Biogeochemical Response of Forest Ecosystems to Simulated Chronic Nitrogen Deposition

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.

Landscape and Regional Scale Studies of Nitrogen Gas Fluxes

“…Previous studies at this site also found low N 2 O fluxes (Bowden et al 1991, Magill et al 1997. Our new NO data suggest that N gas fluxes are larger, and more responsive to N deposition, than previously thought.…”

Effects of nitrogen deposition and fertilization on N transformations in forest soils: a review

“…However, more and more attention is being paid to denitrification process in forests under increasing N deposition due to the high global warming potential of N 2 O (Hall and Matson 1999;Levy-Booth et al 2014). Although some studies have shown that N deposition and fertilization to forest ecosystems have minor effects on soil N 2 O emission (Magill et al 1997), there is mounting evidence that excessive N deposition to forest ecosystems, especially to N-unlimited forest ecosystems, has promoted soil N 2 O emission (Butterbach-Bahl et al 1998;Gundersen et al 2012;Hall and Matson 1999;Jassal et al 2011;Pilegaard et al 2006;Zhang et al 2008). For example, Hall and Matson (1999) found that N addition stimulated N 2 O emissions in both phosphorus (P)-limited and N-limited tropical forest soils, with a greater stimulation in the P-limited forest.…”

Carbon dioxide and methane fluxes by a forest soil under laboratory-controlled moisture and temperature conditions

“…Soil material was collected at the Harvard Forest Long-Term Ecological Research Site in north-central Massachusetts, U.S.A., from an approximately 50-y-old mixed hardwood stand established following abandonment from pastures in the early 1900's (Magill et al, 1997). Total biomass in the stand was 111 t ha À1 , and was dominated by black oak (Quercus velutina Lam.…”