Maintenance Of Soil Organic Matter Research Articles

Environmental impacts of organic agriculture in temperate regions.

Abstract Can organic agriculture elaborate a scientifically based, resource-efficient and agroecological approach to low-input farm management? This review examines the literature from temperate regions, with a particular emphasis on Canadian and US studies that relate to environmental and ecological impacts of organic agriculture with respect to (i) soil organic matter storage, (ii) soil quality/soil health, (iii) nutrient loading and risks of off-farm nutrient and agrochemical losses, (iv) biodiversity and (v) energy use and global warming potential. The context and implications of semi-arid conditions and low soil P levels, common to many organic farms in North America, and widespread adoption of genetically engineered crops in conventional production, is also considered. The consensus of the data available to date indicates the distinctiveness of cropping, floral and habitat diversity, soil management regime, nutrient intensity and use efficiency, and energy, and pesticide use in organic farming confer important environmental and ecological benefits. These include maintenance of soil organic matter and added return of carbon to soil, improved soil health, reduced off-farm nitrogen and phosphorus losses, enhanced vegetative and wildlife (bird) biological diversity, extended sometimes to other taxa depending on landscape context, improved support for pollinators and pollination and reduced energy use and improved energy efficiency. The continued evolution of organic agriculture to a more outcomes-based, agroecological production system will require an expanded multi-disciplinary research effort, linked ideally to support from consumers and policy-makers on the basis of renewed understanding of its potential contribution to global environmental sustainability.

Effectiveness of the GAEC standard of cross compliance Crop rotations in maintaining organic matter levels in soils

Our study was conducted in the framework of EFFICOND project, with the aim of evaluating the environmental effectiveness of GAEC (Good Agricultural and Environmental Conditions) standards with particular focus to the maintenance of soil organic matter (SOM) levels through the appropriate crop rotation. The study analyzed the effect of crop rotation on the build-up of soil organic matter in three different areas of Italy, located in the North (Lodi), Centre (Fagna, Firenze) and South (Foggia) of Italy, characterized by different climate, soil, and cropping systems. In the two experiments conducted in the South of Italy, in a dry Mediterranean climate, the stock of C was kept steady in most of the rotations compared with the monoculture of durum wheat. In such environment, with very dry and hot summers, introducing a year of fallow seems to improve SOM content, but these data need further investigation. In the Centre of Italy (Fagna), with less extreme climate than in Foggia, the effect of rotation compared to the monoculture of maize is negligible, but investigation on the soil organic matter composition, showed that in the rotation the SOM appeared to be more stable and, in the long term, probably more resistant to degradation. Eventually, experiments conducted in the North of Italy, showed that the monoculture, despite the application of FYM (Farm Yard Manure) or semi-liquid manure, led to a decrease of SOM. To an increase of the rotation complexity, corresponded an increase in the stock of C in soil. Summarizing, results showed that crop rotation could guarantee the maintenance of SOM level, given that the input of C to the soil is maintained at a good level or, in other word, that productivity of the system is high. Other practices such as conservation tillage, appropriate management of residues, and manure application could enhance the positive effect of rotations. Moreover, preliminary investigation of soil microbial diversity, suggests the positive effect of rotations on soil biological fertility.

