Soil warming and nitrogen deposition alter soil organic matter composition at the molecular-level

Rising temperatures and nitrogen (N) deposition, both aspects of global environmental change, are proposed to alter soil organic matter (SOM) biogeochemistry. For example, increased plant productivity and enhanced microbial decomposition of litter and SOM may reduce soil carbon stocks and fertility. To better understand SOM biogeochemical shifts at the molecular-level, we employed an array of biomarker and nuclear magnetic resonance (NMR) techniques to investigate the composition and degradation of SOM components in the forest floor and mineral soil horizons of warmed (5 A degrees C above average soil temperature) and N fertilized (5 g m(-2) year(-1) N applied in the growing season) plots from the soil warming x nitrogen addition study at the Harvard Forest, MA, USA. Biomarker analyses indicated increased plant-derived inputs into the forest floor under N fertilization. Soil warming promoted the decomposition of plant-derived aliphatic and cyclic compounds in the forest floor. Cutin degradation was observed in the heated forest floor which also exhibited relatively higher microbial activity. Lignin oxidation was also observed but was most pronounced in the mineral horizon of the heated plots. These results suggest that continued soil warming may promote the degradation of lignin- and cuticle-derived SOM. N fertilization also enhanced lignin oxidation but to a lesser extent likely due to a decline in microbial activity. H-1 NMR spectra of the mineral soils revealed enrichment of plant-derived alkyl structures and microbial-derived organic matter with both soil warming and N fertilization. Overall, this study shows that the decomposition and accumulation of molecularly distinct SOM components occurs with soil warming and N amendment and may subsequently alter soil biogeochemical cycling.