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Carbon and nitrogen dynamics in flooded soilsBook is availableCarbon and nitrogen dynamics in flooded soils Proceedings of a Workshop, 19-22 April 1999, Los Baños Laguna, Philippines Edited by G. Kirk and D. Olk Modeling SOM cycling in rice-based production systems Jonathan Arah Puddling, flooding, and an aerenchymatous crop distinguish rice production from other commonly modeled systems. A comprehensive synthetic model of soil organic matter (SOM) cycling within such a system requires a full account of the transport, reaction, and influence of oxygen, but no such account is currently available. Parameterizing such a model calls for controlled incubations, which can also, if labeled with appropriate isotopic tracers, be interpreted analytically (that is, in such a way as to measure real C and N fluxes between real SOM fractions). The flooded and periodically puddled irrigated rice system presents an ideal opportunity for testing the performance of synthetic models against their primary predicands. Achim Dobermann and Christian Witt Changes in soil and crop management practices will occur in intensive rice systems, mainly triggered and driven by socioeconomic changes. These changes will also affect the cycling of carbon (C) and nitrogen (N) in these systems as well as other soil properties related to them. New germplasm, crop diversification and changing tillage, crop establishment, nutrient management, and harvest practices will affect yields and organic matter turnover in intensive rice systems. Greater biomass production is expected to increase C sequestration, but also increase the amount of crop residues that needs to be handled without increasing the emission of greenhouse gases or affecting organic matter pathways negatively. Crop rotations, soil tillage, fallow period, and water management are expected to become important components of manipulating organic matter, both in terms of rates of decomposition and the actual decomposition products. Our understanding of C and N management in such systems, however, has lagged behind. Therefore, we discuss some general research needs. A physical soil fractionation scheme to obtain organic matter fractions suitable for modeling John L. Gaunt, Saran P. Sohi, Jonathan R.M. Arah, Nathalie Mahieu, Haishun Yang, and David S. Powlson A model that is based on measurable pools has distinct advantages, not least that the magnitude of soil organic matter (SOM) pools can be measured in any soil at any time. Such measurements can be used to run the model in a predictive manner, with no previous knowledge of a particular site, or, conversely, to verify model predictions. If such a model is to be parsimonious, the number of pools that should be included, in the model and the associated extraction protocol, is determined by the number of tracers employed. Where a 13C and 15N tracer is employed, it should it possible to infer the transformations, and hence derive the effective reactivity of up to 5 SOM pools. We examined the chemical characteristics of organic matter contained within the model pools. The pools from our physical fractionation procedure that comprise particulate organic matter (i.e., Free and Intra-aggregate fractions) show a strong contrast in chemical composition. This consistency in the chemical characteristics of individual pools across soil type and environment and the significant differences in the chemical composition of the organic matter pools suggest that this extraction protocol and the associated model may have a generic application. G.J.D. Kirk and H.J. Kronzucker Current understanding of the processes involved in N acquisition by rice roots growing in flooded soil is reviewed. Measurements in field experiments and the results of mathematical modelling show that rice can absorb fertilizer N broadcast into the ricefield floodwater very rapidly if the fertilizer application is timed precisely to match crop demand. But the uptake of N from the soil, which is important in the overall efficiency of fertilizer N, is in many cases limited by rates of N absorption and assimilation in roots. Measurements of the kinetics of N absorption and assimilation, made with the tracer 13N, coupled to mathematical models of N transport through the soil to root surfaces, have been used to identify processes that might be manipulated through plant breeding or agronomy to increase uptake efficiency. This has revealed an unusually high efficiency for NO3- absorption by rice, suggesting that uptake of NO3- formed in the rhizosphere and otherwise lost through denitrification, may be much more important than previously thought. Rates of NH4+ absorption and assimilation are greater under mixed NH4+- NO3- nutrition, and plant growth rates may also be greater. Implications for plant breeding and agronomy are discussed. Managing soil organic matter in rice and nonrice soils: agronomic questions D.C. Olk, C. van Kessel, and K.F. Bronson The importance of managing soil organic matter (SOM) for crop production has long been recognized. Various aspects of SOM management have been studied extensively in upland cropping systems and more recently also in irrigated rice-based cropping systems. Some key findings have been that, first, the significance of SOM for specific soil properties varies considerably by soil type. Further, both SOM quantity and quality are potential factors of crop production, although a comprehensive definition of SOM quality remains elusive. Also, the quantity of SOM depends on the SOM level before amendment of organic materials, the type and amounts of the amended materials, and the decomposition rates of SOM and amended materials. These factors can be manipulated through cropping practices. In intensive irrigated lowland rice-based cropping systems, research on SOM management has addressed short-term yield responses to the principal types of organic amendments: crop residues, green manures, and animal manures. Less is known about their long-term effects on SOM properties and crop performance. Crop residues are commonly available in farmers' fields, and their proper use can improve nutrient cycling, especially when coordinated with timely applications of mineral fertilizers. While green manures have been used to increase N availability, their use in favorable lowland rice soils is limited by economic constraints. The use of animal manures in Asia has diminished in recent years. Future research should determine the long-term effects of organic inputs and SOM management for those crop management practices likely to be common in the future, such as increased mechanization, water-saving irrigation techniques, and direct