The role and function of organic matter in tropical soils

Soil organic matter (SOM) has many functions, the relative importance of which differ with soil type, climate, and land use. Commonly the most importantfunction of OM in soil is as a reserve of the nitrogen and other nutrients required by plants, and ultimately by the human population. Other important functions include: the formation of stable aggregates and soil surface protection; maintenance of the vast array of biological functions, including the immobilization and release of nutrients; provision of ion exchange capacity; and storage of terrestrial carbon (C). This paper considers the quantity and quality of SOM of soils in the tropics, which are estimated to contain one quarter of the C in the global pool in terrestrial soils, and supports strongly the use of analytical methods to characterizing labile SOM to develop valuable insights into C dynamics. As in other regions, the transformation of tropical lands for agriculture exploits SOM, and in particular nutrient reserves. The process of exploitation is accelerated in the tropics by the necessity to increase agricultural production, largely through agricultural intensification, to overcome inadequate nutrition, to satisfy population growth, and to cope with the limited reserves of arable land. Poverty has an overriding influence on the exploitation and degradation processes. Areas at greatest risk of land degradation are the infertile acid soils of the tropics, which, invariably, are cultivated by the poor. Soil organic matter has a central role in sustainable land management, but perspectives on the roles of SOM differ widely between farmers, consumers, scientists and policy-makers. Some consider SOM as a source of nutrients to be exploited, whereas others can afford to utilize it as a key component in the management of the chemical, biological, and physical fertility of soils. Still others see SOM as a dumping ground for excess nutrients and toxins, or as a convenient store for fossil fuel emissions, particularly CO₂. Farmers need sustainable land management systems that maintain OM and nutrient reserves. Nevertheless, many available practices, whether based on indigenous or scientific knowledge, do not meet social and economic criteria that govern farmer behaviour. Much scientific knowledge about the various roles of SOM does not reach farmers and other decision-makers in a form that can be used easily. The biggest challenge to researchers is to engage with clients to pinpoint gaps in knowledge and utilize new and existing information to devise decision support Systems tailored to their needs. This revised version was published online in June 2006 with corrections to the Cover Date.     

Response of bulk chemical composition, lignin and carbohydrate signature to grassland conversion in a ley-arable cropping system

Grassland conversion is a common practice in ley-arable cropping systems. The effects of such a disturbance on soil organic matter status and its consequences for biogeochemical cycles in terms of soil organic matter (SOM) dynamics remain poorly understood. We investigated changes occurring in soil organic carbon and nitrogen content, bulk chemical composition and in lignin as well as carbohydrate signature during 2 years after grassland conversion into arable land. Our results showed a rapid SOM decrease in the first few months after the conversion. The bulk chemical composition as seen by solid-state ¹³C NMR spectroscopy was similar under grassland and arable land, whereas different landuse had an impact on the contribution of plant litter compounds to SOM. SOM of arable soil had higher lignin contents and lower contents of non-cellulosic neutral carbohydrates than grassland soil. After grassland conversion, the most prominent change was an increase of the SOM’s content of non-cellulosic carbohydrate above the contents recorded for grassland or arable land. Principal component analysis indicated that SOM chemical characteristics of converted grassland even after 2 years are similar to those of initial grassland. We conclude that the chemical composition of SOM is less susceptible to rapid change and that re-installation of grassland within some years will safeguard the initial SOM status in ley-arable rotations.     

汉中地区不同土壤阳离子交换容量的测定及分析

测定了汉中地区不同土壤阳离子交换容量来判断该地区土壤的保肥能力,结果表明：汉中地区土壤的阳离子交换量(CEC)约为24.1 cmol/kg大于20cmol/kg,因此有较好的肥力水平、保肥能力和缓冲能力,并分析与土壤CEC含量与土壤一些理化性质的相关性。     

Dynamics of soil nutrients and microbial biomass during first year cropping in an 8-year jhum cycle

