The Effects Of Biochar And Alternate Irrigation On Herbaceous Crops Growth,Physiology And Water Use Efficiency

Globally,the increasing climate variability,the scarcity of freshwater resources,and the gradually depleting soil fertility,etc.,have seriously restricted the sustainable development of agriculture.Therefore,developing the efficient water-saving technologies and environment-friendly soil amendment in agriculture is an important strategy to enhance crop water use efficiency,improve soil fertility and increase agricultural productivity.Recently,alternating root zone irrigation（PRD）and biochar are highly recommended strategies for conserving water and improving fertility,yet the mechanisms for regulating crop growth physiology and soil nutrients are unclear and often vary depending on the soil-biochar-crop matching.Accordingly,three pot experiments were conducted in this thesis using three different herbaceous crops,three soil types and three biochar.ExperimentⅠ,tobacco as the target crop,and two typical soil types for tobacco planting（Lou soil-Anthrosol,Acidic red soil-Ferralsol）and 500°C soft wood（SWB）and wheat-straw（WSB）biochar were used.The plants were subjected to three irrigation treatments,i.e.,full irrigation（FI,irrigated daily to 90%of soil water holding capacity（WHC））,deficit irrigation（DI,whole pot irrigated with 70%water amount used in FI）and PRD（irrigation amount same as DI in one side,another side dry to about 10%,then shifting irrigation）.Mainly to investigate the effects of biochar and PRD on tobacco growth,physiology and water use efficiency.ExperimentⅡ,an intercropping system of 1 faba bean（main crop）and 10 ryegrass（secondary crop）was built,and 500°C WSB and 800°C CRB（agricultural cleaning residue biochar）were used.All plants were subjected to three irrigation treatments,i.e.,FI（every 2-3 days irrigated to 95%WHC）,DI（watering with 70%irrigation amount used in FI）and PRD（irrigation amount same as DI in one side,another side drying to about 13%,then shifting irrigation）,mainly to investigate the effect of high temperature biochar and PRD on water use efficiency of intercropping systems.ExperimentⅢ,after the faba bean-ryegrass intercropping experiment was finished and the faba bean plants were harvested,all pots with ryegrass roots were then relocated to the greenhouse for a study of the residual effects of biochar（WSB and CRB）on the ryegrass growth under drying/wetting cycles（DWC,soil progressively dried to ca.10%,re-wetting to WHC and then dried;FI as the control）.The main results of this study are as follows:（1）Revealed the effects of biochar addition and alternate irrigation on crop yield and water use efficiency.The incorporation of WSB improved the growth performances of tobacco and latter-ryegrass and increased the biomass of them,but reduced irrigation regimes inhibited the crops growth and reduced the biomass of tobacco and latter-ryegrass.In intercropping systems,CRB and reduced irrigation reduced biomass of the intercropped faba bean and intercropping systems,but increased that of intercropped ryegrass.The application of biochar reduced water use efficiency in both tobacco and intercropping systems,but for different reasons,the former due to increased water consumption and the latter due to reduced biomass.As a result of the time-effect,biochar and especially WSB significantly increased the water use efficiency of the latter-ryegrass.Under the same amount of irrigation,PRD maintained crop biomass and increased water use efficiency compared to DI.Therefore,WSB and PRD can synergistically increase biomass and water use efficiency in monocrops,while high temperature biochar has a negative effect on biomass and water use efficiency of the intercropped legumes.（2）Revealed the effect of biochar addition and alternate irrigation on crop gas exchange and plant-water relation.Compared to FI,the reduced irrigation treatments（DI,PRD and DWC）inhibited all crops gas exchange and plant water status;due to the pronounced declines in stomatal conductance and transpiration rate than that of photosynthetic rate,the intrinsic（WUE_i）and instantaneous water use efficiency（WUE_n）were significantly increased.Under the same irrigation amount,PRD plants have lower stomatal conductance and transpiration rate but relatively higher photosynthetic rate and plant water status than DI plants,thus PRD had a higher WUE_i and WUE_n than DI.Biochar improved gas exchange and leaf water status of all crops,CRB and especially WSB significantly increased WUE_i and WUE_n of the latter-ryegrass,but WSB reduced the WUE_iand WUE_n of tobacco and CRB reduced that of intercropped faba bean.（3）Revealed the mechanism of biochar and alternate irrigation-mediated ABA signaling regulating stomata.Compared to FI,the reduced irrigation treatments（DI and PRD）increased stomatal density（SD）and leaf ABA concentration but reduced stomatal size（SS）and plant tissueΔ¹³C of tobacco and faba bean;PRD plants had higher SD and ABA concentration but lower SS andΔ¹³C than DI plants,and these changes were further amplified by WSB.Crop stomatal density（SD）increases linearly with increasing leaf ABA concentration,and stomatal conductance andΔ¹³C both were linearly negatively correlated with SD and ABA concentration,but positively correlated with SS.Thus,the increased ABA concentration under PRD and WSB significantly increased SD and decreased SS andΔ¹³C,co-resulting in the decreased stomatal conductance,thereby increasing short-and long-term WUE.（4）Clarified the responses of crop nutrient status to biochar addition and alternate irrigation.The incorporation of WSB increased carbon（C）and phosphorus（P）but reduced nitrogen（N）uptake in tobacco plants;while significantly increased potassium（K）uptake in leaves（116%）,stems（116%）,roots（55%）,shoot（116%）and whole plant（116%）,but reduced C/K（49%）,N/K（56%）and P/K（48%）in leaves of tobacco,but SWB had little effect.Compared to FI,reduced irrigation treatments（DI and PRD）reduced tobacco C,N,P and K uptake,but PRD relatively maintained a higher plant nutrient status than DI.Similarly,WSB and CRB increased C and N uptake and C/N of the latter-ryegrass,especially under the wetting/drying cycle.In the intercropping systems,biochar WSB and especially CRB increased intercropped ryegrass C and N uptake and C/N but reduced that of intercropped faba bean and intercropping system,and which was amplified by the implementation of DI and especially PRD.Therefore,the application of CRB and PRD is not recommended to improve the nutrient status of legume crops in faba bean-ryegrass intercropping system.（5）Explored the preliminary mechanisms of biochar and alternate irrigation in regulating soil nutrient availability.The application of SWB and especially WSB increased specific root length and root length density but reduced root diameter of tobacco plants However,reduced irrigation regimes limited root growth and inhibited the improvement of crop root morphology by biochar;however,PRD increased root length and specific root length and reduced root diameter in tobacco compared to DI,thereby alleviating the limitation of root morphology under water deficit.In addition,WSB and PRD co-increased the p H of Anthrosol,further optimizing the soil environments for improving soil nutrient availability.The application of WSB significantly increased the soil available phosphorus（40%）and potassium content（173%）in the rhizosphere of tobacco plants and this improvement was amplified by PRD.Simultaneously,biochar and PRD coordinately increased the total carbon and total nitrogen content of rhizosphere soils of tobacco and bulk soils of the latter-ryegrass.Thus,the improved crop root morphological characteristics and the increased soil p H under biochar and alternate irrigation can synergistically improve soil nutrient stocks and availability.In conclusion,wheat straw biochar（WSB）coupled with alternate irrigation（PRD and DWC）can synergistically improve yield and WUE of mono-crops,but high temperature biochar（CRB）has negative impacts on the growth and physiology of intercrops containing legume,such as faba bean,thus should not be recommended.In terms of water-saving mechanism,alternate irrigation coupled with WSB induced increased leaf ABA concentration,and that increased stomatal density and reduced stomatal size co-resulting in a decreased stomatal conductance,which could effectively improve crop WUE.