Distribution of nitrogen pools in the soil profile of undisturbed and reseeded grasslands

This study evaluated the effect of cultivation and reseeding on the distribution and fate of soil mineral N (SMN), soluble organic N (SON) and potentially mineralisable N (PMN) in the soil profile of two longterm grasslands in the UK. Cultivation and reseeding significantly increased the total soluble N concentration (SMN plus SON) of the soil profile (0-90 cm), with over 50 mg SON kg(-1) observed. By contrast, the PMN pool was unaffected by cultivation and declined with increasing soil depth. The flush in SON and SMN observed in both soils disappeared within 1 year following cultivation. The fate of SON appeared to be dependent on soil type, with considerably more movement to deeper layers apparent in the profile of a silty clay loam (30% clay) than in a clay loam (49% clay). Mineralisation and/or immobilisation of SON in the topsoil probably accounted for the changes observed in the SON content of the clay learn. SON is an important N pool in grassland soils and cultivation has a significant impact on its release. Measurements of SON Should therefore be included in studies of N cycling in agricultural cropping systems, so that full account may be taken of its potential as a source or sink of mobile N.