A meta-evaluation of nitrapyrin agronomic and environmental effectiveness with emphasis on corn production in the Midwestern USA

The effectiveness of nitrification inhibitors for abatement of N loss from the agroecosystem is difficult to measure at typical agronomic scales, since performance varies at the research-field scale due to complex interactions among crop management, soil properties, length of the trial, and environmental factors. The environmental impact of the nitrification inhibitor nitrapyrin on N losses from agronomic ecosystems was considered with emphasis on the Midwestern USA. A meta-evaluation approach considered the integrated responses to nitrification inhibition found across research trials conducted in diverse environments over many years as measured in side-by-side comparisons of fertilizer N or manure applied with and without nitrapyrin. The resulting distributions of response indices were evaluated with respect to the magnitude and variance of the agronomic and environmental effects that may be achieved when nitrification inhibitors are used regionally over time. The indices considered (1) crop yield, (2) annual or season-long maintenance of inorganic N within the crop root zone, (3) NO₃-N leached past the crop root zone, and (4) greenhouse gas emission from soil. Results showed that on average, the crop yield increased (relative to N fertilization without nitrapyrin) 7% and soil N retention increased by 28%, while N leaching decreased by 16% and greenhouse gas emissions decreased by 51%. In more than 75% of individual comparisons, use of a nitrification inhibitor increased soil N retention and crop yield, and decreased N leaching and volatilization. The potential of nitrification inhibitors for reducing N loss needs to be considered at the scale of a sensitive region, such as a watershed, over a prolonged period of use as well as within the context of overall goals for abatement of N losses from the agroecosystem to the environment.     

GIS-model based estimation of nitrogen leaching from croplands of China

Nitrogen (N) is the most widely used fertilizer nutrient, and its application has increased substantially in recent decades in China. N loss through leaching has been recognized as one of the most common agricultural sources of groundwater contamination. Thus, prediction of N leaching from cropland is crucial for preventing groundwater pollution. This paper quantifies nitrogen leaching from China’s croplands, identifies its spatial distribution under current cropping systems at national scale, and finally puts forward some policies or strategies to reduce rates of N leaching. A computer process simulation model of carbon and nitrogen biogeochemistry in agro-ecosystems (DNDC) was applied to predict nitrogen leaching in the soil layer of agricultural ecosystems at national scale. Data on climate, soil properties, cropping systems, acreage, and management practices at county scale were collected from various sources and integrated into a spatial GIS database to run the model. The total amount of N-leaching was predicted at 4.57 million t N/year, which is equivalent to 48 kg N per ha cropland in 1998. The spatial distribution of N leaching in China showed a sharp discrepancy between the northern and southern counties due to the differences in climatic conditions, soil properties, as well as farm management practices. The study also suggests that applying management alternatives, such as proper fertilizer, crop, water and soil management, could be efficient means for decreasing N leaching rates.     

The fate of nitrogen in agroecosystems: An illustration using Canadian estimates

Agroecosystems rely on inputs of nitrogen (N) to sustain productivity. But added N can leak into adjacent environments, affecting the health of other ecosystems and their inhabitants. Worries about global warming have cast further attention on the N cycle in farmlands because farms are a main source of N₂O, and because carbon sequestration, proposed to help reduce CO₂ loads, requires a build-up of N. Our objective was to estimate, as an illustrative example, the net N balance of Canadian agroecosystems in 1996 and then infer some hypotheses about the routes of N loss, their magnitude, and ways of reducing them. We defined agroecosystems as all agricultural lands in Canada including soil to 1 m depth and all biota, except humans. Only net flows of N across those boundaries were counted in our balance – all others represent internal cycling. Based on our estimates, about 2.35 Tg N entered Canadian agroecosystems from biological fixation, fertilizers, and atmospheric deposition (excluding re-deposited NH₃). In the same year, about 1.03 Tg N were exported in crop products and 0.19 Tg were exported in animals and animal products. Consequently, N inputs exceed exports in products by about 1.13 Tg, a surplus that is either accumulating in agroecosystems or lost to the environment. Because potential soil organic matter gains can account for only a small part of the surplus N, most is probably lost to air or groundwater. Our finding, that N losses amount to almost half of N added, concurs with field experiments that show crop recovery of added N in a given year is often not more than 60%. Better management may reduce the fraction lost somewhat but, because N in ecosystems eventually cycles back to N₂, substantive gains in efficiency may not come easily. As well as trying to reduce losses, research might also focus on steering losses directly to N₂, away from more harmful intermediates. If some of the `missing N' can be assimilated into organic matter, agricultural soils in Canada may need little added N to achieve C sequestration targets.     

