Influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol

In this study, the influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol from north east Scotland was investigated. Firstly, soil cores were incubated at 5, 10 and 15 degreesC f or up to 8 weeks. Net mineralisation was observed at all temperatures with larger rates observed at higher temperatures. In contrast, water extractabl e dissolved organic nitrogen (DON) displayed no clear trend with time and s howed little response to temperature. Secondly, intact cores of the same so il, with and without vegetation, were leached with artificial rain for 6 we eks at 6.5 and 15 degreesC. Temperature acid the presence of vegetation int eracted to have a significant (P < 0.01) effect on the concentration of NO; in leachates, highest concentrations were observed in leachates from cores without vegetation at 15 degreesC, whereas lowest concentrations were obse rved in leachates from cores with vegetation at 6.5"C. In contrast, concent rations of DON and dissolved organic carbon (DOC) were significantly (P < 0 .001) higher in leachates from cores with vegetation than without vegetatio n and were not affected by temperature. The cumulative amounts of DON and D OC leached from the cores with vegetation were 4 and 2.5 times greater, res pectively, than those leached from the cores without vegetation. Comparison of soil solution (extracted by centrifugation at 0-5 and 5-10 cm depth) af ter leaching for 6 weeks, showed that the upper layer contained more than t wice the amount of DON than the 5-10 cm layer and that the difference in co ncentration between the two depths was enhanced in the presence of vegetati on. The results indicate that vegetation is an important source of DON and DOG. However, the removal of vegetation did not lead to an increase in the quantity of total dissolved nitrogen (TDN) in soil water, but resulted in a change in the dominant N fraction from DON to NO;. In addition, the result s show that DON, in both the incubated and leached cores, did not change as inorganic N was mineralised. This suggests that if water extractable DON w as acting as a source of NH: or NO;, then it was being replenished by, and in equilibrium with, a large reserve of organic N. Evidence of such a pool was indirect in the form of additional DON (equivalent to 2 g N m(-2) being extracted by 0.5 M K2SO4. (C) 2001 Elsevier Science Ltd. All rights reserv ed.