Effect Of Biochar Application On Soil Erosion And Nutrient Loss In Losses Sloping Farmland

As a recalcitrant carbon source,the residence time of biochar was estimated to be hundreds to thousands of years.It continuously interacts with the soil and has a profound impact on the stability of the soil and the chemical cycles in soil.In recent years,the prospect of biochar application as a soil amendment has been widely studied.In order to study the effect of biochar application on soil erosion and nutrient loss in loess sloping farmland,the soil with0,7.5,15 and 30 t/ha biochar?0?1.36 wt.%?was used as the research material,and a combination of the simulated rainfall test,scoring test and laboratory agrochemical analysis was used in this study.The effects of biochar amount on soil aggregate stability and nutrient distribution,soil detachment process under scoring test,soil erosion process and nutrient transport under simulated rainfall events were analyzed.The suitable application amount and topographic conditions for biochar were expounded,which was helpful to further clarify the biochar application potential in loess sloping farmland.This study identified the positive effects of biochar application in preventing soil erosion and improving soil fertility and provided scientific basis for the promotion of biochar in loess sloping farmland.The main results were as follows,?1?According to the analysis of the composition and nutrient distribution of the mechanically stable aggregates and water stable aggregates of the control and biochar treatments,it was found that the application of biochar was beneficial to the large-diameter aggregates formation and enhanced the stability of soil aggregates showing as geometric mean diameter?GMD?and mean weight diameter?MWD?increased with the application amount of biochar,and the fractal dimension?D?decreased with the application amount of biochar.Meanwhile,the addition of biochar enriched soil organic carbon?SOC?in<0.25 mm soil particles and>2 mm water stable aggregate,respectively.The content of total nitrogen?TN?and total phosphorus?TP?in small size particles was significantly increased with the application of 15 and 30 t/ha biochar?0.68 wt.%and 1.36 wt.%?.For both mechanically stable aggregates and water stable aggregates,the contribution rate of large-size aggregates to soil nutrients gradually increased with the increase of the applied biochar amount.?2?According to the analysis of the soil detachment process under five flow shear stress conditions?5.66,8.31,12.21,15.55 and 19.15 Pa?for control and biochar treatments,it was found that the application of biochar significantly reduced the soil detachment capacity and the rill erodibility.As the biochar amount increased from 0 to 30 t/ha?1.36 wt%?,the soil detachment capacity and rill erodibility decreased by 48%and 46%,respectively.The rill erodibility decreased as the amount of biochar added increased,while when the amount of biochar was more than 15 t/ha?0.68 wt.%?,the rill erodibility no longer changed significantly.The soil detachment capacity was a non-linear function with the flow shear stress and the amount of biochar,and the relative rill erodibility was a quadratic function relationship with the amount of biochar added.?3?The soil erosion process of the control and biochar treatments under 3 rainfall intensities?60,90 and 120 mm/h?and 5 slope gradients?5°,10°,15°,20°and 25°?were analyzed to find the influence of the factors on the erosion process.The order of the three factors of the experimental design on soil erosion in sloping farmland was rainfall intensity>slope gradient>biochar application amount.Rain intensity and slope gradient were the main factors affecting runoff generation time and runoff rate,while the biochar application amount had no significant effect on both.Adding biochar reduced the sediment loss rate and the sediment content of the slopes while the extent of reduction decreased with the increase of slope gradient.Under the condition of a gentle slope of?15°,biochar had the most significant effect on reducing erosion rate and average sediment content.?4?The application of biochar increased the content of available nutrients in the soil comparing with the control treatment.The contents of ammonium nitrogen?NH₄⁺-N?,nitrate nitrogen?NO₃^--N?and available phosphorus?Olsen-P?in the soil were positively correlated with the biochar application amount.The distribution of NH₄⁺-N?NO₃^--N and Olsen-P in the vertical profile of soil was mainly affected by the sampling depth and had no significant correlation with the biochar application amount,slope gradient and slope position.On the other hand,the soil treatments with 15 and 30 t/ha?0.68 wt.%and 1.36 wt.%?biochar showed a retention effect on the transport of NO₃^--N and Olsen-P,which delayed the transport of NO₃^--N and Olsen-P into the deep soil.NO₃^--N was more likely to move downward with soil water,while NH₄⁺-N was hard to transport to the lower layer with water infiltration.The NH₄⁺-N content in soil surface of both control and biochar treatment was slightly higher than the lower layer soil.The application of biochar had no significant effect on the redistribution of NH₄⁺-N in the tillage layer after rainfall events,but it significantly increased the cumulative stock of NO₃^--N and Olsen-P reserved above the available nutrient peaks.?5?The nutrient loss process of control and biochar treatment measured under 3 rainfall intensities and 5 slope gradients.The soluble nutrients(NH₄⁺-N,NO₃^--N and PO₄^3--P)concentration in runoff was significantly negatively correlated with rainfall time and runoff rate,and positively correlated with sediment loss rate and sediment content.The sediment content was used to estimate the soluble nutrient concentration in runoff.When the sediment content was higher than 48.58 g/L,38.00 g/L and 46.38 g/L,NH₄⁺-N?NO₃^--N and PO₄^3--P concentrations of the biochar treatments were higher than those of control treatment,respectively.Under the condition of gentle slopes??15°?and low or medium rain intensity?60 and 90 mm/h?,biochar reduced the average soluble nutrient loss rate in runoff.Adsorbed nutrients?SOC,TN and TP?were the main forms of nutrient loss on slopes.The application of biochar increased the content of SOC,TN and TP in sediments.Particularly,the content of SOC and TN in sediment showed a logarithmic function relationship with the content of sediment.Meanwhile,while biochar application reduced the selective transport of SOC and TN due to soil detachment,showing as the ER_SOC and ER_TN decreased with increasing biochar application amount.Biochar increased the TP content in sediment under 60 mm/h-5°?25°and90 mm/h-5°conditions but reduced the ER_TP under heavy rainfall conditions.The application of biochar reduced the loss rate of SOC,TN and TP and the relative nutrient loss rate on<15°slopes.The most significant factor affecting nutrient loss was rainfall intensity,followed by slope gradient and biochar content.Multiple nonlinear equations of the soluble and adsorbed nutrient loss rate with rainfall intensity,slope gradient,and biochar content were established respectively with the R² of 0.904?0.957.It could be used to predict the nutrient loss rate on the loess sloping farming with biochar application.The application of biochar improved the physical and chemical properties of soil,affected the soil stability and nutrient distribution of aggregates,enhanced the erosion resistance of soil,and thus affected the soluble and adsorbed nutrients loss in the erosion process.Comprehensively considered the role of biochar in improving soil resistance to erosion,soil fertility in loess sloping farmland and the potential eutrophication threat to downstream water system.Adding 30 t/ha?1.36 wt.%?biochar showed the most significant effect on reducing soil erosion,enhancing nutrient retention and reducing nutrient loss of sloping farmland.The above positive effects decreased with increasing slope gradient,so it was deemed that biochar was more appropriate to be applied to<15°sloping farmland as a soil amendment.