The soil nutrient status and microbial biomass at three stages of firstyear cropping in an 8-year jhum (slash-and-burn agriculture) cycle system weredetermined and compared to an adjacent humid tropical forest in ArunachalPradesh, north-eastern India. Soil pH increased after burning and decreased asthe cultivation progressed in the jhum field. Soil organic carbon, available-P,total Kjeldahl nitrogen, ammonium-N and nitrate-N decreased as the duration ofcultivation increased. Microbial biomass C, N, and P were high in the foreststand. Microbial biomass C increased gradually as cultivation progressed, whilemicrobial biomass N and P showed a post-burn decreasing trend. Bacterial andfungal populations were drastically reduced following slash burning. The studyindicates that first-year cropping may result in temporary patternhomogenization of soil nutrient cycling, but can have drastic effects withcontinued slashing and burning for long-term agriculture.     

Vegetable cultivation under greenhouse conditions leads to rapid accumulation of nutrients,acidification and salinity of soils and groundwater contamination in South-Eastern China

Vegetable cultivation during winter season is economically profitable, but the impact of the intensive production on soil and water quality remains to be studied. The objectives of this study were to investigate the seasonal dynamics of soil nutrients, acidification and salt accumulation in vegetable fields in South-Eastern China. Various vegetables were grown either under open-field conditions or under two different alternating open-field and greenhouse conditions with three replications. Soil samples were collected periodically and analyzed for pH, plant available nitrogen (N), phosphorous (P), potassium (K), electrical conductivity (EC), and urease activity. Water samples from wells located in or near the plots were collected and analyzed for nitrate. Soil nitrate, available phosphate and salt concentrations declined in summer under open-field conditions and significantly increased from December to May under greenhouse conditions. Exchangeable K also decreased in summer season, but did not increase in the spring. Under alternating open-field and greenhouse conditions, nutrient accumulation, soil salinity and acidification were significantly higher for soil used for vegetable cultivation for 2 years (2-y-plot) than that for only half year (0.5-y-plot). The accumulation of nitrate significantly correlated with soil EC and soil acidification. Thirty-two percent of groundwater samples from the 2-y-plot showed a nitrate concentration higher than 50 mg NO₃ l⁻¹. Conversely, no groundwater sample of 0.5-y-plot showed such high nitrate concentration. It can be concluded that the nitrate accumulation in soil used for vegetable cultivation under alternating open-field and greenhouse conditions not only causes soil salinization and soil acidification but also presents a high pollution potential for groundwater.     

Organic matter turnover and management in low input agriculture of NE Brazil

The storage and release of nutrients by soil organic matter (SOM) is the primary determinant of soil fertility in low-input agriculture of semiarid NE Brazil. Traditional shifting cultivation systems have utilised the SOM built up during the fallow phase to supply nutrients for a cultivation phase of some 4–6 years. In this paper we analyse the turnover, stabilisation and quality of organic matter in landuse systems of NE Brazil. This analysis relies on a review of our own and literature data as well as farmers' perceptions recorded in a survey of 240 farms. Components critical for the understanding of SOM balances and transformations are residue inputs under native and agricultural vegetation, rates and controls of SOM mineralisation under cultivation, controls on SOM accretion under fallow vegetation, and the quality of SOM with respect to nutrient supply. While all these factors are known in outline, the detail of understanding that would be required for fine tuning management systems to be sustainable under present (and increasing) production pressure is still lacking. In particular quantity and quality of organic matter inputs from different vegetation types, controls on SOM stabilisation under different cultivation regimes, and the rates and synchrony with plant demand of nutrient release from mineralising organic matter need to be investigated in further detail. This revised version was published online in June 2006 with corrections to the Cover Date.     

