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RESEARCH PAPER
https://doi.org/10.1071/SR20327
Influence of the physical properties of pumice and biochar
amendments on the soil’s mobile and immobile water:
implications for use in saline environments
Chao KongA,* , Marta Camps-ArbestainA and Brent ClothierB
For full list of author affiliations and
declarations see end of paper
*Correspondence to:
Chao Kong
School of Agriculture and Environment,
Private Bag 11222, Massey University,
Palmerston North 4442, New Zealand
Email: 1140586458@qq.com
Handling Editor:
Etelvino Novotny
Received: 21 November 2020
Accepted: 30 September 2021
Published: 17 November 2021
Cite this:
Kong C et al. (2022)
Soil Research, 60(3), 234–241.
doi:10.1071/SR20327
© 2022 The Author(s) (or their
employer(s)). Published by
CSIRO Publishing.
This is an open access article distributed
under the Creative Commons Attribution-
NonCommercial-NoDerivatives 4.0
International License (CC BY-NC-ND).
OPEN ACCESS
ABSTRACT
Context. Biochar and pumice have potential to improve soil water retention and mitigate salinity.
However, little is known about their effect on salt transport in sandy soils. Aims. We investigated
the influence of the porosity and pore size distribution of soil amendments with pumice and biochar
on the mobile water content of a New Zealand sandy soil. Methods. Pumice and biochar (1.5-cm,
3-cm and 6-cm in diameter, Ø) were characterised using scanning electron microscope technology.
The fraction of mobile water present in these amendments, previously added to a sandy soil at
different application rates and particle sizes, was determined using a tracer (Na+) technique.
Key results. (1) Pumice exhibited a wider pore-size span than biochar; and (2) both materials
had a predominance of pores with Ø < 30 μm; but (3) the total porosity in pumice and biochar
was not significantly different; (4) pumice had a significantly larger (P < 0.05) mean absolute
micro-scale porosity than biochar; and (5) a significantly greater (P < 0.05) relative resident Na+
concentration than biochar, irrespective of the particle size. Conclusions. These results reflect
a larger fraction of the mobile water in pumice than that of biochar under near-saturated
conditions, irrespective of the biochar particle size; and this increased as the pumice particle
size increased. Implications. While both materials are expected to contribute to water
retention and thus might alleviate salt-stress by diluting salt concentration, pumice may perform
better than this specific biochar on improving the retention of plant-available water.
Keywords: dilution, miscible displacement, mobile-water fraction, particle size, physical properties,
porosity, salinity, scanning electron microscopy.
Introduction
The sustainability of agriculture in arid regions is challenged by the limited availability of
water, and the need to manage salinity in soils and irrigation-water. This demands long-
term interventions, both economically and environmentally (Alon et al. 2006). Despite
the magnitude of these challenges, there are opportunities to overcome them by further
exploring innovative techniques that alleviate salt and plant-water stress (Abou-Baker
and El-Dardiry 2016). One such potential option is the use of amendments, such as
adding either pumice or biochar to soils. Potentially, these can contribute to improving
the physical, chemical and biological properties of salt-affected soils, while promoting
better plant growth (Saifullah et al. 2018). However, a deeper knowledge of the influence
of the physical properties of the amendments is needed, as well as that of the impact of their
particle size and application rate on soil water retention so that their value in ameliorating
soil salinity is better understood.
Both materials have in common the fact that they are very porous. For biochar, this is
particularly the case when produced from woody material (Waldron 2014). The porosity of
pumice (64–85% by volume) is generated by air bubbles created during its formation,
which give this material a low bulk density (0.35–0.65 g cm−3), and large pore-size span
(from micrometre to millimetre) (Ersoy et al. 2010; Cekova et al. 2013). The physical
www.publish.csiro.au/sr Soil Research
properties of biochar mainly depend on the type of feedstock,
which is influenced by the plant cellular structure, plus the
type of pyrolyser, highest heating temperature of pyrolysis,
residence time (Rasa et al. 2018) and activating agents.
Biochar has been reported to have a bulk density ranging
from 0.06 to 0.7 g cm−3, with a specific surface area from
50 to 630 m2 g−1 (Rajkovich et al. 2012), while its pore size
distribution can vary greatly, ranging from sub-nanometre
to hundreds of microns (Brewer et al. 2014). The physical
properties of these amendments can directly or indirectly
influence soil properties and plant growth.
