Soil-profile distribution of inorganic N during 6 years of integrated crop-livestock management

Excessive accumulation of soil nitrate-N can threaten water and air quality. How integrated crop-livestock systems might influence soil-profile nitrate-N accumulation has not been investigated. Therefore, we determined soil nitrate-N accumulation during 6 years of evaluation of diverse cropping systems on a Typic Kanhapludult in Georgia, USA. Of the total change in soil nitrate-N content that occurred during 6 years (i.e. increase of 14 kg N ha−1 year−1), an average of 60% occurred in the primary rooting zone (0–90-cm depth) and 40% occurred in the zone below typical rooting (90–150-cm depth). Soil nitrate-N accumulation was greater in cropping systems with greater N fertilizer input, while it was surprisingly insensitive to differences in harvested N output. Soil nitrate-N accumulation was greater under conventional tillage than under no tillage at all soil depths (e.g. 5.1 ± 4.2 kg N ha−1 year−1 greater at a depth of 90–150 cm), suggesting soil disturbance was a key factor in mobilizing N and keeping it more disassociated from the organic–inorganic cycling system. Grazing of cover crops had variable effects on soil nitrate-N content: greater soil nitrate-N content in the rooting zone at the end of 1 year (63 vs. 47 kg N ha−1), greater soil nitrate-N content in the zone below typical rooting at the end of 3 and 4 years (91 vs. 70 kg N ha−1), and lower soil nitrate-N content in the rooting zone at the end of 6 years (89 vs. 120 kg N ha−1). These results confirm the beneficial effect of no-tillage management on moderating nitrate-N accumulation in the soil profile and indicate a variable, but mostly neutral effect of cover crop grazing on soil nitrate-N accumulation.