Long-term impact of chronosequential land use change on soil carbon stocks on a Swedish farm

Agricultural practices and land use significantly influence soil carbon storage. The processes that are affected by land use and management are generally understood, but uncertainties in projections are high. In this paper, we investigate the long-term effects of chronosequential land use change from grassland to cropland and vice versa on soil carbon stock dynamics in four fields on a Swedish farm. Between 1850 and 1920, three of the fields were converted from grassland into cropland, and one was converted back to grassland in 1971. The fourth (control) field is a grassland that has never been ploughed. In 1937, the four fields were sampled at 111 points in a regular grid (25 or 50 m) and the dried soil samples were stored at our Department. In 1971 and 2002, the original grid points were revisited and re-sampled. Land use changes affected the soil C stock significantly. In 1937, carbon stocks were significantly smaller in the arable fields than in the grassland soil. In the field that was converted from arable back to grassland, soil C increased significantly at an average rate of about 0.4 Mg ha⁻¹ year⁻¹. A soil C balance model (ICBM) driven by standard meteorological data and soil carbon input estimated from yield records described soil carbon dynamics reasonably well, although the range of simulated relative changes in C stocks between 1937 and 2002 in the four fields (from −7.4 to +8.8%) was narrower than those measured (from −19.5 to +16.5%). There are only few long-term studies in Northern Europe available for quantifying the effect of land use change on soil carbon stocks and the results presented here are therefore useful for improving predictions of changes in soil carbon driven by land use change.     

Effect of conversion of natural grassland to cropland on nitric oxide emissions from Venezuelan savanna soils. A four-year monitoring study

Conversion of “natural” savanna to agricultural soil is proceeding at a very fast rate. In this work, nitric oxide (NO) emissions were measured immediately after conversion of a “natural” grassland to fertilized agricultural fields (corn, sorghum and cultivated pasture), and also during the second and fourth year after conversion. Large fluxes were observed after plowing grassland soil, and almost no difference was observed between ammonium-fertilized and unfertilized plowed grassland soils. Soil water content and pH related negatively to NO emissions. NO emissions were positively correlated with soil nitrate concentrations and carbon content (corn only) whereas little or no correlation was found with ammonium concentrations. This suggests that in these savanna soils, the NO emitted from soil is mainly denitrification-derived. The disturbed soils emitted on average ∼7 times more NO than the original grassland. During the second and fourth year of cultivation the emissions from corn and sorghum fields (plowed every year) were around 10 times higher than from the control grassland soils. The fertilizer-induced emission (FEI) values for NO emission estimated in this work show that similar agricultural practices could lead to dramatically different FEI values depending on the water content of the soils. These FEI results corroborates that modeling approaches to determine the global agriculture-derived NO emissions should take into consideration the main factors that regulate the NO emission at the scale of functional units with similar climate, soil and management conditions instead of expressing it as a percentage of the fertilizer applied. In our sandy loam savanna soils these factors are inorganic nitrogen content, pH and WFPS.     

Effect of long-term application of inorganic fertilizer and organic amendments on soil organic matter and microbial biomass in three subtropical paddy soils

Soil organic matter (SOM), microbial carbon (C_mic), and microbial nitrogen (N_mic) status affected by the application of inorganic fertilizer and organic amendments in subtropical paddy soils were investigated. Soil samples were collected from the plow layer of three long-term (17 years) field experiments at Xinhua, Ningxiang, and Taojiang counties in Hunan Province, China. Results showed that, compared to the control, application of inorganic fertilizer alone showed no significant effect on soil organic C (SOC), total N (N_tot), C_mic and N_mic. The application of inorganic fertilizer along with manure or straw significantly increased SOC and N_tot and soil C_mic and N_mic contents for all three sites, while following an application of inorganic fertilizer along with straw only for two sites. C_mic and N_mic were closely correlated with SOC and N_tot, respectively. In conclusion, application of inorganic fertilizer along with manure or straw is an effective way of enhancing SOM and microbial biomass in subtropical paddy soils.     

