The effect of land use types and climate change on soil moisture profile dynamics

Soil moisture, as a key indicator of soil functionality, is significantly influenced by the evolution of soil-plant systems and the hydrologic cycle. Little long-term data is available about how land use and climate change affect the spatial and temporal distribution of soil water. In particular the variations in deeper subsoil layers are poorly documented. Here, the effects of five land use types, including two croplands with conventional and organic farming (CF and OF) and three grasslands with intensive and extensive meadow (IM and EM), as well as extensive pasture (EP) on soil moisture profiles were investigated at the Global Change Experimental Facility (GCEF), at Bad Lauchstädt in the German dryland belt. The facility harbors two climate treatments. The ambient climate and a future climate with increased temperature by ~0.55 ^◦C across seasons, and the altered precipitation patterns by ~9 % additional irrigation in spring and autumn, and ~21 % reduction in summer. The soil moisture profiles were bi-weekly monitored with a portable probe (TRIME Pico IPH) down to 110 cm for two continuous years.

Soil moisture content in topsoil and subsoil reflected the presence and size of transpiring plants, i.e. from October to next April, the soil water content was lower in grasslands than in croplands, which planted winter crops. During summer, there was a marked decrease in soil water content in the deeper soil layers of grasslands, while the crop on the cropland was already harvested. As a result, the recovery of soil water storage was faster during winter in croplands than in grasslands. Within croplands, OF had higher moisture than CF below 30 cm during the whole growing season and beyond due to less vigorous growth imposed by nutrient deficits. Within grasslands, differences in soil moisture only emerged in deeper soil (> 70 cm). In general, soil moisture in the shallow soil layers (0 - 20 cm) was very similar across land uses and climate scenarios and these clear differences only emerged in deeper soil. In the deeper soil (< 50 cm), croplands and extensively used grasslands showed an obvious increase of soil moisture in future treatment, especially during wet spring and summer.

Our results clearly indicate long-term differences in soil moisture between the land uses. Climate manipulation at the GCEF only manifests itself in the subsoil (> 50 cm), by contrast, topsoil (< 30 cm) was more controlled by short-term dynamics induced by evaporation and precipitation. These findings stress the importance of deep soil moisture monitoring for a more comprehensive assessment of the water budget.