Laird, David (USDA, ARS, NSTL, 2150 Pammel Drive, Ames, IA, 50011; Phone: 515-294-1581; Fax: 515-294-8125; Email: laird@nstl.gov)

Impact of Nitrogen Fertilization and Crop Rotations on Soil Organic Carbon Sequestration and Soil Quality

D.E. Russell, D.A. Laird *

We analyzed the impact on soil organic C (SOC) of four N fertilization rates (0–270 kg N ha-1) and four cropping systems: continuous corn (CC); corn–soybean (CS); corn–corn–oat–alfalfa (CCOA), and corn–oat–alfalfa– alfalfa (COAA). Soils were sampled in 2002, years 23 and 48 of the experiments located in northeast (Nashua) and north-central (Kanawha) Iowa, respectively. The experiments were conducted using a replicated split-plot design under conventional tillage. A native prairie was sampled to provide a reference. Although corn yields increased with N additions, N fertilization increased SOC stocks (0-15 cm) only in the CC system at one site. Evaluated over the whole profile (0-100 cm) the effect was not significant. Considering the C cost for N fertilizer production, N fertilization generally had a net negative effect on C sequestration. Soil pH and available P tended to decrease with increasing levels of N fertilization in the surface soils (0-15 cm) for both sites. Crop rotations that included alfalfa had the highest SOC stocks, whereas the CS system generally had the lowest SOC stocks. Concentrations of SOC increased significantly from 1990 to 2002 in only two (fertilized CC and COAA at Kanawha) of the nine systems for which historical data were available. Indices of soil quality such as total N, particulate organic carbon, and microbial biomass were influenced by cropping system, with CS less than CC less than CCOA. The results support carbon credits for rotations that include alfalfa but not for high levels of N fertilization.