Influence of topography and land management on soil nutrients variability in Northeast China

It is well recognized that soil nutrient content varies across the landscape, but the nature and degree of that variability with respect to landscape position is still poorly understood and documented. Slope steepness and aspect, climate and land management are known to affect soil nutrient distribution in a field, but the relative and cumulative strengths of these effects are less well investigated. Four hundred and thirty-five topsoil samples collected from a typical Mollisol under intensive crop management in Northeast China were used to analyze the influence of landscape position, climate and land management on the spatial variability of soil organic matter (SOM), total nitrogen (TN) and total phosphorus (TP). Both geo-statistics and traditional statistics were used to analyze the data, and significant spatial variability was found for SOM (22.5–86.6 g kg⁻¹), TN (0.98–4.26 g kg⁻¹) and TP (0.26–1.80 g kg⁻¹). The distribution of all 3 nutrients was found to be influenced by human activity and by landscape. When both slope degree and slope aspect were considered, the results differed from when only aspect or steepness was considered independently. In a northern aspect, SOM and TN were significantly higher on slopes of 0–2% than on steeper slopes, in a south-eastern aspect they were significantly higher on slopes of 0–2, 2–3 and 3–4% than on slopes >4% and in a south-western aspect those nutrients on slopes of 2–4% were significantly higher than on slopes of >5%. Cross-slope tillage effectively increased SOM, TN and TP by 33.8, 23.3 and 22.4%, respectively compared to down-slope tillage, indicating the potential for adoption of a nutrient-retaining management practice in the Mollisol region of northeast China.     

Nitrogen pools and fluxes in grassland soils sequestering carbon

Carbon sequestration in agricultural, forest, and grassland soils has been promoted as a means by which substantial amounts of CO₂ may be removed from the atmosphere, but few studies have evaluated the associated impacts on changes in soil N or net global warming potential (GWP). The purpose of this research was to (1) review the literature to examine how changes in grassland management that affect soil C also impact soil N, (2) assess the impact of different types of grassland management on changes in soil N and rates of change, and (3) evaluate changes in N₂O fluxes from differently managed grassland ecosystems to assess net impacts on GWP. Soil C and N stocks either both increased or both decreased for most studies. Soil C and N sequestration were tightly linked, resulting in little change in C:N ratios with changes in management. Within grazing treatments N₂O made a minor contribution to GWP (0.1–4%), but increases in N₂O fluxes offset significant portions of C sequestration gains due to fertilization (10–125%) and conversion (average = 27%). Results from this work demonstrate that even when improved management practices result in considerable rates of C and N sequestration, changes in N₂O fluxes can offset a substantial portion of gains by C sequestration. Even for cases in which C sequestration rates are not entirely offset by increases in N₂O fluxes, small increases in N₂O fluxes can substantially reduce C sequestration benefits. Conversely, reduction of N₂O fluxes in grassland soils brought about by changes in management represents an opportunity to reduce the contribution of grasslands to net greenhouse gas forcing.     

Combined effect of water and organic matter on phosphorus availability in calcareous soils

Phosphorus removal from soil solution is mainly due to adsorption and precipitation. For calcareous soils, with a large reservoir of exchangeable calcium, precipitation of insoluble Ca-P phases is the predominant process that reduces P availability to plants. Soil water content positively affects P-precipitation, while the addition of organic matter (OM) has an opposite effect. Little information on the effect of soil organic matter on P-insolubilisation as a function of soil water contents has prompted this study of the variation of extractable P, after addition of mineral P fertiliser. Columns packed with a calcareous soil were enriched with different levels of OM, extracted from Irish peat, and subjected to different rainfall simulations. After 102 days of experimentation and 171 mm of accumulated rainfall, the Olsen-P was 53% of the initially applied amount in 6.2% OM-enriched soil, 37% in 4.1% OM-enriched soil, and 20% in untreated soil (1.9% of OM). While the curve describing Olsen-P decrease as a function of accumulated rainfall was clearly exponential for untreated soil, the curves for OM-enriched samples were flatter, evidence that OM addition modified P-insolubilisation. The P-insolubilisation, after P-fertilisation, at several constant values of soil moisture for (i) calcareous soil, (ii) calcareous soil after removing carbonates and saturating the exchange complex with Ca, and (iii) calcareous soil after addition of different levels of OM followed first-order kinetics. The K_obss followed the order: Ca-saturated soil > untreated soil > OM-enriched samples. Results from rainfall simulation experiments and kinetics of Olsen-P decrease at several constant soil moisture contents indicated that the soil water amount was the main factor in reducing extractable P after P fertilisation and that the soil OM content was the main factor in keeping P in extractable forms. On the other hand, the addition of OM to calcareous soil increased the extractable P at each soil moisture regime, decreasing P-insolubilisation more effectively at lower soil water contents. P-sorption isotherms of calcareous soil after addition of different levels of OM showed that the presence of OM mainly influences P-insolubilisation, but not the adsorption process.     

