Alfalfa Forage Yield,Soil Water And Nutrient Availability In Response To Plastic Film Mulch With Ridge And Furrow And Phosphorus Fertilization In The Semi-arid Region Of Loess Plateau

Alfalfa（Medicago sativa L.）is an important forage legume in farming and animal husbandary systems in the semi-arid region of Loess Plateau.However,its forage yields are usually very low due to water shortages and low soil phosphorus（P）availability.Improving soil water condition and P availability are critical to increase alfalfa forage yield.Based on this problem,a six-year field experiment was conducted at Zhonglianchuan（36°02′N,104°25′E,above sea level 2400 m）,Yuzhong County,Gansu Province,China from 2011 to 2016 to explore the effects of plastic film mulch and P fertilization on alfalfa forage yield and its components,soil water and P availability,combined with shoot nitrogen（N）and P concentration of alfalfa,soil microbial biomass and soil organic carbon（C）mineralization（laboratory incubation）,in order to interpret the reason for forage yield change.Twenty-four plots were arranged in randomized blocks of split-plot design in the present study,with two film mulching regimes as main plots:no plastic film mulch with flat planting（NM）and plastic film mulch with ridge and furrow（M）,and four P application levels as split-plots:0（P0）,9.73（P1）,19.3（P2）and 28.9 kg P ha^-1（P3）,respectively.Each treatment was replicated three times.Alfalfa forage yield significantly increased with plastic film mulch.Forage yield improvement was related more to plastic film mulch than P fertilization,resulted in higher forage yield for the no-P fertilizer in plastic film mulch（MP0）than for the high P level in no plastic film mulch（NMP3）.Alfalfa forage yield increased with increasing P levels in no plastic film mulch.Forage yield under plastic film mulch was higher in low P level（MP1）than in no-P fertilizer（MP0）,but decreased gradually in high P levels（MP2 and MP3）compared with low P level（MP1）,and reached the maximum at 16.1 kg P ha^-1.Alfalfa forage yield was related more to plant height and plant mass than plant density,shoots per plant,shoots per square meter and shoot mass.Soil water content in the upper 1 m of soil profile in plastic film mulch was 20.4%higher than in no plastic film mulch in April and October from 2012 to 2016,but in the 1-5 m of soil profile was lower than in no plastic film mulch,especially in no-P fertilizer and low P level（P0and P1）.Soil water content in the 1-5 m of soil profile decreased with increasing P levels in no plastic film mulch.Soil water content in the 1-5 m of soil profile under plastic film mulch was lower in low P level（MP1）than in no-P fertilizer（MP0）,but increased gradually in high P levels（MP2 and MP3）compared with low P level（MP1）.Plastic film mulch and P fertilization both helped alfalfa absorb water from the deep soil（1-5 m）,but soil water depletion from the deep soil in high P levels under plastic film mulch decreased,resulting from the lower forage yield.Shoot P concentration of alfalfa in October from 2012 to 2016 was lower than in July,but shoot N concentration and shoot N/P ratio（N concentration/P concentration）were higher than in July,indicating that soil P availability in October was lower than in July,and the relative soil P availability was lower than N in October.Plastic film mulch had no significant effect on shoot N concentration,P concentration and N/P ratio.Shoot N concentration had no significant difference among different P levels,and shoot P concentration increased with increasing P levels,resulted in decreased shoot N/P ratio with increasing P levels,suggesting that soil N availability decreased with increasing P levels.Soil pH in plastic film mulch was lower than in no plastic film mulch,and decreased with increasing P levels.The effects of plastic film mulch and P fertilization on soil pH are as follow,pH=–0.37 M-0.46 P（R²=0.35,P<0.001）,indicating that decreased soil pH was related more to P fertilization than plastic film mulch.Soil pH was significantly negatively correlated with soil available P content（R²=0.17,P<0.001）,suggesting that decreased soil pH was beneficial for improving soil P availability.Therefore,soil P availability increased with increasing P levels.Higher forage yield in plastic film mulch than in no plastic film mulch helped alfalfa absorb more P from soil,and higher soil C/P ratio（soil organic C/available P）than in no plastic film mulch indicated soil microbes competed available P with alfalfa,resulting in lower soil available P content in plastic film mulch than in no plastic film mulch.Alfalfa did not reach the 90%of maximum forage yield in no plastic film mulch,and the critical soil P requirement for 90%of maximum forage yield in plastic film mulch was 3.73 mg kg^-1.Alfalfa P uptake efficiency was decrased by 9.92%from the 90%of maximum forage yield to maximum forage yield.Therefore,high alfalfa forage yield was obtained in plastic film mulch when soil available P content was 3.73mg kg^-1,under which P uptake efficiency was also high.Soil total nitrogen（N）and inorganic-N decreased gradually in P2 and P3 of mulching compared with P1 level.Soil microbial biomass C,microbial biomass C/N ratio and organic C mineralization in plastic film mulch were higher than in no plastic film mulch.Soil microbial biomass C,microbial biomass C/N ratio and organic C mineralization increased with increasing P levels,but decreased at high P levels（MP2 and MP3）compared with low P level（MP1）in plastic film mulch,indicating that soil microbial activities and organic C mineralization were restrained at high P levels in plastic film mulch.Decreased soil microbial activities and organic C mineralization affected soil nutrient supplying capacity,along with decreased shoot N/P ratio and increased soil microbial biomass N,suggesting that shoot N uptake and alfalfa forage yield in high P levels of the mulching field were limited by low soil N availability.In a word,improving soil P availability and soil water condition in the upper soil layer,and depletion of deep soil water are the mechanisms for promoting alfalfa forage yield when using plastic film mulch and P fertilization.High alfalfa forage yield and P uptake efficiency were obtained when soil available P content was 3.73 mg kg^-1 under plastic film mulch.Soil N availability limitation resulted in the decreased forage yield in high P levels of the mulching field,which might need to apply N fertilizer or explore other reasons to boost forage yield further.