Long-term Soil Organic Matter Research Articles

Source or decomposition of soil organic matter: what is more important with increasing forest age in a subalpine setting?

Afforestation has been the dominant land-use change in the Swiss Alps during the last decades which has not only the potential to increase soil organic carbon sequestration, but it has also the potential to alter soil organic matter (SOM) dynamics through the vegetation shift and change in organic matter (OM) input into soils. The effects of afforestation on SOM dynamics, however, are still not fully understood as specific sources of OM and modifications of soil processes influencing decomposition and preservation remain largely unknown on alpine to subalpine slopes. Within this study we aimed to identify the potential sources and the decomposition of OM in a subalpine afforestation chrono-sequence (0–130 years) with Norway spruce (Picea abies L.) on a former pasture by using a multi-proxy molecular marker approach. We observed that leaf-derived OM plays an essential role in the pasture areas, while root-derived OM only plays a minor role in pasture and forest areas. Needle-derived OM represents the dominant source of SOM with increasing forest age, while understory shrubs and moss also contribute to the OM input in younger forest stand ages. However, needle litter and buildup of organic layers and subsequently less input of fresh OM from organic horizons to mineral soil can result in increased OM decomposition in mineral soils rather than contributing to additional SOM stabilization in mineral soils. This was most pronounced in the oldest forest stand (130-year-old) in the investigated afforestation sequence, particularly in deeper soil horizons (10–45 cm). Thereby, our study provides new insights into SOM dynamics following afforestation, especially with respect to the long-term SOM sequestration potential of afforestation of subalpine pasture soils.

Effects of Long-Term Straw Return and Environmental Factors on the Spatiotemporal Variability of Soil Organic Matter in the Black Soil Region: A Case Study

Exploring the effects of straw return and environmental factors on the spatiotemporal variation of soil organic matter (SOM) in black soil regions is essential for soil carbon sequestration research. However, studies seldom quantified the effects of long-term straw return on a long-term SOM variation at a regional scale in typical black soil areas. The case was conducted in one of the three major black soil regions in the Northern Hemisphere, where the straw return policy has been implemented for a long time. The study obtained the SOM spatial distribution in 2007, 2009, 2012, 2015, and 2018 with approximately 9000 samples and analyzed the effects of soil types, texture, elevation, and human management on the spatiotemporal variation. The results indicated that from the 1980s to 2007, before the straw return policy implementation, the mean SOM content decreased from 24.38 g kg−1 to 18.94 g kg−1. In contrast, the mean SOM content gradually increased from 2007 to 2018 after implementing straw return practices. In addition, the area of SOM within 20–30 g kg−1 increased gradually, with 32.2%, 40.5%, 50.2%, 49.4%, and 60.5% in 2007, 2009, 2012, 2015, and 2018, respectively. Surprisingly, the SOM within 30–40 g kg−1 emerged in 2018. The results indicated that returning straw to the field might promote SOM accumulation. However, the SOM contents in Phaezems (19.25–21.82 g kg−1) were lower than that in natural Phaezems (40–60 g kg−1), indicating severe degradation. The clay content positively correlated to SOM and was a major explanatory variable for the response of SOM to straw return. Straw return practices are promising measures in the black soil region and are worth exploring more effective approaches to allow straw return to play a better role.

Modelling long-term yield and soil organic matter dynamics in a maize cropping system

Modelling long-term yield and soil organic matter dynamics in a maize cropping system

Modeling the Dynamics of Soil Organic Matter Accumulation from Leaf Litterfall as Affected by Tillage Practices in Mango Orchard

This study was undertaken to determine long-term soil organic matter (SOM) accumulation dynamics from added mango leaf litter (MLL) into tilled (MT) and non-tilled soil (MnT) through integrated field experiment (bio-physico-chemical edaphic properties) and system dynamics modeling. Field data shows that the coarse (Lf) and fine (Fn) fractions of SOM have no significant difference (p > 0.05) between MT and MnT during the dry season. In contrast, there is a significant difference (p < 0.01) in the Fn fraction between MT and MnT during the wet season. The total N, available P, exchangeable K, as well as the collembola, bacteria, and fungi in the soil were also assessed. Of these parameters, the available P and exchangeable K have a significant difference between the MnT and MT during the dry season. In comparison, the total N and collembola significantly differed during the wet season. The results of the field experiment were used in the parametrization to build the SOM accumulation model. The 20-year simulation showed that the incorporation of MLL into the soil through tillage increased SOM from 3.09% to 3.13%, with an average of 2.78 ± 0.018%. In the non-tilled orchard, SOM also increased from 3.09% to 4.23%, with an average of 3.59 ± 0.035%. This SOM level can sustain the fertility and productivity of the mango agroecosystem. The increase in SOM is accounted for by the coarse fraction both in tilled and non-tilled plots. Given the foregoing, with minimum tillage, mango orchards can be developed into crop-based systems.

