Contribution of microbial activity and vegetation cover to the spatial distribution of soil respiration in mountains

The patterns of change in bioclimatic conditions determine the vegetation cover and soil properties along the altitudinal gradient. Together, these factors control the spatial variability of soil respiration (R_S) in mountainous areas. The underlying mechanisms, which are poorly understood, shape the resulting surface CO₂ flux in these ecosystems. We aimed to investigate the spatial variability of R_S and its drivers on the northeastern slope of the Northwest Caucasus Mountains, Russia (1,260-2,480 m a.s.l.), in mixed, fir, and deciduous forests, as well as subalpine and alpine meadows. R_S was measured simultaneously in each ecosystem at 12 randomly distributed points using the closed static chamber technique. After the measurements, topsoil samples (0-10 cm) were collected under each chamber (n = 60). Several soil physicochemical, microbial, and vegetation indices were assessed as potential drivers of R_S. We tested two hypotheses: (i) the spatial variability of R_S is higher in forests than in grasslands; and (ii) the spatial variability of R_S in forests is mainly due to soil microbial activity, whereas in grasslands, it is mainly due to vegetation characteristics. Unexpectedly, R_S variability was lower in forests than in grasslands, ranging from 1.3-6.5 versus 3.4-12.7 μmol CO₂ m^-1 s^-1, respectively. Spatial variability of R_S in forests was related to microbial functioning through chitinase activity (50% explained variance), whereas in grasslands it was related to vegetation structure, namely graminoid abundance (27% explained variance). Apparently, the chitinase dependence of R_S variability in forests may be related to soil N limitation. This was confirmed by low N content and high C:N ratio compared to grassland soils. The greater sensitivity of grassland R_S to vegetation structure may be related to the essential root C allocation for some grasses. Thus, the first hypothesis concerning the higher spatial variability of R_S in forests than in grasslands was not confirmed, whereas the second hypothesis concerning the crucial role of soil microorganisms in forests and vegetation in grasslands as drivers of R_S spatial variability was confirmed.