Afforestation with trees and shrubs around cropland can effectively decrease soil degradation and avoid sand storms, but subsequent modification of litter quality accelerates the degradation of native organic matter via the soil priming effect (PE). Although carbon accumulation in agricultural soils after afforestation was widely studied, little is known about the extent to which soil organic carbon (SOC) mineralization is induced by complex residue input in agro-forest-grass composite ecosystems. Here, we mixed corn field soil and litter of afforestation tree and shrub species together in a micro-environment to quantify the effects of litter-mixture input on farmland soil priming associated with afforestation. Additionally, we studied the responses of bacterial and fungal species to litter chemistry, with the aim to identify the litter and microbial driver of soil priming. The results showed that soil priming was accelerated by different litter addition which varied from 24 to 74% of SOC mineralization, suggesting that priming intensity was relatively flexible and highly affected by litter quality. We also find that the macro-chemistry (including litter carbon, nitrogen, lignin, and cellulose) directly affects priming intensity, while micro-chemistry (including litter soluble sugar, water-soluble phenol, methanol-soluble phenol, and condensed tannin) indirectly influences priming via alteration to dominant bacterial taxa. The stepwise regression analysis suggested that litter nitrogen and cellulose were the critical litter drivers to soil priming (r2 = 0.279), and the combination of bacterial phylum Proteobacteria, Firmicutes, Bacteroidetes, Acidobacteria, and fungal taxa Eurotiomycetes was a great model to explain the priming intensity (r2 = 0.407).
Litter decomposition is the main driver of nutrient cycling process in terrestrial ecosystems. Afforestation completely altered vegetation composition and litter species, disrupting the long-term carbon balance in grassland ecosystem. However, there is a lack of understanding of how litter mixing effect (LME) affects soil carbon cycling in afforested ecosystem. Here, we investigated the effects of litter richness and quality of tree, shrub, and grass species and their litter mixture on soil CO2 fluxes. The results showed that cumulative soil CO2 flux in the early stage (1–28 days) was 1.75 times higher than that in the late stage (29–113 days), indicating litter decomposition was intensive at first and then decreased with time. Soil carbon flux changed with decomposition stages. In the early-stage of decomposition, soil CO2 flux increased with the concentrations of litter carbon, nitrogen and condense tannin. In the late phase of decomposition, all litter chemical traits were negatively related to the soil carbon flux. Additionally, plant litter richness was negatively correlated to early-stage soil CO2 flux, whereas it was positively related to late-stage soil carbon flux. Our results provide evidence that long-term carbon balance in grassland ecosystems was interrupted by afforestation, and the dominant litter chemical traits that controlling soil carbon cycling changed over time.
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