Assessing Long-Term Nitrogen Use Efficiencies, Sources and Sinks.

A nitrogen budget and assessment of the losses of N and pathways for these losses will be presented for a long-term corn-corn rotation. Since 1999 we have been studying the effects of nitrogen management on irrigated no-till (NT) corn (Zea mays L.) yields in a Fort Collins clay loam soil at the CSU ARDEC near Fort Collins, CO. The NT N fertilizer rates averaged 0, 34, 67, 134, 202 and 246 kg N ha^-1 from 1999 to 2015. Fifteen corn plants were collected at about 146 days after planting (DAP) to determine corn grain, cob, stalk and total aboveground biomass. Harvest grain yields were determined by harvesting 15 m of row at 173 DAP. N content in the aboveground plant compartments and in the 0 – 1.5 m soil profile was measured for every growing season. Our preliminary results suggest that using linear regressions to conduct C and N balances is a strong approach to assess nitrogen balance and changes in soil C and N content. We modeled these changes using PROC MIXED with a first-order autoregressive covariance structure for repeated measures over years on the subject plot, together with lack-of-fit analysis (LOF) and adjusted p-values using PROC MULTTEST with the false discovery rate (FDR) method. These preliminary results suggest that the cycling of N from added fertilizer is larger than expected and net N losses from the system are large, with potential global and regional implications. An N budget will be presented for this corn-corn rotation and an assessment of the losses of N and pathways for these losses will be presented.