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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, many with sampling protocols that are more reliable than in the past. In this paper, we review denitrification rates for agricultural and forest soils that have been reported in the literature, discuss factors that appear to be important in controlling the amount of denitrification that occurs in these 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, denitrification rates have been reported (0–239 kg N/ha.year), with the highest rates typically occurring in irrigated, nitrogen-fertilised soils. However, most annual denitrification rates reported in the literature appear to be fairly 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 process in situ. The literature indicates that the highest rates of denitrification can be expected in nitrogen-fertilised soils, or where site management increases soil nitrate availability. Where nitrate is non-limiting, denitrification rates appear to be highest in irrigated loam soils. The review suggests that it is difficult to predict denitrification rates based on our current understanding, and that pilot studies should still be conducted if soil nitrogen balances are required.

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Bulk density of forest soils from nine locations in New England was closely and inversely related to the organic fraction of the soil. Measured data over the whole range of organic fractions followed the theoretical relationship Db = DbmDbo/[FoDbm + (1 − Fo)Dbo] where Db is the bulk density (Mg/m3), Fo is the organic fraction (kgo/kg), Dbo is the bulk density when Fo = 1, and Dbm is the bulk density when Fo = 0. The relation arises from assuming that (i) Dbo, the bulk density of "pure" organic matter, and Dbm, the bulk density of "pure" mineral matter, are constant and (ii) in a mixture, the volumes occupied by the organic mass and the mineral mass are additive. For forest soils on coarse-textured till in New England, Dbo = 0.11 Mg/m3 and Dbm varied from 1.45 Mg/m3 for sandy loams to 2.19 Mg/m3 for silt loams. When these parameters are known, Db can be estimated from Fo, which is more easily measured. When Fo is greater than 0.1 kgo/kg, the organic mass per unit soil volume (FoDb), or organic density, is approximately constant at 0.1 Mgo/m3. For many nutrients, separate evaluation of the organic density and the amount of nutrient per unit organic mass may facilitate intersite comparisons for studies of nutrient availability and leaching.

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