Dynamic changes and distribution of topsoil moisture during the natural recovery of patchily degraded meadow towards healthy meadow on the Qinghai-Tibet Plateau of West China

Soil moisture is one of the limiting factors for the natural recovery of degraded patches towards healthy meadow. This study aims to understand the impact of interfering factors on soil moisture dynamics, and to determine the critical threshold for the area of degraded patches to exceed the lateral redistribution zone of soil moisture via a dual-factor randomized block design for plateau pika (Ochotona curzoniae) and overgrazing. The dynamic changes of soil moisture in three layers 10 cm, 20 cm and 30 cm below the surface were monitored near the interface between degraded patches and healthy alpine meadows, which was supplemented with the spatial dynamic changes of surface soil moisture tracked from time-series drone images. It is found that the moisture content of the 10 cm layer of healthy meadow was higher than that of the 20 cm and 30 cm layers thanks to the developed plant root system, and had a large range of fluctuation due to evaporation and transpiration. Due to the high variability of soil moisture decline rate, plateau pika and grazing had no significant effect on moisture (P > 0.05). One cause of variability was the much reduced absorption of soil moisture infiltration by degenerated patches, resulting in significant spatial heterogeneity in their absorption of rainwater (P < 0.05). This spatial heterogeneity results in the formation of lateral redistribution zones of soil moisture at the interface between alpine meadows and degraded patches. A high density of plateau pika significantly reduced the zonal width (P < 0.05). Recovered patches require supplemental moisture from the redistribution zone to maintain their water consumption. When the diameter of a degraded patch exceeds the limiting distance of the redistribution zone, the recovery of alpine meadow appears to be hysteretic, degenerating into patchily degraded meadow.