[show abstract][hide abstract] ABSTRACT: Adding biochar to soils and maintaining high earthworm biomasses are potential ways to increase the fertility of tropical soils and the sustainability of crop production in the spirit of agroecology and ecological engineering. However, a thorough functional assessment of biochar effect on plant growth and resource allocations is so far missing. Moreover, earthworms and biochar increase mineral nutrient availability through an increase in mineralization and nutrient retention respectively and are likely to interact through various other mechanisms. They could thus increase plant growth synergistically. This hypothesis was tested for rice in a greenhouse experiment. Besides, the relative effects of biochar and earthworms were compared in three different soil treatments (a nutrient rich soil, a nutrient poor soil, a nutrient poor soil supplemented with fertilization). Biochar and earthworm effects on rice growth and resource allocation highly depended on soil type and were generally additive (no synergy). In the rich soil, there were both clear positive biochar and earthworm effects, while there were generally only positive earthworm effects in the poor soil, and neither earthworm nor biochar effect in the poor soil with fertilization. The analysis of earthworm and biochar effects on different plant traits and soil mineral nitrogen content, confirmed that they act through an increase in nutrient availability. However it also suggested that another mechanism, such as the release in the soil of molecules recognized as phytohormones by plants, is also involved in earthworm action. This mechanism could for example help explaining how earthworms increase rice resource allocation to roots and influence the allocation to grains.
Soil Biology and Biochemistry 04/2010; 42(7):1017-1027. · 3.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: Black carbon (BC) is an important pool of the global C cycle, because it cycles much more slowly than others and may even be managed for C sequestration. Using stable isotope techniques, we investigated the fate of BC applied to a savanna Oxisol in Colombia at rates of 0, 11.6, 23.2 and 116.1 t BC ha−1, as well as its effect on non-BC soil organic C. During the rainy seasons of 2005 and 2006, soil respiration was measured using soda lime traps, particulate and dissolved organic C (POC and DOC) moving by saturated flow was sampled continuously at 0.15 and 0.3 m, and soil was sampled to 2.0 m. Black C was found below the application depth of 0–0.1 m in the 0.15–0.3 m depth interval, with migration rates of 52.4±14.5, 51.8±18.5 and 378.7±196.9 kg C ha−1 yr−1 (±SE) where 11.6, 23.2 and 116.1 t BC ha−1, respectively, had been applied. Over 2 years after application, 2.2% of BC applied at 23.2 t BC ha−1 was lost by respiration, and an even smaller fraction of 1% was mobilized by percolating water. Carbon from BC moved to a greater extent as DOC than POC. The largest flux of BC from the field (20–53% of applied BC) was not accounted for by our measurements and is assumed to have occurred by surface runoff during intense rain events. Black C caused a 189% increase in aboveground biomass production measured 5 months after application (2.4–4.5 t additional dry biomass ha−1 where BC was applied), and this resulted in greater amounts of non-BC being respired, leached and found in soil for the duration of the experiment. These increases can be quantitatively explained by estimates of greater belowground net primary productivity with BC addition.
[show abstract][hide abstract] ABSTRACT: Nitrification, a key process in the global nitrogen cycle that generates nitrate through microbial activity, may enhance losses of fertilizer nitrogen by leaching and denitrification. Certain plants can suppress soil-nitrification by releasing inhibitors from roots, a phenomenon termed biological nitrification inhibition (BNI). Here, we report the discovery of an effective nitrification inhibitor in the root-exudates of the tropical forage grass Brachiaria humidicola (Rendle) Schweick. Named "brachialactone," this inhibitor is a recently discovered cyclic diterpene with a unique 5-8-5-membered ring system and a gamma-lactone ring. It contributed 60-90% of the inhibitory activity released from the roots of this tropical grass. Unlike nitrapyrin (a synthetic nitrification inhibitor), which affects only the ammonia monooxygenase (AMO) pathway, brachialactone appears to block both AMO and hydroxylamine oxidoreductase enzymatic pathways in Nitrosomonas. Release of this inhibitor is a regulated plant function, triggered and sustained by the availability of ammonium (NH(4)(+)) in the root environment. Brachialactone release is restricted to those roots that are directly exposed to NH(4)(+). Within 3 years of establishment, Brachiaria pastures have suppressed soil nitrifier populations (determined as amoA genes; ammonia-oxidizing bacteria and ammonia-oxidizing archaea), along with nitrification and nitrous oxide emissions. These findings provide direct evidence for the existence and active regulation of a nitrification inhibitor (or inhibitors) release from tropical pasture root systems. Exploiting the BNI function could become a powerful strategy toward the development of low-nitrifying agronomic systems, benefiting both agriculture and the environment.