Although the ability of pumice (Malekian et al. 2012) and
biochar (Herath et al. 2013) to retain soil water has been
reported, the mechanistic understanding of how their use
affects water-borne salt transport in soils under arid condi-
tions remains largely unclear (Noland et al. 1992; Lura
et al. 2004; Batista et al. 2018). Given that this is strongly
related to the influence of these materials on the soils mobile–
immobile water fractions, we aimed to evaluate how the
physical properties of a pumice and a biochar (i.e. particle
size and application rate) affected their mobile–immobile
water when added to a sandy soil. Our objectives were
two-fold:
• We first characterised the porosity and pore-size
distribution of a pumice from New Zealand’s central
North Island, and a biochar produced from willow wood
chips at a highest heating temperature of 350°C. We
used a low-temperature biochar to minimise the amount
of ash, given that salts in the ash can contribute to soil
salinity when applied to soils in arid environments.
• We then investigated the amount of mobile water present
in these amendments (previously added to a sandy soil at
different rates and particle sizes) using a tracer (Na+)
during miscible displacement experiments.
Materials and methods
Preparation of the experimental material
Pumice was taken from the Tongariro National Park New
Zealand (39°12 036.5″S, 175°40 055.5″E), and was washed
with deionised water, then dried at 30°C for 72 h to a
constant weight prior to its use. The biochar was produced
from weeping-willow chips (Salix matsudana L.) using a
rotary kiln pyrolyser (25-L retort), at heating rate ca.
10°C min−1 and a highest heating temperature of 350°C,
which was held for 15 min. Both pumice and biochar were
categorised into three different particle sizes (1.5-, 3- and
6-cm in diameter, Ø). The biochar was crushed before the
particle size screening. Both materials were further milled
to achieve a particle size < 0.3 mm for chemical analysis.
The sandy soil (96.6% sand) for the miscible displacement
experiment was obtained from the sand dunes at Himatangi
Beach New Zealand (40°23 054.6″S, 175°13 033.8″E) and
was air dried before use. The mineralogy of the sandy soil
was predominantly quartz and feldspar (Claridge 1961). An
artificial saline solution was prepared using Na
2
SO
4
, CaCl
2
,
NaCl and MgSO
4
salts at the following concentrations
0.285, 0.517, 2.865 and 0.924 g L−1, respectively. The final
solution had an electrical conductivity (EC) of 6.4 dS m−1
and a Na+ concentration of 2300 mg L−1.
Scanning electron microscopy (SEM) analysis
For measurements of porosity (pores > 300 nm Ø) and pore size
distribution of pumice and biochar, surface and cross-section
samples were mounted on 0.5″ (1.27 cm) aluminium
specimen stubs equipped with SEM carbon foils (Agar
scientific, UK). The gold coating was applied to samples after
air drying at 40°C for 24 h. Micrographs were taken on a FEI
Quanta 200 Environmental Scanning Electron Microscope
(Quanta, Oregon, USA) at a magnification ranging from 60×
to 260×, with an acceleration voltage of 20 kV. The SEM
images were manually corrected with the use of Photoshop
CS5 (Adobe) to remove obvious debris and darken pores,
which contained either foreign material or pore sidewalls.
Pore count, porosity and pore size were determined with the
software ImageJ (ver. 1.49s, National Institutes of Health,
http://imagej.nih.gov/ij/) using the gij_Pore Analysis plugin
(Impoco et al. 2006). An arithmetic average of the data from
six different SEM images was taken for each parameter. Pore
sizes were functionally divided into four categories: (1) ultra-
micropores (Ø < 3 μm); (2) micropores (Ø of 3–30 μm);
(3) mesopores (Ø of 30–100 μm); and (4) macropores
(Ø > 100 μm) (Landis et al. 1990; Drzal et al.1999). The
macropores enable soil drainage and aeration, the mesopores
contribute to soil-water conductivity, the micropores provide
soil-water retention, with the water retained in ultra-
micropores being unavailable for plant use (Landis et al. 1990;
Drzal et al. 1999).
