In agricultural systems, optimization of carbon and nitrogen cycling through soil organic matter can improve soil fertility and yields while reducing negative environmental impact. A basic tenet that has guided the management of soil organic matter for decades has been that equilibrium levels of carbon and nitrogen are controlled by their net input and that qualitative differences in these inputs are relatively unimportant1,2,3. This contrasts with natural ecosystems in which there are significant effects of species composition and litter quality on carbon and nitrogen cycling4,5. Here we report the net balances of carbon and nitrogen from a 15-year study in which three distinct maize/soybean agroecosystems are compared. Quantitative differences in net primary productivity and nitrogen balance across agroecosystems do not account for the observed changes in soil carbon and nitrogen. We suggest that the use of low carbon-to-nitrogen organic residues to maintain soil fertility, combined with greater temporal diversity in cropping sequences, significantly increases the retention of soil carbon and nitrogen, which has important implications for regional and global carbon and nitrogen budgets, sustained production, and environmental quality.

Legume-based cropping systems have reduced carbon and nitrogen losses

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Replacing bare fallows with cover crops in fertilizer-intensive cropping systems: A meta-analysis of crop yield and N dynamics.

Agricultural intensification has greatly increased the productive capacity of agroecosystems, but has had unintended environmental consequences including degradation of soil and water resources, and alteration of biogeochemical cycles. Current nutrient management strategies aim to deliver soluble inorganic nutrients directly to crops and have uncoupled carbon, nitrogen, and phosphorus cycles in space and time. As a result, agricultural ecosystems are maintained in a state of nutrient saturation and are inherently leaky because chronic surplus additions of nitrogen and phosphorus are required to meet yield goals. Significant reductions of nutrient surpluses can only be achieved by managing a variety of intrinsic ecosystem processes at multiple scales to recouple elemental cycles. Rather than focusing solely on soluble, inorganic plant‐available pools, an ecosystem‐based approach would seek to optimize organic and mineral reservoirs with longer mean residence times that can be accessed through microbially and plant‐mediated processes. Strategic use of varied nutrient sources, including inorganic fertilizers, combined with increases in plant diversity aimed at expanding the functional roles of plants in agroecosystems will help restore desired agroecosystem functions. To develop crops that can thrive in this environment, selection of cultivars and their associated microorganisms that are able to access a range of nutrient pools will be critical. Integrated management of biogeochemical processes that regulate the cycling of nutrients and carbon combined with increased reservoirs more readily retained in the soil will greatly reduce the need for surplus nutrient additions in agriculture.

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Nutrients in agroecosystems: Rethinking the management paradigm

Denitrification is a microbial respiratory process during which soluble nitrogen oxides are used as an alternative electron acceptor when oxygen is limiting. It results in considerable loss of nitrogen, which is the most limiting nutrient for crop production in agriculture. Denitrification is also of environmental concern, since it is the main biological process responsible for emissions of nitrous oxide, one of the six greenhouse gases considered by the Kyoto protocol. In addition to natural variations, agroecosystems are characterized by the use of numerous practices, such as fertilization and pesticide application, which can influence denitrification rates. This has been widely documented in the literature, illustrating the complexity of the underlying mechanisms regulating this process. In the last decade, however, application of molecular biology approaches has given the opportunity to look behind denitrification rates and to describe genes, transcripts, and enzymes responsible for the process. In order to reduce denitrification in arable soil, it is important to understand how different factors influence denitrification and how the denitrifier community structure is related to in situ activity. This chapter focuses on the impact of natural events as well as agricultural practices on denitrifying microorganisms.

Ecology of Denitrifying Prokaryotes in Agricultural Soil

A 12-week greenhouse experiment was conducted to determine the effect of the polyphenol, lignin and N contents of six legumes on their N mineralization rate in soil and to compare estimates of legume-N release by the difference and 15N-recovery methods Mature tops of alfalfa (Medicago sativa L), round leaf cassia (Cassia rotundifolia Pers, var Wynn), leucaena (Leucaena leucocephala Lam, deWit), Fitzroy stylo (Stylosanthes scabra Vog, var Fitzroy), snail medic (Medicago scutellata L), and vigna (Vigna trilobata L, var verde) were incorporated in soil at the rate of 100 mg legume N kg-1 soil The medic and vigna were labeled with 15N Sorghum-sudan hybrid (Sorghum bicolor, L Moench) was used as the test crop A non-amended treatment was used as a control Net N mineralization after 12 weeks ranged from 11% of added N with cassia to 47% of added N for alfalfa With the two legumes that contained less than 20 g kg-1 of N, stylo and cassia, there was net N immobilization for the first 6 weeks of the experiment The legume (lignin + polyphenol):N ratio was significantly correlated with N mineralization at all sampling dates at the 005 level and at the 001 level at 6 weeks (r2=0866) Legume N, lignin, or polyphenol concentrations or the lignin:N ratio were not significantly correlated with N mineralization at any time The polyphenol:N ratio was only significantly correlated with N mineralization after 9 weeks (r2=0692) The (lignin + polyphenol):N ratio appears to be a good predictor of N mineralization rates of incorporated legumes, but the method for analyzing plant polyphenol needs to be standardized Estimates of legume-N mineralization by the difference and 15N recovery methods were significantly different at all sampling dates for both 15N-labeled legumes After 12 weeks, estimates of legume-N mineralization averaged 20% more with the difference method than with the 15N recovery method This finding suggests that estimates of legume N available to subsequent crops should not be based solely on results from 15N recovery experiments