A five-year assessment of corn stover harvest in central Iowa, USA

Sustainable feedstock harvest strategies are needed to ensure bioenergy production does not irreversibly degrade soil resources. The objective for this study was to document corn ( Zea mays L.) grain and stover fraction yields, plant nutrient removal and replacement costs, feedstock quality, soil-test changes, and soil quality indicator response to four stover harvest strategies for continuous corn and a corn–soybean [ Glycine max. (L.) Merr.] rotation. The treatments included collecting (1) all standing plant material above a stubble height of 10 cm (whole plant), (2) the upper-half by height (ear shank upward), (3) the lower-half by height (from the 10 cm stubble height to just below the earshank), or (4) no removal. Collectable biomass from Treatment 2 averaged 3.9 (±0.8) Mg ha −1 for continuous corn (2005 through 2009), and 4.8 (±0.4) Mg ha −1 for the rotated corn (2005, 2007, and 2009). Compared to harvesting only the grain, collecting stover increased the average N–P–K removal by 29, 3 and 34 kg ha −1 for continuous corn and 42, 3, and 34 kg ha −1 for rotated corn, respectively. Harvesting the lower-half of the corn plant (Treatment 3) required two passes, resulted in frequent plugging of the combine, and provided a feedstock with low quality for conversion to biofuel. Therefore, Treatment 3 was replaced by a “cobs-only” harvest starting in 2009. Structural sugars glucan and xylan accounted for up to 60% of the chemical composition, while galactan, arabinan, and mannose constituted less than 5% of the harvest fractions collected from 2005 through 2008. Soil-test data from samples collected after the first harvest (2005) revealed low to very low plant-available P and K levels which reduced soybean yield in 2006 after harvesting the whole-plant in 2005. Average continuous corn yields were 21% lower than rotated yields with no significant differences due to stover harvest. Rotated corn yields in 2009 showed some significant differences, presumably because soil-test P was again in the low range. A soil quality analysis using the Soil Management Assessment Framework (SMAF) with six indicators showed that soils at the continuous corn and rotated sites were functioning at an average of 93 and 83% of their inherent potential, respectively. With good crop management practices, including routine soil-testing, adequate fertilization, maintenance of soil organic matter, sustained soil structure, and prevention of wind, water or tillage erosion, a portion of the corn stover being produced in central Iowa, USA can be harvested in a sustainable manner.

Soil fertility management: Impacts on soil macrofauna, soil aggregation and soil organic matter allocation

Maintenance of soil organic matter through integrated soil fertility management is important for soil quality and agricultural productivity, and for the persistence of soil faunal diversity and biomass. Little is known about the interactive effects of soil fertility management and soil macrofauna diversity on soil aggregation and SOM dynamics in tropical arable cropping systems. A study was conducted in a long-term trial at Kabete, Central Kenya, to investigate the effects of organic inputs (maize stover or manure) and inorganic fertilizers on soil macrofauna abundance, biomass and taxonomic diversity, water stable aggregation, whole soil and aggregate-associated organic C and N, as well as the relations between these variables. Differently managed arable systems were compared to a long-term green fallow system representing a relatively undisturbed reference. Fallowing, and application of farm yard manure (FYM) in combination with fertilizer, significantly enhanced earthworm diversity and biomass as well as aggregate stability and C and N pools in the top 15 cm of the soil. Earthworm abundance significantly negatively correlated with the percentage of total macroaggregates and microaggregates within macroaggregates, but all earthworm parameters positively correlated with whole soil and aggregate associated C and N, unlike termite parameters. Factor analysis showed that 35.3% of the total sample variation in aggregation and C and N in total soil and aggregate fractions was explained by earthworm parameters, and 25.5% by termite parameters. Multiple regression analysis confirmed this outcome. The negative correlation between earthworm abundance and total macroaggregates and microaggregates within macroaggregate could be linked to the presence of high numbers of Nematogenia lacuum in the arable treatments without organic amendments, an endogeic species that feeds on excrements of other larger epigeic worms and produces small excrements. Under the conditions studied, differences in earthworm abundance, biomass and diversity were more important drivers of management-induced changes in aggregate stability and soil C and N pools than differences in termite populations.

The effect of plantation silviculture on soil organic matter and particle-size fractions in Amazonia

Eucalyptus grandis and other clonal plantations cover about 3.5 million ha in Brazil. The impacts of intensively-managed short-rotation forestry on soil aggregate structure and Carbon (C) dynamics are largely undocumented in tropical ecosystems. Long-term sustainability of these systems is probably in part linked to maintenance of soil organic matter and good soil structure and aggregation, especially in areas with low-fertility soils. This study investigated soil aggregate dynamics on a clay soil and a sandy soil, each with a Eucalyptus plantation and an adjacent primary forest. Silvicultural management did not reduce total C stocks, and did not change soil bulk density. Aggregates of the managed soils did not decrease in mass as hypothesized, which indicates that soil cultivation in 6 year cycles did not cause large decreases in soil aggregation in either soil texture. Silt, clay, and C of the sandy plantation soil shifted to greater aggregate protection, which may represent a decrease in C availability. The organic matter in the clay plantation soil increased in the fractions considered less protected while this shift from C to structural forms considered more protected was not observed.

