VERTICAL TRANSPORT OF DISSOLVED ORGANIC C AND N UNDER LONG-TERM N AMENDMENTS IN PINE AND HARDWOOD FORESTS

At the Harvard Forest, Massachusetts, a long-term effort is under way to study responses in ecosystem biogeochemistry to chronic inputs of N in atmospheric deposition in the region. Since 1988, experimental add itions of NH4NO3 (0, 5 and 15 g N m(-2) yr(-1)) have been made in two forest stands: Pinus resinosa (red pine) and mixed hardwood. In the se venth year of the study, we measured solute concentrations and estimat ed solute fluxes in throughfall and at two soil depths, beneath the fo rest floors (Oa) and beneath the B horizons. Beneath the Oa, concentra tions and fluxes of dissolved organic C and N (DOG and DON) were highe r in the coniferous stand than in the hardwood stand. The mineral soil exerted a strong homogenizing effect on concentrations beneath the B horizons. In reference plots (no N additions), DON composed 56% (pine) and 67% (hardwood) of the total dissolved nitrogen (TDN) transported downward from the forest floor to the mineral soil, and 98% of the TDN exported from the solums. Under N amendments, fluxes of DON from the forest floor correlated positively with rates of N addition, but fluxe s of inorganic N from the Oa exceeded those of DON. Export of DON from the solums appeared unaffected by 7 years of N amendments, but as in the Oa, DON composed smaller fractions of TDN exports under N amendmen ts. DOC fluxes were not strongly related to N amendment rates, but rat ios of DOC:DON often decreased. The hardwood forest floor exhibited a much stronger sink for inorganic N than did the pine forest floor, mak ing the inputs of dissolved N to mineral soil much greater in the pine stand. Under the high-N treatment, exports of inorganic N from the so lum of the pine stand were increased >500-fold over reference (5.2 vs. 0.01 g N m(-2) yr(-1)), consistent with other manifestations of nitro gen saturation. Exports of N from the solum in the pine forest decreas ed in the order NO2-N > NH4-N > DON, with exports of inorganic N 14-fo ld higher than exports of DON. In the hardwood forest, in contrast, in creased sinks for inorganic N under N amendments resulted in exports o f inorganic N that remained lower than DON exports in N-amended plots as well as the reference plot.