No abstract

No abstract

Estimating contributions by root respiration and root litter to total soil respiration is difficult owing to problems in measuring each component separately. In a mixed hardwood forest in Massachusetts, we added or removed aboveground litter and terminated live root activity through construction of trenches and root barriers to determine the contribution of aboveground litter, belowground litter, and root respiration to total soil respiration. Annual soil respiration at control plots, measured by the soda-lime technique, was 371 g C•m−2-year−1. We used aboveground litter inputs (138 g C•m−2year−1) and differences in carbon dioxide effluxes among treatment plots to calculate contributions to total soil respiration by live root respiration (33%) and by organic matter derived from aboveground (37%) and belowground (30%) litter. Newly deposited aboveground litter contributed 31% of the carbon dioxide emitted by total aboveground litter. This estimate is consistent with values published in litter decomposition studies. Nearly two thirds of soil respiration in this forest can be attributed to root activity, comparable with a previous study suggesting that live root respiration plus decomposition of root litter contributes 70–80% of total soil respiration across a wide range of forests.

No abstract

Reported in this paper are foliar chemistry, tree growth (above-and belowground), soil chemistry, nitrogen cycling (net mineralization and nitrification) and soil N 2 O 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 Ϫ1 ·yr Ϫ1 ), high nitrogen (150 kg·ha Ϫ1 ·yr Ϫ1 ), and low nitrogen plus sulfur treatments, with additions occurring in six equal doses over the growing season as NH 4 NO 3 and Na 2 SO 4 . 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 substantially 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 NH 4 was consistently higher in treated plots than in controls in both stands, where extractable NO 3 was higher than controls only in the treated pine plots. Soil extracts yielded Ͻ1.5 kg/ha of NO 3 -N for all plots in the hardwood stand throughout the experiment. Effluxes of N 2 O were consistently greater in the pine high-N plot than in the pine control plot, but there were no observed large-scale increases in N 2 O 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.