| Soil organic carbon(SOC)is an essential indicator of soil fertility,higher SOC contents can increase soil nutrients supply and enhance crop yield.SOC also plays an important role in alleviating climate change,with the potential of increasing carbon sequestration.Soil microorganisms are involved in SOC decomposition and accumulation,thus microbial communities and enzyme activities are key components in ecosystems,contributing to SOC transformation.Tillage is an important management practice in crop production,which has effect on SOC and its labile fractions,enzyme activities and microbial communities.However,studies on the effect of tillage practices on labile SOC fractions and enzyme activities in the whole crop growth stages are rarely reported,especially,interaction between microbial communities and SOC changes is still unclear.Therefore,a long-term(11 years)field experiment was conducted in Weibei dryland.We systemically studied the soil organic carbon fractions and enzyme activities under different tillage practices at the sowing,jointing,filling and harvest stages of winter wheat,and clarified the changes in soil organic carbon fractions and enzyme activities during the growth stages of winter wheat and identified both soil organic carbon fractions and enzyme activities were affected by tillage practices.Furthermore,soil bacteria and fungi communities at the filling stage were investigated to explore the microbial mechanism of tillage practices on SOC fractions changes.Also,we measured the yield of winter wheat and assessed the carbon footprint in winter wheat production,finally identified the best tillage practice for achieving sustainable agricultural production in Weibei dryland.These results provided theoretical and technical support for regulating soil carbon pool.The main results are as follows:(1)Studied the changes in soil labile organic carbon fractions throughout the various growth stages of winter wheat,and clarified the effect of tillage practices on soil labile organic carbon fractions.SOC contents were not significantly different at any growth stage of winter wheat,however,soil labile carbon fractions(microbial biomass carbon,MBC;permanganate oxidizable organic carbon,POXC;particulate organic carbon,POC)changed across growth stages.MBC contents increased from the sowing stage to the jointing stage(reaching a maximum)and then decreased at the filling stage and harvest stage.POXC contents were similar at the sowing,filling,and harvest stages,but declined at the jointing stage.POC contents increased over winter wheat growth.POXC and POC contents both showed the highest values at the harvest stage.No tillage(NT),subsoiling tillage(ST)and rotation tillage(NT/ST,ST/PT,PT/NT)significantly increased SOC and its labile fractions throughout all growth stages of winter wheat when compared with plow tillage(PT).NT/ST significantly increased SOC,MBC,POXC and POC contents at the following depths: 0-10 cm,10-20 cm,10-20 cm and 35-50 cm.NT treatment primarily increased SOC,MBC,POXC and POC contents at depths of0-10 cm and 10-20 cm.ST treatment primarily increased SOC,MBC,POXC and POC contents at depths of 0-10 cm and 35-50 cm.ST/PT rotation significantly increased SOC,MBC,POXC and POC contents at a depth of 0-10 cm.PT/NT rotation significantly increased SOC,MBC,POXC and POC contents at a depth of 10-20 cm.NT/ST and NT treatments exhibited the highest SOC,MBC,POXC and POC contents,which were also significantly different than those exhibited by ST,PT/NT and ST/PT treatments.In terms of the increment of SOC fractions,the sensitive labile SOC fraction to tillage was different across winter wheat growth,exhibited as POXC was the most sensitive at the sowing stage,POC was the most sensitive at the jointing and filling stages,and MBC was the most sensitive at the harvest stage.(2)Clarified the changes in soil enzyme activities throughout the various growth stages of winter wheat,and identified the effect of tillage practices on soil enzyme activities.Cellobiohydrolase(CBH),β-1,4-glucosidase(BG)and β-1,4-xylosidase(BXYL)activities decreased from the sowing stage to the jointing stage,but increased at the filling stage and then decreased at the harvest stage.These enzyme activities exhibited the following order: sowing stage > filling stage > jointing stage > harvest stage.