Effects Of Crucifer-legume Green Manures Returned On Soil Fertility In Hilly Orchard

In the loess hilly region of northern Shaanxi,soil texture is loose,water and fertilizer preservation performance is poor,soil organic matter content is low,soil p H is high,nutrient content and availability is low（especially phosphorus is easy to be fixed by soil）.How to increase organic fertilizer source,improve soil structure,increase soil nutrient reserve and improve nutrient availability according to local conditions are the urgent problems to be solved in apple production in this area.This study by planted rape of crucifer,melilotu and soybean of legume,and combined rape and melilotus mixed（RM）,rape and soybean rotation（RS）of green manure planting patterns,after six consecutive years of rturned tests of green manure in mountain orchards,to discuss the effects of returned crucifer-legume green manures on soil improvement in hilly orchards.The following results were obtained:（1）Returned crucifer-legume green manures had significant effects on soil aggregates content and stability indexes.In the 0-30 cm soil layer,the percentage of soil mechanically-stable aggregate with>2 mm and>0.25mm particle size treated by RM and RS decreased year by year.However,the content of mechanically-stable aggregates with particle size of 2-0.25 mm and the content of water-stable aggregates with particle size of>0.25 mm increased year by year.In addition,the percentage of water-stable aggregates with<0.25 mm particle size decreased year by year,but its content was still significantly higher than that of other particle size.After 2,4 and 6 years of rturned,compared with BK,the soil mechanically-stable aggregate of>0.25 mm in RM and RS treatment decreased by0.62%-2.54%and 0.11%-2.65%respectively,RM and RS treatments increased>0.25 mm aggregates by 19.52%-54.14%and 21.81%-64.15%.The soil stability gradually increased with the increase of returned years.specifically,the content of>0.25mm particle size water-stable aggregates increased year by year,the mean weight diameter and geometric mean diameter increased significantly,and the percentage of aggregate destruction decreased significantly（P<0.05）.（2）Returned crucifer-legume green manures could reduce soil p H in 0-30cm soil layer,and showed a slow decrease year by year.The contents of SOM,TN,NO₃^--N and NH₄⁺-N increased rapidly with the increase of returned years.After 6 years of returned,the contents of SOM,NO₃^--N and NH₄⁺-N in RS treatment were significantly higher than those in RM treatment（P<0.05）.Compared with BK,after 2,4 and 6 years of returned,the contents of SOM,TN,NO₃^--N,NH₄⁺-N increased by 20.76%-30.38%,13.41%-39.46%,37.44%-141.86%,1.29%-35.05%,respectively.the contents of SOM,TN,NO₃^--N,NH₄⁺-N under RM treatments increased by 24.38%～53.44%,14.56%～41.00%,50.93%～169.07%and 4.09%～49.89%,respectively.（3）Returned crucifer-legume green manures could increase the contents of phosphorus fractions in the 0-30 cm layer.In the 0-30 cm soil layer,the contents of TP,TIP and AP in RM and RS treatments were significantly higher than those in BK treatment（P<0.05）.The contents of phosphorus and PAC（%）increased year by year,among which the increase of AP was the largest,followed by TIP,and the increase of TP and TOP was small.In addition,crops available phosphorus sources（Ca₂-P）,slow phosphorus sources（Ca₈-P,Al-P,Fe-P）increased the content of inorganic phosphorus fractions more than others.After 2,4 and 6years of returned,compared with BK,soil TP,TIP,TOP and AP increased by10.47%-39.22%,6.89%-19.53%,18.06%-80.97%and 63.22%-219.54%under RM and RS treatment,respectively.The increase rates of Ca₂-P,Ca₈-P,Ca₁₀-P,Al-P,Fe-P and O-P were 63.41%-223.17%,8.49%-49.03%,4.78%-10.84%,14.47%-22.98%,5.26%-19.39%,2.75%-5.49%,respectively.After 6 years of returned,the contents of TP,TIP,TOP,AP,Ca₂-P,Ca₈-P and Fe-P in RS treatment were significantly higher than those in RM treatment（P<0.05）,which were 1.06,1.04,1.08,1.08,1.07,1.28,1.07 times,respectively.However,there were no significant differences in Ca₁₀-P,Al-P and O-P between RM and RS treatments（P<0.05）.Correlation analysis showed that there was a significant positive correlation between TP,TIP,TOP,AP and inorganic phosphorus fractions in all treatments（P<0.01）.（4）After 6 years of rturned crucifer-legume green manures,in the 0-60cm soil layer,the contents of SOC and DOC in RM and RS treatments were significantly higher than those in BK treatment.The contents of SOC and DOC in RM and RS treatments were as follows:RM>RS>BK.Five components were identified by EEM-PARAFAC analysis,which were humic-like components（C1,C2,C3）and protein-like components（C4,C5）,the percentage of fluorescence intensity indicated that the humic-like components in the soil were mainly increased by returned crucifer-legume green manures.The fluorescence index（FI）of each treatment in the 0-30cm soil layer was 1.8>BK>RM>RS>1.2,respectively,indicating that the main source of DOM in the 0-30cm layer was the mixed source of land plants and microorganisms,main source of BK treatment is microorganisms,and the proportion of microbial sources in the 30-60cm layer gradually increased with the increase of soil depth.HIX and SUVA254 of each treatment were RS>RM>BK,respectively,which indicated that RM and RS treatment could increase the humification degree of DOM and the abundance of aromatic carbon in soil organic matter.S_Rof each treatment were BK>RM>RS,indicating that the remaining macromolecular compounds in the soil treated by RM and RS were difficult to decompose and relatively stable.Our study can provide a scientific basis for the improvement of poor soil in arid and semi-arid regions by returning cruciferus-leguminous green manures,and has important practical significance for solving the shortage of organic fertilizer sources in hilly orchards and improving the quality and efficiency of apple industry in hilly regions.