BIOGEOCHEMICAL RESPONSE OF FOREST ECOSYSTEMS TO SIMULATED CHRONIC NITROGEN DEPOSITION

Reported in this paper are foliar chemistry, tree growth (above- and below- ground), soil chemistry, nitrogen cycling (net mineralization and nitrification) and soil N20 flux responses to the first 6 yr of chronic nitrogen amendments at the Harvard Forest (Massachusetts, USA). A 70-yr-old red pine (Pinus resinosa Ait.) stand and a 50-yr-old mixed hardwood stand received control, low nitrogen (50 kg.ha-'-yr-'), high nitrogen (150 kg.ha-'-yr-'), and low nitrogen plus sulfur treatments, with additions occurring in six equal doses over the growing season as NH4NO3 and Na2SO4. Foliar N concentrations increased up to 25% in the hardwood stand and 67% in the pines, and there was no apparent decrease of N retranslocation due to fertilization. Wood production increased in the hardwood stand in response to fertilization but decreased in the pine stand. Fine-root nitrogen concentrations increased with N additions, and fine roots were a significant sink for added nitrogen. Nitrate leaching losses increased continuously over the 6-yr period in the treated pine stands but remained insignificant in the hardwoods. Annual net N mineralization increased substan- tially in response to treatments in both stands but declined in the pine high-N plot by the end of year six. Net nitrification increased from 17% of net mineralization in 1988 to 51% in 1993 for the pine high-N plot. Only a slight increase in net nitrification was measured in the hardwood stand, and only in 1993. Extractable NH4 was consistently higher in treated plots than in controls in both stands, where extractable NO3 was higher than controls only in the treated pine plots. Soil extracts yielded <1.5 kg/ha of NO3-N for all plots in the hardwood stand throughout the experiment. Effluxes of N2O were consistently greater in the pine high-N plot than in the pine control plot, but there were no observed large-scale increases in N20 emissions immediately following fertilizer application. Calculated nitrogen budgets for the first 6 yr showed extremely high N retention (85-99%). Of the retained N, 50-83% appears to be in the long-term, recalcitrant soil pool. The relative importance of biotic and abiotic mechanisms of N incorporation into soils remains uncertain. Size, kinetics, and uptake capacity of this soil pool are critical and largely unknown factors determining ecosystem response to increased N loading and may be related to land-use history.

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