Soil-moisture-dependent nocturnal water use strategy and its responses to meteorological factors in a seasonal-arid poplar plantation

Nocturnal water uses (Qn) significantly affect global water budgets and diurnal water cycles, which are currently suffering from restrictions of soil drought and soil water depletion caused by climate change and worldwide greening. Recent studies have recognized the vital influence of soil water availability on Qn, but responses of nocturnal water use strategy to soil moisture variation were not fully understood. For example, the effect of soil water content (θ) on the trade-offs between two components of Qn, stem water refilling (Re) and nocturnal transpiration (Tn), was rarely involved and remained unclear. This study analyzed the nocturnal sap flow of typical afforestation species, poplar plantation, under different soil water conditions in a seasonal arid region of northern China. Results indicated that higher θ significantly promoted Qn through enhancing the stomatal conductance but had less influence on the proportion of nocturnal water use to daily water use (Qn%) (7 %∼10 %). With the decline of soil water conditions, significant linear correlations between Qn and θ shifted from deep soil layers to the soil surface. The influence of meteorological factors on Qn depended on soil water condition, and the explanation of nocturnal vapor pressure deficit (VPDn) to Qn became weaker with increasing soil moisture. Negative linear relations between the proportion of stem water refilling to Qn (Re%) and θ were shown within treatments, but a significantly higher Re% was observed under more sufficient soil water. These opposite responses of Re% to θ reflected the different adaptions of Qn to soil water availability on short- and long-scales. This study highlights the crucial role of soil moisture in the nocturnal water use and balance strategy of a seasonal-arid poplar plantation. Our results help better understand the nocturnal transpiration processes in the context of global climate change. •Higher soil water content (θ) significantly promoted nocturnal water use (Qn).•Effects of θ on Qn shifted from deep soil to soil surface with declined soil water.•Influence of meteorological factors on Qn depended on θ.•Stem water refilling/Qn responded differently to θ on short- and long-term scales.