Nitrous oxide emissions in soils cropped with maize under long-term tillage and under permanent pasture in New Zealand

Information on the influence of tillage on nitrous oxide (N2O) emissions is needed by researchers, policy makers and farmers for assessment of global ramifications of warming potential of greenhouse gas N2O. This research evaluates the effects of continuous long-term tillage and seasonal changes in N2O emissions in New Zealand temperate climate. The experiment was conducted in a Kairanga silty clay loam (Eutric Gleysol) where maize (Zea mays L.) was grown continuously for either 17 (K17) or 34 (K34) years, with conventional tillage. An adjacent permanent ryegrass (Lolium perenne L.) and clover (Trifolium sp.) pasture (PP) was used as a control. Mean N2O emissions under K17 and K34 and PP ranged from 0.04 to 1.35, 0.03 to 1.13, and 0.03 to 0.99 g N2O-N/ha/h, respectively. Average annual N2O emissions from K34 were slightly lower than those in the K17 fields although there were no differences in N2O emissions between PP and K17. Depleted organic C, total N and microbial biomass C in K34 as compared with K17, possibly limited denitrification process and decreased N2O emissions. Soil water content (SWC) was a key factor controlling N2O emissions in both pasture and cropped soils. Low emissions occurred when gravimetric SWC content was less than 30%. N2O emissions from grazed pasture were low during summer (dry) period and increased during winter (wet) season. There were no differences in N2O between the K17 and K34 during dry season but N2O emission rates in wet season were lower in the K34. Despite high temporal variability observed in the N2O emissions, our estimates based on low frequency of measurements over 1 year were in general agreement with those in the literature.