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Light use efficiency (LUE) is a crucial indicator used to reflect the ability of terrestrial ecosystems to transform light energy. Understanding the long-term trends in LUE and its influencing factors are essential for determining the future carbon sink and carbon sequestration potential of terrestrial ecosystems. However, the long-term interannual variability of LUE in grasslands in northern China at the ecosystem scale is poorly understood due to the limitations of the year length and the coverage of the site data. In this study, we assessed the long-term LUE trends in the grasslands of northern China from 1982 to 2018 and then revealed the relationships between interannual variability in LUE and climate factors. Our study showed a substantial rising trend for LUE from 1982 to 2018 in the grasslands of northern China (3.42 × 10−3 g C/MJ/yr). Regarding the different grassland types, alpine meadow had the highest growth rate (4.85 × 10−3 g C/MJ/yr), while temperate steppe had the lowest growth rate (1.58 × 10−3 g C/MJ/yr). The climate factors driving LUE dynamics were spatially heterogeneous in grasslands. Increasing precipitation accelerated the interannual growth rate of LUE in temperate steppe, and increasing temperature accelerated the interannual growth rate of LUE in other types. In addition, the temporal dynamic of LUE showed different trends in relation to time scales, and the growth trend slowed down after 1998. Our results should be considered in developing future grassland management measures and predicting carbon cycle–climate interactions. [ABSTRACT FROM AUTHOR]

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]