NITROGEN CYCLING IN CONVENTIONAL AND NO-TILLAGE AGROECOSYSTEMS: ANALYSIS OF PATHWAYS AND PROCESSES (SORGHUM, SOYBEAN)

Nitrogen dynamics in sorghum (1978-1979) and soybean (1980-1981) conventional and no-tillage agroecosystems are compared. Major ecological processes including crop and weed production, litter decomposition, insect consumption, soil and solution chemistry were quantified and provided integrative information on nitrogen cycling in conventional and no-tillage systems. Annual nitrogen budget models of soybean and sorghum systems summarize input, output and internal agroecosystem nitrogen fluxes. Data represent four complete annual farming cycles (two sorghum/rye and two soybean/rye).; Crop residue placement practices (i.e. plowing or its absence) were identified as major regulators of nitrogen cycling in these agroecosystems. Rates of nitrogen transfer and total amount of nitrogen transferred between several agroecosystem components were increased by plowing. Soil tillage was effectively a system energy subsidy. By mixing plant residue with soil, decomposition was accelerated, producing artificially high nitrate-nitrogen levels. Grain yields were insensitive to tillage practices except under drought stress when no-tillage system yields were higher than conventional tillage. Leaching of nitrate-nitrogen was greater in plowed than unplowed soil. Conventional tillage systems consistently supported higher standing stock values, except for weeds and soil organic nitrogen levels, which were higher in no-tillage systems. Both foliar grazing by insects and leaf nitrogen content were higher in conventional than in no-tillage systems, suggesting a possible ecosystem-level interaction occurring between soil tillage practices and rates of insect herbivory. No-tillage soil macro-arthropod nitrogen standing stocks were significantly higher than conventional tillage systems, indicating the potentially important role of soil arthropods in the cycling of nitrogen.; Net annual losses of nitrogen were higher in both sorghum and soybean conventional tillage systems than in either no-tillage system. No-tillage system input to output ratios approached unity, while conventional tillage ratios indicated a greater divergence from steady state. No-tillage systems may cycle nitrogen more slowly than plowed systems. Both sorghum systems had longer turnover times for nitrogen than soybean systems. General conclusion: conventional tillage systems do not cycle nitrogen as efficiently as no-tillage, suggesting a greater degree of biologically mediated nitrogen retaining mechanisms in no-tillage systems.