Biogeochemical cycling of carbon (C) in soils is crucial to the global C cycle. The transformation of C in soil is mediated by microbial metabolism, which influences the accumulation of soil organic carbon (SOC). Variability among ecosystems, compounded by disturbances such as N fertilization, make it challenging to understand what drives increased SOC storage in grassland ecosystems. This study focuses on the coupled dynamics of C chemistry and nitrogen (N) inputs to understand the molecular mechanisms regulating SOC storage in grassland ecosystems. We hypothesized that respiration of decomposable organic molecules will alter the chemical composition of SOC in soil, resulting in decreased abundance of energetically favorable compounds, and increased representation of persistent C classes. We further hypothesized that this effect will be most pronounced in grasslands with soil mineralogy that favors organic matter accumulation. In sites disturbed by N additions, we hypothesized that decomposition will be inhibited resulting in a greater diversity of persistent SOC. We investigated how N deposition and soil mineralogy influence the chemistry of persistent SOC across a network of grassland field fertilization using a lab-scale incubation coupled with high-resolution mass spectrometry of SOC.
Results/Conclusions
Over the course of an 8-month incubation, decomposition of SOC increased the similarity of dissolved organic matter (DOM) chemistry between five grassland soils, but the similarity of chemical composition of persistent organic matter was unchanged. The abundance (# of identified molecules) of energetically favorable compounds (such as, proteins, lipids and carbohydrates) decreased over the incubation, whereas the abundance of molecules resembling lignin, tannin and condensed hydrocarbon increased. Regarding N additions, cumulative respired C during the 8-month incubation was significantly inhibited by N addition; and, reductions of cumulative respired C among sites correlated with soil properties, such as mean annual temperature and C/N ratio, which explained 44% and 39% of the variability. Further, N addition did not significantly influence the similarity of chemical composition among soils; however, it significantly increased the abundance (# of identified molecules) of organic molecules in three out of five sites. Neither the shifts in SOC composition nor the effect of N inputs on C chemistry were regulated by soil mineralogy in this lab-incubation study. This study demonstrated that the coupled dynamics of C chemical compositions and N inputs, providing an advanced understanding of the biogeochemical cycle of C in grasslands.