C:N stoichiometry of stable and labile organic compounds determine priming patterns

Priming effects (PE) can greatly influence global carbon (C) storage in soil and lead to climate feedbacks by accelerating the decomposition of organic matter (OM). Although nitrogen (N) availability can alter the magnitude and direction of priming (stoichiometric constrains), it remains unclear whether additions of NO3 − (nitrate) and NH4 + (ammonium) have distinct effects on the decomposition of various OM. Thus, the aims of this study were to investigate the responses of OM decomposition along a decay continuum (i.e. decreasing decomposition degree) to labile C and N inputs and determine the PE induced by the two N forms. Four OM forms, namely leaf litter, wood litter, organic soil horizon, and mineral soil, with a broad range of C:N ratios were collected along a decay continuum in a typical subtropical forest and incubated for 38 days with labile C (13C labeled glucose) and N (NO3 −) additions. Based on the very broad range of C:N ratios in OM in soil and inputs of labile C and N, we demonstrated the OM decomposition within a decay continuum as well as PE intensities and the thresholds for the switch of PE directions. In contrast to NH4 + additions, NO3 − generally accelerated the decomposition of all OM. Priming of plant litter was dependent on the C:N ratios of the labile inputs. However, leaf litter decomposition was more controlled by N addition than wood litter. Glucose addition greatly increased the priming of OM decomposition in soils, demonstrating energy limitation for microorganisms. Distinct priming patterns were observed between NO3 − and NH4 + additions, both for the individual OM types and for all four types of OM. On a basis of C balance between primed C and the remaining added C, PE induced by labile C and N inputs can increase or reduce C sequestration depending on C:N stoichiometric ratios of labile inputs. Our findings provide important insights into the specific role of NO3 − or NH4 + together with labile C inputs (e.g. from root exudation), and thus changes in the composition of deposited N (atmospheric deposition and fertilization) may induce distinct climate feedbacks.