Mechanistic insights into mixed gravel–soil mulching effects on soil hydrology in arid ecosystems: A field micro-lysimeter study

Gravel mulching plays a vital role in modifying the hydrological cycle in arid and semi-arid areas. Yet, the mechanisms underlying long-term mulching effects on soil evaporation remain poorly understood. To investigate the hydrological effects of mixed gravel–soil mulching (MGSM), we conducted a controlled 39-d soil evaporation experiment (from 22 July to 30 August 2021) using micro-lysimeters at the field experimental site of Ningxia University, China. The soil evaporation rate ( E ), cumulative soil evaporation ( E c ), soil water content (SWC), mulch resistance ( r m ), and micro-meteorological variables were assessed for six mulch treatments, each containing a different proportion of gravel by volume: 100.00% (M1), 80.00% (M2), 60.00% (M3), 40.00% (M4), 20.00% (M5), and 0.00% (M6). The treatments (M2–M6) showed a prolonged soil moisture depletion phase and greater E c (28.71%–83.31%) relative to the gravel-only treatment (M1) ( P <0.050); these effects were primarily attributed to reduced r m . As compa r ed to E c , the SWC showed an inverse response, decreasing as Cg decreased. A robust exponential relationship was observed between E and r m ( P <0.001). Evaporation suppression mediated by r m was particularly p r onounced during the residual evaporation stage (>312 h post-wetting), with the strongest effect occurring in M3, where the mean r m doubled. The SWC, mulch prope r ties, and micro-meteorological parameters (i.e., air relative humidity and surface net radiation flux) were the most important predictors of r m in the mulch treatments. Together, these results suggested that MGSM unexpectedly exacerbated surface soil moisture loss by reducing r m . To mitigate this effect, an optimized mixed gravel–soil mulch, containing 60.00% gravel by volume, might be used; this mixture balances evaporation control with hydrological sustainability and represents a practical strategy for dryland management, offering a compromise between short-term water retention and sustained soil moisture regulation.