Nitrogen Fixation Over the Long-Run: Lessons for Organic and Integrated Grain Production From the Living Field Laboratory.

Long-term research trials provide unique insights into nutrient management at different states, transitional and equilibrium. Research on nitrogen (N) dynamics and agronomic recommendations are often based on limited data sets, which are biased towards transitional states. Yet management practices alter equilibrium, through feedbacks that influence plant N fixation and soil C and N pools over a decade or more. The Living Field Laboratory is a corn-soybean-wheat rotation trial established in 1993 at the W.K. Kellogg Biological Station in Michigan to investigate how organic and conventional management, cover crops and compost practices interact and influence biological processes and cropping system performance. A micro-plot study was initiated to quantify N fixation in soybean over three years, based on nod and non-nod N difference and natural abundance methodology. We compared conventional management with and without cover crops, versus compost plus cover crop management at soil organic C levels of 0.81%, 0.91%, 1.16% with C:N ratios of 9.2, 10.1 and 10.8, respectively. Chemically labile organic matter, and N mineralization were evaluated to determine the extent to which N processes had been altered by management over 16 years. Soybean micro-plots with non- and non-nod isolines were established for three years in the treatments. High N-content in non-nod soybean biomass reflected N mineralization, which was greater in 2006 than in 2007 and 2008. The natural abundance technique showed that the fraction of N fixation in soybean grain was consistently highest under conventional management, 70-80%, whereas it was 60-63% in the treatment with the highest soil organic matter, compost+cover crop. Nitrogen mineralization may be suppressing high rates of soybean N fixation in treatments that build soil organic matter. The influence of weather and soil organic matter pools need to be considered as factors in understanding biological N fixation response.