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FOREST, RANGE & WILDLAND SOILS

Controls on the Spatial Patterns of Carbon and Nitrogen in Adirondack Forest Soils along a Gradient of Nitrogen Deposition

Dep. of Earth and Environmental Science, Univ. of Pennsylvania, 240 S. 33rd St., Philadelphia, PA 19104

^* Corresponding author (jbedison{at}sas.upenn.edu)

We sampled 42 permanent vegetation plots in the Adirondack Mountains, NY to characterize the spatial patterns of, and controls on soil C and N contents. Average C (24 kg m^–2) and N (1.1 kg m^–2) contents of the combined organic and mineral horizons were high for northeastern U.S. forests. Contrary to our expectations, whole-profile C and N amounts were not different among northern hardwood (NH), spruce-fir (SF), and pine-dominated (PW) plots. Most of the C and N were stored in organic horizons in the high-elevation SF plots, and in the mineral horizons in NH and PW plots. Regression analyses of the pooled set of sites revealed that the factors that explained the most variability in soil C and N contents were different for organic and mineral horizons and differed between forest types. Overall, growing season degree-days (GSDD) was the variable most closely correlated with C and N contents in both organic and mineral horizons, and varying combinations of N deposition, conifer importance, and soil texture were the principal secondary influences. Spruce-fir plots received the most atmospheric N deposition, and multivariate regression tree (MRT) analysis indicated that N deposition rate was the environmental variable that explained the most variation in organic horizon C (sums of squares [SS] explained = 54%) and N (79% SS explained) amounts in these high-elevation stands. However, even in the unlikely case of 100% retention of the atmospheric inputs of N deposited over the past 50 yr, this source only accounted for a small portion of soil N, and the differences in N among the plots along the deposition gradient. It is likely that differences in GSDD accounted for most of the differences in C and N amounts.

Abbreviations: ANPP, annual net primary productivity • D_b, bulk density • GIS, Geographic Information System • GSDD, growing season degree-days • MAP, mean annual precipitation • MRT, multivariate regression tree • n, sample size • NEP, net ecosystem productivity • NH, northern hardwood • PW, pine-dominated • s.e., standard error • SF, spruce-fir • SS, sums of squares • T_h, horizon thickness