Limited potential of biosolids application for long-term soil carbon stabilization in coastal dune forests

Whether or not biosolids application facilitates long-term soil organic matter (SOM) stability in coastal dune forests remains unsolved. We conducted a long-term experiment in a coastal radiata pine (Pinus radiata D. Don) forest growing on sandy soils in New Zealand, where biosolids have been applied at moderate (3 t ha−1) and high (6 t ha−1) rates every three years for two decades. We determined total carbon (C) and nitrogen (N) contents, the amounts of iron and aluminum oxides, labile and recalcitrant C and N fractions and the chemical composition of SOM in the topsoil, and explore their relations to plant, microbial and geochemical controls. Biosolids application had increased total, labile and recalcitrant C by 6%–94%, 6%–56% and 0%–55%, respectively; increased total, labile and recalcitrant N by 14%–124%, 5%–214% and 0%–173%, respectively. Biosolids application had no significant effects on free and amorphous iron oxides, but significantly increased aluminum oxides with enhanced crystallization of amorphous aluminum. The metal/recalcitrant-C ratios, the recalcitrant/total-C ratios and the alkyl/O-alkyl-C ratios remained unchanged, indicating both the potential of mineral protection and the chemical recalcitrance of SOM were not reinforced. Decreased ratios of microbial lipid biomass to recalcitrant C further indicated biosolids application may limit microbial contribution to SOM stabilization. These limitations to confer higher stability to SOM under biosolids application is likely due to reduced belowground C allocation (root biomass and mycorrhizal associations) and microbial C limitation (higher C- versus nutrient-acquiring enzyme activities) and stress response (higher ratio of cyclopropyl-to-precursor fatty acids). Given unchanged mineral protection to SOM, decreased mycorrhizal associations coupled with increases in Gram-positive bacteria and microbial metabolic quotient may facilitate the decomposition of recalcitrant SOM. Hence, although biosolids application usually promoted immediate C accumulation, weak mineral protection and altered plant-microbial interactions may undermine its potential for long-term soil C sequestration in coastal dune forests.

315 Formation, persistence, and function of soil organic matter: how recent scientific advances apply in agroecosystems

Abstract Soil organic matter is fundamental to healthy and productive soils and building it is an effective means by which to draw down atmospheric greenhouse gas concentrations with added co-benefits. Scientific understanding of soil organic matter dynamics is constantly evolving, and the past decade has seen major advances and paradigm shifts. Soil organic matter creation from decaying plant litter is now thought to occur under two separate pathways, yielding two functionally different types: predominantly plant-derived, unprotected particulate organic matter (POM) and predominantly microbially-derived, mineral-associated organic matter (MAOM). The idea of naturally-occurring humic substances in soils has been largely abandoned, and long-term soil organic matter persistence is now understood to be driven mainly by mineral association. We will present the research behind these paradigm shifts, and show how considering POM and MAOM separately is key to understanding the mechanisms driving carbon accrual and persistence in soil, and therefore to guiding policy and management for soil carbon sequestration. We will present drivers of POM and MAOM contents, from individual fields to continents, including their capacity for sequestration and saturation in agricultural soils of the USA, and their responses to common management practices in agroecosystems.