Proceedings of the National Academy of Sciences 09/2009; 106(41):17302-7. · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: We compared differences in soil phosphorus fractions between large earthworm casts (Family Glossoscolecidae) and surrounding soils, i.e., Oxisols in 10 year-old upland agroforestry system (AGR), pasture (PAS), and secondary forest (SEC) in the Central Brazilian Amazon. AGR and PAS both received low-input fertilization and SEC received no fertilization. We found that earthworm casts had higher levels of organic hydroxide P than surrounding soils, whereas fertilization increased inorganic hydroxide P. Inorganic P was increased by fertilization, and organic P was increased by earthworm gut passage and/or selection of ingested materials, which increased available P (sum of resin and bicarbonate fractions) and moderately available P (sum of hydroxide and dilute acid fractions), and P fertilizer application and land-use increased available P. The use of a modified sequential P fractionation produced fewer differences between earthworm casts and soils than were expected. We suggest the use of a condensed extraction procedure with three fractions (Available P, Moderately Available P, and Resistant P) that provide an ecologically based understanding of the P availability in soil. Earthworm casts were estimated to constitute 41.0, 38.2, and 26.0 kg ha-1 of total available P stocks (sum of resin and bicarbonate fractions) in the agroforestry system, pasture, and secondary forest, respectively.
[show abstract][hide abstract] ABSTRACT: Maintaining an appropriate level of soil organic matter and biological cycling of nutrients is crucial to the success of any soil management in the humid tropics. Cover crops, mulches, compost, or manure additions have been used successfully, supplying nutrients to crops, supporting rapid nutrient cycling through microbial biomass, and helping to retain applied mineral fertilizers better (Goyal et al., 1999; Trujillo, 2002). The benefits of such amendments are, however, often short-lived, especially in the tropics, since decomposition rates are high (Jenkinson and Ayanaba, 1977) and the added organic matter is usually mineralized to CO2 within only a few cropping seasons (Bol et al., 2000). Organic amendments therefore have to be applied each year to sustain soil productivity.
Management of black carbon (C) — increasingly referred to as bio-char — may overcome some of those limitations and provide an additional soil management option. This is a highly aromatic form of organic matter that is present in most soils to varying extents (Schmidt and Noack, 2000; Skjemstad et al., 2002). Interest in and application of biomass-derived black carbon — using incompletely combusted organic matter such as charcoal (Glaser et al., 2002) — was prompted by studies of soils found in the Amazon Basin, referred to as Terra Preta de Indio (Lehmann et al., 2003c). These Amazonian Dark Earths are anthropic soils that were created by Amerindian populations between 500 and
2500 years ago. They have maintained high amounts of organic carbon, and their high fertility, even several thousand years after they were abandoned by the indigenous population, contrasts distinctly with the low fertility of the adjacent acid upland soils (Lehmann et al., 2003b).
The reasons for these soils’ high fertility are multiple, but the source of the large amounts of organic matter and their high nutrient retention has been attributed to the extraordinarily high proportions of black carbon (Glaser et al., 2001). Such large amounts of black carbon can only originate from incompletely combusted biomass carbon, such as wood from kitchen fires or possibly from in-field burning (Smith, 1980; Hecht, 2003). This chapter considers the beneficial effects of this bio-char soil management system and discusses opportunities for applying such management within a sustainable system that can be called “slash-and-char,” as well as within other smallholder agricultural systems.
