Long-term effects of fertilization and rotation on denitrification and soil carbon

Various long-term crop management strategies are known to hare differing ef fects on soil organic C. This laboratory study explored the effect of long- term (35 yr) fertilization and crop rotation on soil organic C and denitrif ication capacity at different depths of a Brookston clay loam soil (fine-lo amy, mixed, mesic Typic Argiaquoll). We related denitrification capacity to soil biochemical (CO2 production, organic C, microbial biomass C, soluble organic C) and soil structural properties. Denitrification capacity was det ermined as the increase in N2O that occurred when NO3--amended soils were i ncubated anaerobically in the presence of acetylene, Treatments included fe rtilized and nonfertilized plots of continuous corn (Zea mays L.), continuo us bluegrass (Poa pratensis L.), and rotation corn (corn-oat [Avena sativa L.]-alfalfa [Medicago sativa L.]-alfalfa). Soils from an adjacent mixed dec iduous woodlot were also sampled. Soils from the woodlot had higher denitri fication capacities than the continuous or rotation corn treatments. Among the agricultural treatments, the soil under bluegrass had the greatest deni trification capacity followed by the soil under corn rotation, with the con tinuous corn having the los est capacity. Long-term fertilization resulted in 35% higher denitrification capacity and 65% higher CO2 production than n onfertilized soils. Denitrification capacity across all depths in the agric ultural soils was correlated with CO2 production (r(2) = 0.76), microbial b iomass C (r(2) = 0.60), and organic C (r(2) = 0.54); however, the relations hip between denitrification rapacity and soil structure was not as strong ( r(2) = 0.28).