Mulching the soil surface with a layer of plant residue is considered an effective method of conserving water and soil because it increases water infiltration into the soil,reduces surface runoff and the soil erosion,and reduces flow velocity and the sediment carrying capacity of overland flow.However,application of plant residues increases operational costs and so optimal levels of mulch in order to prevent soil and/or water losses should be used according to the soil type and rainfall and slope conditions.In this study,the effect of wheat straw mulch rate on the total runoff and total soil losses from 60-mm simulated rainstorms was assessed for two intensive rainfalls (90 and 180mmh-1) on three slope gradients typical conditions on the Loess Plateau of China and elsewhere.For short slopes (1 m),the optimal mulch rate to save water for a silt loam and a loam soil was 0.4 kg m-2.However,for a clay loam soil the mulch rate of 0.4 kg m-2 would be optimal only under the 90 mm h-1 rainfall;0.8 kg m-2 was required for the 180 mm h-1.In order to save soil,a mulch rate of 0.2 kg m-2 on the silt loam slopes prevented 60％-80％ of the soil losses.For the loam soil,mulch at the rate of 0.4 kg m-2 was essential in most cases in order to reduce soil losses substantially.For the clay loam,0.4 kg m-2 may be optimal under the 90 mm h-1 rain,but 0.8 kg m-2 may be required for the 180 mm h-1 rainstorm.These optimal values would also need to be considered alongside other factors since the mulch may have value if used elsewhere.Hence doubling the optimal mulch rate for the silt loam soil from 0.2 kg m-2 or the clay loam soil under 90 mm h-1 rainfall from 0.4 kg m-2 in order to achieve a further 10％ reduction in soil loss needs to be assessed in that context.Therefore,Optimal mulch rate can be an effective approach to virtually reduce costs or to maximize the area that can be treated.Meantime,soil conservationist should be aware that levels of mulch for short slopes might not be suitable for long slopes.
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