[Effects of Biochar Application on Soil Microbial Nutrient Limitations and Carbon Use Efficiency in Lou Soil]

Soil microbial metabolism is vital for nutrient cycling and stability of an ecosystem. To elucidate the long-term effects of biochar application on nutrient limitations and carbon use efficiency (CUE) of soil microbial metabolisms, biochars pyrolyzed at 450℃ from trunks and branches of fruit trees under an oxygen-limited condition were mixed with the top Lou soils (0-20 cm) with application amounts of 0, 20, 40, 60, and 80 t·hm^-2 in 2012. Corn-wheat rotation was carried out afterwards for seven years. The nutrient limitations of soil microbial metabolisms were analyzed quantitatively through ecoenzymatic stoichiometry in 2019. The results indicated that:① With an increase in the biochar application amount, soil moisture, organic carbon, total nitrogen, C:N, C:P, and N:P significantly increased, whereas there were no clear patterns for the active components of carbon, nitrogen, and phosphorus, microbial biomass carbon, nitrogen, phosphorus and total phosphorus. In contrast, the activities of five extracellular enzymes (β-1,4-glucosidase, cellobiohydrolase, leucine aminopeptidase, β-1,4-N-acetylglucosaminidase, and phosphatase) were significantly reduced. ② The soil microorganisms suffered from the phosphorus limitation under all treatments in this study. In the treatments of biochar application, the carbon and phosphorus limitations of microbial metabolisms increased significantly with increasing application amount, whereas the microbial CUE decreased significantly. When the application amount was 20 t·hm^-2, the carbon limitation (0.625±0.022) and phosphorus limitation (62.153°±0.892°) were lowest, and the microorganism CUE (0.511±0.007) was highest. ③ Partial least-squares path modeling (PLS-PM) showed that soil carbon, nitrogen, phosphorus, and their stoichiometry had a very direct positive effect on phosphorus limitation (P<0.01), and there was a positive correlation between carbon limitation and phosphorus limitation (R²=0.242, P<0.001); in contrast, the carbon and phosphorus limitations had a very significant negative effect on CUE (P<0.001). It was revealed that the excessive application of biochar had caused a soil element stoichiometry imbalance, which deteriorated the phosphorus limitation of the soil microbial metabolism and further led to carbon limitation and reduction of CUE. When the biochar application amount was 20 t·hm^-2, C and P limitations were lowest, and microbial CUE was highest. Therefore, 20 t·hm^-2 was optimal for regulating soil microbial metabolism, maintaining ecological functions, and reducing carbon dioxide emission produced by microbial metabolism.