Maize root biomass and architecture depend on site but not on variety: Consequences for prediction of C inputs and spread in topsoil based on root-to-shoot ratios

Abstract Knowledge about the response of root biomass and root system architecture (RSA) to soil conditions would increase efficiency in selecting high performance crop varieties and predicting the impact of rotations on long-term soil organic matter (SOM) stock. We evaluated the effect of variety and site on maize (Zea mays L.) root growth. Two-dimensional (2D) and 3D techniques were used to quantify changes of RSA. Commercial maize varieties were grown in three sites with typical soil types for maize cultivation in Belgium. We observed that at Merelbeke, above- and belowground biomass and root-to-shoot ratios (R/S) were larger compared to those at Ravels. Both factors interactively influenced an index of biological stability (ISB) based on biochemical quality of the roots. There were also more fine roots at Merelbeke than at Bassevelde and Ravels. Together with a lower ISB value, this suggests that site had an inherent effect on root biochemical quality. We conclude that a strong impact of site exists on both above- and belowground biomass; however, they are not necessarily correlated. With a fixed R/S, predictions of root biomass were misestimated in one half of the cases. This complex response of root phenotyping to varying soil conditions should not be overlooked.

Ten-year legacy of organic carbon in non-agricultural (brownfield) soils restored using green waste compost exceeds 4 per mille per annum: Benefits and trade-offs of a circular economy approach

Soil organic carbon (SOC) was re-analysed 10 years after application of source-segregated green waste compost at a 1 ha previously-developed UK site to compare with the increases suggested by the 4 per mille initiative proposed at COP21 in Paris. Compost prepared to PAS100 standard had been incorporated once at rates of 250, 500 and 750 t·ha−1 in 2007 in clay subsoil prior to planting of perennial energy crops. Our results show statistically significant differential increases in SOC, total nutrients N and P, or contaminants Zn, Pb, Cu, As and B, remain from the compost application after a decade. For the 500 or 750 t·ha−1 compost rates the SOC increments in the upper 10–15 cm were 0.85% or 1.6% over the 4.9% developed from a baseline of 1.8% in control areas by a decade of natural regeneration. Calculation of the elemental loadings from the compost analyses compared to the present-day levels suggests SOC declines after application at an average annual rate approaching 10%, compared to 5% for the nutrients or contaminants, roughly equivalent to half-lives of 5 or 10 years respectively. The study demonstrates the long-term soil organic matter (SOM) additions, fertility benefits and technical feasibility of a one-off, high-rate application of waste-derived compost to improve urban soils, compared to the potential trade-off of adding to PTE loadings. This longevity of SOC addition, previously unrecognised in brownfield soils, may be inferred for other areas where further cultivation is precluded, as is typical after landscape restoration or under perennial energy crops for the production of biomass. This unprecedented result has wider implications for marginal land use for bioenergy and the opportunities therein for SOC management using anthropogenic organic wastes to mitigate greenhouse gas emissions.

Roots of non-woody perennials accelerated long-term soil organic matter decomposition through biological and physical mechanisms

The rhizosphere priming effect (RPE) is increasingly recognized as an important factor in mediating soil organic matter (SOM) decomposition, which influences the CO2 release from terrestrial systems to the atmosphere. However, little is known about the long-term RPE of non-woody perennial species and the physical mechanisms underlying the RPE. Here the RPEs of three non-woody perennials (Stipa grandis, Leymus chinensis, Medicago sativa) differing in root traits and exudation were quantified using a natural 13C tracer method (C3 plant - C4 soil system) during a 476-day experiment. The results indicated that all plant species showed positive RPEs, with M. sativa showing the largest RPE and S. grandis the smallest RPE. Differences in the RPEs between species are likely associated with differences in root biomass, length, and exudates. Furthermore, the positive RPEs of the three species increased continuously with sampling time, indicating the long-term nature of the RPE. Results from structural equation modeling indicated that plant roots accelerated SOM decomposition via three mechanisms. The first mechanism (biological) was through the positive effect on dissolved organic C and microbial biomass C by the presence of the rhizosphere, consistent with the microbial activation hypothesis. The second mechanism (biological) was where plant roots significantly reduced soil mineral N and in turn promoted SOM decomposition, providing support for the microbial N-mining hypothesis. The third mechanism (physical) was where plant roots caused a net positive effect on SOM decomposition through net destruction of macroaggregates possibly as a result of high root length density and the associated drying-rewetting cycles. Overall, this study provides the first experimental evidence for the aggregate destruction hypothesis as a physical mechanism causing the RPE and highlights the importance of multiple mechanisms of the RPE on long-term SOM decomposition.

