Effect And Mechanism Of Cover Crop Combined With Different Fertilization On Apple Yield And Quality As Well As Soil Water And Nutrient Use Efficiency

In the process of apple production in the dryland orchard of Loess Plateau,water and nutrients are usually two key factors for the increase of tree growth,apple yield and quality.Cover crop and chemical fertilizer combined with manure are common management practice and fertilization in the apple orchard.The cover crop will compete soil water and nutrients with apple tree,but conserve soil water and increase organic matter after mowing and mulching.Additionally,chemical fertilizer combined with manure is a kind of reasonable fertilization compared with application of chemical fertilizer alone.Otherwise,a reasonable fertilization can compensate the competition of soil nutrients of cover crop.However,the mechanism for the effect of long-term application of cover crop and chemical fertilizer combined with manure on apple yield and quality is still not very clear.Thus,this study showed the effects of cover crop and different fertilization?no fertilizer,chemical fertilizer,manure,chemical fertilizer combined with manure?on the growth of apple tree,apple yield and apple quality.From the point of soil water change?we also compared the cover crop combined with plastic mulch and plastic mulch alone on soil water conservation?and mechanism of soil fertility increase,we study the effects of long-term cover crop and chemical fertilizer combined with manure on water use of apple tree,soil nutrients,soil enzyme activities,soil microbial community and soil functional genes,and showed the water and fertility mechanism for the effects of long-term cover crop and chemical fertilizer combined with manure on apple yield and quality.The main results and conclusions are as follows:?1?Compared to other treatments,long-term cover crop combined with chemical fertilizer and manure could increase the growth of apple tree?the growth of leaf and spring shoot,etc.?,fruit weight and fruit number?in 2013 and 2014,under the cover crop condition,the fruit weight of NPKM was 23.81%,10.42% and 6.32% higher than CK,M and NPK;the fruit number of NPKM was 48.80%,29.34% and 26.48%?,thereby enhanced apple yield?the apple yield of NPKM was averagely 87.28%,44.33% and 33.41% higher than CK,M and NPK?.Meanwhile,apple quality,especially for Vc content,was also increased.The double covered model?cover crop combined with plastic mulch?increased the apple yield and quality by optimize the soil cover measurement,compared to applied plastic mulch alone?2013-2015,the apple yield of double covered model was 14.10%,18.84% and 26.69%increased in 2013,2014 and 2015,respectively?.?2?Cover crop could conserve more soil water in the hot rainy season for transpiration during the fruit expansion stage of apple trees,particularly in the dry year?73%?.Thus,the apple yield and water use efficiency were increased.However,the effect of cover crop on soil water was not obvious when enough precipitation prevented water stress,compared to no cover crop.Additionally,in comparison with other fertilization treatments,chemical fertilizer combined with manure could compensate the water and nutrients consumed by cover crop.Hence,cover crop combined with chemical fertilizer and manure resulted in the highest yield and water use efficiency of the tested treatments.?3?Compared with the use of plastic mulch alone,the combined application of plastic mulch and a cover crop increased the inter-row soil water storage after the harvest and decreased the summer evaporation in the inter-row areas.Moreover,the summer water use efficiency was increased under cover crop combined with plastic mulch during the experimental years compared with plastic mulch alone.The total water use efficiency was also increased by 13.8% and 11.7% in 2013 and 2014,respectively,although the effect was not obvious in 2015.In addition,the application of the cover crop during a hot summer modified the high temperature due to strong solar radiation and plastic mulch.Hence,the application of plastic mulch combined with a cover crop increased apple yield,compared to plastic mulch alone.?4?After mowing and mulching,cover crop increased soil water content,nitrate nitrogen,available phosphorus and phosphatase?PHOS?,?-N-acetylglucosaminidase?NAG?,?-glucosidase?BG?,cellobiohydrolase?CBH?and ?-xylosidase?BXYL?.Compared to no cover crop,cover crop increased soil organic carbon content,total nitrogen and PHOS,NAG,BG,CBH and BXYL activities,no matter after or before mowing and mulching,but the soil water content,nitrate nitrogen and available phosphorus were decreased.Under cover crop,the soil organic carbon,total nitrogen,available phosphorus of treatments with application of manure?manure and chemical fertilizer combined with manure?were higher than no fertilizer and chemical fertilizer,and PHOS,NAG,BG,CBH and BXYL of chemical fertilizer combined with manure were higher than chemical fertilizer treatment.Overall,cover crop combined with chemical fertilizer and manure could increase soil fertility and enzyme activity effectively.?5?The cover crop had a greater impact on the composition of the soil microbial community than the chemical fertilizer.The cover crop increased the relative abundances of taxa related to the breakdown of plant polymers,thereby affected soil microbial community composition.The soil microbial network also changed with the change of the composition ofthe microbial community.The competition between bacterial and fungal taxa increased.The cooperation among bacterial taxa also increased.These changes mainly occurred at the intra phyla level?mainly at Proteobacteria,Firmicutes,Actinobacteria and Bacteroidetes?.Moreover,the long-term growth of the cover crop increased the soil nutrient levels?e.g.,organic carbon and total nitrogen?,which affected the status of keystone taxa in the network,such as the phylum Thaumarchaeota,unidentified_Nitrospiraceae,unidentified_Nitrosomonadaceae,Faecalibacterium,Coprococcus₃ and Ruminococcaceae_NK4A214_group.Consequently,the changes in the composition of soil microbial community and network increased the specific genes involved in the degradation of cellulose,hemicellulose and cello-oligosaccharides,which enhanced the soil enzyme activities and contributed to increases in the soil organic matter contents.?6?The microbial composition and network in the macroaggregate were more similar to those in the whole soil compared with the microaggregate and silt+clay.The distributions of microbes in the soil aggregates were influenced by the soil structure.In addition,the annual cover crop altered the microbial community composition,where the changes varied among the different aggregate fractions.The relationships between fungi and bacteria in the different aggregate fractions were changed by the annual cover crop.The competition between fungi and bacteria in the macroaggregate as well as the cooperation between fungi and bacteria in the silt+clay fraction was increased.The relationships between Ascomycota and Actinobacteria/Alphaproteobacteria indicated similar trends.The keystone species in the different aggregate fractions were changed by the cover crop.Compared with the GC treatment,species that had significant correlations with soil organic carbon,total nitrogen,and the enzyme activities tended to be more influential in the networks obtained for the no cover crop treatment,with lower nutrient levels in the whole soil and different aggregate fractions.?7?Nitrogen cycling and forms of soil organic matter metabolism were improved in the soil in cover crop combined with chemical fertilizer.The relative abundance of genes related to nitrate reduction,nitrogen fixation,nitrification,phosphorus cycling,and plant degradation were also abundant in the soil in cover crop combined with chemical fertilizer.Network analysis based on different type of metabolism indicated the key roles for nitrogen and phosphorus cycling in soil organic matter turnover,which were also supported by network analysis based on genes.Thus,changes in the relative abundance of genes affected the soil properties,and genes involved with nitrogen and phosphorus cycling explained moreof the variations in the soil properties compared with genes involved with plant degradation.Compared to chemical fertilizer and cover crop alone,cover crop combined with chemical fertilizer supported higher available nitrogen and phosphorus for the construction of carbon,nitrogen and phosphorus cycling.Thus,the application of the cover crop combined with fertilizer enhanced the soil properties and apple yield via the integrated adjustment of nitrogen and phosphorus cycling and soil organic matter turnover.