[show abstract][hide abstract] ABSTRACT: Nitrification, a microbial process, is a key component and integral part of the nitrogen (N) cycle. Soil N is in a constant state of flux, moving and changing chemical forms. During nitrification, a relatively immobile N-form (NH4 ) is converted into highly mobile nitrate-N (NO3 ). The nitrate formed is susceptible to losses via leaching and conversion to gaseous forms via denitrification. Often less than 30% of the applied N fertilizer is recovered in intensive agricultural systems, largely due to losses associated with and following nitrification. Nitrogen-use efficiency (NUE) is defined as the biomass produced per unit of assimilated N and is a conservative function in most biological systems. A better alternative is to define NUE as the dry matter produced per unit N applied and strive for improvements in agronomic yields through N recovery. Suppressing nitrification along with its associated N losses is potentially a key part in any strategy to improve N recovery and agronomic NUE. In many mature N-limited ecosystems, nitrification is reduced to a relatively minor flux. In such systems there is a high degree of internal N cycling with minimal loss of N. In contrast, in most high-production agricultural systems nitrification is a major process in N cycling with the resulting N losses and inefficiencies. This review presents the current state of knowledge on nitrification and associated N losses, and discusses strategies for controlling nitrification in agricultural systems. Limitations of the currently available nitrification inhibitors are highlighted. The concept of biological nitrification inhibition (BNI) is proposed for controlling nitrification in agricultural systems utilizing traits found in natural ecosystems. It is emphasized that suppression of nitrification in agricultural systems is a critical step required for improving agronomic NUE and maintaining environmental quality.
[show abstract][hide abstract] ABSTRACT: Andean hillsides dominate the landscape of a considerable proportion of Cauca Department in Colombia. The typical cropping cycle in the region includes monocrops or intercrops of maize (Zea mays L.), beans (Phaseolus vulgaris L.) and/or cassava (Manihot esculenta Crantz). Cassava is usually the last crop before local farmers leave plots to natural fallow until soil fertility is recovered and a new cropping phase can be initiated. Previous studies on land use in the Río Cabuyal watershed (6500 ha) show that a considerable proportion of land (about 25–30%) remains under natural fallow every year. The focus of our studies is on systems of accelerated regeneration of soil fertility, or improved fallow systems, as an alternative to the natural regeneration by the native flora. Fallow improvement studies were conducted on plots following cassava cultivation. The potential for soil fertility recovery after 12 and 28 months was evaluated with two fast growing trees, Calliandra calothyrsus Meissn (CAL) and Indigofera constricta L.(IND), and one shrub, Tithonia diversifolia (Hemsl.) Gray (TTH), as slash/mulch fallow systems compared to the natural fallow (NAT). All planted slash/mulch fallow systems produced greater biomass than the natural fallow. Greatest dry biomass (16.4 Mg ha−1 year−1) was produced by TTH. Other planted fallows (CAL and IND) produced about 40% less biomass than TTH and the control (NAT) about 75% less. Nutrient levels in the biomass were especially high for TTH, followed by IND, CAL, and NAT. The impact of fallow management on soil chemical, physical and biological parameters related to residual soil fertility during the cropping phase was evaluated. Soil parameters most affected by slash/mulch fallow systems included soil total N, available N (ammonium and nitrate), exchangeable cations (K, Ca, Mg and Al), amount of P in light fraction, soil bulk density and air permeability, and soil macrofauna diversity. Results from field studies suggest that the Tithonia slash/mulch fallow system could be the best option to regenerate soil fertility of degraded volcanic-ash soils of the Andean hillsides.
