Winter Cover Crops Root Characteristics Influence Aggregate Stability And Drought Resistance Of Summer Maize In Clayey Red Soil

The red soils in the southern region of China have poor soil physical properties（high percentage of microaggregate,high clay content,and low soil organic matter）,and these properties coupled with low rainfall and high temperature in summer/autumn,are prone to attenuate seasonal drought in the rain-fed agriculture area.Temporally,the red soil region’s seasonal drought is characterized by an intermittent short-term water shortage（usually weeks）in late summer and early fall（July to September）.Spatially,the red soil water shortage mainly occurs at the shallow soil layer,but the high amount of soil water in subsoils is difficult to be used by shallow-rooting crops due to poor subsoil aggregate structure in the seasonal drought period.Therefore,using winter cover crops with a strong root penetration ability to improve the aggregate properties will possibly promote subsequent season crop root growth to pursue deep layer soil water and improve their drought resistance.This dissertation focuses on the mechanism of winter cover crop root traits to change soil aggregate properties and their potential to alleviate the seasonal drought of subsequent season summer maize in a clayey red soil.The objectives of the current study were to（i）evaluate different winter cover crop root traits（root diameter（RD）,root volume density（RLD）,root surface area density（RSD）,and root volume density（RVD）,etc.）and its drilling ability in soils,（ii）assess the winter cover crop root trait’s effects on soil aggregate properties in a macroscale（root size scale）,（iii）determine the influence of root characteristic and root-derived soil organic carbon on aggregate stability through soil internal forces（microscale）,（iv）to study the soil aggregates relationship with the drought resistance of subsequent season summer maize after winter cover crop applications.The field trials were conducted with five winter cover crop-summer maize rotations,including common-rooted rapeseed（Rape＿C）,deep-rooted rapeseed（Rape＿D）,lucerne,one-year-old vetiver,and six-year-old vetiver（Vet＿6Y）and one control（without winter cover crop）from 2019 to 2022 in a research station affiliated with the Huazhong Agricultural University.Winter cover crop root traits（morphological and chemical traits）,soil aggregate properties,soil internal force,soil drought index,and maize hydrological traits were determined.The main findings of this study were as below:1)Compared with tap-rooted crops（lucerne and rapeseed cultivars）,the fibrous-rooted Vet＿6Y had higher root diameter（RD）,root length density（RLD）,higher root surface area density（RSD）from 2019 to 2022.The root of Vet＿6Y was distributed to the highest soil depth of 80 cm compared to 60 cm for lucerne,40 cm for Rape＿D,and 30 cm for Rape＿C.The higher RSA and RLD distribution in the soil profile was responsible for deeper root distribution in red soil which was confirmed by positive correlation analysis,and this was the case for perennial fibrous-rooted vetiver with higher RLD.2)The same winter cover crops were continued to be analyzed in the second field trial in 2021 and 2022 to investigate how their root traits（morphology,lignin,and cellulose）,and soil organic carbon（SOC）)influence aggregate properties in different soil depths（macroscale）.The perennial fibrous-rooted vetiver has higher RLD,higher percentage of fine roots（0.2–0.5 mm）,and the lower root lignin/cellulose ratio than taproot crops,and these root traits also resulted in their higher SOC than taproot crops and control.Consistently,vetiver’s root properties improved the soil macroaggregate（>5 mm and 5-2 mm）percentage and MWD in a higher degree compared to taproot crops and control in both years.The structural equation modeling（SEM）showed the contribution of winter cover crop root traits（RD,RLD,RSD,RVD）,SOC,and clay content on aggregate stability（MWD）and aggregate size distribution.The SEM results showed that the root traits and SOC all positively affect soil aggregate,and the RSD was the most important factor on aggregate（path coefficient=0.39）,followed by the second factor of root-derived SOC on soil aggregate.3)This study also further compared three selected winter cover crops（deep-rooted rapeseed,lucerne,and Vet＿6Y）and two control（fallow with no crop in winter）and fallow with SOC removal（fallow-SOC-R）in the laboratory to determine the root-derived SOC effect on soil aggregation through soil internal forces（SIFs）in a microscale.The SIFs include electrostatic repulsive force(P_erf),van der Waals attractive force(P_vdw),and surface hydration repulsive force（P_h）,and a large net repulsive force(sum of P_vdw,P_erf,and P_h)will impair aggregate stability.The perennial crop（Vet＿6Y）showed the highest root number density（RND）(0.21 counts cm^-2),second highest root mass density（RMD）(1.80 mg cm^-3),and the lowest root lignin/cellulose ratio（0.69）,and thus contributed to its higher SOC content and polysaccharide-C SOC functional groups than annual crops（rapeseed and lucerne）.Such SOC traits of perennial vetiver resulted in a higher soil specific surface area（SSA）,lower surface charge density（σ₀）,and surface