Impact of soil and water conservation structures on the spatial variability of topsoil moisture content and crop productivity in semi-arid Ethiopia

Summary

Plant available soil moisture and its spatial variability is among the most important limiting factors for crop and livestock productivity in semi-arid Ethiopia. The objectives of this study were therefore: 1) to measure the effects of land use and slope gradient on spatial variability of topsoil moisture content. 2) to analyze the variability of topsoil moisture content and crop responses under different soil and water conservation (SWC) structures and 3) to analyze whether topsoil moisture content, grain and biomass yields vary in relation to SWC structures. In total, 21 large runoff plots (600–1000 m²) were installed and treated with stone bunds, trenches, stone bunds with trenches and a control plot at each site. Plot sites were located on gentle (5%), medium (12%) and steep (16%) slopes of cropland and rangeland. Topsoil (0–15 cm) moisture content for plots were monitored daily using soil moisture probes (Theta-Probes) and weekly using manual sampling over two growing seasons. Crop performance was monitored during two growing seasons and grain and biomass yield were measured at harvest. Results show that average gravimetric moisture content of the topsoil during growing seasons is remarkably higher for cropland (33–25 g/100 g) compared to rangelands (24–20 g/100 g). This is due to tillage-induced runoff retention and infiltration. Soil moisture content decreases with increasing slope gradients, owing to increased drainage. Considering the effects of SWC structures, average topsoil moisture contents at accumulation and erosion zones are generally higher as compared to middle zone between two successive SWC structures. Average seasonal volumetric soil moisture content of 38.2 (cm³/100 cm³) in 2011 and 39.4 (cm³/100 cm³) in 2012 for accumulation zone of plots with stone bunds with trenchers are significantly higher than at other sampling locations. Crop growth performance, grain and biomass yield varied among plots and depended on slope gradients and SWC structures. Grain yields of wheat in 2011 and barley in 2012 were significantly higher for plots with SWC structures as compared to control plots and are highly correlated to topsoil moisture content (r = 0.64; p < 0.04 in 2011 and r = 0.59, p < 0.01 in 2012). Even when taking into account the cropland area occupied by SWC structures, grain yield increased by (4.2%−33%) for plots with SWC structures as compared to control plots. Overall, our results show that implemented measures may not only reduce runoff and soil loss, but also increase crop productivity by enhancing soil moisture and thus can significantly contribute to sustainability of agro-ecosystems in semi-arid environments.