[show abstract][hide abstract] ABSTRACT: Over the past three decades, large expanses of forest in the Amazon Basin were converted to pasture, many of which later degraded to woody fallows and were abandoned. While the majority of tropical secondary forest (SF) Studies have examined post-deforestation or post-agricultural succession, we examined post-pasture forest recovery in 10 forests ranging in age from 0 to 14 years since abandonment. We measured aboveground biomass and soil nutrients to 45 cm depth and computed total site carbon (C) and nutrient stocks to gain an understanding of the dynamics of nutrient and C buildup in regenerating SF in central Amazonia. Aboveground biomass accrual was rapid, 11.0 Mgha(-1.)yr(-1), in the young SFs. Within 12-14 yr, they accumulated up to 128.1 Mg/ha of dry aboveground biomass, equivalent to 25-50% of primary forest biomass in the region. Wood nitrogen (N) and phosphorus (P) concentrations decreased with forest age. Aboveground P and calcium (Ca) stocks accumulated at a rate of 1.2 and 29.4 kgha(-1.)y
[show abstract][hide abstract] ABSTRACT: In the Amazon, approximately 35 million hectares of primary forest that was converted to pasture is now being abandoned. This represents about 70% of all pastureland that was previously established. The dynamics of reconversion of this land to secondary forest is of interest because the length of time required for pasture to convert to secondary forest will impact net primary productivity and the amount of carbon being stored on abandoned pastures. In addition, the length of time required for pasture to convert to secondary forest may depend on the size of the pasture, whether it is surrounded by primary or secondary forest, and on pasture productivity at the time of abandonment. Pasture productivity at the time of abandonment will depend primarily on the age structure of the pasture grasses and on weediness, which are influenced by grazing and fire history. Also, an understanding of the dynamics of conversion of pastureland to forest can serve as the basis for management strategies to inhibit pasture conversion. A spatial, dynamic model of the conversion of pasture to secondary forest was developed using the PCRaster Dynamic Modeling Package. This software provides a computer language specially developed for modeling temporal and spatial processes in a GIS, and is well suited for the development of ecological, dynamic models. The model of pasture conversion is implemented for the central Amazon. We assume that succession involves only three plant types: pasture grass, weeds and woody plants. The pasture grass is parameterized for Brachiaria (brizantha, humidicola), the weeds for Borreria and Rolandra, and the woody plants for Vismia spp. The model uses a 1m x 1m grid and 2-month time step. Each initial plant and each surviving propagule is referred to as a plant and only occupies one grid cell. A number of values are calculated for each grid cell for each time-step. These include whether vegetation is present and, if so, which species, the age of the species, the current standing biomass of the cell, the productivity of the cell for the current time step, and, in the case of woody plants, the height of the plant. The spatial distribution of these variables is available for every time step, as well as values that are integrated over the entire area of the simulation. In this simulation study, low grazing resulted in rapid decline of NPP of Brachiaria and rapid weed invasion resulting in an acceleration of dominance by Vismia. This led to higher standing biomass at the end of 14 years relative to other management scenarios. With high grazing, weeds are never as dominant as in low grazing systems because more Brachiaria remain vigorous. With frequent burning (2, 3 and 4 years) and high grazing, Brachiaria covers 50% of the pasture even after 8 years. But by 12 years, there is only a small amount of grass and weed coverage in any of the systems. The model predicts that frequent burning will result in more bare patches even in the later stages of succession.
American Geophysical Union Spring Meeting - Biogeochemistry SectionAmerican Geophysical Union Spring Meeting - Biogeochemistry Section; 01/2001
[show abstract][hide abstract] ABSTRACT: Nutrient leaching in highly weathered tropical soils often poses a challenge for crop production. We investigated the effects of applying 20 t ha biochar (BC) to a Colombian savanna Oxisol on soil hydrology and nutrient leaching in field experiments. Measurements were made over the third and fourth years after a single BC application. Nutrient contents in the soil solution were measured under one maize and one soybean crop each year that were routinely fertilized with mineral fertilizers. Leaching by unsaturated water flux was calculated using soil solution sampled with suction cup lysimeters and water flux estimates generated by the model HYDRUS 1-D. No significant difference ( > 0.05) was observed in surface-saturated hydraulic conductivity or soil water retention curves, resulting in no relevant changes in water percolation after BC additions in the studied soils. However, due to differences in soil solution concentrations, leaching of inorganic N, Ca, Mg, and K measured up to a depth of 0.6 m increased ( < 0.05), whereas P leaching decreased, and leaching of all nutrients (except P) at a depth of 1.2 m was significantly reduced with BC application. Changes in leaching at 2.0 m depth with BC additions were about one order of magnitude lower than at other depths, except for P. Biochar applications increased soil solution concentrations and downward movement of nutrients in the root zone and decreased leaching of Ca, Mg, and Sr at 1.2 m, possibly by a combination of retention and crop nutrient uptake.
