| Dry farming areas are important food production areas and play a vital role in ensuring food security.In these areas,long-term conventional tillage and single cultivation patterns are likely to cause a decline in soil fertility and micro-ecological imbalances.Conservation tillage combined with Leguminous-Graminaceous crop rotation can improve soil fertility and crop rhizosphere microenvironment,increasing crop nutrient absorption capacity.However,the mechanisms of soil microbial action for efficient nitrogen utilization of winter wheat in conservation tillage and soybean-wheat rotation systems are not clear.Therefore,this study based on the ecological conditions of dry farming on China’s Loess Plateau,and took the center of"crop-microbe-soil",three tillage practices(chisel plow tillage:CPT;zero tillage:ZT;plow tillage:PT)and two crop rotation patterns(soybean-wheat rotation:SW;maize-wheat rotation:MW)were conducted to investigate the soil bacterial communities and winter wheat growth characteristics in soil tillage and crop rotation systems by using 16S r RNA amplicon sequencing and metabolomics.Moreover,together with soil physicochemical properties to clarify the responses of soil bacterial communities to soil tillage and crop rotation during winter wheat development.The potential relationships between rhizosphere bacterial communities and nitrogen utilization in winter wheat were analyzed from a multi-omics perspective,to clarify the rhizosphere bacterial mechanism on winter wheat nitrogen utilization in conservation tillage and soybean-wheat rotation systems.The main findings of this study are as follows:(1)Soil tillage regulates the growth characteristics and soil bacterial communities of the preceding crops in the rotation systems by improving the soil environment.Compared with conventional tillage(PT)practice,Conservation tillage(CPT and ZT)practices significantly increased the contents of soil moisture(3.62–13.13%),soil organic carbon(29.19–39.67%),and nitrogen(38.69–60.23%)contents and decreased soil p H value(1.15–3.76%)of the preceding crops.CPT increased maize and soybean biomass(2.39%and 3.58%)and nitrogen accumulation(5.53%and 7.03%)compared to PT,while ZT decreased their biomass(4.21%and 8.88%)and nitrogen accumulation(6.83%and 5.56%).The rhizosphere bacterial community composition varied consistently with the growth of the previous crop.Rhizosphere effect decreased the competition among rhizosphere bacterial communities(the edges of rhizosphere and bulk soil bacterial community network were 926 and 1095,respectively;the densities of rhizosphere and bulk soil bacterial community network were 0.157 and 0.184,respectively)and increased the structural complexity of the community network(the modularity of community network were 0.857 and 0.556,respectively);moreover,conservation tillage and soybean cultivation increased community network complexity(the modularity of community network were 0.879,0.742,and 0.495 under CPT,ZT,and PT,respectively,and were 0.525 and 0.888 in maize and soybean field,respectively).Although the phylogenetic clustering of rhizosphere bacterial communities caused by the rhizosphere effect was lower than that of bulk soil bacterial communities(2.13%),it had no significant influence on rhizosphere or bulk soil bacterial community assembly,and deterministic processes dominated the formation of soil bacterial community structure.Partial least squares path modeling analysis showed that temporal variations in the rhizosphere bacterial communities(explained variations were 94.7%and 92.7%,respectively)were regulated by soil tillage and growth stage,which to some extent contributed to the growth of the previous crops.(2)The results of the study on soil tillage and crop rotation systems affecting rhizosphere bacterial communities during winter wheat development showed that the relationships between rhizosphere bacterial communities were closely related to the changes in edaphic properties caused by tillage and crop rotation.The phylogenetic diversity and taxonomic composition of the rhizosphere bacterial communities were significantly influenced by winter wheat growth stage(R~2:0.2130,P=0.001;R~2:0.2067,P=0.001),soil tillage(R~2:0.1277,P=0.001;R~2:0.12617,P=0.001),and crop rotation(R~2:0.0241,P=0.001;R~2:0.0392,P=0.001).Rhizosphere bacterial community similarity decreased