不同侵蚀部位微生物变异性研究

土壤是一个由生物和非生物组成的复杂综合体,肥力是土壤的本质和属性。土壤养分含量和土壤微生物特征是表示土壤肥力高低的重要指标,水土流失不仅导致土壤养分含量下降,而且对土壤微生物特征产生明显的影响,进而影响到土壤养分的循环和转化。本研究对华南赤红壤丘陵坡地不同侵蚀部位微生物特性的变异性进行了对比分析。结果表明：随着土壤侵蚀程度的加剧和植被的破坏,土壤微生物总数逐渐减少,土壤基础呼吸和土壤诱导呼吸显著下降。在没有草本植物的情况下,仅有人工荷木并不能起到明显的水土保持作用,近地表覆盖的草本植物应是水土保持植被建设必不可少植物类群。以此为华南赤红壤丘陵区土壤生态环境修复和水土保持提供科学依据。     

Farmers' perceptions and management of soil organic matter – a case study from West Africa

A farm survey was carried out in 155 Ghanaian villages covering parts of the forest and savanna zones of West Africa to assess farmers' views on ‘soil organic matter’ (SOM) and its management. The results of a closed questionnaire accompanied by open discussions showed that most farmers are well aware of SOM and its importance for crop yields. In southern Ghana, farmers perceive SOM generally by its colour, while in northern Ghana, it is mostly assessed by the density and kind of vegetation. Farmers' perception of the properties of SOM was directed at its main functions as a primary provider of plant nutrients and its ability to conserve water. Other properties mentioned were the improvement of soil aeration and drainage, the loosening of soil structure as well as its impact on soil temperature. The major strategies farmers used in maintaining or augmenting SOM levels were: manure application, mulching with crop residues, slashing weeds without burning, composting, and shifting cultivation (natural fallow). Promoted technologies, such as green manuring, no tillage, or agroforestry were used only by a few of the farmers interviewed. The differences between farmers' views and strategies in the two zones as well as farmers' constraints in SOM management are discussed. It appears that the level of farmers' commitment to excellent soil management can vary with biophysical as well as socio-economic conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

The impact of historic land use and modern forestry on nutrient relations of Central European forest ecosystems

In the past forests in Central Europe were not only sources of timber and fuelwood but also sources of nutrients to sustain the human population. The use of forests as pastures is still common in some areas while other formerly widespread practices such as litter raking or pollarding have been abandoned. Harvesting of wood, a material of extremely low mineral nutrient content and of wide C/N-ratio depletes nutrients and acid neutralizing capacity at only moderates rates, harvesting of other biomass fractions has a much more severe impact on forest ecosystems. Soil acidification from intensive biomass harvesting of historic land use equaled or exceeded present soil acidification due to the deposition of air pollutants. As a result of historic land use the majority of Central European forest ecosystems was severely depleted of nutrients and acid neutralizing capacity when modern long-rotation forestry became the dominant form of forest land use. At present high deposition rates of acidifying air pollutants prevent the recovery of forest ecosystems in Central Europe. It has to be noted that ecosystem degradation due to excessive biomass harvesting led to systems which were depleted both in nitrogen and acid neutralizing capacity, while high nitrogen deposition rates from Central European air pollution cause a novel combination of progressive soil acidification and concurrent nitrogen saturation. This combination has a high potential for aggravating mineral nutrient deficiencies and nutritional disorders in forest ecosystems.     

