Sources and transformations of organic nitrogen in intensively managed grassland soils

Abstract

Large amounts of nitrogen (200–350 kgN ha⁻¹ yr⁻¹) are returned to the soil under intensively managed grassland through the death and decay of unharvested herbage and of roots. With grazed swards, the return of animal excreta may contribute an additional 150–300kgN ha⁻¹ yr⁻¹, but this is distributed unevenly. Earthworms are important in the early stages of decomposition of plant residues and faeces. The balance between the mineralisation and immobilisation of N during decomposition is influenced largely by the C:N ratio. In many grassland soils, the rate of addition of organic matter exceeds the rate of decomposition but, when long-term grassland is subjected to intensive management, this pattern may be reversed. Soils differ considerably in the proportion of the total soil N that is mineralised, and hence becomes available for uptake, during a growing season. Available soil N may be predicted by chemical analysis based on the measurement of NH⁺ ₄-N + NO⁻ ₃-N extracted by boiling with l M KCl. In the field, prediction is improved when the results of soil analysis are adjusted by a factor, calculated for the individual site, derived from mean values of temperature and either soil water status or rainfall.