The effects of long-term nitrogen enrichment and acidification on soil carbon, nitrogen, and phosphorus dynamics at the Bear Brook Watershed in Maine.

摘要

Nitrogen (N) pollution is a global concern particularly in areas of the world where industrial activities and fertilizer use have enriched the environment with N. While N in the environment can act as a pollutant, it is also a commonly limiting nutrient in terrestrial ecosystems. We studied the effects of N enrichment and acidification soil carbon (C), N, and phosphorus (P) dynamics at the Bear Brook Watershed in Maine (BBWM). The BBWM is a long-term paired watershed experiment, where the West Bear (WB) watershed has been treated with (NH4)2SO4 (ambient + treatment N deposition ≈ 36 kg N ha-1yr-1) since 1989. The adjacent East Bear watershed serves as a biogeochemical reference, receiving only ambient deposition (total N deposition ≈ 8 kg N ha-1 yr-1). Each watershed contains two distinct forest types: northern hardwoods at lower elevation and predominantly red spruce at higher elevation. Soil C, N, and P dynamics at BBWM were characterized using measurements of soil solution chemistry, microbial enzyme activities, and analysis of ecosystem C:N:P stoichiometry. Decadal trends in soil solution chemistry suggested greater leaching and export of N associated with treatment. However, differences in soil solution PO4-P by forest type suggested that treatment also enhanced the bioavailability of P in hardwoods, but not in softwoods. Evidence for higher P bioavailability in treated hardwoods was supported by trends in microbial response to inorganic P additions that was consistent with lower microbial P limitation by inorganic P in WB hardwoods compared to WB softwoods. Thus, N enrichment and acidification appear to have increased the extent of microbial P limitation in softwoods at the BBWM. Finally, patterns in C:N, N:P and C:P stoichiometry provided evidence that foliage and soils were strongly regulated by homeostatic mechanisms. There was less evidence of strong homeostatic regulation in litter, roots, soil solution and stream water, where biotic homeostatic as well as abiotic ecosystem processes govern C:N:P stoichiometry. These findings provide novel insights into the nature of microbial nutrient limitation and the prevalence of homeostatic regulation of C, N, and P stoichiometry that can inform our mechanistic understanding of forest response to N enrichment and acidification.