Effects of long-term fertilization and mulch on soil fertility in contour hedgerow systems: A case study on steeplands from the Three Gorges Area,China

The use of contour hedgerows is widely advocated to sustain crop production and reduce soil loss on steeplands in the Three Gorges Area of China. However, little is known about the effects of soil management on soil fertility within these systems, or about the spatial gradients in soil nutrients that may develop in terraces formed behind the vegetative barriers. Therefore, we carried out a study on the effects of various long-term soil management practices on soil fertility and spatial variation of fertility between hedgerows. At a site in the Three Gorges Area, China, we applied five treatments to a contour hedgerow system: control (no fertilizer and manure); chemical fertilizer (CF); chemical fertilizer and mulch (CF + MU); pig manure (PM); and mulch, pig manure, and chemical fertilizer (CF + PM + MU). Soil samples were collected from the topsoil horizon (0–20 cm) of the selected five treatments in 2006 after 11 crop cycles, and physical and chemical properties were analyzed. The results showed that chemical fertilizer clearly improves nutrient status of the topsoil, while pig manure also increased the amount of soil organic matter. This increase in organic matter was associated with an increase in soil aggregate stability, a reduction in bulk density, and reduced penetration resistance of the soil. Mulch with pig manure and chemical fertilizer was the best management practice for improving soil quality and crop yields in the Three Gorges Area. Further, mulch and pig manure addition also decreased the magnitude of the spatial variation, but did not offset the soil fertility gradients because tillage resulted in significant movement of soil. More favorable soil properties were found at the lower positions within each alley, regardless of the management practice applied.     

Methane oxidation in soils with different textures and land use

Intact core samples from soils with different textures and land use were tested for their capacity to oxidise methane. The soil cores were taken from arable land, grassland and forest. It was found that coarse textured soils (6.74–16.38 μg CH₄ m^-2 h^-1) showed a higher methane uptake rate than fine textured soils (4.66–5.34 μg CH₄ m^-2 h^-1). Increasing soil tortuosity was thought to reduce the methane oxidation rate in fine textured soils. The oxidation rate of forest soils (16.32–16.38 μg CH₄ m^-2 h^-1), even with a pH below 4.5, was very pronounced and higher than arable land (11.40–14.47 μg CH₄ m^-2 h^-1) and grassland (6.74–9.30 μg CH₄ m^-2 h^-1). Within the same textural class arable land showed a faster methane uptake rate than grassland. In grassland with a fine texture, even methane production was observed. Nitrogen availability and turnover in these land use systems were thought to cause the different oxidation rates. Decreasing the moisture content slowed down the oxidation rate in all soils. This could be caused by an increased N turnover and a starvation of the methanotrophic bacteria. This revised version was published online in August 2006 with corrections to the Cover Date.     

Agronomic and economic performance of reservoir sediment for rehabilitating degraded soils in Northern Ethiopia

Nutrient export through sediment is, often, not given due attention in improving the fertility of soils. A Randomized Complete Block Design (RCBD) (replicated three times) was conducted to assess the effect of sediment, collected from a microdam, on the yield of a local wheat cultivar (Triticum aestivum) grown on a Luvisol and a Vertisol during cropping season of 2003 and 2004 in Tigray, Ethiopia. In the first season, the treatments consisted of applying 34.3 Mg sediment ha⁻¹ (T1), 0.1 Mg urea ha⁻¹ (T2), 4 Mg manure ha⁻¹ (T3) and a control plot (T4). The experiment was repeated in the 2004 cropping season using the same indicator crop but without the use of T1 and T3 inputs to evaluate the residual effect of these treatments on crop productivity. Plant height, plant density, spike length, straw and grain yield were compared among the treatments. After harvest, surface soil (0–25 cm) samples were taken and analyzed for total Nitrogen (Ntot), available Phosphorous (P), Soil Organic Carbon (SOC), Cation Exchange Capacity (CEC), and porosity to examine treatments effect on these soil properties. Higher CEC, exchangeable bases, SOC, Ntot, available P and micronutrients except Cu characterized sediment compared to the two soils used. Application of sediment increased the average grain and straw yield of wheat by 72% and 12%, respectively, in the Luvisol, and by 38% and 23%, respectively, in the Vertisol relative to the control. However, the application of sediment didn’t change the physicochemical properties of the Vertisol, but in the Luvisol CEC and available P were increased significantly. Like manure, sediment provided marginal rate of return above the minimum acceptable value (100%) by farmers, demonstrating the potential of these resources to improve degraded soils and boost farmers’ income.     

