The role of organic matter quality in nitrogen cycling and yield trends in intensively cropped paddy soils

The sustainability of intensive lowland rice (Orzyza sativa, L.) cropping in tropical Asia has been evaluated in on-station field experiments of doubleor triple-cropped rice that have run for up to 30+ years. In those experiments managed at initially high yield levels, availability of soil nitrogen (N) and grain yields declined across several years, despite maintained quantities of total soil N and availability of fertilizer N. To explain the decrease in soil N availability, we investigated the quality, or chemical nature, of soil N under intensive rice cropping. Chemical characterization of soil organic matter (SOM) fractions extracted from the long-running field experiments associated intensive rice cropping with accumulation of incompletely decomposed materials derived from crop residues, including phenolic compounds. Under controlled conditions phenols chemically stabilize N, suggesting a mechanism for decreased availability of soil N. In order to further test this hypothesis under field mitigation options, in a new field experiment SOM quality and N cycling were compared for anaerobic decomposition of crop residues the conventional practice in lowland rice cropping versus aerobic decomposition. During four years, anaerobic decomposition led to gradual enrichment of young SOM with phenolic compounds and decreased N mineralization from young SOM during the growing season, as compared to aerobic decomposition. In-season N mineralization from one SOM fraction was inhibited by anaerobic decomposition relative to aerobic decomposition by up to 22 kg N ha. The inhibition became most pronounced in midto late-season. It was best correlated with phenol enrichment at high biomass of incorporated crop residues, and the correlation strengthened across consecutive years. N-labeled fertilizer was applied at different splits. By harvest, of the N that had been immobilized, little was remineralized following anaerobic decomposition, but 45% was remineralized following aerobic decomposition. The responses of crop N uptake and grain yield to residue management were in most years consistent with trends in soil N mineralization, but causality cannot be established in this experiment. Nitrogen fertilizer was applied in synchrony with crop N demand, which should have maintained an adequate N supply to the crop despite decreased availability of soil N. Also, a zinc deficiency affected crop growth in a pattern confounded with that of residue management. Although the duration of this experiment did not suffice to allow a definitive explanation for the decrease in grain yield and soil N availability of the long-running field experiments, observations from all experiments are consistent with the hypothesis that anaerobic decomposition of crop residues amended at high rates promotes an accumulation of phenols. These compounds may OLK & CASSMAN 17 WCSS, 14-21 August 2002, Thailand 1355-2 chemically stabilize soil N into forms that remain less available during the growing period, leading to late-season deficiency of soil N. Results suggest that aerobic decomposition of crop residues will over the long-term promote greater mineralization of soil N during the growing season and hence recovery by the crop. Aerobic decomposition could be accomplished by incorporating crop residues during the fallow or by draining floodwaters during critical portions of the growing season.