Runoff and soil properties following cattle-dung manuring and dry-grass mulching and erosion-aggregation relationships for well-drained Ultisols

The inherent fairly high structure stability of tropical soils notwithstanding, poor aggregation still contributes appreciably to their water erosion. To understand the role of soil and water management in this regard, a field trial was conducted on runoff plots to assess the effects of cattle-dung (CD) manure and dry-grass mulch (DGM) on aggregate stability and permeability of well-drained Ultisols in southeastern Nigeria. The trial involved incorporating CD manure into topsoil at 10 tha-1 and/or surface-applying DGM at 20 t/ha. Water erosion monitored during 1-7 months after manuring was related to soil aggregation and hydraulic behaviour at 3 and 7 months. Treatment had no effects on runoff and soil loss but affected few soil properties; degree of aggregation (3 months), proportion of water-stable aggregates, clay dispersion ratio and cation exchange (7 months). Runoff and soil loss were lower in DGM than no-mulch plots. Proportion of water-stable aggregates increased by 18% due to CD and 27% due to DGM at 7 months, when DGM similarly increased degree of aggregation (from 56.40% to 75.80%) and state of aggregation (from 49.50% to 66.40%) including aggregates mean-weight diameter (from 1.60 to 2.25 mm) and percolation stability (from 236 to 298 cm3). Neither CD nor DGM affected soil organic matter, steady-state infiltration rate and internal hydraulic properties of soil bulk density, total porosity and saturated hydraulic conductivity. Infiltration capacity seemingly increased with reduced leaching and hence soil acidity and enhanced soil fertility, macro-aggregation and macro-favoured porosity. Soil loss mirrored its runoff and mid-season aggregates mean-weight diameter and microporosity (R2 = 0.94). For soil conservation from a broader perspective, CD+DGM is suggested for well-drained humid tropical soils; however, for only soil stability against erosion, CD or preferably DGM alone is suggested. Runoff-based soil loss predictions could benefit from macro-aggregation and water retention data.