charge number（Q_s）compared to annual crops（rapeseed and lucerne）and controls,and the relationship between roots,SOC and soil surface properties（i.e.,SSA）was further confirmed by significant positive correlation（r ranging from 0.76 to 0.97）（P<0.05）.Ultimately,these soil surface charge properties after winter cover crops increased P_vdw,decreased P_erf,and thus decreased repulsive net force(P_net)with the average repulsive P_net displayed in an order of fallow-SOC-R（2.77 MPa）>fallow（2.11 MPa）>lucerne（2.02 MPa）>rapeseed（1.78 MPa）>Vet＿6Y（1.45 MPa）at a distance of 1 nm.Smaller repulsive P_net after winter cover crop treatments resulted in smaller mass percentage of released small particles（<20,<10,and<2μm）w（<d）%from aggregate compared to controls.4)After an improvement in soil aggregates,the study further selected three different drought years to investigate the winter cover crop roots effect on soil hydrological properties（SHP）and subsequent season maize drought resistance in 2019（dry year）,2021（normal-wet year）,and 2022（severe dry year）.The higher soil aggregate stability and macroaggregate（>2 mm）percentage after winter cover crop treatment improved soil water content while decreased the soil drought degree（D）compared to control.The average surface soil water content（SWC）of three years followed in an order of vetiver（0.19 cm³/cm³,0.25 cm³/cm³,0.22 cm³/cm³）>lucerne（0.15 cm³/cm³,0.22 cm³/cm³,0.19 cm³/cm³）>Rape＿D（0.15cm³/cm³,0.19 cm³/cm³,0.14 cm³/cm³）>Rape＿C（0.14 cm³/cm³,0.19 cm³/cm³,0.14cm³/cm³）>fallow（0.11 cm³/cm³,0.14 cm³/cm³,0.10 cm³/cm³）.The average soil drought degree at the surface showed in reverse order between treatments,with vetiver（0.17 to 0.41,0.30 to 0.40,and 0.42 to 0.74）<lucerne（0.19 to 0.55,0.23 to 0.52,and 0.41 to 0.82）<Rape＿D（0.25 to 0.54,0.26 to 0.51,and 0.41 to 0.85）<Rape＿C（0.27 to 0.66,0.28 to 0.53,and 0.44 to 0.86）<fallow（0.31 to 0.78,0.36 to 0.68,and 0.45 to 0.96）in 2019,2021 and2022.Meanwhile,larger gradient between maize leaf and root water potential（Ψ_l,Ψr）,and lower crop water stress index（CWSI）was found after fibrous-rooted vetiver than taproot（rapeseed cultivars and lucerne）and fallow in different degree of drought years.In severe drought year（2022）,winter cover crops displayed a more obvious positive effect on the improvement in soil water properties and maize growth than other years which was indicated by a larger gradient between maize leaf and root water potential（Ψ_l,Ψr）,lower CWSI,and higher maize root distribution and maize yield than other years.Correlation analysis showed that aggregates（>5 and 5-2 mm）played positive role with soil water content and negative role with D value（r from–0.54 to–0.91）,which explained why vetiver treatment with high macroaggregate stability resulted in high soil water retention and low D.The aggregate（>0.25 mm）also played negative role with maize root water potential at all growth stages,indicating that improvement in macroaggregate percentage decreased the maize root water potential and will accelerate the water movement from soil into maize root and then maize leaf.The better effect of vetiver on alleviating maize drought than taproot crops was further confirmed by principal component analysis（PCA）,where SWC,winter cover crop root trait,and maize leaf and root water potential gradients are the dominant approaches adjusted by winter cover crops to alleviate the maize drought.Ultimately,winter cover crop treatment improved maize drought resistance by improvement in SHP,gradient between maize root and leaf water potential,and better maize root and aboveground growth（plant height,stem diameter,and leaf area）and finally reduced the yield loss compared to control.This drought alleviation effect after winter cover crops was the most obvious for fibrous-rooted vetiver and in severe drought year.In summary,winter cover crop practices with optimal root characteristics improved aggregate stability in macroscale（root size scale）and microscale（nano-scale internal force）in a clayey red soil.The perennial fibrous-rooted vetiver with the deepest root distribution improved the soil aggregate stability in a larger degree than taproot crops and control treatment.In the macroscale,winter cover crop root traits,especially RSD,played the first order in aggregate stability,followed by SOC,and the highest RSD and SOC from vetiver treatment was consistent with its highest aggregate stability.In the microscale,the highest winter crop-derived SOC content and polysaccharide-C functional groups from fibrous-rooted vetiver determined the lowest repulsive soil internal forces by changing soil surface properties and resulted in the highest aggregate stability.The improved aggregate properties through winter cover crop root characteristics alleviated the seasonal drought of subsequent season maize by enhancing the soil hydrological properties and hydraulic structure of maize plant（gradient of maize root and leaf water potential,CWSI）and thus enhanced the maize root and aboveground growth and reduced the yield loss during drought.Our findings suggest that fibrous-rooted winter cover crops are suitable for long-term,sustainable agricultural productivity in seasonal drought by improvement in soil aggregation in the clayey red soil.