Effect of human activities on forest ecosystems: N cycle and soil fertility

Forests are important terrestrial ecosystems, with particular nutrient cycling mechanisms to maintain structure and functions. Nitrogen is essential for forest growth and development, and commonly limited for the forest productivity. N cycles in forest ecosystems are frequently disturbed by intensive human activities. Based on a variety of research results, some potentially important human disturbances are discussed and their effects on forest ecosystems are reviewed. Precipitation is a considerable N input to forest ecosystems. However acid precipitation is detrimental to the ecosystems in the long run. Acidification causes remarkable reduction in forest productivity in the world, due to the harmful effect of acid on plant physiology and more importantly to the reduction in soil fertility by lowering mineralization and increasing N loss by runoff and leaching. The most important nutrient cycling mechanism in forest ecosystems is litterfall. Removal of trunks only for commercial use will not affect N cycle in forest ecosystems significantly, but attention on the intensity and rotation times of harvest should be paid. Clear-cutting should be prevented in forest harvesting. It deserves more attention that the change of environment after clear-cutting will affect the N cycling processes in forest ecosystems, which substantially influence soil fertility and forest productivity. Ammonification and nitrification processes are stimulated after harvesting, by which N is becoming more moveable. Unfortunately in the situation of no assimilation after clear-cutting, much of N will be lost out of the ecosystems and soil fertility will be diminished. The N pool in forest floor and underlying mineral soil is big, but forest productivity is generally low in natural conditions. Forest management is needed to meet the increasing demand for forest products. Optimization of stands structure is the most economic way to increase soil fertility and forest productivity. Mixed coniferous-broad leaved forest is recommended for plantation practice. Addition of fertilizer N effectively promotes forest productivity and may compensate for the N loss from the systems by harvesting.     

Factors affecting the use of fertilizers and manure by smallholders: the case of Vihiga,western Kenya

Sub-Saharan Africa faces huge food supply challenges due to increasing human population, limited opportunities to increase arable land, and declining yields associated with continuously declining soil fertility. To cater for their food requirements, smallholders use only modest levels of inorganic fertilizers and rely to a large extent on manure, which is generally of low quality. To explore factors influencing fertilizer and manure use at the farm level, 253 farm households in Vihiga district of western Kenya were sampled. A pair of Tobit models was used to relate amounts of manure and fertilizer used to household variables. The results indicate that the use of both manure and fertilizer reciprocally influence each other and are strongly influenced by household factors, and also imply that manure and fertilizer uses are endogenous. Policy changes are required to (1) reduce the burden on farming alone in rural areas; (2) promote the use of higher-cost, higher-value inputs such as fertilizers; (3) improve access to input and output markets; and (4) encourage farmer education so as to promote sustainable soil fertility management. Improved understanding of the biophysical and socioeconomic environment of smallholder systems can help target sustainable soil fertility interventions more appropriately.

The role of inorganic fertilizers and their management practices

Nutrient management is the key issue in sustainable soil fertility. N, P, K fertilization aims not only for a high economic return of the investment through optimized yield and quality, but also for minimum environmental hazards. The basic concept underlying integrated plant nutrition systems, is the maintenance and possible increase of soil fertility for sustaining enhanced crop productivity through optimal use of all sources of plant nutrients, particularly inorganic fertilizer, in an integrated manner and as appropriate to each specific ecological, social and economic situation. Much research has established the importance of fertilizers in increasing the fertility of soil and in influencing its productivity. It has been observed that applying fertilizers causes many changes in the soil, including chemical changes, that can positively or negatively influence its productiveness. Only a fraction of the fertilizer applied to the soil is taken up by the crop, the rest either remains in the soil or is lost through leaching, physical wash-off, fixation by the soil, or release to the atmosphere through chemical and microbiological processes. The critical information on the relative merits of different fertilization practices such as method of fertilizer placement, time and rate of application and type of fertilizers, is essential. Results from different field and laboratory experiments which helped to achieve maximum efficiency, in the most economical and sustainable way of fertilizer use to reduce the nutrient losses and production costs to the farmers and prevent environmental pollution are presented in the paper.     

