Dissolved organic nitrogen in contrasting agricultural ecosystems

Dissolved organic nitrogen (DON) is increasingly being recognized as a major component of the terrestrial nitrogen cycle, however, the factors that regulate its behaviour in soil remain poorly understood. The aim of this study was to investigate the impact of agricultural land use on the amount of DON in soil. At 94 sites, representing seven contrasting agricultural land use types, we extracted soil solution during the growing season. DON was high in all land use types constituting 57 +/- 8% of the total dissolved N (TDN) pool and generally followed the series citrus > vegetable > forest = arable > grassland = wetland > heathland. The TDN pool was dominated by DON in less intensive agricultural systems. In relative contrast to DON, the amount of dissolved inorganic N (DIN) varied widely upon land use with intensive agricultural systems being dominated by NO3- and low input systems dominated by NH4+. We conclude that DON represents a significant N pool in all agroecosystems but its concentration is less sensitive to land use system than DIN. (c) 2005 Elsevier Ltd. All rights reserved.