Land use effects on soil hydraulic properties and the contribution of soil organic carbon

Soil hydraulic properties (SHP) control water movement and storage and thus affect a wide range of biogeo-chemical processes and ecosystem services. The objectives of this study were to identify the interactions between long-term land use and soil type on SHP (e.g., available water content [AWC], hydraulic conductivity [K]), and the effects of soil organic carbon (SOC) content on these properties. Soil water release curves and (near) saturated hydraulic conductivity at three soil depths (0-7.5, 7.5-15, and 15-30 cm) were measured under three long-term (>20 years) land uses (irrigated pasture [IP], dryland pasture [DP], and irrigated cropping [IC]) in Canterbury, New Zealand. For each land use, three soil types with contrasting drainage characteristics were selected: well-drained Lismore (LIS) stony silt loam, moderately well-drained Templeton (TEM) silt loam, and poorlydrained Waterton/Temuka (WAT) clay loam. Compared to DP, the IP and IC soils had lower AWC and K. Significant interactions between land use and soil type were found for AWC but not for K. Bulk density, SOC, and clay content could explain half the variation in AWC but had limited influence on K. More variance in AWC (6-13%) and K (5-27%) could be explained by including categorical variables (i.e., land use, soil type, and soil depth) and their interactions with continuous variables, indicating the potential benefit of including these categorical variables when developing pedotransfer functions for AWC and K. This study suggests that many macropores (>30 mu m) were poorly connected and contributed more to water storage than to conducting water. The commonly used soil matric potential of -10 kPa was suitable for defining field capacity in DP and IC soils but a lower matric potential (-33 kPa) was more appropriate for IP soil affected by livestock treading during grazing. Soil organic carbon mainly increased AWC by affecting water retention at soil matric potentials of between -40 and -1500 kPa (equivalent to pore size 0.2-7.5 mu m). Therefore, soil carbon sequestration may be important to alleviating water stress in dry environments. Our study also suggests that the effects of land use and its interaction with soil type, soil matric potential at field capacity, and soil organic carbon on SHP need to be considered during soil parameterization of hydrological models.