Annual denitrification rates in agricultural and forest soils: a review

Denitrification is an important soil process for assessing nitrogen cycling and controlling nitrogen pollution in the environment. Numerous studies of denitrification rates in soils have been reported over the last decade, ma ny with sampling protocols that are more reliable than in the past. In this paper, we review denitrification rates for agricultural and forest soils t hat have been reported in the literature, discuss factors that appear to be important in controlling the amount of denitrification that occurs in thes e soils, and summarise modelling approaches that have been used to predict annual denitrification rates. Most studies of in situ denitrification in upland soils have been conducted in agricultural grassland and forest ecosystems, with a paucity of studies reported from other ecosystems. A large range of annual, in situ, denitrif ication rates have been reported (0-239 kg N/ha.year), with the highest rat es typically occurring in irrigated, nitrogen-fertilised soils. However, mo st annual denitrification rates reported in the literature appear to be fai rly low, with over half of the rates in forest soils being <1 kg N/ha.year (mean of 1.9 kg N/ha.year). Rates of denitrification in agricultural soils tend to be higher than in forest soils, with 85% of rates reported being >1 kg N/ha.year, and a mean rate of 13 kg N/ha.year. Numerous soil, site, and management factors have been reported to affect the denitrification proces s in situ. The literature indicates that the highest rates of denitrificati on can be expected in nitrogen-fertilised soils, or where site management i ncreases soil nitrate availability. Where nitrate is non-limiting, denitrif ication rates appear to be highest in irrigated loam soils. The review sugg ests that it is difficult to predict denitrification rates based on our cur rent understanding, and that pilot studies should still be conducted if soi l nitrogen balances are required.