Bovine urine inhibits microbial function and increases urea turnover in dairy grazed soils

Effects of bovine urine on microbial functional attributes within the carbon (C) cycle have not previously been investigated. The magnitude of urine effects on microbial populations may be mediated by the ability of a soil to buffer changes to pH and electrical conductivity (EC) in response to urine. We examined changes in the metabolism of C substrates by microbial communities subsequent to treatment with dairy cow urine in 27 dairy grazed soils across four soil orders. Untreated soils (baseline) and soil treated with urine or water were incubated (25°C) for 21 days then assessed for microbial function using MicroResp™. Urine addition decreased functional capacity, microbial diversity, and microbial biomass C at 21 days after urine addition, but did not affect basal respiration, compared with the water control. Urine addition also led to a shift in community-level physiological profiles. There were no indirect effects of soil pH or EC buffering capacity on the functional microbial parameters measured. Urine addition increased the utilisation of urea and may be a factor in losses of fertiliser nitrogen in dairy systems. The length of time that urine depresses catabolic function could have important implications for long-term soil organic matter cycling under urine patches.

Aggregation controls the stability of lignin and lipids in clay-sized particulate and mineral associated organic matter

Physical separation of soil into different soil organic matter (SOM) fractions is widely used to identify organic carbon pools that are differently stabilized and have distinct chemical composition. However, the mechanisms underlying these differences in stability and chemical composition are only partly understood. To provide new insights into the stabilization of different chemical compound classes in physically-separated SOM fractions, we assessed shifts in the biomolecular composition of bulk soils and individual particle size fractions that were incubated in the laboratory for 345 days. After the incubation, also the incubated bulk soil was fractionated. The chemical composition of organic matter in bulk soils and fractions was characterized by 13C-CPMAS nuclear magnetic resonance spectroscopy and sequential chemical extraction followed by GC/MS measurements. Plant-derived lipids and lignin were abundant in particulate organic matter (POM) fractions of sand-, silt-, and clay-size and the mineral-bound, clay-sized organic matter. These results indicate that recent conceptualizations of SOM stabilization probably understate the contribution of plant-derived organic matter to stable SOM pools. Although our data indicate that inherent recalcitrance could be important in soils with limited aggregation, organo-mineral interactions and aggregation were responsible for long-term SOM stabilization. In particular, we observed consistently higher concentrations of plant-derived lipids in POM fractions that were incubated individually, where aggregates were disrupted, as compared to those incubated as bulk soil, where aggregates stayed intact. This finding emphasizes the importance of aggregation for the stabilization of less ‘recalcitrant’ biomolecules in the POM fractions. Because also the abundance of lipids and lignin in clay-sized, mineral-associated SOM was substantially influenced by aggregation, the bioavailability of mineral-associated SOM likely increases after the destruction of intact soil structures.

Evidence for Losses From Strongly Bound SOM Pools After Clear Cutting in a Northern Hardwood Forest

Forest soils in the northeastern United States store considerable amounts of carbon (C). With the increasing utilization of biomass as a “C-neutral” form of energy in the United States, these forests are susceptible to clear cutting and large losses of soil organic matter (SOM) to the atmosphere as carbon dioxide (CO2). The relative stability versus susceptibility of SOM to degradation can be approximated, in part, through the strength of organo-mineral interactions, that is, the strength of binding between SOM and mineral surfaces in the soil. This study investigated differences in SOM organo-mineral binding between northern hardwood forest stands with varying clear-cutting histories in Bartlett Experimental Forest in Bartlett, New Hampshire. Sequential chemical extractions were performed to quantify SOM storage in organo-mineral pools of various binding strength. In this case study, soils from Mature forest stands stored significantly more SOM in strongly mineral-bound and stable C pools than soils from Cut stands did. Differences in the relative distribution of C in organo-mineral pools in Mature and Cut forests may inform our understanding of SOM bioavailability, microbial decomposition, and CO2 production in ecosystems after clear cutting. These findings should contribute to discussions on long-term SOM stability in northeastern U.S. soils.

