Fifty Years of Over- and Under-Application of Nitrogen: Effects On Soil Carbon and Soil Nitrogen.

There are uncertainties about the sustainability of long-term monoculture and nitrogen (N) fertilizer use in corn production. Long-term, replicated research plots exist near Arlington, WI that allows us to examine the effects of 50 years (1958-2007) of continuous corn and N fertilizer use on corn yield, N use efficiency, soil pH, and soil organic carbon. Corn was harvested for grain with residues returned annually since 1958 on a Plano silt loam soil. The experimental design includes three N fertilizer rates (0, 140 and 280 kg N ha^-1) and two lime treatments (imposed in 1985) with four replications. Soil pH and organic matter content were measured periodically during the experiment. Average corn yields in N fertilized treatments increased dramatically (100%) over time, as did apparent N use efficiency (kg grain kg^-1 N fertilizer), thus higher yields in recent years have not required greater N fertilizer use. Results suggest that both hybrid genetic improvement and improved management techniques contributed to the long-term yield gain. Soil organic matter (SOM) content (0-20 cm) was maintained or increased with long-term N additions. Deep soil cores, 0-100 cm, were collected in the spring of 2011 and are being processed to evaluate deep carbon storage. Based on yield and routine soil test results, no decline in productivity or SOM has been observed, even with continual over-applications of N. If managed properly, continuous corn cropping systems may be sustainable in similar landscapes.