The environmentally-sound management of agricultural phosphorus

Freshwater eutrophication is often accelerated by increased phosphorus (P) inputs, a greater share of which now come from agricultural nonpoint sources than two decades ago. Maintenance of soil P at levels sufficient for crop needs is an essential part of sustainable agriculture. However, in areas of intensive crop and livestock production in Europe and the U.S.A., P has accumulated in soils to levels that are a long-term eutrophication rather than agronomic concern. Also, changes in land management in Europe and the U.S.A. have increased the potential for P loss in surface runoff and drainage. There is, thus, a need for information on how these factors influence the loss of P in agricultural runoff. The processes controlling the build-up of P in soil, its transport in surface and subsurface drainage in dissolved and particulate forms, and their biological availability in freshwater systems, are discussed in terms of environmentally sound P management. Such management will involve identifying P sources within watersheds; targeting cost-effective remedial measures to minimize P losses; and accounting for different water quality objectives within watersheds. The means by which this can be achieved are identified and include developing soil tests to determine the relative potential for P enrichment of agricultural runoff to occur; establishing threshold soil P levels which are of environmental concern; finding alternative uses for animal manures to decrease land area limitations for application; and adopting management systems integrating measures to reduce P sources as well as runoff and erosion potential.     

Available technologies to replenish soil fertility in East Africa

Low inherent soil fertility in the highly weathered and leached soils largely accounts for low and unsustained crop yields in most African countries. But in particular, the major nutrients, nitrogen (N) and phosphorus (P), are commonly deficient in these soils. This scenario of nutrient depletion is reflected in food deficits and hence the food aid received continuously, specifically in sub-Saharan Africa. Undoubtedly, substantial efforts have been made in the continent to replenish the fertility of degraded soils in attempts to raise crop yields, towards self-sufficiency and export. Such efforts consist of applications of both organic and inorganic resources to improve the nutrient status of soils and enhanced nutrient uptake by crops, provided that soil moisture is adequate. Overall, positive crop responses to these materials have been obtained. Thus in the East African region, maize (staple) yields have been raised in one growing season from below 0.5 t/ha without nutrient inputs, to 3–5 t/ha from various nutrient amendments at the smallhold farm level. However, in spite of the positive crop responses to nutrient inputs, farmers are generally slow to adopt the soil fertility management technologies. In this paper we review the impact of some technologies, focussing the use of nutrient resources of different characteristics (qualities) in relation to improved crop yields, with an overall goal to enhance technology adoption. Thus, inorganic resources or fertilizers often give immediate crop responses, but their use or adoption is rather restricted to large-scale farmers who can afford to buy these materials. Organic resources, which include crop residues, water hyacinth and agroforestry shrubs and trees, are widely distributed, but they are generally of low quality, reflecting the need to apply large quantities to meet crop nutrient demands. Moreover, most organics will add N mainly to soils. On the other hand, phosphate rocks of varying reactivity are found widely in Africa and are refined elsewhere to supply soluble P sources. The recently developed soil fertility management options in East Africa have targeted the efficient use of N and P by crops and the integrated nutrient management approach. Some people have also felt that the repackaging of inputs in small, affordable quantities, such as the PREP-PAC described in this paper, may be an avenue to attract smallhold farmers to use nutrient inputs. Nonetheless, crop responses to nutrient inputs vary widely within and across agroecozones (AEZs), suggesting specificity in recommendations. We highlight this observation in a case study whereby eight soil fertility management options, developed independently, are being tested side-by-side at on-farm level. Farmers will be empowered to identify technologies from their own choices that are agronomically effective and economically friendly. This approach of technology testing and subsequent adoption is recommended for technology development in future.     