Cereal straw management: a trade-off between energy and agronomic fate

Climate change mitigation is the most important driving force for bioenergy development. Consequently, the environmental design of bioenergy value chains should address the actual savings of both primary energy demand and greenhouse gases (GHG) emissions. According to the EU Renewable Energy Directive (2009/28/EC), no direct impacts and no GHG emissions should be attributed to crop residues (like cereal straws) when they are removed from agricultural land for the purpose of bioenergy utilisation. The carbon neutral assumption applied to crop residues is, however, a rough simplification. Crop residues, indeed, should not be viewed simply as a waste to be disposed, because they play a critical role in sustaining soil organic matter and therefore have an inherent C-capturing value. Moreover, considering straws as an energy feedstock, its status of co-product is clearly recognised and its availability could be obtained according to different cropping systems, corresponding to different primary energy costs and GHG emissions. This paper highlights some hidden features in the assessment of agricultural energy and carbon balance, still very difficult to be detected and accounted for. Although they are frequently disregarded, these features (such as long term dynamic trend of soil organic carbon and annual nitrous oxide emissions from the soil) should be carefully considered in assembling the energy and emission balance. By using a crop simulation model, the long-term soil organic matter and annual N2O soil emissions were estimated. Consequently, a comprehensive energy and GHG balance was determined in accordance with the life cycle assessment methodology. Contrasting methods of straw management and wheat cultivation were compared: straw retention vs removal from the soil; conventional vs conservation tillage; wheat cropping system as a single-crop or in rotation. The resulting carbon footprint of straws has different magnitudes with respect to the several experimental conditions. By selecting the best agricultural practices, energy from straw can be optimally coupled with grain productions, without detrimental effects on soil fertility. An improved and specifically tailored cropping system is designed to obtain an optimal trade-off.

Contribution of roots and amendments to soil carbon accumulation within the soil profile in a long-term field experiment in Sweden

The contribution of different C inputs to organic carbon accumulation within the soil profile in the Ultuna long-term continuous soil organic matter experiment, established in 1956, was determined. Until 1999, C3-crops were grown at the site, but since then maize (C4) has been the only crop. The effect of a total of 10 different inorganic nitrogen and organic amendment treatments (4Mg Cha−1yr−1) on SOC in topsoil and subsoil after 53 years was evaluated and the contribution from maize roots to SOC after 10 years of cultivation was estimated.Soil organic carbon (SOC) and δ13C signature were measured down to 50cm depth. The C content in the topsoil (0–20cm depth) was 1.5% at the start of the experiment. After 53 years of treatments, the average topsoil C content varied between 0.9 and 3.8% of soil dry weight, with the open fallow having the lowest and the peat amended the highest value. Nitrogen seemed to promote C accumulation in the topsoil treatment effects were smaller below 20cm depth and only two of the amendments (peat and sewage sludge) significantly affected SOC content down to 35cm depth. Despite this, penetrometer measurements showed significant treatment differences of compaction below 41cm depth, and although we could not explain these differences this presented some evidence of an initial treatment-induced subsoil differentiation. Ten years of maize growth affected the δ13C of SOC down to 22.5cm depth, where it varied between −25.16 and −26.33(‰), and an isotopic mass balance calculation suggested that maize C accounted for 4–8% of total SOC in the topsoil. Until less than 2500 years ago the site was a post-glacial sea floor and the 14C data suggest that marine sediment C still dominates the SOC in deeper soil layers. Overall, the results suggest that 53 years of treatments has caused dramatic changes on the stored C in the topsoil in several of the treatments, while the changes in the subsoil is much less dramatic and a small C accumulation in the upper subsoil was found in two of the treatments.The contribution from roots to SOC accumulation was generally equal to or greater than the contribution from amendments. The retention coefficient of root-derived C in the topsoil was on average 0.30±0.09, which is higher than usually reported in the literature for plant residues but confirms previous findings for the same experiment using another approach. This strengthens the conclusion that root-derived SOC contributed more to SOC than above-ground crop residues.

