Warming But Not Straw Application Increased Microbial Biomass Carbon and Microbial Biomass Carbon/Nitrogen: Importance of Soil Moisture

Investigating the effects of warming and straw application on soil microbial biomass carbon (C), nitrogen (N) and C/N is crucial for better understanding the responses of soil microorganisms to global warming. Soil microbial biomass C, N and C/N under different warming and straw application regimes were measured in a winter wheat–soybean rotation cropland. The split–plot experiment with four blocks included two main plots, i.e., warming (coded as W) and unwarmed (coded as U) treatments, with three straw application levels (0, 0.6 and 1.2 kg m−2, coded as S1, S2 and S3, respectively). The ranges of microbial biomass C were 250.0–1328.5, 228.1–932.4, 210.6–960.1, 177.9–710.8, 262.9–804.0 and 139.5–547.6 mg kg−1 in the WS1, WS2, WS3, US1, US2 and US3 plots, respectively, over the four crop-growing seasons, and the ranges of microbial biomass N were 9.7–86.2, 7.9–80.0, 14.7–75.3, 13.4–115.8, 13.1–98.7 and 15.5–110.3 mg kg−1 in the WS1, WS2, WS3, US1, US2 and US3 plots, respectively. Repeated-measures ANOVA indicated that warming induced a significant increase in microbial biomass C (P = 0.015) and C/N (P = 0.009) over the four growing seasons, while straw application did not significantly (P > 0.05) affect microbial biomass C, N or C/N. Soil microbial biomass C, N and C/N changed under the different soil moisture conditions. Warming-associated water losses also played a key role in influencing microbial biomass C, N and C/N. The results indicate that soil microorganisms in agroecosystems under the projected warmer climate conditions in the future may accumulate more C than those under the current climate conditions.