The largest terrestrial pool of carbon, soil organic matter (SOM), may be particularly sensitive to environmental changes that could cause large feedbacks on the atmosphere and climate. Forests in the northeast US experienced a history of elevated nitrogen deposition, which can alter leaf litter chemistry and decomposition, but we do not fully understand the cascading effects of this response on the formation or destabilization of SOM. We hypothesize that chronic nitrogen deposition reduces the decomposition rate of recalcitrant plant tissue, such as lignin, and leads to an accumulation of particulate organic matter. To test this hypothesis, we coupled a two-year litter decomposition experiment with a soil density fractionation study at a long-term, whole-watershed fertilization experiment at the Fernow Experimental Forest in West Virginia. Leaf litter from four dominant species in the fertilized and reference watersheds were decomposed reciprocally to test the effects of both litter quality and soil properties on leaf litter decay rates. Soil fractions were separated into three pools representing increasing levels of microbial processing: light particulate, heavy particulate, and mineral-associated fractions.
Results/Conclusions
After three and six months, the leaf litter deployed in the fertilized watershed decayed more rapidly than leaves deployed in the reference watershed, regardless of watershed of origin. In later stages of decomposition, leaf litter that originated from the fertilized watershed decayed more slowly, regardless of the watershed in which it was deployed. Thus, initial stages of decomposition are apparently influenced by soil properties that allow for rapid decay of labile and simple components of leaf tissue, while in later stages of decomposition, litter chemistry more strongly influences decomposition rates. From soil density fractionation, we found that SOM in the fertilized watershed had a greater light particulate fraction and lesser heavy particulate fraction than SOM in the reference watershed. The proportion of SOM in the mineral-associated fraction did not differ between watersheds. These results suggest that more plant inputs bypass microbial decomposition in the fertilized watershed, resulting in a greater fraction of labile SOM and less SOM physically protected by aggregates and microbial byproducts. Collectively, these studies suggest that chronic nitrogen deposition may alter the pathway of SOM formation, leading to SOM that could be more vulnerable to loss under future conditions of reduced nitrogen deposition and climate change.