seeding. Properties of chemically extracted soil organic matter in intensively cropped lowland rice soils D.C. Olk and N. Senesi Soil organic matter (SOM) appears to play significant roles in crop nutrient uptake in intensively cropped lowland rice soils, but insufficient process-level knowledge exists on how the submerged conditions of this unique cropping system affect SOM properties and possibly nutrient cycling and crop uptake. Several researchers have extracted organic matter from lowland rice soils to determine its chemical properties, and this report emphasizes recent results using a chemical extraction method. The labile mobile humic acid (MHA) fraction and more recalcitrant calcium humate (CaHA) fraction were analyzed by several leading spectroscopic methods, including 13C and 31P nuclear magnetic resonance (NMR), Fourier-Transform, electron spin resonance, and fluorescence spectroscopies. All analyses indicated an increasingly less humified nature of SOM with increasing intensity of irrigated rice cropping and soil submergence. Results suggest the hypothesis that the chemical nature of newly forming SOM is primarily determined by soil aeration status during decomposition of the dominant organic inputs, most commonly crop residues. More evidence is needed to test this hypothesis. Promising lines of future organic matter research include further use of the modern spectroscopic methods listed above and other advanced physicochemical techniques, the nature and stability of metal-SOM complexes, the type and extent of interactions of soil mineral matter with SOM, and the macromolecular size, shape, and aggregation processes of SOM. Several knowledge gaps exist, including relating new information on the chemical nature of SOM to trends in nutrient cycling within one cropping period, under various cropping strategies, and across different sites, whether for continuous irrigated rice cropping or for rice-wheat cropping systems. Long-term soil organic matter dynamics D.S. Powlson and D. Olk It is generally acknowledged that maintenance of adequate soil organic matter (SOM) content is an essential requirement for the sustainability of upland cropping systems. In part, this is because SOM plays a key role in nutrient cycling, although other possible influences on soil properties are less well characterized. Compared with upland cropping systems, less is known about the significance of SOM in lowland rice-based soils. Transformations of SOM are gradual, and their contributions to sustainability are best studied in long-term experiments (LTE). Concepts of SOM dynamics gathered from LTEs and discussed here include the factors of equilibrium SOM levels, C budgets for rice and nonrice soils, nutrient-related benefits associated with SOM, and non-nutrient-related benefits and their possible causes. These concepts provide a starting point for studying SOM functions in the more complex and variable cropping environments typically found on-farm. The utility of LTEs in identifying the respective roles of SOM in different cropping systems can be enhanced by sharing their results and interpretations through networking among interested scientists. Microbial processes in C and N dynamics W. Reichardt, K. Inubushi, and J. Tiedje Organic carbon balances in tropical wetlands remain positive, in contrast to dryland agricultural soils, apparently because of additional biomass production via phototrophic and chemoautotrophic pathways. Retarded remineralization of the more recalcitrant organic matter constituents also plays a role. Formation and remineralization of soil organic matter depend on biogeochemical pathways of C and N that are governed by soil microbial biomass and soil enzymes. Mainly prokaryotic microbial biomass constitutes the bulk fraction of soil biota in flooded soils. As it functions simultaneously as a sink and source of plant nutrients, environmental changes caused by rice soil and water management practices ought to have a tangible impact on the predominance of either function. While pool sizes of microbial biomass in rice soils account for only 2-4% of total C, this most labile fraction of soil organic matter is turned over rapidly. Measures related to the functional profiles and status of microbial biomass in rice soils promise a better insight into biogeochemical N cycle dynamics than mere biomass pools. Both N-immobilizing and N-remobilizing functions of soil biota are influenced by organic C and fertilizer inputs as well as by controlling the redox potential via water management. As rice roots leak O2, an oxygenated rhizosphere favors the microbial oxidation of ammonium to nitrate. Hence, N supply at the uptake sites of rice roots is likely to be different from N availability in anoxic bulk soil. Major research gaps for flooded rice-cropping systems have been identified in the measurement of gross N mineralization rates and their linkage with C:N ratios of the substrates, in gross assessment of production rates of soil microbial biomass, and in a universal quantification of energy flow in agricultural wetlands to explain an apparent limitation of C and energy in intensive rice-cropping systems. Also, the relevance of sequential pathways of energy production (such as respiratory ferric iron reduction in particular) remains to be assessed for different types of rice soils. Extracellular soil enzymatic processes govern rate-limiting steps in the decomposition of particulate organic matter, as well as part of the immobilization and remobilization of N in flooded soils, but are still poorly understood. Profiles of microbial groups with significant functions in the biogeochemistry of C and N can be obtained from (signature lipid) biomarker and genetic fingerprinting and from cultivation-based assays. Despite some insights obtained, soil-specific responses to water, nutrient, and pest management practices require further, more systematic analyses. In view of current and future water-saving practices, earlier measures of nitrification-denitrification losses may have to be reassessed. By the same token, major knowledge gaps also exist for the combined effects of modern pest and weed protection measures on key pathways of N cycling and N supply in particular, and on soil quality and ecotoxicity in general.
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Modeling SOM cycling in rice-based production systems A physical soil fractionation scheme to obtain organic matter fractions suitable for modeling Managing soil organic matter in rice and nonrice soils: agronomic questions Properties of chemically extracted soil organic matter in intensively cropped lowland rice soils |
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