with winter wheat development,and both conservation tillage and SW reduced the decay rate of rhizobacteria.The co-occurrence network analysis of rhizosphere bacterial communities showed that the differential modules among the rhizosphere bacterial communities decreased with wheat development in the MW and SW.The results of random forest analysis indicated that p H value,soil moisture,total nitrogen,and soil organic carbon contents were the most important environmental factors affecting the diversity and assembly process of rhizosphere bacterial communities.The ecological processes of rhizosphere bacterial community structural formation were significantly correlated with soil moisture,total nitrogen,p H value,and rhizosphere bacterial community diversity(P<0.05).(3)Further analysis of the effects of soil tillage and crop rotation on the growth characteristics of winter wheat revealed that CPT and ZT significantly increased soil moisture(6.25–9.99%and 5.95–11.43%,respectively),soil organic carbon(14.99–17.62%and 14.73–15.38%,respectively),and total nitrogen(30.77–37.30%and26.51–32.99%,respectively)levels compared to PT;SW increased soil organic carbon(0.38–0.60%)and total nitrogen(0.34–2.56%)levels compared to MW.Tillage practice had significant effects on the growth characteristics of wheat(P<0.05),whereas crop rotation pattern only had a greater effect on the nitrogen concentration of wheat.Comparing CPT and PT,ZT significantly decreased wheat nitrogen fertilizer productivity(7.52–14.57%)and nitrogen uptake efficiency(10.44–24.62%),while wheat nitrogen use efficiency showed the opposite results.Additionally,CPT increased grain yield(3.28%)and economic benefit(5.41%)of wheat compared with PT,while ZT decreased grain yield(9.69%)and economic benefit(7.41%);SW increased grain yield(1.12%)and economic benefit(1.83%)compared to MW under CPT,and reached its highest under the SW＿CPT treatment.(4)By analyzing the rhizosphere bacterial mechanism on nitrogen use efficiency of winter wheat,it was found that CPT increased nitrogen accumulation(2.15%and3.38%)and protein content(4.75%and 2.64%)of winter wheat compared to PT in MW and SW systems,while ZT showed the opposite results;SW increased nitrogen accumulation(4.54%)and protein content(9.20%)of winter wheat compared to MW.Wheat root glutamate dehydrogenase(GDH)and glutamine synthetase(GS)activities increased with winter wheat growth,while glutamate synthase(GOGAT),nitrate reductase(NR),and nitrite reductase(Ni R)activities decreased;CPT had a tendency to increase GOGAT,GDH,and GS activities and decrease NR and Ni R activities compared to PT,and ZT showed the opposite result;SW increased GOGAT,GDH,and GS activities and decreased NR and Ni R activities compared to MW.Changes in rhizosphere bacterial community diversity(PERMANOVA,P<0.05)caused by soil tillage and crop rotation were important biological factors affecting nitrogen utilization in winter wheat.Soil tillage and crop rotation caused changes in the relative abundances of dominant phyla(Proteobacteria,Actinobacteria,etc.)in the differential module of the rhizosphere bacterial co-occurrence networks altering the community metabolites.The upregulations of Tetrahydrofolic acid and PC(14:1(9Z)/20:2(11Z,14Z)in conservation tillage and SW systems were related to the process of soil nitrogen metabolism.The changes in the relative abundances of Streptomyces,Phyllobacterium,and Steroidobacter were closely related to the metabolic functional diversity of rhizosphere bacterial communities,resulting in differential metabolites,which affected the nitrogen uptake and utilization of winter wheat.In summary,CPT and SW could affect soil bacterial community diversity and structure formation process by improving soil microecological environment,and had a tendency to improve crop growth characteristics and productivity.Additionally,CPT and SW induced changes in the abundance of rhizosphere bacteria and their differential metabolites,and promoted the absorption and utilization of nitrogen in winter wheat by regulating the activities of enzymes related to nitrogen metabolism and their corresponding processes.Therefore,combining SW with CPT can be an ideal green ecological agronomic management practice to improve crop-microbe-soil interactions in dry farming areas. |
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