Effects Of Mulching On Soil Hydrothermal Regimes And Nutrient Utilization Of Dryland Wheat And Mechanisms For Yield Increase

Straw strip mulching（SM）is a new mulching technology.In response to the lack of systematic comparative studies on the effects of SM and the full-field plastic film mulching（PM）on soil temperature,soil moisture and utilization of nutrients in dryland wheat（Triticum aestivum L.）over a longer period of time and under the multi-dimensional environments,and the unclear mechanism of yield increase by mulching practices,field comparative trials with multi-years,multi-locations and multi-cultivars of wheat were conducted in semiarid rainfed areas of northwest China from 2012 to 2020.Each experiment consisted of the three treatments:SM,PM and no mulching with wheat sown in rows as the control（CK）.In the same experiment,the rate of sowing and fertilizer application were the same for each treatment,so SM had to increase the rate of sowing per row.The effects of different treatments on soil moisture,soil temperature and soil nutrients were investigated,and the mechanisms associated with the influence of mulching treatments on grain yield were dissected around the structure and formation sequence of the grain yield components.The main results are as follows:1.The mulching treatments had yield-increasing effect by the Meta-analysis of 56 field trials（P<0.05）.SM could achieve an average yield similar to that of PM（P>0.05）when the percentage of the field area mulched in SM reached 59%.The correlation analysis showed that the mulching treatments increased yield mainly by increasing spikes ha^-1（r=0.62**,p<0.01,the same as below）and total sink capacity(grains ha^-1)（r=0.91**）,promoting vegetative growth（r=0.77**）and increasing the contribution percentage by 5.4%～12.7%through transferring from pre-anthesis photosynthate to grain yield,among which the contribution from the transfer of stem was the largest.2.Mulching treatments not only affected the soil water supply ability,but also could change the water use ways.Compared with CK,SM and PM have similar increasing moisture effect based on the average moisture across the growing stages.In the range of 42%to 59%area mulched,the effects of increasing moisture were similar among different patterns of SM.Nevertheless,there were spatial and temporal differences in increasing moisture effects of mulching treatments over CK.At all growth stages and 0～200 cm soil depth,SM had a general effect of increasing moisture.The increasing moisture effect by PM from overwintering to jointing in 0～60 cm soil depth was significantly higher than that by SM,but it was significantly lower than that by SM after booting.Especially,during the period from grain filling to maturity,PM showed a general effect of decreasing moisture relative to CK.The total water consumptions（or evapotranspiration,ET）were similar（P>0.05）among SM,PM and CK.However,compared with CK,both PM and SM significantly reduced the water consumptions in the early growing phase（from sowing to jointing）and their ratios to the total water consumption of whole growth period（ET）;PM increased water consumptions in the middle growing phase（from jointing to flowering）and their ratios to the ET compared with CK and SM;SM increased the water consumptions in the later growing phase（from flowering to maturity）and their ratios to the ET compared with CK and PM.The enhancement of water consumption in the early phase was beneficial to promoting vegetative growth（r=0.40**）and increasing spikes ha^-1（r=0.33**）;the enhancement of water consumption in the middle phase was beneficial to promoting vegetative growth（r=0.51**）,increasing grains per spike（r=0.57**）and the total sink capacity（r=0.43**）;however,in the later growing phase,the enhancement of water consumption was beneficial to increasing grain weight（r=0.27**）.The reason for more water consumption of CK in the early phase with less spikes ha^-1was that the proportion of soil ineffective evaporation in the early phase caused by bare surface was higher,but the transpiration used really for growth and yield formation was not much.Although mulching practices could improve soil moisture to some extent,the main reasons for increasing yield by mulching were still the optimization of water use,i.e.,while increasing the proportion of transpiration in the whole growth period,they provided more sufficient water supply and increased the corresponding water consumption for the key period of yield component formation and vigorous vegetative growth,thus significantly increasing water use efficiency by13.6%～18.4%.3.Compared with CK,SM had a general cooling effect,while PM had the opposite.With the decrease of the area mulched,the cooling effects by SM were weakened gradually.However,with changes of growth stages,both PM and SM showed the dual effects of warming and cooling.During the phase of soil temperature≤0℃in winter,SM also had a warming effect compared with CK,but after regreening stage,there was a general cooling effect.PM had a prominent warming effect before jointing,but a general cooling effect also occurred after jointing.During the overwintering period,the