| The yellow brown soil region is an important crop production area and a major economic forest producing area in China because of the abundant food and economic crops.Due to the characteristics of yellow brown soil and improper field management,the food production safety is facing a severe test.In order to improve the soil characteristics,increase soil fertility and grain yield,and explore soil improvement under different depths of straw returning to the field,a three-year field expeirments was conducted to study the effects of straw on soil physical characteristic(soil hydraulic properties,aggregate distribution,soil bulk density and total porosity),soil nutrient content(soil organic matter,nitrogen and available phosphorus)and enzyme activity in different soil layers.Four different depth treatments(straw surface coverage,straw 10-20 cm returning field,straw 20-30 cm returning field and straw 30-40 cm returning field,labeled as T0,T10,T20 and T30)were established.Principal component analysis was used to explore the improvement effects of different depths of straw returning on the soil,to find the optimum depth of returning to the filed,and to provide scientific basis for straw returning to improve yellow-brown soil,soil and water conservation and promote sustainable development of agriculture.The research results were as follows:(1)Straw returning can improve soil hydraulic properties,increase soil water stability and large aggregate content,increase soil porosity and reduce soil bulk density,and the inceasement scaled with the extension of returning time.In the three years of straw returning to the field,the soil bulk density of straw treated in the field was significantly reduced.The soil bulk density of T10 treatment decreased by 4.92%,4.65%and 4.85%,respectively,and the total soil porosity of T10 treatment was increased by 7.93%.7.19%and 7.36%in three years.The hydraulic properties of T10 and T20 treatments in the first year of straw returning were better than that of other treatments,and the effect of T10 treatment was more apparent with the increase of returning time.With the continuation of straw returning,the water agglomeration content(particle size>0.25 mm)in the soil increased significantly.The water-stable aggregates content of the soil reached the highest when the straw was returned to the field at 10-20 cm.The soil aggregates MWD and GMD were significantly increased with straw returing to the field.The hydraulic properties of straw returning to the field for three consecutive years showed that the improvement of straw returning to the field(T10)in 10-20 cm soil layer was the best.In the third year of straw returning,the soil saturated water content,field capacity,available water content and saturated hydraulic conductivity increased by 15.24%,16.26%,35.66%and 21.41%,respectively,and the wilting coefficient decreased by 13.26%.(2)Returning straw to the field can increase soil nutrient content.The content of soil organic matter increased significantly after adding straw.The content of organic matter in each treatment ranked as CK<T30<T20<T0<T10.The content of organic matter in T10 treatment was the highest as well as the increase rate.The soil organic matter content after straw returning to the field increased year by year.In the second year of straw returning,the organic matter content of T0,10,T20 and T30 treatment increased by 12.61%,14.12%,12.24%,and 16.89%,respectively compared with that of in the first year of straw returning.In the third year of straw returning,the organic matter content of T0,T10,T20 and T30 treatment increased by 1.27 g·kg-1,1.25 g·kg-1,1.33 g·kg-1,1.28 g-kg-1,respectively,and the differences reached significant levels(p<0.05).The soil nitrogen level increased significantly after the straw was returned to the field,and the nitrogen levels of the upper soil(0-10 cm and 10-20 cm)was higher than the deep layer.With the increase of the returning years,the nitrogen level of the middle and lower layer increased gradually,and the improvement effect gradually appeared.The trends of soil total nitrogen and nitrate nitrogen were the same,both showed an increasing trend year by year.The content of total nitrogen and nitrate nitrogen in the third year of straw returning increased by 12.04%?15.02%and 11.24%-13.17%,respectively.The straw returning to the field for three consecutive years showed that the 10-20 cm soil layer straw returning treatment(T10)had the highest total nitrogen and nitrate nitrogen content.The content of ammonium nitrogen decreased year by year,and the content of ammonium nitrogen in the third year of straw returning decreased by 1.51 mg·kg-1 to 2.07 mg·kg-1 compared with that of in the first year.The avaliable phosphorus content of soil treated with straw returned to the field increased significantly for three consecutive years.The available phosphorus content of soil treated in the third year of straw returning to the field increased by 20.41%-28.08%compared with the first year of straw returning.In the three years,the treatment of straw returning to the field had the highest increase in T10 treatment,which was 18.73%,14.85%and 17.06%.(3)Straw returning has a significant effect on soil enzyme activity.After adding straw to soil,the activities of soil invertase,urease,catalase,dehydrogenase and acid phosphatase increased s ignificantly,and the enzyme activity increased with the continuous straw returning period upward trend.In the third year of returning to the field,the activities of soil sucrase,urease,catalase,dehydrogenase and acid phosphatase increased by 27.63%?34.14%,27.66%?37.69%,45.65%-46.92%,26.17%?37.30%,24.21%?25.83%,respectively,compared with that of in the first year.The effects of different straw returning depth on soil enzyme activity were different.When the straw was returned to the field at 10-20 cm depth,the soil enzyme activity increased obviously.(4)Principal component analysis showed that there were some differences in the soil properties of different treatments after returning straw to different soil depths,the order of comprehensive score in different treatments is:CK<T30<T0<T20<T10.The soil hydraulics and other physical properties of T10 treatment and T20 treatment were better than other treatments.The nutrient content and enzyme activity were higher than other treatments,which T10 treatment was the highest,while CK treatment was the lowest.The results indicated that the straw burying depth can effectively improve the hydraulic properties,optimize soil physical properties,increase soil nutrient content and enhance enzyme activity,moreover,the returning straw at depths of 10-20 cm and 20-30 cm has a significant effect on soil improvement. |
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