Black oat cover crop management effects on soil temperature and biological properties on a Mollisol in Texas, USA

Management of a black oat ( Avena strigosa [Schreb.]) cover crop by mowing method (none, flail mowing, or sickle bar mowing) affected soil micro environmental conditions and soil microbial and chemical properties. Soil temperatures at depths of 0, 5, 10 and 20 cm were highest in flail mowed treatment plots (up to near 45 °C at 5 cm depth), followed by sickle bar mowed plots (averaging 10 °C lower at 5 cm depth). Lowest soil temperatures were maintained in plots that were not mowed; averaging 5 °C lower at 5 cm depth than the sickle bar mowed plots. Increasing temperature convergence was observed below 5 cm, with the highest temperature (flail mowed treatment) at 32 °C and the lowest (not mowed) at 27 °C. Microbial biomass increased significantly ( P < 0.05) from < 100 mg C kg − 1 soil in the late spring, to > 150 mg C kg − 1 in the early fall. β-glucosidase and alkaline phosphatase activities decreased slowly throughout the experiment, and were significantly greater ( P < 0.05) only when comparing the not mowed treatment to both mowing treatments. Dissolved organic C (DOC) decreased sharply, perhaps due to rainfall. Similarly, soil inorganic N decreased sharply, from approximately 0.025 mg N kg − 1 soil in the not mowed plots, to approximately 0.005 mg N kg − 1 during a rainy period. Immobilization of N during rapid microbial biomass growth could also explain the decrease in inorganic N. While resin-extractable P was significantly higher in the not mowed treatment, few differences in extractable P were detected between either mowing treatment during the experiment. Standing oat residues maintained higher levels of soil enzyme activities and the lowest microbial biomass, suggesting that controlling contact of crop residues with the soil may introduce a rate limiting factor for residue decomposition. The timing of residue decomposition in this system may be delayed by leaving most of the plant biomass above the ground (i.e. not mowed or sickle bar mowed). These residue management factors may be important in controlling soil nutrient transformations and soil organic matter maintenance in subtropical climates.

USE OF WATER HYACINTH (Eichornia crassipes) IN AMELIORATION OF A SANDY SOIL

Soil fertility management is a key issue for sustaining agricultural production in dry lands. Organic resources are important for nutrient avail-ability and maintenance of soil organic matter. Sustaining fertility of arid soils necessitates in-creasing inputs of organic residues. Two experi-ments with water hyacinth were carried out to monitor (in the first; field study with litter bags) decomposition and changes in N, P, K, Ca and Mg as influenced by method of application where, in the second (greenhouse study) short-term effect on some properties of a sandy soil and early estab-lishment of fodder sorghum (Sorghum bicolor). Results of the first experiment showed that resi-dues incorporated in the top 0-30cm soil depth had significantly (P≤0.01) increased loss of dry matter weight compared to surface application. Decom-position rate constants of surface and sub-surface application were 0.029% week-1 and 0.041% week-1, respectively. Potassium was the fastest nutrient released with 50% loss attained after the 2nd and 6th week from incorporation and surface application, respectively. Nitrogen, P, Ca, and Mg releases from water hyacinth incorporated in the sub-soil were significantly (P≤ range from 0.0001 to 0.008) high compared to surface application. Nutrients loss was in the order of K =Mg > P > N > Ca. However, results of the second experi-ment showed that incorporation of water hyacinth residues significantly (P ≤ 0.001) decreased pH (by about 27%) and hydraulic conductivity (by 40-46%). Also, cation exchange capacity and organic C were significantly increased by about 23 and 100%, respectively. The study concluded that ap-plication of water hyacinth could be one of the good alternatives for improving quality of arid soils.

Organic matter quality and management effects on enrichment of soil organic matter fractions in contrasting soils in Zimbabwe