Impacts of organic amendments on carbon stocks of an agricultural soil — Comparison of model-simulations to measurements

Organic amendments such as straw, green manure or farmyard manure are used to mitigate the soil carbon (C) losses from cultivated soils. We investigated the role of various organic amendments with different C quality for development of soil C stocks, by simulating the Ultuna long-term soil organic matter experiment in Sweden with the Yasso07 model. The aim was to evaluate the performance of the Yasso07 soil carbon model in predicting changes in soil C stocks by comparing modeled C stocks to measurements between years 1956–1991. Uncertainty bounds were calculated from the estimated uncertainty in the C inputs and model parameters. The model performance was assessed in terms of regression coefficient (R2), root mean square error (RMSE) and model efficiency (ME). The model could very accurately predict the decrease in soil C stock in bare fallow, and in treatments receiving crop litter inputs and N fertilization. Yasso07 could also predict the increase in C stocks due to different organic matter applications, based on the varying quantity and quality of these C inputs. These results support the use of the model for testing the long-term effects of different agricultural measures aiming to mitigate soil C losses.

Soil microbial community structure affected by 53 years of nitrogen fertilisation and different organic amendments

The Ultuna long-term soil organic matter experiment in Sweden (59′82° N, 17′65° E) was started in 1956 to study the effects of different N fertilisers and organic amendments on soil properties. In this study, samples were taken from 11 of the treatments, including unfertilised bare fallow and cropped fallow, straw with and without N addition, green manure, peat, farmyard manure, sawdust, sewage sludge, calcium nitrate and ammonium sulphate, with n = 4 for each treatment. Samples were taken from topsoil (0–20 cm) and subsoil (27–40 cm depth) and analysed for concentrations of phospholipid fatty acids (PLFAs), organic C, total N and pH. The results showed that the subsoil samples reflected the total PLFA content of the topsoil, but not the microbial community structure. Total PLFA content was well correlated with total organic C and total N in both topsoil and subsoil. Total PLFA content in topsoil samples was highest in the sewage sludge treatment (89 ± 22 nmol PLFA g dw−1). This contradicts earlier findings on microbial biomass in this sewage sludge-treated soil, which indicated inhibition of microorganisms, probably by heavy metals added with sludge. A switch towards microbial growth and faster decomposition of organic matter occurred around 2000, coinciding with lowered heavy metal content in the sludge. According to the PLFA data, the microbial community in the sewage sludge treatment is now dominated by Gram-positive bacteria. A lack of Gram-negative bacteria was also observed for the ammonium sulphate treatment, obviously caused by a drop in pH to 4.2.

Fungi mediate long term sequestration of carbon and nitrogen in soil through their priming effect

It is increasingly recognized that soil microbes have the ability to decompose old recalcitrant soil organic matter (SOM) by using fresh carbon as a source of energy, a phenomena called priming effect (PE). However, efforts to determine the consequences of this PE for soil carbon and nitrogen dynamics are in their early stage. Moreover, little is known about the microbial populations involved. Here we explore the consequences of PE for SOM dynamics and mineral nitrogen availability in a soil incubation experiment (161 days), combining the supply of dual-labeled (13C and 14C) cellulose and mineral nutrients. The microbial groups involved in PE were investigated using molecular fingerprinting techniques (FAMEs and B- and F-ARISA). We show that mean residence time of SOM pool controlled by the PE decreased from 3130 years in the subsoil, where the availability of fresh carbon is very low, to 17–39 years in the surface layer. This result suggests that the decomposition of this recalcitrant soil C pool is strictly dependent on the presence of fresh C and is not an energetically viable mean of accessing C for soil microbes. We also suggest that fungi are the predominant actors of cellulose decomposition and induced PE and they adjust their degradation activity to nutrient availability. The predominant role of fungi can be explained by their ability to grow as mycelium which allows them to explore soil space and mine large reserve of SOM. Finally, our results support the existence of a bank mechanism that regulates nutrient and carbon sequestration in soil: PE is low when nutrient availability is high, allowing sequestration of nutrients and carbon; in contrast, microbes release nutrients from SOM when nutrient availability is low. This bank mechanism may help to synchronize the availability of soluble nutrients to plant requirement and contribute to long-term SOM accumulation in ecosystems.

