Influence of soil carbon content on denitrification from fallow land during autumn

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Abstract
Denitrification in autumn removes nitrate from soil which may limit subsequent crop growth but also lessens the nitrate available for leaching. Autumn denitrification may therefore play a significant role in determining water quality because autumn and winter leaching of nitrate probably provides the greater part of the nitrate reaching groundwater from arable agriculture. The influence of soil carbon content on rates of denitrification during October and November 1987 was investigated in field plots of a continuous spring barley experiment which had large differences in soil organic matter content. Denitrification occurred between early autumn, when the warm soil became saturated with water, and late autumn when its temperature dropped to 6‐8°C. Gaseous N losses from plots with a low carbon content and given inorganic nitrogen were equivalent to 4.5 kg N ha−1, but those from farmyard‐manure‐treated plots with a high carbon content were 29 kg N ha−1, approximately six times greater. Measurements indicated that the high carbon content plots contained twice the denitrifying biomass of the low carbon content plots. The initial denitrification rate (Phase 1) of a laboratory measurement of potential denitrification gave a good estimate of denitrification in the field.