Soil water dynamics and availability for citrus and peanut along a hillslope at the Sunjia Red Soil Critical Zone Observatory (CZO)

The hillslopes of red soils (Ultisols) in southern China are intensively cultivated for cash crops and fruit trees. During the rainy monsoon, soil erosion is prevalent, whereas a summer/autumn dry season induces drought stress. Crops respond differently to these stresses, and have different effects on soil water regime. This study used a combination of field observation and HYDRUS-2D modeling to assess the soil water dynamics and plant available water for peanut (Arachis hypogaea) and citrus (Citrus sinensis) at Sunjia Red Soil Critical Zone Observatory (CZO). Between April 1, 2012 and March 31, 2014, surface runoff and moisture content at 5, 20, 40, and 80 cm depths in the soil under both land uses were monitored at up, middle and foot slope positions along a hillslope. Results indicate that the citrus plot had higher soil water content at 5 cm depth during the dry season, and lower at 20, 40 and 80 cm depths throughout the year than the peanut plot. As expected, the soil water content was higher at foot slope, compared to up slope, and in deeper soil than near surface. We observed limited soil water availability to peanut during mid-July to August, and to citrus from mid-July to mid-November. Compared to the peanut plot, the citrus plot generally showed 12-28% greater evapotranspiration, 3-4 times less runoff, and 2-57% greater deep drainage. These differences were greater at the up slope position. Our data and HYDRUS-2D simulation suggest that the deep-rooted citrus reduced runoff during the rainy season by increasing macropore flow and canopy interception, and minimized the soil water stress during the dry season by utilizing water from deeper soil. Thus, we recommend trench planting of citrus along with peanut intercropping on hilly red soils as sustainable agricultural practices.