PARENT SESSION
Oral Session # 14: Agro-Ecology I.
Presiding: C Salo
Tuesday, August 5. 8:00 AM to 11:30 AM, SITCC Meeting Room 100.

Long-term effects of nitrogen fertilization and crop rotation on soil carbon in corn-belt agroecosystems.

Russell, Ann^{*,1, 2}, Laird, David¹, Mallarino, Antonio³, ¹ USDA-ARS National Soil Tilth Laboratory, Ames, IA, USA² Iowa State University, Ames, IA, 50011³ Iowa State University, Ames, IA, USA

ABSTRACT- We investigated effects of rate of N fertilization and crop-rotation type on soil properties in Iowa agroecosystems, in comparison with an undisturbed prairie. Our two rain-fed experimental sites at Nashua and Kanawha, IA had been tile-drained and were under conventional tillage. The split-plot experimental design for the past 23 y (Nashua) and 18 y (Kanawha) included three rotations: 1) continuous corn for grain (CC); 2) corn-soy (CS); and 3) corn-corn-oats-alfalfa (CCOA). Each rotation had been subjected to four N levels: 0, 90, 180 and 270 kg ha^-1 y^-1. We measured the following soil properties in 2002 at six depth intervals over 0-100 cm: total soil organic carbon (TSOC); soil inorganic carbon (SIC); total soil N; bulk density; and pH. We measured other soil properties more intensively in the 0-15-cm depth at Nashua and the native prairie soil in 2001 and 2002. N fertilization significantly lowered soil pH in all rotations (0-15 cm), but fertilization had no significant effects on other soil properties. Rotation type did have significant effects on soil carbon in the 0-15 cm layer. TSOC was significantly higher (P=0.05) under CCOA rotation (4194 g m^-2) than CC and CS rotations (3922 and 3690 g m^-2, respectively at Nashua). All crop rotations had significantly lower TSOC, ranging from 20.4 to 23.4 g kg^-1 over all treatments, compared to 64.7 g kg^-1 in native prairie. Under CCOA, the following soil properties were higher compared to the other rotations: microbial activity (P=0.10); microbial biomass C (P=0.01); particulate organic matter C (P=0.20); and resistant soil C (P=0.05). Our results indicated that higher N fertilization did not result in increased soil carbon storage under any crop rotation in these systems. Type of crop rotation, however, did influence soil carbon sequestration and biological activity of the soil, hence its capacity to supply nutrients for plant growth.

Key words: soil carbon sequestration, N fertilization, corn-belt agroecosystems, crop rotations