Dissolved organic carbon and nitrogen relationships in forest litter as affected by nitrogen deposition

Dissolved forms of carbon and nitrogen have become recognized for their importance in forest nutrient cycling. The role of dissolved organic carbon (DOC) as an energy source for microbial metabolism is of particular interest. A laboratory decomposition experiment was conducted to examine the relationship between potential increased N inputs (via acid deposition) and DOC production in the forest litter layer and subsequent effects on DOC availability in the forest floor. Air-dried leaf litter (seven species) was treated with nitrogen (nitrate or ammonium) or deionized (DI) water at weekly intervals throughout 15 weeks and leached with DI water at 1 or 2 week intervals. Leachate was analyzed for DOC, inorganic nitrogen (NO3--N and NH4+-N) and dissolved organic nitrogen (DON). Litter was analyzed for percent C, percent N, weight loss and percent cellulose and lignin. Nitrogen treatments did not greatly affect DOC,concentrations in litter leachate. Differences in DOC concentrations were primarily due to a wide range of initial litter chemistries, where species with high extractives and low lignin had the highest DOC leachate concentrations. Nitrogen treated samples showed greater weightless than controls although nitrate and ammonium treatments were not significantly different. Between 6 and 39% of total carbon loss was leached as DOC. These findings suggest that different forest types could vary greatly in the quantity of carbon consumed or released and that nitrogen inputs appear to affect this overall cycle by increasing respiration (as measured by weight loss), rather than increasing DOC release into the soil solution. Further examination of the fate of DOC as it moves down in the soil profile and measurements of CO2 evolution during laboratory decomposition, are necessary to better understand these processes. (C) 2000 Elsevier Science Ltd. All rights reserved.