Maintenance of soil organic matter (SOM) at levels that sustain optimal supply of soil nutrients and enhance efficiency of externally added fertilizers is a major challenge for smallholder farming systems of southern Africa. A study was conducted to quantify the interactive effects of organic resource quality and management on SOM formation and subsequent maize yields under contrasting soil types. Crotalaria juncea L., Calliandra calothyrsus Meissn., cattle manure, maize (Zea mays L.) stover and Pinus patula Schiede and Schltdl. and Cham. sawdust were applied at 1.2 and 4 t C ha−1 at Domboshawa and Makoholi Experimental Stations, simulating some of the soil amendments commonly available on smallholder farms. Soils at Domboshawa are sandy–clay loams with 220 g clay kg−1 while the sandy soils at Makoholi had <100 g clay kg−1. At 12–14 weeks after incorporation, organic resource quality effects on particulate organic matter (POM) C enrichment were most significant (p < 0.01) in the macro-POM (250–2,000 μm diameter) fraction of both soil types constituting 15–30% of total soil C on coarse sand soil and 5–10% on sandy clay loam soils. The highest increases were under C. calothyrsus, manure and sawdust treatments. There was evidence of sub-soil enrichment under these two treatments on sandy soils at different sites. While no significant treatment effects were observed on the size of organo-mineral fraction, there was a significant (p < 0.05) separation of treatments in terms of potential mineralizable N from the same fraction. On coarse sands, organo-mineral fraction under medium to high-quality materials such as manure and C. juncea released ∼50 mg N kg−1, compared to 8–18 mg N kg−1 from sawdust and maize stover, suggesting that such materials enhanced the N-supply capacity of this fraction without necessarily increasing its size. The same trends were observed under sandy clay loams although, in contrast to coarse sands, the high-quality materials released no more than 25 mg N kg−1, suggesting that the added C was protected against short-term mineralization. These contrasting properties were also reflected in maize yield patterns. On sandy clay loams, a significant linear relationship between maize yield and the amount of mineralizable N in the macro-POM fraction (R2 = 0.50; p < 0.01) was evident, while the best predictor for maize yield on coarse sands was the amount of mineralizable N from the organo-mineral fraction (R2 = 0.86). We concluded that maize productivity on contrasting soil types hinges on different soil organic fractions and therefore require different management strategies. Sustainability of cropping on sandy soils is likely to depend on a regular supply of high-quality C materials, which enhance the nutrient supply capacity of the small organo-mineral fraction. Under the relatively C protective sandy clay loams, it is apparently the size of the macro-POM fraction which largely determines crop yields in the short-term.

Tillage and Manure Effects on Soil and Aggregate‐Associated Carbon and Nitrogen

In agricultural systems, maintenance of soil organic matter (SOM) has long been recognized as a strategy to reduce soil degradation. No‐tillage and manure amendments are management practices that can increase SOM content and improve soil aggregation. We investigated the effects of 10‐yr of different tillage systems and N sources on soil aggregate‐size distribution and aggregate‐associated C and N. The study was a split‐plot design replicated four times. The main plot treatment was tillage (no‐tillage, NT; conventional tillage, CT) and the subplot treatment was N source (manure, M; NH4NO3 fertilizer, F). The experiment was established in 1990 on a moderately well‐drained Kennebec silt loam (Fine‐silty, mixed, superactive mesic Cumulic Hapludoll) with continuous corn (Zea mays L.). In 1999, soil samples were collected (0‐ to 5‐cm depth) from the field treatments and separated into four aggregate‐size classes (&gt;2000, 250–2000, 53–250, and 20–53 μm) by wet sieving. Labile C and N content of all aggregate‐size fractions were measured using 28‐d laboratory incubations of intact and crushed aggregates. No‐tillage and M treatments significantly increased total C and N and the formation of macroaggregates. Conventional tillage in comparison with NT significantly reduced macroaggregates with a significant redistribution of aggregates into microaggregates. Aggregate protected labile C and N were significantly greater for macroaggregates, (&gt;2000 and 250–2000 μm) than microaggregates (53–250 and 20–53 μm) and greater for M than F indicating physical protection of labile C within macroaggregates. No‐tillage and M a lone each significantly increased soil aggregation and aggregate‐associated C and N; however, NT and M together further improved soil aggregation and aggregate‐protected C and N.