β-1,4-N-acetylglucosamines(NAG)and alkaline phosphatase(AP)exhibited a decreasing trend over the course of growth,and both showed the following order: sowing stage >jointing stage > filling stage > harvest stage.Greater soil enzyme activity was observed after NT,ST,NT/ST,ST/PT,PT/NT treatments than after PT treatment over the entire growth process of winter wheat.Compared with PT treatment,NT/ST,NT,ST,and ST/PT treatments significantly enhanced CBH,BG,BXYL,NAG,and AP activities in the 0-10 cm layer,while PT/NT treatment significantly increased CBH,BG,BXYL,NAG,and AP activities in the 10-20 cm layer.The highest soil enzyme activities were associated with NT/ST and NT treatments,which were also significantly greater than ST,ST/PT,and PT/NT treatments.In addition,NT/ST,NT,ST,ST/PT and PT/NT treatments regulated the soil ecoenzymatic stoichiometry by reducing the ratio of Ln(CBH+BG+BXYL):Ln(NAG)and increasing the ratios of Ln(CBH+BG+ BXYL):Ln(AP)and Ln(NAG):Ln(AP)compared with PT.We also found that soil enzyme activities were positively associated with SOC and its labile fractions.Redundancy analyses showed that the key factors driving the changes in soil enzyme activities were POXC and SOC at the sowing stage,SOC and MOC at the jointing and harvest stages,and SOC and POC at the filling stage.(3)The study found that tillage practices significantly influenced soil bacteria communities,but weakly influenced soil fungal communities.The increase of Poteobacteria play vital roles in soil organic carbon changes.The dominant bacterial phyla across all soil samples were Poteobacteria,Actinobacteria,and Bacteroidetes.NT/ST,NT and ST soils showed significant increases in the abundance of Proteobacteria,with the increment of 212.9%,215.0% and 195.8% in the0-10 cm layer and 210.7%,233.8% and 190.6% in the 10-20 cm layer.Similarly,NT/ST,NT and ST soils had higher abundances of Gammaproteobacteria,Xanthomonadales,Xanthomonadaceae and Stenotrophomonas.Proteobacteria can produce C cycling enzymes(CBH,BG,BXYL)to break down plant biomass,and thus promoting the transformation of SOC.On the other hand,the abundance of Poteobacteria was positively correlated with labile carbon fractions and enzyme activities,and was mainly influenced by the combination effect of MBC,POC,CBH and BXYL,thus made Poteobacteria fast-growing in rich carbon soils.It is a virtuous cycle of Poteobacteria,enzyme activities and soil organic carbon.The dominant fungal phyla across all soil samples were Ascomycota,Basidiomycota,and Mortierellomycota.NT soils showed significant decreases in the abundance of Ascomycota,with the decrement of 12.0% and 37.4% in the 0-10 cm and 10-20 cm layer,respectively.NT/ST,ST,ST/PT and PT/NT treatments had no significant difference in the abundance of Ascomycota compared with PT treatment.(4)Verified spikes and grain yield of winter wheat were affected by tillage practices,and assessed carbon footprint in winter wheat production,finally optimized the best tillage practices.NT/ST,ST/PT and ST treatments significantly increased spikes by 13.0%,5.8%,12.2%,and increased grain yield by 11.9%,10.1%,11.4% when compared with PT treatment.However,PT/NT and NT treatments did not significantly improved spikes and grain yield.So,increasing spikes is the key to increase the grain yield of winter wheat.NT/ST treatment exhibited the highest spikes and grain yield among all tillage treatments.NT/ST,ST/PT,PT/NT,NT and ST treatments significantly decreased carbon footprint by 145.4%,73.4%,104.3%,125.6% and 119.1%,respectively,and also decreased yield-scaled carbon footprint by 118.2%,104.3%,54.5%,125.6% and 119.1% when compared with PT treatment.After 11 years of straw returning to the field,the increase of SOC storage could give a great carbon footprint reduction,made all tillage practices served as carbon sinks.NT/ST treatment exhibited the lowest carbon footprint among all tillage treatments.Above all,these results of this study showed NT/ST rotation was associated with the highest grain yield,soil organic carbon contents,soil enzyme activities and abundance of Proteobacteria,also produced the lowest carbon footprint,and was thus determined to be the best tillage practice for balancing sustainable production in the Weibei dryland of Loess Plateau. |
