Accelerating soil water recovery in alfalfa-converted cropland: Critical roles of fallow season mulch, crop selection, and precipitation

Alfalfa (Medicago sativa L.) crops rotation has been proposed as a sustainable strategy for dryland farming. However, limited understanding of the spatio-temporal dynamics of deep soil water recovery following alfalfa conversion constrains broader adoption of this practice. This study investigates soil water recovery in alfalfa-converted cropland (AC) compared to conventional cropland (CC) under a plastic-mulched maize-potato rotation over 12 years (2010–2021). We further examined variations in soil water recharge across crop types (maize vs. potato) and seasons (growing vs. fallow). At alfalfa conversion, the water deficit (DS) in the 0–500 cm profile was −0.37 (the relative change in soil water content in AC compared to CC). Following conversion, DS increased exponentially with conversion duration. Soil water in the upper 60 cm recovered within 2 years, while deeper layers (0–500 cm) recovered after 12 years. Most importantly, rapid recovery in the upper 60 cm enabled AC to achieve equivalent evapotranspiration and crop water productivity compared to CC. Soil water recharge in the 0–500 cm profile was similar during growing and fallow seasons, demonstrating the importance of precipitation storage under plastic mulch during fallow periods despite much lower precipitation. Potato cropping-years resulted in significantly greater soil water recharge than maize years, suggesting that increasing potato frequency in crop rotations could further accelerate soil water recovery. Soil water recharge showed a strong linear relationship with precipitation. A minimum annual precipitation threshold of 322 mm was identified for positive recharge, with fallow season precipitation contributing disproportionately to deep-layer replenishment. Based on these findings, we recommend implementing alfalfa rotation in regions with > 322 mm annual precipitation, prioritizing potato in rotations, and optimizing water-saving management during fallow periods to maximize water capture. These findings advance strategies for reconciling agricultural productivity with hydrological sustainability in water-limited ecosystems.