The nitrogen mineralization rate of legume residues in soil as influenced by their polyphenol, lignin, and nitrogen contents

A pot experiment was conducted to study the transformations of organic and inorganic N in soil and its availability to maize plants. Inorganic N was in the form of15N labelled ammonium sulphate (As) and15N labelledSesbania aculeata (Sa), a legume, was used as organic N source. Plants utilized 20% of the N applied as As; presence of Sa reduced the uptake to 14%. Only 5% of the Sa-N was taken up by the plants and As had no effect on the availability of N from Sa. Losses of N from As were found to be 40% which were reduced to 20% in presence of Sa. Losses of N were also observed from Sa which increased in the presence of As. Application of As had no effect on the availability of soil or Sa-N. However, more As-N was transported into microbial biomass and humus components in the presence of Sa.

Transformations in soil and availability to plants of15N applied as inorganic fertilizer and legume residues

Denitrification and total N losses were quantified from an irrigated field cropped to maize and wheat, each receiving urea at 100 kg N ha-1. During the maize growing season (60 days), the denitrification loss measured directly by acetylene inhibition-soil cover method amounted 2.72 kg N ha-1 whereas total N loss measured by 15N balance was 39 kg ha-1. Most (87%) of the denitrification loss under maize occurred during the first two irrigation cycles. During the wheat growing season (150 days), the denitrification loss directly measured by acetylene inhibition-soil cover and acetylene inhibition-soil core methods was 1.14 and 3.39 kg N ha-1, respectively in contrast to 33 kg N ha-1 loss measured by 15N balance. Most (70-88%) of the denitrification loss under wheat occurred during the first three irrigation cycles. Soil moisture and NO
 3
 -
 -N were the major factors limiting denitrification under both crops. Higher N losses measured by 15N balance than C2H2 inhibition method were perhaps due to underestimation of denitrification by C2H2 inhibition method and losses other than denitrification, most probably NH3 volatilization.

Denitrification and total N losses from an irrigated sandy-clay loam under maize-wheat cropping system

In a 2-year field study, denitrification loss was measured from an irrigated sandy-clay loam under cotton receiving urea-N at 158–173 kg ha–1. An acetylene inhibition-soil core method was employed for the direct measurement of denitrification, considering also the N2O entrapped in the soil. Taking into account the N2O evolved from soil cores and that entrapped in the soil, a total of 65.7 kg N ha–1 and 64.4 kg N ha–1 was lost due to denitrification during the 1995 and 1996 cotton-growing seasons, respectively. Most (>70%) of the denitrification loss occurred during June–August, a period characterized by high soil temperatures and heavy monsoon rains. On average, 35% of the denitrification-N2O was found entrapped in the soil and the amount of entrapped N2O was significantly correlated with head space N2O concentration and with water-filled pore space. 15N-balance during the 1996 growing season revealed a loss of 71.8 kg N ha–1. It was concluded that a substantial proportion of the fertilizer-N applied to irrigated cotton is lost under the semiarid subtropical climatic conditions prevailing in the Central Punjab region of Pakistan and that denitrification is the major N loss process under irrigated cotton in this region.

Denitrification and total fertilizer-N losses from an irrigated cotton field

Saline-sodic soils are characterized by a very low nitrogen and organic matter content and thus are practically non fertile. However under these conditions, certain plants have been found to grow luxuriantly. One of such plants, Leptochloa fusca (Kallar grass) has exhibited nitrogenase activity associated with its roots as determined by acetylene reduction assay (ARA). Quantification of such nitrogen fixation was also carried out using 15N isotope dilution technique.

Associative N2-fixation in plants growing in saline sodic soils and its relative quantification based on 15N natural abundance.

Experiments to determine the contribution of biological nitrogen fixation associated with roots of Kallar grass have been carried out both in pot and field using15N isotopic dilution method. In the absence of any appropriate reference plant, the uninoculated treatment was used as a control in the pot experiment. It was found that as a result of inoculation withKlebsiella sp. (NIAB-I) andBeijerinckia sp (Iso-2) 20–26% of the N in the plant was derived from the atmosphere. In the field experiment, a treatment receiving higher application of nitrogenous fertilizer to inhibit nitrogenase activity was kept as a control. Kallar grass was grown in 1 m2 microplots for 9 months and harvested thrice. The estimation of N fixed was made by calculating the ‘A’ value for different treatments at each harvest. It was found that the rate of N fixation increased with the plant growth. At the 2nd harvest, which was after monsoon, it was estimated that 32 kg N ha−1 was fixed. The survival of inoculated bacteria was also studied using fluorescent antibodies prepared against the inoculated bacterium. Survival and proliferation of these bacteria were found only where Kallar grass was growing, thus indicating a beneficial influence of the plant on bacterial growth.

M. I. Sajjad

Papers

Transformations in soil and availability to plants of15N applied as inorganic fertilizer and legume residues

Denitrification and total N losses from an irrigated sandy-clay loam under maize-wheat cropping system

Denitrification and total fertilizer-N losses from an irrigated cotton field

Associative N2-fixation in plants growing in saline sodic soils and its relative quantification based on 15N natural abundance.

Quantification of N2-fixation and survival of inoculated diazotrophs associated with roots of Kallar grass