Effect Of Partial Root Drying Irrigation On The Stability Of Alfalfa Forage Yield And The Availability Of Nitrogen And Phosphorus

Water shortage is a key limiting factor of a holistic approach to conservation of mountains,rivers,forests,farmlands,lakes and grasslands in the arid and semi-arid areas.Therefore,application of water saving technique is necessary to improve the high-efficiency production of alfalfa pasture as less water as possible,which is an important way to improve the high-quality development of prataculture,and is a key measurement to solve the contradiction between ecological water use and productive water use.Partial root drying irrigation(PRDI),a water saving irrigation technique,has been widely used in many crops throughout the world due to its low input,easy promotion and higher water efficiency.Alfalfa is a main crop that is used for establishing pasture in northern China,and its pasture areas is increasing,accompanying by the adjustment of planting structure and construction of eco-environment,and three major alfalfa planting regions are formed in Hexi Corridor,Hetao Irrigation Area and Horqin Grassland,respectively.Shortage of water resources in the arid and semi-arid areas enable alfalfa pasture production to be saved water resoureces in northern china.Previous studies have used the data from one growing season with three harvest times and two growing seasons with one harvest time to verify that PRDI can be used for alfalfa production.However,alfalfa pasture often can harvest from 5 growing seasons to 10 growing seasons with many harvest times in a growing season,it,therefore,is necessary to determine the effect of PRDI on alfalfa yield stability.PRDI often induces soil to experience dry and wetting cycling,which usually alters soil biopsychical processes,and further influences elements uptake by alfalfa plant and soil nutrients availability.In northern china,soil nitrogen(N)and phosphorus(P)is lackage but potassium is rich,thus,it is an important issue to understand the effect of partial root drying irrigation on soil N and P availability,which may contribute to alfalfa yield stability.This study involved two factors(irrigation mode and irrigation volume)in a split-plot design,in which irrigation modes were considered as main plot and irrigation volumes were considered as subplot.Irrigation modes consist of PRDI and conventional furrow irrigation,and irrigation volumes were set based on alfalfa water requirement(ETc):0.70 ETc,0.85 ETc,1.00 ETc and 1.15ETc.A field experiment was conducted from 2017 to 2019 to investigate the effect of PRDI on alfalfa biomass allocation,root characteristics,N and P allocation and its stoichiometric ratio,and soil N and P availability,and these results can disclose the ways of PRDI maintaining alfalfa yield stability and water saving economics,find the optimal combination between irrigation modes and irrigation volumes.The findings of this study can present a pattern of how PRDI influences on perennial forages,and provide the scientific information for optimizing water-saving,N and P management for alfalfa pasture.Mainly conclusions of this study were following:1.PRDI can maintain alfalfa yield stability and improve water saving benefit by comparing the alfalfa yield at each harvesting,whole growth season and water saving benefit between PRDI and CFI.PRDI had no effect on alfalfa yield at each harvesting,but significantly increased water use efficiency by 71.41%,PRDI significantly increased irrigation water productivity,total water using productivity and economic water productivity by 117%,87.56%and 87.33%with the average of three-year data.PRDI significantly decreased total input by 16.92%with the average of three-year data,but increased total output,gross output and net benefit ratio by 8.73%,17.40%and 6.76%with the average of three-year data.These results indicated PRDI maintained alfalfa yield stability and improved water productivity and economic benefits,further verified that PRDI was a potential water saving irrigation technique that can be generalized to farmers'alfalfa production.2.PRDI increased the stem and root biomass but decreased leaf biomass by comparing the leaf,stem and root biomass between PRDI and CFI.PRDI increased alfalfa stem biomass at each harvesting times and whole growing season,and root biomass at whole growing season,whereas it decreased leaf biomass at each harvesting times and whole growing season.PRDI encouraged alfalfa plants to allocate more biomass to root in the view of