Miscible displacement analysis
Measurements of the pore volume of materials
The pore volume (PV) of the materials (Table 1)were
estimated by immersing a known volume of the amendments
(v
i
) in water for 72 h and looking at the corresponding
increases in weight (m
i
). In this experiment, since the volume
of the amendment only accounts for a small part of the total
soil volume, we used the PV of sandy soil to represent the PV
of the soil after amendment addition hereafter.
Measurement of the mobile water fraction in
pumice and biochar
The mobile water fraction (θ
m
) of pumice and biochar
during near-saturated flow was determined following the
tracer technique proposed by Clothier et al. (1992) with
some modifications as detailed below, and in the
235
C. Kong et al. Soil Research
Table 1. Pore volume and Na+ concentration of pumice, biochar, and
sandy soil.
Pore volume (v/v) Na+ (mg cm−3)
Pumice
Pu-1.5 0.16 ± 0.02b 0.01 ± 0.01a
Pu-3 0.19 ± 0.02a 0.01 ± 0.01a
Pu-6 0.22 ± 0.01a 0.02 ± 0.01a
Biochar
Bi-1.5 0.35 ± 0.06a 0.03 ± 0.01a
Bi-3 0.29 ± 0.04a 0.04 ± 0.01a
Bi-6 0.31 ± 0.03a 0.04 ± 0.01a
Sandy soil
S 0.31 ± 0.03 0.50 ± 0.02
Note: values are mean ± s.d. of three replicates. Mean values with different
letters indicate significant differences within the same material (Duncan’s test,
P < 0.05).
Pu-1.5, pumice with 1.5-cm Ø; Pu-3, pumice with 3-cm Ø; Pu-6, pumice with
6-cm Ø; Bi-1.5, biochar with 1.5-cm Ø; Bi-3, biochar with 3-cm Ø; Bi-6,
biochar with 6-cm Ø; S, sandy soil.
Supplementary information. Briefly, pumice and biochar of
three particle sizes (1.5-, 3- and 6-cm Ø) were separately
added to the sandy soil at three application rates (3, 6 and
12%, v/v basis). Thereafter, 1 L of mixed sandy soil and
amendment (in triplicate) was added to a 2.3-L plastic
container (16.5 × 15.5 × 9cm
3) with free-water drainage at
its bottom. Initial charging of the immobile fraction θ
im
was
achieved by first wetting the soil with two PV of deionised
water until near-saturated conditions prevailed. The system
was then rapidly re-wet with an artificial saline solution
containing a tracer (Na+) at the C
m
concentration of
2300 mg L−1. A total volume of eight PV of saline solution
was supplied to the soil. Subsequently the bottom 5-cm of
soil was sampled. Pumice and biochar were completely
separated from the sandy soil and then dried at 30°Cfor
72 h to a constant weight. Prior to chemical characterisation,
pumice and biochar particles, at the beginning and at the end
of the experiment, were ground to a size < 0.25 mm. After
homogenisation, deionised water at a 1:5 w/v solid:water
ratio was added to the ground material. The suspension
was then shaken on an end-to-end shaker for 2 h and stood
overnight. Thereafter, it was filtered through a Whatman
no. 42 filter paper. The concentration of Na+ in the water
soluble-extract (hereafter referred to as C*) was measured
using a Microwave Plasma Atomic Emission Spectrometer
(MP-AES). The ratio of the measured resident solute concen-
tration C* to applied solution concentration C
m
, C*/C
m
, was
the fraction of material’s water that was effectively mobile.
The concentration of Na+ of pumice, biochar and the sandy
soil at the beginning of the experiment are reported in
Table 1, where it is shown that concentrations, on volume
basis, were > 10 times smaller in the amendments than in
the sandy soil. With this methodology, field irrigation was
simulated using a burette as the wetting system through
which the amount of water added to the soil and the rate of
soil wetting could be accurately controlled.
Data processing and statistical analysis
Data processing was performed with Microsoft Excel 2019.
Statistical analyses were carried out using the SPSS ver.
14.0 software package (IBM, Armonk, New York, USA) and
GraphPad Prism 8 software. A one-way ANOVA with
Duncan’s test was used to detect significant differences
(at P < 0.05) between the treatment means for parametric
data (non-SEM data). The Kruskal–Wallis and Nemenyi tests
were used to detect significant differences (at P < 0.05)
between the treatment means for SEM data.