Responses of bacterial and archaeal ammonia oxidizers to soil organic and fertilizer amendments under long-term management

Ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) co-exist in soil, but their relative distribution may vary depending on the environmental conditions. Effects of changes in soil organic matter and nutrient content on the AOB and AOA are poorly understood. Our aim was to compare effects of long-term soil organic matter depletion and amendments with labile (straw) and more recalcitrant (peat) organic matter, with and without easily plant-available nitrogen, on the activities, abundances and community structures of AOB and AOA. Soil was sampled from a long-term field site in Sweden that was established in 1956. The potential ammonia oxidation rates, the AOB and AOA amoA gene abundances and the community structures of both groups based on T-RFLP of amoA genes were determined. Straw amendment during 50 years had not altered any of the measured soil parameters, while the addition of peat resulted in a significant increase of soil organic carbon as well as a decrease in pH. Nitrogen fertilization alone resulted in a small decrease in soil pH, organic carbon and total nitrogen, but an increase in primary production. Type and amount of organic matter had an impact on the AOB and AOA community structures and the AOA abundance. Our findings confirmed that AOA are abundant in soil, but showed that under certain conditions the AOB dominate, suggesting niche differentiation between the two groups at the field site. The large differences in potential rates between treatments correlated to the AOA community size, indicating that they were functionally more important in the nitrification process than the AOB. The AOA abundance was positively related to addition of labile organic carbon, which supports the idea that AOA could have alternative growth strategies using organic carbon. The AOB community size varied little in contrast to that of the AOA. This indicates that the bacterial ammonia oxidizers as a group have a greater ecophysiological diversity and potentially cover a broader range of habitats.

Trade-offs between the short- and long-term effects of residue quality on soil C and N dynamics

The decline of soil organic matter (SOM) and its associated fertility is one of the most important constraints to enhanced crop productivity in sub-Saharan Africa. Integrated soil fertility management recognizes the potential benefits of the combined use of organic residue and mineral fertilizer inputs for improved crop yield and SOM build up. However, these benefits may be controlled by residue quality. We examined the short- to long-term C and N dynamics following application of different quality residues with and without N fertilizer in a series of experiments comprising different timescales of measurement in a Kenyan Humic Nitisol. The combined results of these studies indicate that residue quality and fertilizer additions alter short-term C and N mineralization. Combining low quality residue and fertilizer inputs immobilized a greater amount of fertilizer-N than high quality residue. Under field conditions, this reduction in available N induced by the combination of low quality residue and fertilizer reduced environmental N losses and created a positive interactive effect on crop N uptake. While input management manipulated short-term nutrient dynamics, it did not influence long-term SOM stabilization. The input of residue, regardless of quality, contributed to long-term soil fertility improvement. In conclusion, organic residue quality can be manipulated to optimize short-term nutrient dynamics while still conferring the same benefits to long-term SOM contents.

Assessing the sustainability of wheat-based cropping systems using APSIM: model parameterisation and evaluation

Assessing the sustainability of crop and soil management practices in wheat-based rotations requires a well-tested model with the demonstrated ability to sensibly predict crop productivity and changes in the soil resource. The Agricultural Production Systems Simulator (APSIM) suite of models was parameterised and subsequently used to predict biomass production, yield, crop water and nitrogen (N) use, as well as long-term soil water and organic matter dynamics in wheat/chickpea systems at Tel Hadya, north-western Syria. The model satisfactorily simulated the productivity and water and N use of wheat and chickpea crops grown under different N and/or water supply levels in the 1998–99 and 1999–2000 experimental seasons. Analysis of soil-water dynamics showed that the 2-stage soil evaporation model in APSIM’s cascading water-balance module did not sufficiently explain the actual soil drying following crop harvest under conditions where unused water remained in the soil profile. This might have been related to evaporation from soil cracks in the montmorillonitic clay soil, a process not explicitly simulated by APSIM. Soil-water dynamics in wheat–fallow and wheat–chickpea rotations (1987–98) were nevertheless well simulated when the soil water content in 0–0.45 m soil depth was set to ‘air dry’ at the end of the growing season each year. The model satisfactorily simulated the amounts of NO3-N in the soil, whereas it underestimated the amounts of NH4-N. Ammonium fixation might be part of the soil mineral-N dynamics at the study site because montmorillonite is the major clay mineral. This process is not simulated by APSIM’s nitrogen module. APSIM was capable of predicting long-term trends (1985–98) in soil organic matter in wheat–fallow and wheat–chickpea rotations at Tel Hadya as reported in literature. Overall, results showed that the model is generic and mature enough to be extended to this set of environmental conditions and can therefore be applied to assess the sustainability of wheat–chickpea rotations at Tel Hadya.