Effects Of N And P Fertilizations On Leaf Nutrient Resorption Of Alfalfa At Different Growth Stages In The Loess Plateau

There are serious limitations on crop growth and production by loose soil texture,severe soil erosion and low soil fertility in the Loess Plateau of China.Fertilization is one of the main measures to improve crop productivity,but excessive fertilization would cause soil nutrient imbalance and environmental problems,further affecting soil quality and long-term sustainable production.Therefore,in recent years,more and more attention has been paid to the reduction and optimization of fertilization.The fertilization changes not only the nutrient cycling and nutrient limitation of the whole ecosystem,but also inevitably causes the change in internal nutrient cycling of the plant.Nutrient resorption is the process of nutrient re-transportation from senesced leaves to other tissues.It is an important part of nutrient cycling in plants and one of the key mechanisms to preserve nutrients.Past studies indicated that the effect of fertilization on plant nutrient resorption was regulated by plant genetic charactertistics and environmental factors.However,there are few studies on the effects of dynamic changes in soil nutrients on nutrient resorption and plant adaptability after the combined N and P application and one-time P application.Notably there is still lack of detailed exploration on the differential responses of perennial forage crop at different growth stages when nutrient demand and utilization vary a lot.The experiments were carried out at two experimental sites,Dingxi and Qingyang,Gansu Province in the Loess Plateau of China.The changes in nitrogen(N),phosphorus(P)and potassium(K)resorption efficiency(NRE,PRE,KRE)from senesced leaves of alfalfa(Medicago sativa L.)at different ages,cuts and growth stages-in-a-cut were studied under N and P fertilizer applications.The main objectives were to analyze the coupling relationship between leaf and soil nutrient content and leaf nutrient resorption,and to explore the changes in aboveground biomass and quality of forage and their relationship with nutrient resorption.The following are mainly results obtained:1)With the increase of alfalfa age(1-3 year),leaf NRE and PRE gradually increased at both experimental sites,while KRE changed inconsistantly at different sites.The effect of fertilization on NRE and PRE weakened with age,while had inconsisitant effects on KRE.At Dingxi site,the fertilization had no effect on NRE.Fertilization increased PRE at age 1,while N or N×P(combined N and P fertilizers)fertilization decreased PRE at age 2.The fertilization increased KRE at age 1,while had inconsistent influences on KRE at age 2 and 3.At Qingyang site,N fertilization had no effect on NRE,while P or N×P fertilization increased NRE at age 1 and 2.The fertilization had inconsisitent effects on PRE at age 1,while decreased PRE at age 2.The fertilization increased KRE at age 1,while had inconsistent effects on KRE at age 2 and 3.Topdressing P fertilization,NRE and PRE of 4 year alfalfa at Qingyang increased,while barely changed at Dingxi.Leaf nutrient resorptions had significant correlations with soil nitrate nitrogen(NN).PRE was correlated with soil available phosphorus(AP)and available potassium(AK).These correlations varied with the age of alfalfa.The effect of fertilization on leaf nutrient resorption at Qingyang was stronger than that at Dingxi,and leaf nutrient resorption was more correlated with soil nutrients at Qingyang.2)Leaf NRE,PRE and KRE of alfalfa at the first cut were higher than those at the second cut.Fertilization variously affected nutrient resorptions in a cut-specific way.At Dingxi site,the fertilization had no effec on NRE.P fertilization had no effect on PRE,while N or N×P fertilization reduced PRE at the first cut and increased PRE at the second cut.KRE at both cuts tended to decrease after fertilization.At Qingyang site,the fertilization had no effect on NRE and KRE at the first cut,but increased NRE and KRE at the second cut.The fertilization decreased PRE at the first cut,while N or N×P fertilization increased PRE at the second cut.Leaf nutrient resorptions had significant correlations with soil NN,NRE and PRE were also correlated with soil ammonium nitrogen(AN)and AP respectively,which varied a lot with experimental site.3)Leaf NRE,PRE and KRE of alfalfa at the budding stage were significantly higher than those at the early flowering stage.Fertilization improved NRE and PRE at the budding stage and PRE at the early flowering stage,and reduced KRE at both stages.Leaf nutrient resorptions had strong correlations with soil AN and NN,which varied a lot with growth stage in a cut of alfalfa.4)With the increase of alfalfa age(1-3 year),the aboveground biomass(hereinafter referred to as biomass)increased,and the forage quality firstly reduced and then increased.Compared to the first cut,the biomass significantly reduced and the quality increased at the second cut.As the alfalfa grew older in one cut,the biomass increased and the quality reduced.The effect of fertilization on the biomass and forage quality was weakened with the age,cut and growth stage in a cut of alfalfa.With the growth of alfalfa,there were some close relationships of leaf nutrient resorptions with the biomass,which tended to be enhanced with the prolonged growth.At Dingxi site,PRE had negative correlations with the biomass at age 2 and 3,while KRE was negatively correlated with the biomass only at age 2.PRE and KRE had negative correlations with the biomass at the first cut,and PRE was positively correlated with the biomass at the second cut.At Qingyang site,NRE was positively correlated with the biomass at age 1,while PRE was negatively correlated with the biomass at age 2,and positively correlated at age 3;KRE was negatively correlated with the biomass only at age 3.PRE was negatively correlated with the biomass at the first cut,and positively correlated at the second cut.In addition,PRE was positively correlated with the biomass only at the early flowering stage,while KRE had negative correlations with the biomass both at the budding and early flowering stages.Generally,with the prolonged growth of alfalfa,the fertilization had a weakened effect on leaf nutrient resorption of alfalfa.There were few effects of P fertilization on the resorption at the third year after one-time P application.Soil NN played a key role in influencing leaf nutrient resorption.With alfalfa gowth(at age,cut and stage-in-acut scales),the increase of biomass was more dependent on PRE than other nutrient resorptions,so P supplement would increase leaf nutrient resorption efficiency of alfalfa,which further confirmed that the growth of alfalfa was more restricted by soil P availability in the Loess Pleteau of China.According to soil fertility status,applications of N and P fertilizers at the beginning of planting alfalfa could be carried out to adjust nutrient resorption capacity and to enhance the adaptation of alfalfa.In the production of alfalfa,we also recommended that N fertilization combined with one-time P fertilization could be practical and topdressing P fertilizer at the third or fourth year should be applied to improve nutrient utilization efficiency and grassland production.