Results
Pore characteristics of pumice and biochar
Pumice exhibited a pore-size span ranging from 0.5 to
13 000 μm(Table 2). The maximum pore sizes observed
under SEM followed the order of Pu-1.5 (5 mm) < Pu-3
(10 mm) < Pu-6 (13 mm). The pore-size span of biochar
was smaller than that of the pumice and ranged from 0.3 to
651 μm(Table 2). The maximum pore size seen under SEM
followed the order of Bi-1.5 (0.4 mm) < Bi-3 (0.5 mm) <
Bi-6 (0.7 mm). There were no evident differences in the
minimum pore sizes of either pumice, or biochar, between
the three particle sizes. Significant differences (P < 0.05) in
average pore sizes of the pumice were only found between
Table 2. Pore size span and average pore size of pumice and biochar
under three different particle sizes (1.5-, 3- and 6-cm in diameter) based
on the scanning electron microscope inspections.
Pore size span (μm) Average pore size (μm) n
Pumice
Pu-1.5 0.5–4992 497.1 ± 144.2b 3140
Pu-3 1.7–9625 1004.1 ± 557.5ab 3804
Pu-6 0.8–13 308 1843.9 ± 837.4a 3340
Biochar
Bi-1.5 0.3–368 20.9 ± 13.9a 1732
Bi-3 0.3–453 18.5 ± 11.7a 2069
Bi-6 0.5–651 21.2 ± 12.6a 1950
Note: n is the number of pores. Data were analysed by Kruskal–Wallis
and Nemenyi tests using the SPSS ver. 14.0 software package (IBM, Armonk,
New York, USA) and expressed as mean ± s.d. Mean values with different
letters indicate significant differences within the same material (P < 0.05).
Pu-1.5, pumice with 1.5-cm Ø; Pu-3, pumice with 3-cm Ø; Pu-6, pumice with
6-cm Ø; Bi-1.5, biochar with 1.5-cm Ø; Bi-3, biochar with 3-cm Ø; Bi-6,
biochar with 6-cm Ø.
236
(a) (b)
Pu-1.5 Pu-3 Pu-6
(c) (d)
50 70
a
Ultramicro
Micro
Meso
Macro
Bi-1.5 Bi-3 Bi-6
60 80 Ultramicro
Absolute porosity in different size
(%)
Total porosity (%)
Absolute porosity in different size
(%)
a Micro
50 a 75 Meso
ab40 ba
65
a ab
Total porosity (%)
Macro
Total porosity %
40 a 70
30 65
ab b a
20 60
b b
30 a
bc c 60
20
55
10 b aa
cb
10 55
ac bb aa
0 50
c
0 50
Pu-1.5 Pu-3 Pu-6 Bi-1.5 Bi-3 Bi-6
www.publish.csiro.au/sr Soil Research
Pu-1.5 (492 μm) and Pu-6 (1844 μm) (Table 2). For biochar,
no significant differences in the average pore sizes were
detected between the three particle sizes. The average
pore size value under each particle size was always higher
in pumice (range 492–1844 μm) than in biochar (range
19–22 μm) (Table 2).
In the pumice, the mean relative proportion of pores in
each pore size group out of the total pore volume followed
the order ultra-micropore > micropore > mesopore >
macropore, irrespective of the pumice particle size. All
differences were significant at P < 0.05. In this material,
the proportion of ultra-micropores plus micropores (87%)
were more than six-fold that of the mesopores plus
macropores (13%), reflecting that pumice mainly consists
of pores <30 μm(Fig. 1a). Similar patterns of pore-size
distribution were observed for biochar but the mean
relative proportion of ultra-micropores plus micropores
(95%) was significantly greater (P < 0.05) than those of
pumice (87%). The relative proportion of micropores,
which are those responsible for the retention of plant-
available water, was significantly smaller (P < 0.05) in
biochar (31%) than in pumice (41%), irrespective of their
particle size. The relative proportion of mesopores plus
macropores in pumice increased from Pu-1.5 (8%) to Pu-3
(12%) to Pu-6 (19%). All differences were significant at
P < 0.05. There was no particle size effect in the pore size
distribution of biochar (Fig. 1a, b).
