Stoichiometric Control Of Organic Carbon-Nitrate Relationships From Soils To The Sea
Taylor, Philip G 1 ; Townsend, Alan R 2
1 University of Colorado
2 University of Colorado
Human creation of reactive nitrogen has risen an order of magnitude since the dawn of the Industrial Revolution. This dramatic reorganization of a global biogeochemical cycle has brought substantial benefits, but increasingly causes detrimental outcomes for both people and ecosystems. One such problem is the accumulation of nitrate (NO3-) in both freshwater and coastal marine ecosystems. Here we establish that ecosystem NO3- accrual exhibits consistent and negative nonlinear correlations with organic carbon (C) availability along a hydrologic continuum from soils, through freshwaters and coastal margins, to the open ocean (Figure 1). The trend also prevails in ecosystems subject to substantial human alteration. Across this diversity of environments, we find evidence that resource stoichiometry (organic C:NO3-) strongly influences NO3- accumulation by regulating a suite of microbial processes which couple DOC and NO3- cycling. Through meta-analysis, we show that heterotrophic microbes maintain low NO3- concentrations when organic C:NO3- ratios match the stoichiometric demands of microbial anabolism (Figure 2A). However, when resource ratios drop below the minimum C:N ratio of microbial biomass, the onset of C limitation appears to drive rapid NO3- accrual, which may then be further enhanced by nitrification (Figure 2B). At low organic C:NO3- ratios, denitrification appears to constrain the extent of NO3- accretion once organic C and NO3- availability approach the 1:1 stoichiometry of this catabolic process (Figure 2C). Collectively, these microbial processes express themselves on local to global scales by restricting the threshold ratios underlying NO3- accrual to a constrained stoichiometric window. Our findings indicate that ecological stoichiometry can help explain the fate of NO3- across disparate environments and in the face of human disturbance, which has significant implications for the management of a rapidly changing N cycle.