| The planting area of citrus on sloping land in Northwest Guangxi has expanded rapidly.Artificial disturbance of new orange orchard is intense and soil erosion is serious.Soil development is different under different parent rock conditions in Northwest Guangxi.The main soil types are zonal soil red soil and limestone soil on Karst slope.There are obvious differences in soil layer thickness and soil physical and chemical properties between the two soils.Straw mulching is a common water and soil conservation measure on slope land,but its effect on reducing runoff and sediment in orange orchard on slope land in Northwest Guangxi,especially in limestone orange orchard on Karst slope land,is still unclear.In order to compare and analyze the erosion process of young orange orchard on red soil and lime soil slope land and the difference of water and soil conservation effect of rice straw cover on two soil environments under different slope gradients and different rainfall intensities,this paper takes young orange orchard on red soil and lime soil slope land in karst area of Northwest Guangxi as research object,and through indoor artificial rainfall simulation test,establishes2 rain intensities(60 mm/h and 120 mm/h),2 slopes(5 and 15 and 4 kinds of rice straw coverage(0,20%,50% and 80%)were set.The following main conclusions are drawn:(1)The time of delaying the initial runoff production of limestone land surface runoff under straw mulching is greatly affected by slope.At low slope(5 degree),straw mulching could delay the initial runoff generation time of surface runoff of young orange orchard on lime soil slope,while at medium slope(15 degree),the initial runoff generation time was advanced by straw mulching except for 80% straw coverage under heavy rain.At low slope(5 degree),straw coverage can effectively reduce the ground runoff coefficient of orange orchard on limestone slope,and it will decrease with the increase of straw coverage.At mid-slope,straw cover has a great influence on surface runoff coefficient due to rainfall intensity.Straw cover effectively reduces surface runoff coefficient at moderate rainfall intensity,while only 20%straw cover can significantly reduce surface runoff coefficient and increase runoff coefficient in soil at heavy rainfall intensity.Straw mulching can significantly reduce the total amount of surface erosion in limestone orange orchard.Except for 50% straw coverage at low slope and heavy rain,the total amount of surface erosion in other conditions is inversely proportional to the straw coverage.Under low slope conditions,the total erosion under 20%,50% and 80%straw coverage under moderate rain intensity was 26.7%,18.0% and 9.1% respectively under no-measures conditions,and 41.4%,62.5% and 5.3% under heavy rain intensity.Under moderate slope conditions,the total erosion of 20%,50% and 80% straw coverage under moderate rain intensity was 49.7%,13.9% and 3.9% respectively under no-measures conditions,and 55.6%,34.2% and 23.4% under heavy rain intensity.The reason why straw mulching reduces the surface runoff and sediment carrying capacity of young orange orchard on lime soil slope is greatly affected by rainfall intensity.Under medium rainfall intensity,the reduction of runoff and sediment carrying capacity is mainly due to the dual effects of restriction of runoff transportation and restriction of soil stripping.Under heavy rain intensity,the restriction of runoff transportation is weakened,mainly due to restriction of soil stripping.(2)Straw mulching significantly delayed the initial runoff generation time of young orange orchard on red soil slope.Under the same slope condition,the delaying effect of the initial runoff generation time of surface runoff became weaker with the increase of rainfall intensity;under the same rainfall intensity condition,the delaying effect of the initial runoff generation time of surface runoff became weaker with the increase of slope.Straw mulching had no significant effect on runoff coefficient of each Runoff-Producing part of young orange orchard on red soil slope,but it could reduce runoff which precipitation directly contacted with soil surface.Straw mulching significantly reduced the surface erosion of young orange orchard on red soil slope.Except for 50% straw coverage on medium slope and heavy rain,the total surface erosion of other conditions was inversely proportional to the straw coverage.Under low slope conditions,the total erosion under 20%,50% and 80% straw coverage under moderate rain intensity was 39.3%,8.5% and 2.1% respectively under no-measures conditions,and 96.2%,75.3% and 24.4% under heavy rain intensity.Under moderate slope conditions,the total erosion of 20%,50% and 80% straw coverage under moderate rain intensity was 27.1%,14.0% and 4.5% respectively under no-measures conditions,and 50.9%,76.6% and 12.5% under heavy rain intensity.Straw mulching can effectively reduce the sediment carrying capacity of surface runoff of young orange orchard on red soil slopes.The main reason for the decrease of low sediment carrying capacity of each straw coverage on any slope and rainfall intensity is limited soil stripping,of which 80% straw coverage has the lowest sediment carrying capacity.(3)The surface runoff reduction rate of young orange orchard covered with straw on lime soil slope was significantly higher than that of red soil.Under the conditions of high slope and heavy rain,only 20% straw coverage can reduce the flow of young orange orchard on limestone soil and red soil slope.Under the conditions of low or medium slope,the effect of straw mulching on reducing sediment in orange orchard on red soil and limestone soil slope can reach a high level under medium rain intensity,and the difference is not significant,and the sediment reduction rate is proportional to straw coverage.However,under low slope and heavy rain,the effect of straw mulching on sediment reduction of young orange orchard on lime soil slope is better than that of red soil area.Under the same straw coverage,the runoff sediment concentration of orange orchard on red soil slope was higher than that of limestone soil to varying degrees.Under the same rainfall intensity and straw coverage,the average runoff sediment concentration of orange orchard on red soil slope is higher than that of limestone soil with the increase of slope.Under the condition of rainfall intensity(60 mm/h)at low slope(5 degrees),the difference of runoff sediment concentration between red soil and limestone soil gradually narrows with the increase of straw coverage. |