Tillage and Manure Effects on Soil and Aggregate-Associated Carbon and Nitrogen

In agricultural systems, maintenance of soil organic matter (SOM) has long been recognized as a strategy to reduce soil degradation. No-tillage and manure amendments are management practices that can increase SOM content and improve soil aggregation. We investigated the effects of 10-yr of different tillage systems and N sources on soil aggregate-size distribution and aggregate-associated C and N. The study was a split-plot design replicated four times. The main plot treatment was tillage (no-tillage, NT; conventional tillage, CT) and the subplot treatment was N source (manure, M; NH4NO3 fertilizer, F). The experiment was established in 1990 on a moderately well-drained Kennebec silt loam (Fine-silty, mixed, superactive mesic Cumulic Hapludoll) with continuous corn (Zea mays L.). In 1999, soil samples were collected (0- to 5-cm depth) from the field treatments and separated into four aggregate-size classes (>2000, 250–2000, 53–250, and 20–53 μm) by wet sieving. Labile C and N content of all aggregate-size fractions were measured using 28-d laboratory incubations of intact and crushed aggregates. No-tillage and M treatments significantly increased total C and N and the formation of macroaggregates. Conventional tillage in comparison with NT significantly reduced macroaggregates with a significant redistribution of aggregates into microaggregates. Aggregate protected labile C and N were significantly greater for macroaggregates, (>2000 and 250–2000 μm) than microaggregates (53–250 and 20–53 μm) and greater for M than F indicating physical protection of labile C within macroaggregates. No-tillage and M a lone each significantly increased soil aggregation and aggregate-associated C and N; however, NT and M together further improved soil aggregation and aggregate-protected C and N.

Sustainable Crop Production Through Management of Soil Organic Carbon in Semiarid and Tropical India

ABSTRACT Enhancing the long-term sustainability in productivity of crops and cropping systems is directly related to maintenance of an adequate level of soil organic matter. Indian soils contain about 0.1 to 1% organic carbon. The maintenance of soil organic matter in agricultural soils, particularly of semiarid and subtropical regions of India, is generally governed by annual temperature, precipitation and many interacting factors such as soil types, tillage, application of fertilizers, quality and quantity of organics returned to soil, and the method of residue management. In India most residues used are as feed and fuel and one-third of crop residues is used as a source of organic matter. Continuous use of the same crops in the cropping system, unbalanced and inappropriate use of chemical fertilizer, and minimum or no use of organic matter year after year are a major constraints to soil organic carbon accumulation. Alternate land use systems, viz., agro-forestry, agro-horticultural, agro-pastoral, and agro-silvipasture are more effective for soil organic matter restoration than monocropping systems. The constraints for organic matter management are reviewed, possible solution are presented, and future research needed are outlined.

Agricultural sustainability in Turkey: integrating food, environmental and energy securities

AbstractAgricultural production in Turkey is not sustainable due to degradation and loss of croplands, rapid population growth, and inequitable economic growth (poverty and overconsumption). Degrading land uses and management practices disturb the life‐supporting biogeochemical cycles within croplands and between croplands and natural ecosystems by increasing emissions of greenhouse gases (GHGs: CO2, CH4, N2O, CFCs, and tropospheric O3), pollution of water, soil and air, loss of soil organic matter and biodiversity, erosion, salinization and desertification. Sustainability‐oriented management practices in croplands include maintenance of soil organic matter by conservation tillage and residue management, windbreaks, selection of crops ecologically adapted to local climate regimes, efficient crop rotation, enhancement of agrobiodiversity (e.g. intercropping and agroforestry), and adoption of proper drainage techniques. Implementation of these preventive and mitigative measures necessitates internalization of ecological principles into agricultural policy and management processes. This study explores the opportunities and limitations of agricultural sustainability in Turkey in a holistic manner. A multiple linear regression (MLR) model was developed to relate CO2 emissions to energy intensity (energy use/gross domestic product), affluence (gross domestic product/population) and population growth. Our MLR model with a high R2 of 97 per cent revealed that stabilization of human population growth, and increasing energy efficiency in economic growth are essential to decreasing GHG emissions and enhancing environmental quality. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Grassland Management and Conversion into Grassland: Effects on Soil Carbon

Grasslands are heavily relied upon for food and forage production. A key component for sustaining production in grassland ecosystems is the maintenance of soil organic matter (SOM), which can be strongly influenced by management. Many management techniques intended to increase forage production may potentially increase SOM, thus sequestering atmospheric carbon (C). Further, conversion from either cultivation or native vegetation into grassland could also sequester atmospheric carbon. We reviewed studies examining the influence of improved grassland management practices and conversion into grasslands on soil C worldwide to assess the potential for C sequestration. Results from 115 studies containing over 300 data points were analyzed. Management improvements included fertilization (39%), improved grazing management (24%), conversion from cultivation (15%) and native vegetation (15%), sowing of legumes (4%) and grasses (2%), earthworm introduction (1%), and irrigation (1%). Soil C content and concentration increased with improved management in 74% of the studies, and mean soil C increased with all types of improvement. Carbon sequestration rates were highest during the first 40 yr after treatments began and tended to be greatest in the top 10 cm of soil. Impacts were greater in woodland and grassland biomes than in forest, desert, rain forest, or shrubland biomes. Conversion from cultivation, the introduction of earthworms, and irrigation resulted in the largest increases. Rates of C sequestration by type of improvement ranged from 0.11 3.04 Mg C.ha(-1) yr(-1), with a mean of 0.54 Mg C.ha(-1).yr(-1) and were highly influenced by biome type and climate. We conclude that grasslands can act as a significant carbon sink with the implementation of improved management.