Values of total SEM porosity in pumice were found to be
significantly smaller (P < 0.05) in Pu-6 (58.5%) than in
Pu-1.5 (65.6%). Differences in the absolute porosity, namely
the volume of pores out of total volume of amendment, in the
different pore size groups considered of the three pumice
particle sizes were significantly different (P < 0.05). The
highest values for both ultramicro- and micro-scale porosity
were found in Pu-1.5 (32.7 and 27.2%), followed by Pu-3
(28.3 and 24.9%), and then Pu-6 (25.0 and 22.7%). An
opposite pattern was observed in both meso- and macro-
scale porosity with the lowest being under Pu-1.5 (3.5 and
2.1%), followed by Pu-3 (5.1 and 2.6%), and the highest in
Pu-6 (6.4 and 4.5%) (Fig. 1c).
For biochar, the absolute porosity in different pore size
groups followed the order ultramicro- (44.2%) > micropore
(20.7%) > mesopore (1.8%), and > macropore-scale
porosity (1.1%), irrespective of the biochar particle size.
Absolute porosity in the different pore size groups of
biochar varied narrowly between the three particle sizes,
with the most relevant difference found in the 3-cm biochar
particle size, which had a significantly greater (P < 0.05)
ultramicro-scale porosity than the 6-cm biochar. The
opposite pattern was observed for micropore, mesopore and
macropore-scale porosity (Fig. 1d).
Fig. 1. The mean relative proportion of pores in each pore size group out of total pore volume in (a) pumice and (b) biochar (n = 6). Mean
values of pumice pore parameters (absolute porosity for each pore size group out of total volume of the amendment and total porosity) in
(c) pumice and (d) biochar (n = 6). Ultramicropores represent vesicles with Ø smaller than 3 μm; micropores represent vesicles with Ø of
3–30 μm; mesopores represent vesicles with Ø of 30–100 μm; macropores represent vesicles with Ø larger than 100 μm. Pu-1.5, pumice
with 1.5-cm Ø; Pu-3, pumice with 3-cm Ø; Pu-6, pumice with 6-cm Ø; Bi-1.5, biochar with 1.5-cm Ø; Bi-3, biochar with 3-cm Ø; Bi-6,
biochar with 6-cm Ø. Data were analysed by Kruskal–Wallis and Nemenyi tests using the SPSS version 14.0 software package (IBM,
Armonk, New York, USA). Different letters in the same colour block indicate significant differences between the treatments (P < 0.05).
237
(a) (b)
Relative Na
+
concentration, C*/C
m
0.3 ***
n.s. ***
0.2
0.1
0.0
Pu-1.5 Pu-3 Pu-6
Relative Na
+
concentration, C*/C
m
0.3 n.s.
n.s. n.s.
0.2
0.1
0.0
3% Pu 6% Pu 12% Pu
(c) (d)
Relative Na
+
concentration, C*/C
m
0.3 *
n.s. n.s.
0.2
0.1
0.0
Bi-1.5 Bi-3 Bi-6
Relative Na
+
concentration, C*/C
m
0.3 n.s.
n.s. n.s.
0.2
0.1
0.0
3% Bi 6% Bi 12% Bi
C. Kong et al. Soil Research
Mobile water fraction (θ
m
) in pumice and biochar particle size, the mobile water fraction showed a small
decrease with an increasing particle size (from 0.085 to
0.080) and was only significant (P < 0.05) between Bi-1.5
and Bi-6 treatments. When averaging the relative Na+
concentration from the treatments grouped by biochar
The θ
m
of the pumice- and biochar-amended sandy soil, when
subsequently leached, was estimated based on the Na+
concentration at the end of the experiment, relative to the
amount added (C*/C
m
)(Fig. 2). It should be noted that the
amount of water-soluble Na+ in the amendments before
the experiment was <0.05 mg cm−3, and no significant
differences between particle sizes were observed in either
the pumice or the biochar (Table 1). When averaging C*/C
m
from the treatments grouped by pumice particle size, there
was an increase in the mobile water fraction (from 0.14 to
0.24) with increasing particle size of pumice. The differences
between treatments were significant at P < 0.05, except
between Pu-1.5 and Pu-3. When averaging the relative Na+
concentration from the treatments grouped by the pumice
application rate, no significant differences were observed
between the three application rates.
With biochar, when averaging the relative Na+ concen-
tration from the different treatments grouped by biochar
application rate, no significant differences were detected
between the three application rates (Fig. 2).
