Meta-analysis of field scale spatial variability of grassland soil CO2 efflux: Interaction of biotic and abiotic drivers

- Goodness of soil respiration variogram model fit is probably under biotic control.
- Open vegetation is not favourable for the appearance of spatial structures.
- Patterns of soil moisture and temperature constrain patch size of respiration.
- Future less soil moisture may uncouple spatial links of drivers and respiration.

In this study eight temperate grassland sites were monitored for soil CO2 efflux (Rs) and the spatial covariate soil water content (SWC) and soil temperature (Ts) at fine scale in over 77 measurement campaigns. The goals of this multisite study were to explore the correlations between environmental gradients and spatial patterns of Rs, SWC and Ts, which are not site-specific and to quantify the relevance of biotic and abiotic controls over spatial patterns along increasing vegetation structural complexity. These patterns in water-limited ecosystems in East-Central Europe are likely to be influenced by summer droughts caused by the changing climate.A consistent experimental setup was applied at the study sites including 75 sampling locations along 15 m circular transects. Spatial data processing was mainly based on variography. Two proxy variables were introduced to relate the site characteristics in terms of soils, water status and vegetation. Normalised SWC (SWCn) reconciled site-specific soil water regimes while normalised day of year integrated temperature and vegetation phenology.A principal component analysis revealed that the progressing closure of vegetation in combination with large Rs and SWCn values, as well as low Ts and Rs variability support the detectability of spatial patterns found in both the abiotic and biotic variables. Our results showed that apart from SWC the pattern of soil temperature also had an effect on spatial structures. We detected that when the spatially structured variability of Ts was low, a strong negative correlation existed between SWCn and the spatial autocorrelation length of Rs with r = 0.66 (p < 0.001). However, for high spatially structured variability of Ts, occurring presumably at low Ts in spring and autumn, the correlation did not exist and it was difficult to quantify the spatial autocorrelation of Rs. Our results are indicative of a potential shift from homogeneity and dominance of biotic processes to an increased heterogeneity and abiotic regulation in drought prone ecosystems under conditions of decreasing soil moisture.