Effects Of Conservation Tillage On Soil Hydrothermal Conditions And Carbon And Nitrogen Conversion In Weibei Arid Plateau

Weibei arid Plateau is the main rainfed agricultural area and an important grain-producing area on the Loess Plateau.Water and nutrient scarcity are the main limiting factors for the development of agriculture in the Weibei arid Plateau.How to construct reasonable field management measures to maintain the balance between ensuring food security and protecting the ecological environment is an urgent problem to be solved.Conservation tilage（e.g.,no-tilage and mulching）is an effective soil and water conservation measure in dryland agriculture.In this paper,we conducted a three-year field experiment based on over two decades of conservation tilage positioning trials.Through a combination of field in-situ monitoring,indoor culture and analysis,various tilage methods（conventional tilage:CT,no-tillage:NT）and mulching methods（absence of mulching:NM,mulching film:PM,and straw mulching:NM）and their interactions（CTSM,CTPM,NTSM and NTPM）were evaluated the effect on maize growth,soil structure,soil hydrothermal conditions,carbon and nitrogen mineralization,and the properties of the microbial community.The aim of this study was to reveal the contributions of soil hydrothermal spatiotemporal transport,soil carbon and nitrogen transformation,and soil microbial community changes to yield under no-tillage and mulching treatments.From the perspective of microorganisms,the understanding of the impact of long-term conservation tilage on soil environment and agricultural productivity has been deepened,which provides a scientific basis for the sustainable development of rainfed agriculture in dryland.The main results of this paper are as follows:（1）The effects of mulching on maize growth index（plant height,leaf area index,dry matter accumulation（SMD）,and growth rate（GR））and yield were greater than those of tillage,and the effects of both weakened with the growth process.In the early growth period,compared with NM,the effect of PM on the growth index was greater than that of SM.The increase of SMD under PM was 55.8-142.1%higher than that under SM.However,the increase of SM increased 90 days after planting（DAP）.The inhibitory effect of no-tillage on maize growth mainly occurred in the early growth period（0-30DAP）,and NTPM alleviated this phenomenon.For example,NTPM increased GR_0-30DAP by 133.0%compared with CTNM.Mulching significantly increased maize yield and the three-year average increase order was NTPM（31.2%）>CTPM（28.4%）>CTSM（20.7%）>NTSM（15.6%）.Random forest analysis showed that the contribution of growth indices to yield was greater at the jointing and filling stages than at other stages.Additionally,growth indexes under mulching treatments during the jointing and fil ing stage were higher than those under NM treatment,which might be the significant mechanism for mulching to increase yield.Furthermore,in Weibei arid Plateau,the spring maize yield of the conservation tilage system may be accurately predicted by the DSSAT model,with the highest simulation accuracy under NTSM treatment.（2）No-tillage increased soil bulk density and compactness（SPR）in the top-soil layer（0-20 cm）.While,the effect on SPR decreased with the growth process in the 0-10 cm soil layer.The SPR increased in 2021 from 102.3%at the planting stage to 47.3%during harvest.While the plow bottom（20-30 cm）formed by traditional tilage for many years still existed in the late growth stage.With NTSM,the plow bottom issue brought on by traditional til ing is resolved,top-soil compaction from prolonged no-tillage is lessened,and the structure of the0-40 cm soil layer is improved.Plastic film mulching raised the average daily soil temperature(ST_0-80)in 0-80 cm soil layer and effective accumulated temperature(SAT_0-80),with the most significant rise occurring during the early growth period(e.g.SAT_0-80 under PM at 30DAP was10.0～15.4%higher than that under no mulching treatments),the influence of straw mulching showed an opposite trend.However,as the growth period advanced and the soil layer deepened,the effect became less pronounced.The interaction of no-tillage and no mulching（NTNM）and straw mulching（NTSM）significantly reduced ST and SAT in the early growth stage,and the low temperature effect continued to deeper soil layers（80 cm）.While,No-tillage and mulching significantly improved soil water content（SWC）and water storage（SWS）in 0-300cm soil layer during the spring maize growing period,especially at the planting stage（SWC3.6-9.7%;SWS 4.8%-8.5%）,alleviating the water stress caused by drought in early spring.NTPM was most effective in improving water use efficiency（WUE:27.3%）and rainfall use efficiency（RUE:32.2%）.No-tillage combined with mulching improved the soil structure and coordinated the water and heat relationship in the key growth stage,especially in early spring cold and dry periods.