Land use change and soil organic carbon dynamics

Historically, soils have lost 40–90 Pg carbon (C) globally through cultivation and disturbance with current rates of C loss due to land use change of about 1.6 ± 0.8 Pg C y⁻¹, mainly in the tropics. Since soils contain more than twice the C found in the atmosphere, loss of C from soils can have a significant effect of atmospheric CO₂ concentration, and thereby on climate. Halting land-use conversion would be an effective mechanism to reduce soil C losses, but with a growing population and changing dietary preferences in the developing world, more land is likely to be required for agriculture. Maximizing the productivity of existing agricultural land and applying best management practices to that land would slow the loss of, or is some cases restore, soil C. There are, however, many barriers to implementing best management practices, the most significant of which in developing countries are driven by poverty. Management practices that also improve food security and profitability are most likely to be adopted. Soil C management needs to considered within a broader framework of sustainable development. Policies to encourage fair trade, reduced subsidies for agriculture in developed countries and less onerous interest on loans and foreign debt would encourage sustainable development, which in turn would encourage the adoption of successful soil C management in developing countries. If soil management is to be used to help address the problem of global warming, priority needs to be given to implementing such policies.     

Impacts of population growth,changing food preferences and agricultural practices on the nitrogen cycle in East Asia

While increasing population and changing food preferences have changed agriculture in some East Asian countries to high input systems with greater use of fertilizer nitrogen and greater numbers of animals, the changes and the effects on the environment in the different countries have varied considerably. Many areas still do not use sufficient nitrogen to maximize crop yields. In China, fertilizer nitrogen input has increased from 0.54 Tg in 1961 to 28 Tg in 2005, and the animal population increased dramatically, from 27 to 1,013 million. As a result 13 Tg N was lost to the environment in 2005 as nitrous oxide, ammonia or nitrate. In Mongolia, no fertilizer nitrogen was recorded as having been used until 1970, and current use is only ∼4 Gg. The animal population has increased from 23 million in 1961 to 28 million in 2005 and adverse effects on the environment are small (96 Gg N lost). However, a combination of over-ploughing and overgrazing has resulted in soil erosion from wind and rain in both countries and loss of soil nitrogen. These and other effects of changing agricultural systems on the nitrogen cycle in East Asian countries and some approaches to reduce the impact of nitrogen on the environment are reported in this paper.     

Nitrogen use and losses in agriculture in subtropical Australia

This review examines the use of nitrogen (N) fertilizer on sugar-cane, summer and winter grain crops, cotton, tropical fruit crops and pastoral areas in the four subtropical zones in eastern Australia. The pathways for N loss from the various crops grown in these zones are also examined and estimates of N loss given.Sugar-cane is the most important crop grown in the subtropical humid northern and southern zones, using 77% of all N fertilizer applied in 1988–89. Urea is the most widely used form of N fertilizer with about 50% of the applied N often lost via ammonia volatilization, denitrification and leaching. Losses of N via ammonia volatilization can be reduced by either irrigating after application, applying urea in subsurface bands or delaying application until after canopy development. Denitrification losses of 20% of applied N have been measured on clay soils in sugar- cane areas while leaching losses may occur by movement of solutes down preferential pathways (e.g. soil fauna, root channels and structural weaknesses in the soil profile). Tropical fruit crops also make a significant contribution to the economy of the humid northern and southern zones. The livestock industry is well established in the subtropical northern zones, with beef and dairy production relying on leguminous as well as N fertilized pastures. Urea is again the most widely used form of N and is susceptible to large losses via ammonia volatilization. Over a 12 month period, losses of between 9% and 42% of the N applied were recorded from a subtropical pasture.Wheat is the major winter crop of the sub-humid northern and southern zones with grain sorghum the main summer crop. Urea is the principal form of N fertilizer applied to both crops and is essential for increasing or maintaining economic yields from both regions. This decrease in soil fertility in grain producing areas is due mainly to a decrease in the amount of soil organic matter available for mineralization. Cotton is another major crop of both areas and relies heavily on N fertilizer application. Nitrogen fertilizer losses have been recorded from all cropping areas, although nitrification inhibitors such as wax coated calcium carbide and 2-ethynylpyridine have reduced denitrification losses from soils growing wheat and cotton respectively.Subtropical agriculture relies heavily on N fertilizer, principally urea, to maintain and increase crop yields. Losses of N from soils sown to crops and from native and sown pasture occur although management practices are being developed to help minimize this loss.     

