Hydrologic gradient changes of soil respiration in typical steppes of Eurasia

Soil respiration (R_S) is affected by many factors and shows significant diurnal and seasonal changes at different spatial and temporal scales. However, in a semi-arid steppe, the mechanism of the dynamic influence of environmental factors on R_S is not clear, and the effect of subtle changes of soil water on R_S under drought stress is yet to be explored. Therefore, Xilin River Basin, was the study area and a hydrological gradient on the four ecosystems for R_S and hydrometeorological monitoring was selected. We proposed the use of dynamic sunrise and sunset time to distinguish day from night and determine related statistics. Additionally, we analyzed the temporal variation of R_S and its response process and mechanism for hydrometeorological factors during the growing season and at daily scales. Further, we quantitatively simulated the R_S of 594 scenarios in different growing season stages, ecosystems, and precipitation patterns. Results showed that: (1) in the hydrological gradient belt, different ecosystems exhibited the same trend but different characteristics of R_S regulation. From May to November 2020, R_S was 2.34-3.89, 1.89-5.97, 1.90-5.27 and 2.29-3.45 gC m^-2 day^-1 for the four ecosystems. (2) The use of dynamic sunrise and sunset time to distinguish day and night can more accurately describe the statistical value of each variable, which exhibits remarkable feasibility in daily scale research. (3) The changes in R_S were adequately explained by temperature at various time scales, and the photosynthetically active radiation was positively correlated with R_S at the daily scale. The special soil water content (M_S) condition in the study area was not sufficient to explain R_S. (4) Precipitation can affect R_S by changing soil and air; however, only when precipitation exceeds the effective precipitation threshold of 0.6 ± 0.3 mm, it significantly affects R_S.