Discussion
Both pumice and biochar contributed to water
retention and dilution of salinity when applied to
a sandy soil
Porosity in terms of pore size and pore distribution greatly
influence soil properties, particularly aeration, drainage and
water retention (Klug and Cashman 1996). The wide pore
size range observed in the pumice under study is consistent
Fig. 2. The relative Na+ concentration of pumice and biochar at the end of the experiment. Bar
charts showing: (a) when averaging the relative Na+ from the treatments grouped by pumice
particle size (1.5-, 3-, and 6-cm), (b) when averaging the relative Na+ from the treatments
grouped by pumice application rates (3%, 6%, and 12%, v/v, basis), (c) when averaging the
relative Na+ from the treatments grouped by biochar particle size (1.5-, 3-, and 6-cm), (d) when
averaging the relative Na+ from the treatments grouped by biochar application rates (3%, 6%,
and 12%, v/v, basis). Data were analysed by one-way ANOVA using Graph pad prism 8
software and expressed as mean ± s.d. * P < 0.05, ** P < 0.01, *** P < 0.001. Differences were
considered significant if P < 0.05.
238
www.publish.csiro.au/sr Soil Research
Fig. 3. Representative SEM images of the studied pumice and biochar. Images showing (a) pumice with 1.5-cm Ø, (b) pumice with 3-cm Ø,
(c) pumice with 6-cm Ø, (d) biochar with 1.5-cm Ø, (e) biochar with 3-cm Ø, (f ) biochar with 6-cm Ø.
with the observations of Ersoy et al. (2010) working with
pumice (of particle size ranging between 0.5- and 4-cm Ø)
from the Tatvan region of Turkey. This large variety of pore
sizes is been attributed to the rapid release of pressure
during volcanic eruptions, which leads to gas expansion
and the formation of multiple bubbles (Whitham and
Sparks 1986). The elongation of micropores occurs due to
the ductile elongation in the volcanic conduit (Fig. 3a–c)or
in the case of pumiceous lavas, during flow (Papadopoulos
et al. 2008). Particle size of the pumice had an influence in
the size of those pores, where the average pore size of Pu-6
(1.8 mm) was significantly higher than that of Pu-1.5
(0.5 mm). This is likely attributed to the differences in total
volatile content of the magmas between the different
particle size pumice. More volatiles and faster ascent results
in more and larger vesicles (Mitchell et al. 2019). Our
pumice samples were taken from the Tongariro National
Park and originated from the 1994–1995 Mount Ruapehu
eruption. This had a predominance of ultra-micropores
(<3 μm) and micropores (3–30 μm), under the three given
particle sizes. This is probably associated to the rapid
release of gas as a result of a large explosive force during
the eruption (Pardo et al. 2012). These findings agree
with a study of von Lichtan et al. (2016) who found the
SEM-observed peak in vesicle abundance at 25 μm in all
pumice samples taken from the most recent eruption
(1.8 ka) of the Taupo Volcano (New Zealand).
Total porosity of pumice, as estimated from segmented
areas of pixels (ranging from 58.5 to 65.6%), was larger
than that estimated by von Lichtan et al. (2016) using the
mercury intrusion porosimetry in pumice from the nearby
region (Taupo 1.8 ka eruption) (ranging from 41.9 to
53.8%). The reason for this difference is probably the distinct
geological formation conditions, as well as the different
methods used for their measurement (Lubda et al. 2005;
Ersoy et al. 2010).
The greater micro-scale porosity and the smaller macro-
scale porosity in 1.5-cm Ø pumice is consistent with the
results from the mobile water fraction analyses. These
showed that when compared to pumice of large particle
sizes, smaller particle-sized pumice had a smaller volume of
mobile water, due to its larger water holding porosity
(microporosity) and larger hydraulic conductivity (Raviv
et al. 2002) than the coarser pumice. However, pumice of
large particle size (e.g. 6-cm) had a larger volume of
air-filled porosity compared with the smaller particle sizes
studied, and this may contribute to soil aeration (Raviv
et al. 2002). Considering that pores >100 μm drain easily,
239
(a) (b) (c)
(d) (e) (f)
C. Kong et al. Soil Research
and those between 3 and 30 μm retain plant-available water
(Landis et al. 1990), the pumice of all particle sizes under
study could suit as amendment to enhance the permeability
of clayey soils, as well as to improve the water holding
capacity of sandy soils.
