-term effects on tree ring widths and stand basal area growth and how the effects differ from those of a conventional fast-release N fertilizer, ammonium nitrate (AN). We also attempted to determine whether the fertilizer additions changed soil organic matter properties in the long term. The study site was an unfertile Scots pine (Pinus sylvestris L.) stand with study plots with both fertilizer treatments that were performed 47 years ago and unfertilized control plots. As expected, AN addition resulted in a sharp increase in tree ring widths and the annual basal area increment that lasted for approximately seven years. UF addition caused a slower increase that lasted much longer, approximately 17 years. In addition, the UF addition showed indications that it slightly enhanced the growth even during the last decade. Our results also suggested that different fertilization treatments resulted in differences in tree size distribution. After almost a half century, some signs of UF-induced effects were still visible in soil organic matter; for instance, the humus layer in the UF treatment had a lower C-to-N ratio by 7 units than that in the humus layer of the AN treatment. In conclusion, slow-release ureaformaldehyde has a considerably longer effect on tree growth than conventional ammonium-nitrate fertilizer and, after almost a half century, there were signs that it still increased nitrogen availability for trees.
At present, excessive fertilization in vegetable production not only leads to the decline of vegetable quality and yield, but also causes environmental pollution. In this study, a liquid urea-formaldehyde slow release fertilizer (LUFF) was synthesized by using formaldehyde, urea and diammonium hydrogen phosphate. The effects of LUFF on yield, quality, root growth, antioxidant enzyme activity and nutrient absorption of spinach were studied in a pot experiment. Five treatments with four replicates each were established as follows: (1) Control, no fertilizer; (2) CWSF1, conventional water-soluble fertilizer; (3) CWSF2, conventional water-soluble fertilizer reduced by 25%; (4) LUFF1, liquid urea-formaldehyde slow release fertilizer; (5) LUFF2, liquid urea-formaldehyde slow release fertilizer reduced by 25%. Normal N-P2O5-K2O application rate of 2.83-2.73-3.94 g/pot and 25% reduction N-P2O5-K2O of rate (2.12-2.05-2.96 g/pot) were set up. The fertilizer was applied five times-split for CWSF while the equivalent rates of LUFF used as twice-split fertigation. The results showed that the yield of LUFF treatments was significantly increased by 31.81%-77.31% compared with CWSF. Root indexes, such as root length, root projection area and root surface area, were significantly improved by LUFF treatments compared with CWSF. The vitamin C (Vc) concentration and superoxide dismutase (SOD) activity of spinach in LUFF were significantly increased by 7.34%-30.07% and 23.83%-42.52% compared with CWSF. It can be observed from the above results, the LUFF could increase the yield of spinach, improve the quality of spinach, while reducing the frequency of fertigation. Therefore, LUFF may have high application value in agriculture.
Full-text available
This bulletin reviews the status of research and development on slow-release and controlled-release nitrogen fertilizers. The discussion encompasses manufacturing processes, chemistry and mechanism, commercially available materials and agronomic responses. Different groups of these fertilizers have been broadly divided as (i) condensation polymers (including urea formaldehydes, IBDU, CDU), (ii) coated fertilizers (sulphur/sulphur-polymer, polymer, neem, degradable polymer, latex, clay and gypsum coated), (iv) gel based materials, (v) supergranules and compacted materials, (vi) zeolite based materials and (vii) stabilised nitrogen products (nitrifi cation inhibitors, urease inhibitors and natural inhibitors). A list of major manufacturers and information on their products is also provided.
Slow-release nitrogen (N) fertilizers offer many potential benefits for vegetable production. In sandy soils, their use may lessen N leaching. If the slowrelease fertilizer has a release pattern that matches crop needs, N uptake by the growing crop may become more efficient. Additionally, if slow-release fertilizers can be applied as a preplant application, production costs could be lessened, eliminating the need for multiple applications of soluble N fertilizer. Synthetic slow-release fertilizers can be separated into two general groups: those that are slow release as a byproduct of a chemical reaction (such as urea-formaldehyde), and those that are slow release via a sulfur, wax, or resin coating around the fertilizer prill. In vegetable crop research, much of the available literature has focused on use of sulfur coat urea and urea-formaldehyde, as they have been in the fertilizer market for 40 years. Newer research has evaluated resin-coated products. In most studies, use of slowreleaseNfertilizers a a preplant treatment did not decrease crop yield, but yield was rarely increased when compared with standard split applications of soluble N. Based on available research, the benefits of using slow-release N fertilizers in vegetable crop production will come from reduced environmental risk and savings in production costs.
Loss of N from fertilized agricultural soils is a serious problem that can negatively affect environmental quality. Nitrogen loss can be moderated by using slow-release fertilizer (SRF) products (e.g., those created through condensation of urea and formaldehyde) in place of 100% water-soluble N. Release of N from SRFs is affected by the soil environment. To evaluate soil effects on N release, we conducted an incubation study in which temperature and soil type were varied. Four SRFs were studied: liquid Nitamin 30L (L30), liquid Nitamin RUAG 521G30 (G30), granular Nitamin 42G (N42) (all from Georgia Pacific Chemicals, Decatur, GA), and granular Nitroform (NF) (Agrium Advanced Technologies, Loveland, CO). The fertilizers were incubated for 78 d at 20, 25, and 30 degrees C in a sandy soil and at 25 degrees C in a loamy soil. Differential N release kinetics of the N sources were determined by measuring NH(4)-N and NO(3)-N concentrations throughout the incubation. Net N released as a percentage of total N in the fertilizer was significantly affected by N source, temperature, time, and soil type. Increasing temperature increased net N release. The N release rate decreased in the order N42 > G30 > L30 > NF in the sandy soil and G30 > N42 > L30 > NF in the loamy soil. Overall, the release rates of these fertilizers were greater in the loamy soil. The N release characteristics determined in this study can help in the selection of the appropriate SRF source for crops grown under different soil and climatic conditions to improve N use efficiency and minimize N loss to the environment.
