[Effect of Environmental Factors on Variation Characteristics of Soil Microbial Respiration and Its Temperature...

Studying the effect of environmental factors on the variation of soil microbial respiration and its temperature sensitivity (Q₁₀) at different time scales under field conditions is of great significance for accurately understanding the region's climate warming potential. From March 2008 to November 2013, in situ soil microbial respiration rates were determined using an automated CO₂ flux system (Li~8100) in long-term bare fallow soil at the Changwu State Key Agro-Ecosystem Experimental Station, Shaanxi, China, for studying the effect of environmental factors on the variation of soil microbial respiration and Q₁₀ at different time scales. At diurnal time scales, the daily variation of soil microbial respiration rates showed a single-peak curve, which was closely related to soil temperature (P<0.05); whereas the daily mean soil microbial respiration rate and Q₁₀ varied with soil moisture, with both showing the order of moderate soil moisture conditions > higher soil moisture conditions > lower soil moisture conditions[daily mean soil microbial respiration rate:1.20 μmol·(m²·s)^-1 vs. 0.95 μmol·(m²·s)^-1 vs. 0.79 μmol·(m²·s)^-1; Q₁₀:2.12 vs. 1.93 vs. 1.59]. At seasonal time scales, both the seasonal mean soil microbial respiration rate and Q₁₀ showed the order of rainy season > non-rainy season[seasonal mean soil microbial respiration rate:1.11 μmol·(m²·s)^-1vs. 0.90 μmol·(m²·s)^-1; Q₁₀:1.96 vs. 1.59], which was consistent with the trend of soil temperature and moisture (soil temperature:20.39 vs. 14.50℃; soil moisture:49.2% vs. 38.6%). The bivariate model of soil temperature and soil moisture could explain the greater seasonal variability of the soil microbial respiration rate than did the univariate model of soil temperature or soil moisture (R²:0.45-0.82 vs. 0.32-0.67 vs. 0.35-0.86; the fitting coefficient between the simulated and measured soil microbial respiration rates:0.76 vs. 0.64 vs. 0.58). At annual time scales, the annual cumulative soil microbial respiration ranged from 226 to 298 g·(m²·a)^-1, with an average of 253 g·(m²·a)^-1, and the annual Q₁₀ ranged from 1.48 to 1.94, with an average of 1.70. The annual cumulative soil microbial respiration and Q₁₀ showed a negative quadratic correlation with annual mean soil moisture (P<0.05), with the annual mean soil moisture explaining 39% and 54% of the annual variability of annual cumulative soil microbial respiration and Q₁₀, respectively. In the bare soil treatment, the soil organic carbon decreased from 6.5 g·kg^-1 at the beginning of the experiment to 5.5 g·kg^-1 at present; whereas, the annual cumulative soil microbial respiration was up to 255 g·(m²·a)^-1 and the loss of annual cumulative soil microbial respiration was 20 times larger than the loss of soil organic carbon in the Loess Plateau region, China.