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

Various long-term crop management strategies are known to have differing effects on soil organic C. This laboratory study explored the effect of long-term (35 yr) fertilization and crop rotation on soil organic C and denitrification capacity at different depths of a Brookston clay loam soil (fine-loamy, 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 determined as the increase in N2O that occurred when NO3(-)-amended soils were incubated anaerobically in the presence acetylene. Treatments included fertilized and nonfertilized plots of continuous corn (Zea mays L.), continuous bluegrass (Poa pratensis L.) and rotation corn (corn-oat [Avena sativa L.]-alfalfa [Medicago sativa L.]-alfalfa). Soils from an adjacent mixed deciduous woodlot were also sampled. Soils from the woodlot had higher denitrification capacities than the continuous or rotation corn treatments. Among the agricultural treatments the soil under bluegrass had the greatest denitrification capacity followed by the soil under corn rotation, with the continuous corn having the lowest capacity. Long-term fertilization resulted in 35% higher denitrification capacity and 65% higher CO2 production than nonfertilized soils. Denitrification capacity across all depths in the agricultural soils was correlated with CO2 production (r2 = 0.76) microbial biomass C (r2 = 0.60) and organic C (r2 = 0.54): however, the relationship between denitrification capacity and soil structure was not as strong (r2 = 0.28)