Journal of Environmental Quality 41(4):1076-86. · 2.35 Impact Factor
[show abstract][hide abstract] ABSTRACT: The application of biochar (biomass-derived black carbon) to soil has been shown to improve crop yields, but the reasons for
this are often not clearly demonstrated. Here, we studied the effect of a single application of 0, 8 and 20tha−1 of biochar to a Colombian savanna Oxisol for 4years (2003–2006), under a maize-soybean rotation. Soil sampling to 30cm
was carried out after maize harvest in all years but 2005, maize tissue samples were collected and crop biomass was measured
at harvest. Maize grain yield did not significantly increase in the first year, but increases in the 20tha−1 plots over the control were 28, 30 and 140% for 2004, 2005 and 2006, respectively. The availability of nutrients such as
Ca and Mg was greater with biochar, and crop tissue analyses showed that Ca and Mg were limiting in this system. Soil pH increased,
and exchangeable acidity showed a decreasing trend with biochar application. We attribute the greater crop yield and nutrient
uptake primarily to the 77–320% greater available Ca and Mg in soil where biochar was applied.
KeywordsBiochar-Colombia-Crop yield-Exchangeable acidity-Maize-Oxisol-Tropical savannas
Plant and Soil 333(1):117-128. · 2.64 Impact Factor
[show abstract][hide abstract] ABSTRACT: Regulating nitrification could be a key strategy in improving nitrogen (N) recovery and agronomic N-use efficiency in situations
where the loss of N following nitrification is significant. A highly sensitive bioassay using recombinant luminescent Nitrosomonas europaea, has been developed that can detect and quantify the amount of nitrification inhibitors produced by plants (hereafter referred
to as BNI activity). A number of species including tropical and temperate pastures, cereals and legumes were tested for BNI
in their root exudate. There was a wide range in BNI capacity among the 18 species tested; specific BNI (AT units activity
g−1 root dry wt) ranged from 0 (i.e. no detectable activity) to 18.3AT units. Among the tested cereal and legume crops, sorghum
[Sorghum bicolor (L.)], pearl millet [Pennisetum glaucum (L.) R. Br.], and groundnut [Arachis hypogaea (L.)] showed detectable BNI in root exudate. Among pasture grasses, Brachiaria humidicola (Rendle) Schweick, B. decumbens Stapf showed the highest BNI capacity. Several high- and low-BNI genotypes were identified within the B. humidicola species. Soil collected from field plots of 10year-old high-BNI genotypes of B. humidicola, showed a near total suppression (>90%) of nitrification; most of the soil inorganic N remained in the NH4+ form after 30days of incubation. In contrast, soils collected from low-BNI genotypes did not show any inhibitory effect;
most of the soil inorganic N was converted to NO3– after 30days of incubation. In both the high- and low-BNI genotypes, BNI was detected in root exudate only when plants were
grown with NH4+, but not when grown with NO3– as the sole source of N. BNI compounds when added to the soil inhibited nitrification and the relationship was linear (r
2=0.92**; n=12). The BNI from high- and low-BNI types when added to N. europaea in pure culture, blocked both the ammonia monooxygenase (AMO) and the hydroxylamine oxidoreductase (HAO) pathways. Our results
indicated that BNI capacity varies widely among and within species; and that some degree of BNI capacity is likely a widespread
phenomenon in tropical pasture grasses. We suggest that the BNI capacity could either be managed and/or introduced into pastures/crops
with an expression of this phenomenon, via genetic improvement approaches that combine high productivity along with some capacity
to regulate soil nitrification process.