Nitrous oxide emissions from arable soil: Effects of crop rotation, tillage and manure management

Summary

Soil carbon storage and nitrous oxide (N2O) emissions are both important for the greenhouse gas balance of agricultural soil, but difficult to verify under field conditions due to high spatial and temporal variability. Carbon stock changes are particularly elusive because they occur over decades, and future climate or management changes may revert current trends. Emissions of N2O from arable soil are derived mainly from the short-term (<1 yr) turnover of crop residues, fertilizers and manure, which indicates that mitigation options may be found with a better understanding of management effects on soil C and N cycling and N2O emissions. This presentation will describe N2O studies within long-term crop rotation experiments that allow side-by-side comparisons of contrasting management strategies. A tillage experiment on a sandy loam soil, established in 2002, has been used to study effects of residue management and tillage on N2O emissions. With removal of crop residues there was no difference between three tillage strategies, but with residue retention there was significantly higher N2O emission from ploughed soil compared to non-inversion tillage. Cover crops are particularly needed in organic farming systems where the N supply is limited. Another long-term experiment with eight four-crop rotations was established on three soil types in 1996 to investigate strategies to improve crop yields. Estimates of N2O emission from the same crop (winter wheat) in different rotations, and from all crops of a single rotation, suggest that emissions should be considered at the crop rotation level. Although short-term N2O emissions appear to be driven by organic inputs and fertilizers, there are also long-term effects of crop rotation that may interact with environmental drivers such as rainfall or freeze-thaw events. Laboratory results with intact soil cores from four rotations will be used to discuss the relative importance of carbon availability and soil gas diffusivity.