Climate change has increased drought frequency, necessitating strategies to reduce water stress, increase water use efficiency, and improve agricultural productivity. The impacts of different fall/winter cover crops on alleviating maize drought in the subsequent season by changing soil water content remain unclear in Ultisol. Our aim was to investigate whether different fall/winter cover crops (taproot and fibrous root) can improve soil water content to attenuate subsequent season maize drought and its underlying mechanism. This research compared four fall/winter cover crops (2 rapeseed cultivars, lucerne and vetiver) and fallow control (no cover crop in winter) to investigate their root traits, root effect on subsequent season soil water content (SWC), soil drought degree (D), maize leaf and root water potential (Ψl, Ψr), maize crop water stress index (CWSI), and maize growth components (aboveground plant and root parameters) in 2019 (a dry year), 2021 (a normal wet year), and 2022 (a severe dry year). The fibrous-rooted vetiver displayed higher average root diameter and higher average root length density, leading to increased SWC (17%, 15%, 15%), and lower D values in all three years. Additionally, it exhibited a more substantial gradient between Ψl, and Ψr and lower CWSI than taproot (rapeseed cultivars and lucerne) and fallow treatment. Furthermore, the cover crop treatments enhanced the gradient between maize Ψl and Ψr to varying degrees in different drought years. Ultimately, the improvement in SWC resulting from cover crop treatments increased maize aboveground growth (plant height, stem diameter, and leaf area) and maize root development, ultimately improving maize yield. This study introduces a new perspective on investigating the role of cover crops in alleviating seasonal drought in subsequent crops, especially by changing soil water properties in the subtropical climate.

Poor soil hydraulic properties (SPH) limit the productivity of clayey Utisol (red soil) under a humid subtropical climate in southern China. The roots performance of rotation crops was compared, and the effects on SPH were investigated in the field. Five treatments (two cultivars of oilseed rape, one-year-old lucerne, and one- and fiveyear-old vetiver) were employed as pre-crops before summer maize, and their root performances were examined in spring. The soil-water characteristic curve (SWCC) and soil porosity were measured with soil cores, saturated hydraulic conductivity (Ks), and near-saturated hydraulic conductivity with a tension infiltrometer in situ. The two rape cultivars had the thinnest root diameter at an average of 0.45-0.50 mm and the shallowest distribution at a depth of 40 cm, while two vetivers had the thickest mean root diameter at 0.79 mm and the deepest distribution 80 cm. Taproot crops of rape and lucerne did not show stronger rooting ability than fibrous root crops of vetivers. All crops increased soil total and aeration porosity, but no bio-pores larger than 2 mm were formed. Rapes were beneficial for increasing soil aeration porosity (diameter >30 mu m), while vetivers increased active capillary porosity (30-0.2 mu m). The SHP, namely, SWCC and Ks, was improved by all crops, with the best effects in the five-year old vetiver due to its higher root length density (RLD) and the balanced distribution between diameter classes. Ks was significantly correlated with RLD (p < 0.001) but insignificantly correlated with root diameter (p = 0.9929). The SHP was improved mainly by RLD and influenced by root diameter distribution, suggesting that fine roots played a key role in forming aeration and active capillary pores.