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

This study evaluated the effect of cultivation and reseeding on the distrib ution and fate of soil mineral N (SMN), soluble organic N (SON) and potenti ally mineralisable N (PMN) in the soil profile of two longterm grasslands i n the UK. Cultivation and reseeding significantly increased the total solub le 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 cult ivation and declined with increasing soil depth. The flush in SON and SMN o bserved in both soils disappeared within 1 year following cultivation. The fate of SON appeared to be dependent on soil type, with considerably more m ovement 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 SO N 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 SO N Should therefore be included in studies of N cycling in agricultural crop ping systems, so that full account may be taken of its potential as a sourc e or sink of mobile N.