Verifying the nitrification to immobilisation ratio (N/I) as a key determinant of potential nitrate loss in grassland and arable soils

The relative dominance of the competing pathways for ammonium, namely, the microbial processes of nitrification (N) and immobilisation (I), has been suggested as a major factor in controlling nitrogen losses from soils. In this paper we bring together data from four studies in arable and grassland soils to establish whether the ratio N/I is correlated with measured or modelled nitrate loss in drainage water. Individually, measurements of gross nitrogen transformations did not explain the variation in nitrate leaching across all sites. However, amounts of nitrate lost by leaching were well correlated with N/I for all sites. The relative importance of the pathways competing for mineralised nitrogen ( expressed as N/I) was changed by management and this appears to be an important factor in controlling N loss in arable and grassland soils.