Nitrogen release from organic N sources is controlled by the soil environment. Soil incubation was conducted to evaluate the effects of soil moisture (50, 70, and 90% of water holding capacity) and temperature (15/10, 20/15, and 25/20 degrees C [14/10 h]) on N release from four organic N sources. Differential N release kinetics of the N sources were determined by measuring ammonium- and nitrate-N contents periodically over 12 wk. Net N released, as a percentage of organic N, was greatest in the order: urea (91-96%) > blood meal (BM) (56-61%) > alfalfa pellets (AP) (41-52%) > partially composted chicken manure (CM) (37-45%). Increasing soil moisture increased net N released from AP and CM by 12 and 21%, respectively, but did not significantly affect net N released from urea and BM. Increasing temperature increased net N released from AP, BM, and CM by 25, 10, and 13%, respectively, but did not significantly affect net N released from urea. The results indicate that soil moisture and temperature influence N availability from organic N materials differently depending on source of N. In greenhouse production systems, where irrigation and temperature can be controlled, fertilizer management that considers both source of N and soil environment may improve the effectiveness of organic N materials.
An enzyme hydrolyzing the condensation products of urea and formaldehyde (ureaform) was purified and characterized from a bacterium isolated from soil and described as Ochrobactrum anthropi UF4. The enzyme designated as methylenediurea amidinohydrolase (methylenediurea deiminase) hydrolyzed ureaform condensation products of different length (methylenediurea, dimethylenetriurea, trimethylenetetraurea) to ammonium, formaldehyde, and urea at molar ratios of 2:1:1 (methylenediurea), 4:2:1 (dimethylenetriurea), and 6:3:1 (trimethylenetetraurea). Two other substrates, ureidoglycolate and allantoate, were also hydrolyzed, yielding glyoxylate and urea (ureidoglycolate) and glyoxylate, urea, and ammonium (allantoate), respectively. The molecular mass of the enzyme was determined by size exclusion chromatography to be 140 ± 25 kDa; the enzyme was composed of identical subunits of 38 ± 5 kDa, indicating that the native enzyme has a tetrameric structure. Growth of the bacterium in the presence of ureaform specifically induced the methylenediurea deiminase and no complete repression of enzyme synthesis by ammonium was observed.Key words: ureaformaldehyde, methylenediurea deiminase, fertilizer, Ochrobactrum anthropi.
Application of soluble forms of nitrogen (N) fertilizers to sandy soils may cause leaching of nitrate N (NO3‐N) resulting in contamination of groundwater. The leaching loss of N may be reduced to a certain extent by the use of controlled‐release N formulations. A leaching column study was conducted to evaluate the leaching of urea, ammonium N (NH4‐N), and NO3‐N forms from selected urea‐based controlled‐release formulations (Meister, Osmocote, and Poly‐S) and uncoated urea under eight cycles of intermittent leaching and dry conditions. Following leaching of 1,760 mL of water (equivalent to 40 cm rainfall) through the soil columns, the recovery of total N (sum of all forms) in the leachate accounted for 28, 12, 6, or 5% of the total N applied as urea, Poly‐S, Meister, and Osmocote, respectively. Loss of urea‐N from all fertilizer sources was pronounced during the initial leaching events (with the exception of Meister). Cumulative leaching of urea‐N was 10% for uncoated urea while
Ammonia (NH3) volatilization is the major pathway for mineral nitrogen (N) loss from N sources applied to soils. The information on NH3 volatilization from slow-release N fertilizers is limited. Ammonia volatilization, over a 78-d period, from four slow-release N fertilizers with different proportions of urea and urea polymer [Nitamin 30L (liquid) (L30), Nitamin RUAG 521G30 (liquid) (G30), Nitamin 42G (granular) (N42), and Nitroform (granular) (NF)] applied to a sandy loamy soil was evaluated. An increase in temperature from 20 to 30 °C increased cumulative NH3 volatilization loss in the sandy soil by 1.4-, 1.7-, and 1.8-fold for N42, L30, and G30, respectively. Increasing the proportion of urea in the slow-release fertilizer increased NH3 volatilization loss. At 30 °C, the cumulative NH3 volatilization over 78 d from a sandy soil accounted for 45.6%, 43.9%, 22.4%, and <1% of total N applied as N42, L30, G30, and NF, respectively. The corresponding losses in a loamy soil were 9.2%, 3.1%, and 1.7%. There was a significantly positive correlation between NH3 volatilization rate and concentration of NH4-N released from all fertilizers, except for NF (n = 132; r = 0.359, P = 0.017 for N42; r = 0.410, P = 0.006 for L30; and r = 0.377, P < 0.012 for G30). Lower cumulative NH3 volatilization from a loamy soil as compared to that from a sandy soil appeared to be related to rapid nitrification of NH4-N in the former soil than that in the latter soil. These results indicate the composition of slow-release fertilizer, soil temperature, and soil type are main factors to dominate NH3 volatilization from slow- release fertilizers.