Dissolved organic nitrogen dynamics in a Mediterranean vineyard soil

Dissolved organic carbon (DOC) and nitrogen (DON) have been hypothesized to play a central role in nutrient cycling in agricultural soils. The aim of this study was to investigate the annual dynamics of DOC and DON in a Greek vineyard soil and to assess the potential role of DON in supplying N to the vines. Our results indicated that significant quantities of DOC and DON existed in soil throughout the year and that peaks in concentration appeared to correlate with discrete agronomic events (e.g. onset of irrigation and plowing). Both field and laboratory experiments showed that free amino acids were rapidly mineralized in soil and that consequently free amino acids represented only a small proportion of the soil's total soluble N. Due to rapid nitrification the soil solution N was dominated by NO₃^-. Based upon the calculation of a plant-soil N budget and previous studies on N uptake in Vitis vinifera L., it is likely that DON uptake does not directly supply significant amounts of N to the plant. As the soil was not N limited we hypothesize that amino acids are used by the microbial community more as a source of C rather than a source of N. While we conclude that DON constitutes a significant N pool in vineyard soils further work is required to chemically characterize its constituent units and their relative bioavailability so that their overall role in N cycling can be determined.