Fifty Years of Nitrogen Fertilization: Effects On Soil Carbon and Nitrogen.

Increased interest in intensive corn production due to its value as a biofuel feedstock raises questions about the sustainability of yield and maintenance of soil quality in such systems – particularly continuous corn receiving N fertilizer.  In addition to production factors, the ability of agricultural soils to sequester C and thereby mitigate greenhouse gas emissions is of great interest, especially among the most prevalent cropping systems that cover the majority agricultural lands.  A long-term study has been conducted of continuous corn cultivation with variable N rates on a Typic Argiudoll in Wisconsin.  Over the course of 50 years, yields have been seen to increase continuously for medium and high N treatments, and high rates of N fertilization have had no negative impact on SOM in the top 15-20 cm of soil, presumably due to annual incorporation of corn stalk residues.  However, several studies have recently questioned the reliability of limiting sampling to the tillage zone when investigating soil C dynamics.  We have therefore analyzed C and N content down to 1m in these same long-term trials in order to examine impacts of continuous corn and variable N rates on total C content at the field level.  Results of 1m studies are presented here, along with comparison of A horizon depth, field subsidence, and compaction across N rate treatments.