The larger relative proportion of pores able to retain
plant-available water (41 vs 31%) of pumice at a similar
total porosity than that of the specific biochar under study
(Fig. 1a, b) suggests that this pumice may perform better
on improving plant-available water retention capacity,
compared with this biochar produced from willow at a
highest heating temperature of 350°C. Additionally, it
should be noted the properties of pumice and biochar differ
in other aspects, such as the fact that biochar has been
promoted as a technology to sequester carbon, for provision
of nutrients, and as a liming material (Camps-Arbestain
et al. 2015). Therefore, the selection of one material, or the
other, should be based on the desired impact.
Pumice was shown to be better at contributing
to water mobility than biochar under near-
saturated conditions
The mobile water fraction study supports the pattern that is
commonly observed in pumice- and biochar-amended soils,
with a larger soil moisture content where these amendments
have been applied, as compared with the unamended soils
(Downie et al. 2009; Malekian et al. 2012; Burrell et al.
2016). This characteristic has commonly been attributed to
the porous nature of these materials and, in particular, to
their small ‘mean’ pore size (Clothier et al. 1995; Sahin
et al. 2005). In fact, in this study, the fraction of immobile
water of pumice was nearly four-fold that of mobile
water (0.82 vs 0.18), and this ratio was 11-fold in biochar
(0.92 vs 0.08). A higher mobile water fraction in a larger
particle sized pumice agrees with its larger proportion of
macro-scale and meso-scale porosity. Thus, the application
of these two materials could contribute to alleviate salt and
plant-water stress by retaining more water and diluting salt
concentration in the soil solution under arid conditions,
when a biochar with a small ash fraction is used.
The fact that there were no obvious differences in the
mobile water fraction between the three particle sizes
biochar under study could be explained by the fact that
they originated from the same biochar, which was crushed
before the particle screening, and the overall small contri-
bution of biochar to the water mobility. Furthermore, in a
previous study carried out with the same pumice and
biochar (Kong et al. 2021), accumulation of Na+ within
biochar was one fifth that of pumice (70 vs 380 meq L−1)
reflecting the lower ability of biochar to retain Na+
. Here,
we only considered the effect of porosity and pore size
distribution on the mobile water fraction of pumice and
biochar. Other aspects such as the influence of differences
in pore morphology characteristics, homogeneity of pores
and pore sizes, and the connectivity of the pore structure
(Sahin et al. 2005; Ersoy et al. 2010; Fauria et al. 2017;
Liu et al. 2017) cannot be discarded. But these were not
investigated in this study and deserve further research.
Conclusions
The findings of our study have offered an insight into the
relationship between the physical properties of the porosity
and pore size distribution of a pumice and a biochar and
the fraction of mobile–immobile water present in these
materials when added to a sandy soil. The results
emphasise a predominance of pores with Ø < 30 μm and
relatively high total porosity under the three given particle
sizes of both materials, which are expected to contribute to
water retention when these amendments are used in sandy
soil, and to the dilution of salinity in salt-affected sandy
soils assuming a low ash biochar is used. The overall larger
contribution of pumice to the water mobility than that of
biochar under near-saturated conditions could be related to
its relatively higher levels of macro-scale plus meso-scale
porosity, and this increased as the pumice particle size
increased. The knowledge generated in this study provides
an enhanced understanding of the relationship between the
pore characteristics and mobile water fractions of pumice
and biochar, and their implications for potential use in
saline environments. In future studies, Nuclear Magnetic
Resonance (NMR) technology could also be used as this has
been proven to be a non-destructive tool for characterisation
of the pore size distribution of porous samples (de Pierri et al.
2022). The application of this technique might help further
validate the results from our study.
Supplementary material
Supplementary material is available online.
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Data availability. The data that support this study will be shared upon reasonable request to the corresponding author.
Conflicts of interest. The authors declare no conflicts of interest.
Declaration of funding. The authors acknowledge the China Scholarship Council for facilitating and supporting this research.
Acknowledgements. The authors acknowledge Stanislav A. Garbuz from Massey University for helping in the supply of biochar.
Author affiliations
ASchool of Agriculture and Environment, Massey University, Private Bag 11222, Palmerston North 4442, New Zealand.
BPlant and Food Research, Palmerston North 4442, New Zealand.
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