A conceptual model for conducting nutrient audits at national, regional, and global scales

Mining of nutrients from the soil, particularly in developing countries, is a major problem, causing soil degradation and threatening long-term food production. This paper develops a methodology for carrying out nutrient audits, which includes the calculation of nutrient balances and an evaluation of trends in nutrient depletion/enrichment. Nutrient balances for arable farming are constructed for 197 countries for 1996 and for the world and two specific countries – a developed/enriching country (Japan) and a developing/depleting country (Kenya) for the period 1961 – 1996.The results indicate that nutrient efficiency is approximately 50% for N, 40% for P, and 75% for K. In some countries in Western Europe and in Japan and the Republic of Korea, with large, mixed farming systems, there is a surplus of N, P, and K. However, in almost all other countries, food production is currently dependent on depleting large quantities of nutrients from soil reserves and this is likely to continue. The world average soil depletion of nutrients in 1996 was estimated to be 12.1 kg N ha^–1, 4.5 kg P ha^–1, and 20.2 kg K ha^–1. The depletion of K is particularly severe and could ultimately lead to a serious loss of crop productivity in several countries. There is an urgent need to investigate this issue further. Analytical tools, such as the nutrient audit model described, can play an important role in assessing the problem, and in developing sustainable nutrient management policies, strategies, and practices.     

Fate of fertilizer 15N in intensive ridge cultivation with plastic mulching under a monsoon climate

Reducing nitrogen (N) leaching to groundwater requires an improved understanding of the effect of microtopography on N fate. Because of the heterogeneity between positions, ridge tilled fields, frequently used in intensive agriculture, should be treated as two distinct management units. In this study, we measured N dynamics in plastic-mulched ridges and bare furrows with the goal of developing more sustainable agricultural practices with optimal gains, namely crop production versus limited impacts on water quality. We investigated: (1) biomass production; (2) crop N uptake; (3) N retention in soil; and (4) N leaching using ¹⁵N fertilizer in a radish crop. Broadcast mineral N fertilizer application prior to planting resulted in high total leaching losses (of up to 390 N kg ha^?1). The application of plastic mulch in combination with local fertilizer management did not help to reduce N leaching. At all fertilizer N rates, the mean NO₃ ^? concentrations in seepage water were found to be above the WHO drinking water standard of 50 mg NO₃ ^? l^?1. To reduce NO₃ ^? leaching, we recommend: (1) decreasing the fertilizer N rates to a maximum of 150 kg N ha^?1; (2) applying fertilizer N in 3–4 split applications according to the plant’s N needs; (3) applying fertilizer N to the ridges (after their formation) to avoid losses from the furrows; and (4) increasing the soil organic matter content to enhance the water and nutrient retention by covering the furrows with plant residues.     

Intensity cultivation induced effects on soil organic carbon dynamic in the western cotton area of Burkina Faso

The soil organic carbon (SOC) dynamic is a key element of soil fertility in savannah ecosystems that form the key agricultural lands in sub-Saharan Africa. In the western part of Burkina Faso, the land use is mostly linked to cotton-based cropping systems. Use of mechanization, pesticides, and herbicides has induced modifications of the traditional shifting cultivation and increased the need for sustainable soil fertility management. The SOC dynamic was assessed based on a large typology of land cultivation intensity at Bondoukui. Thus, 102 farm plots were sampled at a soil depth of 0–15 cm, considering field–fallow successions, the cultivation phase duration, tillage intensity, and soil texture. Physical fractionation of SOC was carried out by separating the following particle size classes: 2,000–200, 200–50, 50–20, and 0–20 μm. The results exhibited an increase in SOC stock, and a lower depletion rate with increase in clay content. After a long-term fallow period, the land cultivation led to an annual loss of 31.5 g m⁻² (2%) of its organic carbon during the first 20 years. The different fractions of SOC content were affected by this depletion depending on cultivation intensity. The coarse SOC fraction (2,000–200 μm) was the most depleted. The ploughing-in of organic matter (manure, crop residues) and the low frequency of the tillage system produced low soil carbon loss compared with annual ploughing. Human-induced disturbances (wildfire, overgrazing, fuel wood collection, decreasing fallow duration, increasing crop duration) in savannah land did not permit the SOC levels to reach those of the shifting cultivation system.     

