Long-term N fertilization effects on soil carbon balances in experimental agroecosystems in Iowa, USA

Soil carbon (C) balances were measured in two sites, Nashua and Kanawha (Iowa, USA), to evaluate the effects of N fertilization and cropping system. The experimental design was split-plot randomized block, with cropping system in the main plot, and N fertilization (four levels: 0, 90, 180 and 270 kg ha-1) in the subplot. Both sites contained the following cropping systems: continuous corn (CC); corn-soybean (CS); and corn-corn-oats-alfalfa (CCOA). At Kanawha, the fourth cropping system was COAA; at Nashua it was continuous soybean (SS). The experimental unit was the N-fertilization sub-plot. These experiments started in 1979 at Nashua and 1954 at Kanawha. Variables measured included: inputs of organic C, decay rates of organic C, and soil organic C stocks after 23 and 48 years respectively at Nashua and Kanawha. Inputs of organic C, Aboveground plus Belowground Net Primary Productivity (ANPP plus BNPP) minus harvested grain, were determined for corn by measuring total aboveground and belowground biomass in this annual crop at maturity. Aboveground organic matter (OM) inputs were thus calculated as the sum of leaves, stalks, husks, cobs, tassels, aboveground prop roots, weeds, and detritus (current crop only). For estimates of BNPP, biomass of fine roots, belowground prop roots and belowground stem was sampled for each of five depth increments, 0-15, 15-30, 30-50, 50-85, and 85-110 cm. For soybean, oats, and alfalfa, OM inputs were estimated based on measured long-term yield data and published allometric relationships. Plant tissue samples for all crops were analyzed for C content to convert from an OM to a C basis. Organic C decay was evaluated by 1) a mass-balance approach in all cropping systems at both sites; and 2) an isotopic mixing model to assess decay rates of the C3- and C4-derived C inputs to the system in the CS system at Nashua. Soil organic C was measured in all N treatments in the corn phase of all cropping systems in 2002 during a single post-harvest sampling (in October) prior to fall tillage. Soil OC was determined for each of the following depth increments: 0-5; 5-15; 15-30; 30-50; 50-75; and 75-100 cm.