Wednesday, November 15, 2006 - 10:45 AM
282-7
Legume Nitrogen Fixation as an Internal Regulator of Nitrogen Cycling in Agroecosystems.
Soils are inherently heterogeneous and plant plasticity is one key mechanism that helps optimize nutrient uptake within this environment. Conventional agroecosystem management practices largely avoid nutrient heterogeneity by providing nutrients at levels that saturate the system, resulting in extensive losses to the surrounding environment. Legume-based cropping systems have the advantage of internally regulating nitrogen (N) inputs because as soil N availability increases, nitrogen fixation is down-regulated. The relationship between endogenous soil N reservoirs and nitrogen fixation has rarely been studied because inorganic N additions are typically used to produce N-fertility gradients. We used a gradient of endogenous soil N levels to investigate nitrogen fixation responses that occur when fertility is driven by soil organic matter dynamics. Soybean nitrogen fixation was quantified using the 15N natural abundance method across 13 farms representing a fertility gradient due to differences in management histories. Field-scale N mass balances were calculated for 24 vegetable and grain farms in the northeast. The combined results support our hypothesis that legume-based systems maintain tighter balances between N inputs and exports while accumulating soil organic N over time. Average annual N surplus additions were significantly higher in conventionally managed systems than legume-based systems. The reliance on legume nitrogen fixation as a fraction of total N inputs is inversely related to annual N surpluses. Soybean nitrogen fixation ranged from 44 to 209 kg N/ha and decreased with time under organic, legume-based management. Total soybean N uptake was similar across field sites. Understanding plant responses to heterogeneous nutrient availability is critical for optimizing nutrient cycling to maintain production while reducing negative environmental impacts.
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