Effect Of Straw Mulching On Sediment Aggregates And Element Enrichment Characteristics

As an effective measure of soil and water conservation,straw mulching can not only maintain soil fertility,improve soil physical and chemical properties,improve soil quality,but also protect the surface soil,reduce the kinetic energy of raindrops,reduce soil fragmentation and thus reduce the amount of soil erosion.The purple soil area is characterized by concentrated rainfall,abundant water and heat resources,loose soil structure and low anti-erodibility,which make the soil and water loss very serious.The eroded sediment particles will carry C,N,P and other nutrients,which will not only degrade soil quality and reduce productivity,but also cause further non-point source pollution and ecological environment deterioration.Nowadays,the research on sediment particles and their nutrients in the process of erosion has attracted much attention,but the systematic research on the nutrients of aggregates with different particle sizes in the purple soil region is still rare.In order to study the rule of sediment separation and the process of nutrient loss under straw mulching,,three slopes gradients（10°,15°,20°）and five straw mulching treatments(0 kg·m^-2、0.2 kg·m^-2、0.4 kg·m^-2、0.6 kg·m^-2、0.8kg·m^-2)were designed with the experiment carried out in a designed soil tank under the artificial rainfall simulation,taking the soil in purple soil region as the research object.The study focuses on the effects of different straw mulching treatments on runoff and sediment yield,particle size of soil aggregates and nutrient loss.Meanwhile,the relationship between straw mulching and runoff yield,sediment yield,sediment concentration,and nutrient content of each grain size was also analyzed.A theoretical basis for the rule of sediment and nutrient loss on sloping land of purple soil can be provided by our study.The main results were as follows:（1）The straw mulching has a significant effect on the runoff and sediment yield.The runoff yield of 0.8 kg·m^-2 was lower than that of the other four returns during the whole rainfall process.The runoff yield increased first and then stabilized,and reached the maximum in 10 minutes.Under the condition of bare slope,the fluctuation of sediment concentration was the largest and the value was relatively high,and the initial sediment concentration was also significantly higher than the other 4 treatments.Under the condition of maximum mulching,the sediment concentration is at a lower level and the fluctuation is minimum.As a whole,the sediment concentration increased with the increase of slope and decreased with the increase of straw mulching.There was a significant correlation between sediment concentration and runoff yield under bare slope condition（P<0.05）,but no significant correlation under four conditions（P>0.05）.The relationship between sediment yield and runoff yield is similar.There was a significant negative correlation between return yield and runoff yield,sediment concentration and sediment yield（P<0.05）.With the increase of return yield,runoff yield,sediment concentration and sediment yield decreased.（2）The straw mulching affected the loss content of sediment aggregates.The mean particle size of 0.2 kg·m^-2 was significantly larger than that of the other four treatments（P<0.05）.The content of<0.002 mm agglomerates showed no obvious change with the increase of rainfall duration,the content of 0.05-0.002 mm agglomerates decreased gradually,and the content of 2-0.05 mm agglomerates increased gradually.The content of<0.002 mm in 0.8 kg·m^-2 was significantly lower than that in other four straw mulching.The difference of enrichment rate of<0.002 mm agglomerates(ER_<0.002)was the same as that of<0.002 mm.The enrichment rate of 0.05-0.002 mm agglomerates(ER_0.05-0.002)under 0.2 kg·m^-2 was significantly lower than that under other covering conditions（P<0.05）.ER_0.05-0.002 showed an increasing trend as a whole with the increase of returned land The aggregate enrichment rate of 2-0.05 mm(ER_2-0.05)of 0.2 kg·m^-2 under different slopes was significantly higher than that of ER_2-0.05 under other mulching conditions（P<0.05）.With the increase of rainfall duration,the 0.05-0.002 mm aggregates were enriched,indicating that the 0.05-0.002 mm aggregates were most easily eroded.There was a significant negative correlation between the grain content of<0.002 mm and the amount of mulching field（P<0.01）.（3）The distribution of C,N and P in eroded sediment was significantly affected by straw mulching,and the response of each index under 0.25-0.05 mm aggregate was more obvious.The contents of TOC,TN and TP of different size aggregate and different amount of straw mulching were significantly different（P<0.05）.The content of TOC,TN and TP in fine aggregate was significantly higher than that in coarse aggregate,and the content of TOC,TN and TP decreased significantly with the increase of straw mulching.There were significant differences in C,N and P eco-stoichiometry between different amounts of straw mulching and different size sediment aggregate（P<0.05）.Straw mulching significantly reduced the TOC:TN of each aggregate,but the TOC:TN of different size was not significant.There were significant differences in TN:TP and TOC:TP among different sizes（P<0.05）,but straw mulching only significantly reduced TN:TP and TOC:TP by 0.25-0.05 mm.TOC_R was less than 20%and TN_R was less than 10%under each covering condition,which indicated that each aggregate was in N-limited and C-limited states.In addition,the finer the sediment,the higher the TOC_R and TN_R,but the lower the TP_R.ER_TOC and ER_TN of 0.25-0.05 mm agglomerates were significantly higher than those of 0.05-0.002 mm and<0.002 mm agglomerates,and ER_TP of 0.05-0.002 mm and<0.002 mm agglomerates were significantly higher than those of 0.25-0.05 mm agglomerates.Straw mulching had significant effect on ER_TOC and ER_TN of 0.25-0.05 mm aggregates,but had little effect on 0.05-0.002 mm and<0.002 mm aggregates.The ER_TOCand ER_TN of 0.25-0.05 mm sediment aggregates were larger than 1,but ER_TP was smaller than 1.The ER_TOC and ER_TN of 0.05-0.002 mm and<0.002 mm sediment aggregates were smaller than 1 or slightly larger than 1,but ER_TP was obviously larger than 1.The results showed that TOC and TN were enriched in coarse sediment aggregates,but TP was enriched in fine sediment aggregates.In conclusion,straw mulching has significant effects on runoff and sediment yield,sediment aggregates and nutrient transport,and there is a significant correlation between the amount of straw mulching and each index.The results will further deepen the understanding of the law of slope erosion and loss,and provide a theoretical basis for the optimization of tillage measures in purple soil region.