Agronomic performance of experimental fertilizers on spinach (<Emphasis Type="Italic">Spinacia oleracea</Emphasis> L.) in organic farming

In organic farming, soil application of processed agro-industrial by-products could sustain soil fertility for vegetables, which have short cropping cycles. Therefore, the objectives of this 2-year research on organic spinach crop were to assess the productive performance of different experimental fertilizers, the effects on soil fertility, and investigate the dynamics of some soil properties and the N balance. Two types of olive pomace mixtures, with a different initial C/N ratio, were composted and both stopped at the active phase (A1 and B1) and processed until maturation (A2 and B2). Also an anaerobic digestate (DA), and the B2 applied as amendment (B2A) were studied. The four composts, DA, and B2A were compared with a commercial organic fertilizer (Org), and a control (N0). The Org resulted as not sustainable in maintaining soil fertility in the long-term, mainly due to reduction in the soil of total organic carbon by 32 %, compared to the average of the other treatments. Conversely, choosing stage of maturity and adequate C/N of starting mixtures was among the best practices for compost use in spinach crop. The great content of nutrients (N and K higher by 102 and 86 % than Org, respectively), and N surplus (1431 kg ha^?1) in the B2A plots would suggest that they could accumulate after subsequent soil applications, with the risk of losses in the environment. The DA appeared to be the most suitable fertilizer to get a favorable trade-off among yield, quality and N-use efficiency, when applied according to best agronomic practices.     

Microcosmic study of soil microarthropod and earthworm interaction in litter decomposition and nutrient turnover

A microcosm experiment was set up under laboratory conditions and verified under field conditions with the objective of investigating the interaction of soil microarthropods and earthworms in litter decomposition, nutrient release, and uptake by maize crop. The treatments included: soil alone (control), soil with leaf litter (Senna siamea leaves), soil with leaf litter and soil microarthropods, soil with leaf litter and earthworms (Hyperiodrilus africanus), and soil with litter and both of the soil faunal groups. After an 8-week incubation period, the amounts of litter decomposed and N, P, K, Ca, and Mg released followed the order: with microarthropods and earthworms > with earthworms > with microarthropods > no faunal addition. The presence of microarthropods and earthworms also increased the net level of mineral N in the incubated soil. The additive roles of soil microarthropods and earthworms were observed on decomposition and nutrient release. Such faunal interactions resulted in an increased N uptake by maize in the incubated soil. Despite their lower biomass, soil microarthropods contributed significantly to nutrient turnover in the presence of earthworms. This study emphasizes the need to quantify and devise ways of controlling and regulating the abundance and activities of soil fauna for effective nutrient cycling and, consequently, for better crop yields in low-input tropical agricultural ecosystems.     

The availability of nitrogen from sugarcane trash on contrasting soils in the wet tropics of North Queensland

Sugarcane crop residues (‘trash’) have the potential to supply nitrogen (N) to crops when they are retained on the soil surface after harvest. Farmers should account for the contribution of this N to crop requirements in order to avoid over-fertilisation. In very wet tropical locations, the climate may increase the rate of trash decomposition as well as the amount of N lost from the soil–plant system due to leaching or denitrification. A field experiment was conducted on Hydrosol and Ferrosol soils in the wet tropics of northern Australia using ¹⁵N-labelled trash either applied to the soil surface or incorporated. Labelled urea fertiliser was also applied with unlabelled surface trash. The objective of the experiment was to investigate the contribution of trash to crop N nutrition in wet tropical climates, the timing of N mineralisation from trash, and the retention of trash N in contrasting soils. Less than 6% of the N in trash was recovered in the first crop and the recovery was not affected by trash incorporation. Around 6% of the N in fertiliser was also recovered in the first crop, which was less than previously measured in temperate areas (20–40%). Leaf samples taken at the end of the second crop contined 2–3% of N from trash and fertilizer applied at the beginning of the experiment. Although most N was recovered in the 0–1.5 m soil layer there was some evidence of movement of N below this depth. The results showed that trash supplies N slowly and in small amounts to the succeeding crop in wet tropics sugarcane growing areas regardless of trash placement (on the soil surface or incorporated) or soil type, and so N mineralisation from a single trash blanket is not important for sugarcane production in the wet tropics.     

