Lability of C in temperate forest soils: Assessing the role of nitrogen addition and tree species composition

Understanding how atmospheric nitrogen (N) deposition affects carbon (C) stabilization in forest soils has become an important focus as humans continue to alter global C and N cycles. Recent reviews have found a positive effect of increased N inputs on C stabilization in soils of temperate forest ecosystems. However, there is still uncertainty about the role and magnitude of the effect of chronic N inputs on forest soil C sequestration and how different tree species can modulate this effect. We evaluated the response of soil C lability to experimental N additions across plots with different dominant tree species (Acer saccharum, Fagus grandifolia, Betula alleghaniensis, Tsuga canadensis and Quercus rubra). We used a 14-year N addition experiment with a single-species, paired-plot design, and several measurements to estimate C lability, including soil laboratory incubations and density fractionation. Our two principal measures of C lability showed statistically significant interactive effects of N treatment and tree species composition: soils from maple (Acer) stands showed the greatest effect of added N on the light fraction mass in the mineral horizon (a 69% increase), and soils from beech (Fagus) stands showed the greatest N effect on potentially mineralizable C (a 23% decrease). Decreases in soil decomposition and respiration rates in organic and mineral horizons in response to N addition across all five species suggest a significant suppression of C mineralization, particularly in the first few weeks of the incubation, with the strongest responses in beech and oak (Quercus) stands. Our results confirm that increased N additions significantly reduce soil organic matter decomposition rates and the lability of soil C for some tree species, and indicate that mechanisms other than organo-mineral associations could play an important role in the stabilization of C in these soils. Further, our research illustrates the need to consider varying responses among different tree species when predicting future consequences of N inputs on soil C storage.