warming effect to soil was beneficial to reducing the mortality rate and maintaining a slow root growth.Although the cooling effect of soil from regreening to jointing was not conducive to forming more grains per spike（r=0.44*,p<0.05 the same as below）,it was beneficial to increasing the spikes ha^-1（r=-0.48**）,and had little effect on the total sink capacity（r=-0.16）.During the middle phase（from jointing to flowering）,the cooling effect had no significant effect on the grains per spike,but it was beneficial to increasing the spikes ha^-1（r=-0.58**）and the total sink capacity（r=-0.58**）,and promoting vegetative growth（r=-0.36*）.During the later phase（from flowering to maturity）,the cooling effect was beneficial to increasing grain weight（r=-0.66**）.In the middle and later phases with higher atmospheric temperature,the cooling effect by mulching also helps to reduce the evapotranspiration intensity,luxury water consumption and injury to plants by high temperature.Meanwhile,compared with CK,SM and PM significantly reduced the diurnal temperature range at the topsoil of 0 20 cm by 1.91℃and 0.70℃,respectively.Reducing diurnal temperature range was overall beneficial to the formation of higher yield.The effects of reducing the diurnal temperature range of soil in different growing phases on yield formation and vegetative growth were consistent with the cooling effect.4.Mulching increased nutrient utilization.Compared with CK,SM increased the total aboveground uptake of nitrogen（N）,phosphorus（P）and potassium（K）by 16.1%～24.5%.Mulching increased the uptake of N and P in straw more significantly than that in grains.Mulching increased the uptake efficiency and partial productivity of N and P but did not increase the requirements of N and P and the concentrations of N,P,K in grains.The main reason for increased nutrient uptake by mulching was due to higher biomass resulting in more nutrients consumption.The total aboveground uptake of N,P and K was highly positively correlated with biomass,grain yield ha^-1and straw yield ha^-1（r=0.66**～0.84**）,respectively.Due to more nutrient uptake,mulching decreased the residual nutrients and leaching loss toward deeper soil at harvest.Under typical fertilizer application(N,120 kg ha^-1;P₂O₅,90 kg ha^-1),PM and SM reduced NO₃-N residues of 0～100 cm soil by 32%and 49%,total mineralized N accumulation by 51%and 33%,respectively,as well as leaching loss of NO₃-N below 40 cm soil.Meamwhile,mulching also reduced the contents of total N,available P and available K in0～40 cm soil profile,and the reduction in PM was greater than that in SM.Under the uniform fertilization on the whole ground,the contents of N and P of 0～40 cm soil in mulching strips of SM at maturity was lower than those in planting strips,indicating that the environments in mulching strips of SM were more favorable for the uptake of N and P.Although mulching increased nutrients consumption,it will not lead to the exhaustion of farmland nutrients.Through the analysis of the input and output of nutrients in wheat fields,it was shown that,under the existing grain yield level and typical fertilization,with or without the return of wheat straw to the field,PM,SM and CK could maintain nutrient balance,and produced N and P surpluses in soil and had fertilizer saving potentials,in which the fertilizer saving potential in CK was greater than those in SM and PM,and the saving potential of P was much greater than that of N.SM and PM increased urease activities in topsoil by 12.0%and 7.6%,alkaline phosphatase activities by 9.5%and 6.1%,respectively,but had no significant effect on soil p H.Increasing the activity of the two enzymes could accelerate the effective transformation of N and P,and then promote the uptake to N and P by plants.Both SM and PM altered the community structures of bacteria and fungi in soil.The alteration of soil microbial community structures by mulching may affect the formation of available forms and movement of N in soil,which resulted in promoting the uptake and utilization to N by plants,meanwhile,affecting its accumulations and distribution in the soil profile.Negative correlations（r=-0.65*～-0.82**）were found between the accumulation of NO3-N in 0～20 cm soil layer at maturity and the abundance of Acidobacteriota or Actinobacteriota,between the accumulation of NO₃-N in0～100cm soil at maturity and the abundance of Mortierellomycota or Mortierella,as well as between the content of NH₄-N 0～20 cm or 0～100 cm soil at maturity and the abundance of Chloroflexi,respectively.However,the change of abundances in soil microbial communities had no significant effect on the accumulations of available P and K in soil profile at maturity.In conclusion,under the environmental background of semiarid rainfed region in northwest China,the hydrothermal adjustment and coupling effects by mulching provided a suitable soil hydrothermal regimes for increasing yield,and mulching promoted the efficient utilization of nutrients.Through the optimization of soil hydrothermal regimes and nutrient environment,mulching increased vegetative growth and sink capacity,which was the main reason for the yield increase of SM and PM.