Near-Surface Soil Carbon, Carbon/Nitrogen Ratio, and Tree Species Are Tightly Linked across Northeastern United States Watersheds

Abstract

Forest soils hold large stores of carbon, with the highest concentrations in the surface horizons. In these horizons, both the total C mass and the C/N ratio may respond more rapidly to changes in tree species than lower horizons. We measured C and C/N ratios in the Oa or A horizon from 12 watersheds at 8 established forested research sites in the northeastern United States. The dominant tree species included Acer saccharum, Betula alleghaniensis, Fagus grandifolia, Picea rubens, and Tsuga canadensis. In 710 plots, both soil C (50–530 g kg−1) and the C/N ratio (11.6–45.3) had a wide range. In all but the Cone Pond watershed, both N concentration and the C/N ratio were strongly related to C content. For these 11 watersheds, the average C/N ratio = 9.5 + 0.030 × C g kg−1 (R2 = 0.97, P < 0.001). Ratios at Cone Pond were much higher than would be predicted from this equation and charcoal was found in numerous samples, suggesting a source of recalcitrant C. Averaged by watershed, C concentration was also significantly related to C pools. The carbon concentration of the horizons sampled was negatively related to A. saccharum + B. alleghaniensis dominance and positively related to conifer + F. grandifolia dominance. The strong relationships between C, C/N ratio, and species suggest predictable patterns in C accumulation in near-surface soils.

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