GRASSLAND MANAGEMENT AND CONVERSION INTO GRASSLAND: EFFECTS ON SOIL CARBON

Grasslands are heavily relied upon for food and forage production. A key component for sustaining production in grassland ecosystems is the maintenance of soil organic matter (SOM), which can be strongly influenced by management. Many management techniques intended to increase forage production may potentially increase SOM, thus sequestering atmospheric carbon (C). Further, conversion from either cultivation or native vegetation into grassland could also sequester atmospheric carbon. We reviewed studies examining the influence of improved grassland management practices and conversion into grasslands on soil C worldwide to assess the potential for C sequestration. Results from 115 studies containing over 300 data points were analyzed. Management improvements included fertilization (39%), improved grazing management (24%), conversion from cultivation (15%) and native vegetation (15%), sowing of legumes (4%) and grasses (2%), earthworm introduction (1%), and irrigation (1%). Soil C content and concentration increased with improved management in 74% of the studies, and mean soil C increased with all types of improvement. Carbon sequestration rates were highest during the first 40 yr after treatments began and tended to be greatest in the top 10 cm of soil. Impacts were greater in woodland and grassland biomes than in forest, desert, rain forest, or shrubland biomes. Conversion from cultivation, the introduction of earthworms, and irrigation resulted in the largest increases. Rates of C sequestration by type of improvement ranged from 0.11 to 3.04 Mg C·ha−1 yr−1, with a mean of 0.54 Mg C·ha−1·yr−1, and were highly influenced by biome type and climate. We conclude that grasslands can act as a significant carbon sink with the implementation of improved management.

Organic inputs for soil fertility management in tropical agroecosystems: application of an organic resource database

Organic resources play a critical role in both short-term nutrient availability and longer-term maintenance of soil organic matter in most smaller holder farming systems in the tropics. Despite this importance, there is little predictive understanding for the management of organic inputs in tropical agroecosystems. In this paper, an organic resource database (ORD) is introduced that contains information on organic resource quality parameters including macronutrient, lignin and polyphenol contents of fresh leaves, litter, stems and/or roots from almost 300 species found in tropical agroecosystems. Data on the soil and climate from where the material was collected are also included, as are decomposition and nutrient release rates of many of the organic inputs. Examples of uses of ORD are provided in the paper: (1) nutrient contents (including median values and ranges) and other resource quality parameters of farmyard manure and crop residues are compared to that of alternative nutrient sources such as different plant parts and plant types; (2) nutrient stocks found in farm boundary hedges are estimated and evaluated as a source of nutrients for soil fertility management; (3) hypotheses regarding the indices and critical values of N, lignin, and polyphenol contents for predicting N release rates are tested; (4) organic materials for soil fertility management experiments are selected. This database, when coupled with models and decision support tools, will help advance organic matter management for soil fertility improvement from an empirical to a predictive practice.

Social, economic and policy dimensions of soil organic matter management in sub-Sahara Africa: challenges and opportunities