Soil N mineralization in a dairy production system with grass and forage crops

This paper describes the dynamics of soil N mineralization in the experimental intensive dairy farming system ‘De Marke’ on a dry sandy soil in the Netherlands. We hypothesized that knowledge of the effects of crop rotation on soil N mineralization and of the spatial and temporal variability of soil N mineralization in a farming system can be used to better synchronize N application with crop N requirements, and hence to increase the recovery of applied N and to reduce N losses. Soil N mineralization was recorded continuously in the soil layer 0–0.30 m, from 1992 to 2005, using a sequential in situ coring technique on six observation plots, of which two were located in permanent grassland and four in crop rotations with a 3 year grassland phase and an arable phase of 3 or 5 years, dominated by maize. Average annual soil N mineralization was highest under permanent grassland: 381 kg ha^?1 and lowest under ≥3rd years arable crops: 184 kg ha^?1. In temporary grassland, soil N mineralization increased in the order: 1st year, 2nd year, 3rd year grassland and in arable crops after grassland mineralization decreased in the order: 1st year, 2nd year, ≥3rd years. Total mineral N input, i.e. the sum of N mineralization and mineral N supply to soil, exceeded crop N requirements in 1st year maize and was lower than the requirements in 1st year temporary grassland. N mineralization in winter, outside the growing season, was 77 kg ha^?1 in maize and 60 kg ha^?1 in grassland. This points at the importance of a suitable catch crop to reduce the susceptibility to N leaching. Temporal and spatial variability of soil N mineralization was high and could not be related to known field conditions. This limits the extent to which N fertilization can be adjusted to soil N mineralization. Variability increased with the magnitude of soil N mineralization. Hence, situations with high soil N mineralization may be associated with high risks for N losses and to reduce these risks a strong build-up of soil organic N should be avoided. This might be achieved, for instance, by fermenting slurry before application on farmland to enhance the fraction mineral N in slurry at the expense of organic N.     

Management of organic matter in the tropics: translating theory into practice

Inputs of organic materials play a central role in the productivity of many tropical farming systems by providing nutrients through decomposition and substrate for synthesis of soil organic matter (SOM). The organic inputs in many tropical farming systems such as crop residues, manures, and natural fallows are currently of low quality and insufficient quantity to maintain soil fertility hence there is need to find alternative or supplementary sources of nutrients. Knowledge gained over the past decade on the role of organic resource quality in influencing soil nutrient availability patterns (Synchrony Principle) and SOM maintenance (SOM Principle) provides a strong scientific basis on which to develop management tools. This scientific information must be linked with farmer knowledge and circumstances to provide a realistic approach to soil fertility and SOM management in the tropics. A decision tree has been developed for testing hypotheses about the resource quality parameters that affect nitrogen release patterns and rates. The decision tree is linked to an Organic Resource Database (ORD) with detailed information on the resource quality of agroforestry trees and leguminous cover crops providing a systematic means of selecting organic resources for soil fertility management. The decision tree has also been translated into a practical field guide for use with farmers in evaluating organic materials. The longer-term effects of organic inputs on SOM might also be addressed through the decision tree and database. It is generally believed that materials good for short-term soil fertility will not build or maintain SOM; if true then it is difficult to imagine practical means of maintaining SOM in the African context where short-term fertility issues will take precedence over longer-term maintenance of SOM. This revised version was published online in June 2006 with corrections to the Cover Date.