Effects of chronic nitrogen amendments on production of dissolved organic carbon and nitrogen in forest soils

Chronic N deposition has been hypothesized to affect DOC production in forest soils due to the carbon demand exerted by microbial immobilization of inorganic N. We tested this hypothesis in field experiments at the Harvard Forest, Petersham, Massachusetts, USA. During four years of sampling soil solution collected beneath the forest floor in zero-tension lysimeters, we observed little change in DOC concentrations (10-30 % increase, not statistically significant) associated with elevated N inputs, but did observe significant increases in DON concentrations. Both DOC and DON varied seasonally with highest concentrations in summer and autumn. Mean DON concentrations increased 200-300 % with the highest rate of inorganic N fertilization, and concentrations of DON were highest in samples with high inorganic N concentrations, We conclude that the organic chemistry of soil solution undergoes qualitative changes as a result of long-term N amendment at this site, with small changes in DOC, large increases in DON, and a decline in the C:N ratio of dissolved organic matter.