In recent years and in some situations the status of soil organic matter (SOM) has deteriorated considerably due to long periods of continuous cultivation and limited external inputs in the form of mineral fertilizers. Deterioration of SOM varies by agro-ecological zones, by soil types and by cropping patterns. It is more intense in East Africa, followed by coastal West Africa and Southern Africa and least intensive in the Sahel and Central Africa. It is also more serious in areas under low-input agriculture irrespective of the prevailing cropping system. The major consequence of the decrease in SOM in the tropics is lower agricultural productivity with a direct negative effect on food security. While biophysical dynamics of SOM have been extensively covered in the literature, social considerations have not received similar attention. This paper examines the social, economic and policy factors associated with the management of tropical soil organic matter. Empirical data from a range of environments in Africa show that SOM improvement options yield a positive return to land as well as labour. However, there are a number of constraints. Social constraints are related to the large quantities of organic matter that are required (case of farmyard manure), the competitive uses for the material (case of crop residues), land and labour requirements, and gender-related issues. From a policy stand point, unsecured tenure rights together with price distortions and other market failures may be important constraints. Challenges for sustainable management of SOM are identified. These include management conflicts, land tenure arrangements, the divergence in goals between individuals and society, land and labour requirements, inadequate support systems for land users, profitability issues, the role of subsidies, and the absence of national action plans. A number of opportunities are identified that could enhance the improvement or maintenance of SOM. These include: exploring the need and potential role of community-based SOM management practices; development of an integrated plant nutrient management strategy involving both organic and inorganic inputs; and development of concrete national action plans. It is argued that because externalities of SOM improvement or maintenance extend beyond the farmer’s fields, SOM investment may require cost sharing between individuals and the society. Policies on subsidies need to be reconsidered. Research priorities are identified that require closer collaboration between scientists from a variety of disciplines.

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.

The management of rice straw, fertilisers and leaf litters in rice cropping systems in Northeast Thailand.

The maintenance of soil organic matter (SOM) and the balancing of nutrient flows into and out of the rainfed rice cropping systems in Northeast Thailand is of paramount importance to arresting the decline in soil fertility and crop yields. A system where small applications of leaf litters from locally grown trees are applied annually to rice paddy soils prior to transplanting is described. The annual application of 1500 kg/ha of Cajanus cajan, Acacia auriculiformis, Phyllanthus taxodifolius and Samanea saman for five seasons resulted in increases in rice grain yield of 48, 35, 32 and 23% above the no-leaf litter control, respectively. Average annual nutrient inputs from the leaf litters, in kg/ha, ranged from 62.7 N, 3.9 P, 17.9 K, and 3.5 S for Cajanus cajan to 24.5 N, 1.5 P, 8.1 K and 2.0 S for Acacia auriculiformis. Nutrient balances, determined by the difference between the inputs (fertiliser and added leaf litters) and outputs (grain and straw) indicated net positive N and P balances of up to 457 and 60 kg/ha. respectively, after five seasons of leaf litter applications. Sulfur and potassium balances resulted in net deficits of up to −3 and −52 kg S and K/ha, respectively, where no leaf litter was applied and rice straw was removed following harvest. Calculated apparent nutrient recoveries reflected the decomposition rate of the added residues and were highest for P and K, reflecting their higher soil residual value than mobile nutrients such as N and S. Sustainable farming systems will require that crop yields are stable through the maintenance of soil fertility and the balance of nutrients in the system.

Adoption of Agroforestry for Soil Conservation in Akwa Ibom State, Nigeria

ABSTRACT By its location on a lowland coastal plain, Akwa Ibom State should not be menaced by soil degradation through erosion. But some of its component areas are characterised with hill ranges which are intensively cultivated. With a high mean annual rainfall of 4020 mm and its attendant impact on the generally sandy loam soils, these areas have become victims of environmental degradation and require soil conservation. The age old techniques of soil conservation such as earth works and mechanically constructed waterways are known to be of high initial outlay with little or no return to investment capital through crop yield. On the contrary, agroforestry is looked upon as inexpensive but with the potential to achieve more than mechanical structures in erosion control, maintenance of soil organic matter and physical properties as well as in the promotion of efficient nutrient cycling and high yield. It is also seen as an important farming package for sloping lands and an easily adaptable innovation in soil conservation. A change in soil conservation policy is recommended.

Decomposition and nitrogen release patterns of tree prunings and litter

Many studies have shown that agroforestry tree prunings can supply sufficient nutrients to meet crop demand, with the exception of phosphorus. The potential of these organic inputs to supply nutrients depends on their resource quality. Various indices have been developed to predict decomposition and nitrogen release patterns of tree prunings. To date the (lignin + polyphenol):N ratio seems to be the most robust ratio for predicting mass loss and nitrogen release. However, no critical value can be given because of the different methods used to analyze polyphenols. Suggested areas of future research include development of robust indices for predicting plant litter quality, decomposition patterns of belowground litter (roots), residual effects of tree biomass additions, and effects of adding mixtures of organic materials of contrasting quality. The overall challenge is to develop ways of managing organic matter decomposition to optimize short- and long-term release of nutrients and the maintenance of soil organic matter.