Soil organic carbon management for sustainable land use in Sudano-Sahelian West Africa

Judged by their negative nutrient balances, low soil cover and low productivity, the predominant agro-pastoral farming systems in the Sudano-Sahelian zone of West Africa are highly unsustainable for crop production intensification. With kaolinite as the main clay type, the cation exchange capacity of the soils in this region, often less than 1 cmol_c kg⁻¹soil, depends heavily on the organic carbon (Corg) content. However, due to low carbon sequestration and to the microbe, termite and temperature-induced rapid turnover rates of organic material in the present land-use systems, Corg contents of the topsoil are very low, ranging between 1 and 8 g kg⁻¹ in most soils. For sustainable food production, the availability of phosphorus (P) and nitrogen (N) has to be increased considerably in combination with an improvement in soil physical properties. Therefore, the adoption of innovative management options that help to stop or even reverse the decline in Corg typically observed after cultivating bush or rangeland is of utmost importance. To maintain food production for a rapidly growing population, targeted applications of mineral fertilisers and the effective recycling of organic amendments as crop residues and manure are essential. Any increase in soil cover has large effects in reducing topsoil erosion by wind and water and favours the accumulation of wind-blown dust high in bases which in turn improves P availability. In the future decision support systems, based on GIS, modelling and simulation should be used to combine (i) available fertiliser response data from on-station and on-farm research, (ii) results on soil productivity restoration with the application of mineral and organic amendments and (iii) our present understanding of the cause-effect relationships governing the prevailing soil degradation processes. This will help to predict the effectiveness of regionally differentiated soil fertility management approaches to maintain or even increase soil Corg levels. This revised version was published online in June 2006 with corrections to the Cover Date.     

土壤多环芳烃的生物修复研究

多环芳烃(polycyclic aromatic hydrocarbons,PAHs)是环境中普遍存在的具有代表性的一类重要持久性有机污染物,具"三致性"、难降解性,在土壤环境中不断积累,严重危害着土壤的生产和生态功能、农产品质量和人类健康。修复土壤多环芳烃污染已成为研究的焦点。生物修复技术因具有成本低、无二次污染、可大面积应用等独特优点而越来越受到人们的重视,是目前最具潜力的土壤修复技术之一。     

How winter cover crops and tillage intensities affect nitrogen availability in eggplant

This study evaluates the fate of nitrogen (N) content in winter cover crops under different tillage intensities. Field trials were conducted over a 2-year period in a Mediterranean environment adopting a cover crop–eggplant sequence. The treatments were: three cover crops (hairy vetch, oat and oilseed rape); three tillage intensities (residue left on soil surface, shallow green manure and deep green manure). The measurements included: cover crop and eggplant characteristics, N mineralization from cover crops, soil inorganic N and soil CO₂ emission. At cover crop termination, N accumulated in the cover crops was 207, 77 and 77 kg N ha^?1 in hairy vetch, oat and oilseed rape, respectively. Tillage intensity affected biomass decomposition and N mineralization from cover crop residues which were slower when residues were left on soil surface (54 and 71%, respectively) than when incorporated into the soil (66 and 79%, respectively). Hairy vetch showed a greater ability to supply N to eggplant (151 kg N ha^?1), due to the fast decay of its residues, consequently, the N balance index was always high after hairy vetch throughout eggplant cultivation. N mineralized by cover crops was positively correlated with total soil CO₂ emission and soil inorganic N. Placing cover crop residues on soil surface enhances synchronization between N mineralized and eggplant N demand in hairy vetch, while in oat it appears to mitigate the shortage of soil inorganic N for the following vegetable. These findings may also be extended to other summer vegetables which have similar requirements to the eggplant.