whole plants,and PRDI can allocate more biomass to stem in the view of shoots,suggesting that PRDI maintained alfalfa forage yield by trading off leaf,stem and root biomass rather than had no effect on leaf,stem and root biomass.3.PRDI was beneficial to the growth of root crown and root system,and the physiological activities of root system.PRDI increased alfalfa crown diameter,crown buds,taproot diameter,lateral length,lateral number,total root length and nodule numbers,and also enhanced root activity and increased root activity absorbing areas,suggesting that PRDI promoted the compensatory growth of root system,which ensures the ability of alfalfa plant uptake water and nutrients from soil,and contributes to the stability of alfalfa forage yield and high-water productivity.4.PRDI increased alfalfa leaf,stem and root N and P uptake,and encouraged alfalfa plant allocate more N to leaf and more P to roots.PRDI increased the N concentration in alfalfa leaf and stem at each harvesting times,and N concentration in alfalfa root at whole growing season;PRDI also increased N and P uptake in alfalfa leaf and stem at each harvesting times,and N and P uptake in alfalfa root at whole growing season.PRDI encouraged alfalfa plant to allocate more N to be deposited in alfalfa leaf,and more P to be deposited in alfalfa root.These results suggested that PRDI changed N and P uptake and allocation of different organs in alfalfa plant,which is one possible approach that PRDI maintained alfalfa yield stability and improve water productivity.5.PRDI increased N:P stoichiometric ratio in leaf,stem and root,and coefficient variation and path coefficient of N:P stoichiometric ratio was higher in root than stem and leaves.PRDI increased the N:P stoichiometric ratio in alfalfa leaf,stem and roots.During 2017 to2019,PRDI increased the N:P stoichiometric ratio in alfalfa leaf from 14.54 to 16.24,14.21 to 16.20,and 14.67 to 16.32,these results indicated that PRDI encouraged alfalfa growth from N and P colimitation to P limitation.In addition,variation coefficient and path coefficient of N:P stoichiometric ratio in root were higher than those in stem and leaves,indicating that N:P stoichiometric ratio in root is better than leaf and stem to identify the N and/or P limitation for alfalfa growth under PRDI.6.PRDI increased the ratio of NO₃^--N to NH₄⁺-N and inorganic N stock,whereas decreased the soil total N stock.PRDI increased the ratio of NO₃^--N to NH₄⁺-N and inorganic stock,whereas PRDI decreased soil total N stock,indicating that PRDI could increase soil N availability,but decrease the potential soil N availability.These results indicated that PRDI had different impacts on the soil N availability to alfalfa when soil N availability was estimated by the NO₃^--N/NH₄⁺-N ratio,inorganic N stock and total N stock,respectively,which can provide accurate information for soil N management in alfalfa pasture when PRDI is applied.7.PRDI decreased the available P concentration and stock,but had no effect on soil total P concentration and stock.The variable coefficient was similar between soil P concentration and soil P stock,demonstrating that soil P concentration or stock both can be used to estimate soil P availability.PRDI decreased the soil available P concentration and stock,but had no effect on soil total P concentration and stock,suggesting that PRDI decreased the soil P availability but had no effect on potential soil P availability.Although structural equation model showed that soil available P played important role in maintaining alfalfa forage yield stability,more attention still should be paid for potential soil P availability.8.This study found that there were different optimal combinations between irrigation modes and irrigation volume for given goals.Based on the regression model with Matlab 2017a and further found optimal value with the fminsearch function for the model,there were different optimal combinations between irrigation modes and irrigation volumes for a given goals.The optimal combination for high yield were PRDI and 1.06ETc,and the optimal combinations for high water use efficiency,water productivity,total water use productivity and economic benefit were PRDI+0.70ETc,PRDI+0.70ETc,PRDI+0.86ETc and PRDI+0.87ETc,respectively.The optimal combinations for high N concentration and uptake in roots and P uptake was PRDI+0.98ETc,PRDI+0.99ETc and PRDI+0.98ETc,respectively.The optimal combinations for soil NO₃^--N concentration and inorganic N stock were PRDI+1.08ETc and PRDI+0.98ETc.