Priming effect of litter mineralization: the role of root exudate depends on its interactions with litter quality and soil condition.

Background and aims: Root exudation can prime microbial synthesis of additional exoenzymes and consequently accelerate organic carbon (C) and nitrogen (N) mineralization. Such exudate induced priming effect (EPE) has been hypothesized to depend on exudate rate and stoichiometry. Little is known about how EPE would affect litter decomposition. We employed a microcosm experiment to evaluate the influence of root exudate on litter nutrient release and microbial enzyme functions. Methods: Leaf litters of Pinus massoniana, Quercus variabilis and Robinia pseudoacacia were incubated under two soil conditions (fertile versus barren). Solutions of chemicals often found in root exudates with contrasting C:N ratios were inoculated frequently into the microcosms to simulate exudation. By comparing with a water control, exudate effect was determined. Results: In barren soils, exudates with C:N ratio of 10 significantly decelerated C loss of R. pseudoacacia, all N-containing exudates significantly enhanced the N-cycling related enzymes in decomposing Q. variabilis, while C-only exudate accelerated N loss of P. massoniana. In fertile soils, C-only exudate promoted the N-cycling related enzymes in decomposing R. pseudoacacia. Conclusions: A stoichiometric C:N constraint on microbial utilization of exudates arose in decomposing recalcitrant litters in barren soil. EPE and its stoichiometric constraint depend on interactions with litter quality and soil condition. The findings arouse the consequences of exudate rate and stoichiometry changes in determining soil nutrient balance. [ABSTRACT FROM AUTHOR]