（3）The tilage and mulching effects and their interactions significantly affected the dynamic changes of soil carbon and nitrogen content and its mineralization potential.In terms of interannual variation,straw mulching increased soil organic carbon（SOC）and total nitrogen（TN）contents in the plough layer,and NTSM increased the most（35.5%and 15.9%,respectively）after continuous cropping for nearly 20 years.While these in PM treatment showed a decreasing trend,the decrease was NTPM<CTPM.The mineral nitrogen（AN）content increased under PM（NO₃^--N:93.7%,NH₄⁺-N:153%）.Regarding growth period dynamics,SOC and TN contents of non-rhizosphere and rhizosphere soils in most periods were as follows:SM>NM>PM,NT>CT.In the middle and late stages of maize growth,the SOC and TN contents of non-rhizosphere soil showed a decreasing trend under PM treatment.In contrast,the contents of SOC and TN were mainly increased under SM during the whole growing period.The rhizosphere effect increased the seasonal variation of carbon and nitrogen contents of each treatment.Furthermore,PM increased AN content during the entire growth period compared to NM.While,SM lowered AN from the jointing to the fil ing period,and demonstrated an increasing tendency during the early growth stages and harvest,indicating that SM could balance soil carbon and nitrogen consumption and storage.In the vertical profile,the effect of SM on increasing carbon and nitrogen content was greater in the 0-20 cm layer than in the 20-40 cm layer,but the effect of NT in the interlayer was the opposite.In addition,the rhizosphere effect and straw mulching significantly increased the activity of carbon and nitrogen cycle-reated enzymes,and the activity at 90DAP was the highest under the rhizosphere effect.In contrast,the impact of NT on enzyme activity was uncertain.SM,PM,CT,and rhizosphere effect promoted carbon and nitrogen mineralization,while NT was the opposite.Compared with SM,PM decreased the cumulative mineralization of organic carbon(C_min)and nitrogen(N_min).Still,PM increased their biodegradability(C_min/C₀ and N_min/N₀).The maximum C_min/C₀ and N_min/N₀ of the rhizosphere and non-rhizosphere soils under PM were reached at the flowering period,which may be related to increased enzyme activity at this stage.Therefore,PM had the risk of excessive consumption of soil carbon and nitrogen.In contrast,SM increased the plough carbon and nitrogen content,and NTSM was more stable in carbon and nitrogen sequestration.（4）The effects of mulching and tillage on the microbial communities in the soil varied significantly.Theα-diversity of bacterial communities was mainly affected by tillage practices.In contrast to traditional tilage,no-tillage increased bacterial Sobs（3.4%）and Chao＿1（4.3%）.The mulching effect had a greater impact on soil microbial community structure than the tilage effect,which was confirmed by the distribution of mulching treatment-sensitive OTUs（ts OTU）in the co-occurrence network.Moreover,mulching also increased the importance of stochasticity processes in soil bacterial communities,and the soil environmental variables（e.g.,NO₃^--N）were critical to the assembly processes of bacterial communities.PM increased ST and the abundance of ts OTU（e.g.,Sphingomonadales and Sordariales）,promoting the microbial-mediated conversion of total nutrients（SOC,TN）to available nutrients（NO₃^--N）.The increase of NO₃^--N content was the main reason for the yield increase of PM（direct effect=0.96）.Straw mulching positively affected the microbial community by increasing SWC（7.8%）and TN（13.0%）.However,low soil temperature（0.87 of the total effect）inhibited the positive impact of organic matter addition on yield.Furthermore,as seen by the concentrated distribution of keystone taxa,higher average clustering coefficient,and modularity,SM increased the stability of the microbial co-occurrence network.Therefore,the PM had a greater influence on yield than SM,but it also required more input because it used more soil nutrients.Straw mulching enhanced the microbial community’s stability and diversity at the low input level,and increased maize yields.In summary,the conservation tilage method combining no-tillage and mulching improved the production of spring maize by coordinating the water-heat relationship during the key growth period of maize in the Weibei arid Plateau.Regarding enhancing hydrothermal use efficiency,and promoting maize growth and yield,NTPM was superior to NTSM.Simultaneously,NTSM played a more significant role in promoting long-term soil carbon and nitrogen sequestration,improving soil structure and microbial community,and also showing a trend of increasing soil moisture conditions and maize yields.It is suggested that the traditional tilage management mode be replaced in dryland farming of the Loess Plateau with the conservation tilage mode,which combined no-tillage and mulching.