Study On Leaf Trait Responses And Mechanisms Of Phosphatases Regulating Leaf P Resorption Of Lucerne Under P And K Fertilization With Water Supply

Phosphorus(P)is one of the important elements regulating the productivity of agricultural system.However,there are always problems such as inefficient utilization by plants and insufficient global P resource.With the global precipitation pattern greatly changing and nitrogen deposition intensifying,the supply and utilization of soil P also change accordingly in the ecosystem,and P restriction has appeared or intensified in various ecosystems.Potassium(K)is indispensable for maintaining plant function and adapting to adverse environment.Compared with P,soil K resource is relatively abundant,while with the change in the availability of other nutrients in ecosystems,K availability is also greatly affected.Leaf trait is one of the survival strategies and resource utilization abilities of a plant under specific environment.Many studies have shown that leaf traits are seriously affected by genetic,spatial and temporal factors,therefore it is necessary to explore how leaf trait changes in diverse environments to further reveal its role in plant adaptation.Leaf nutrient resorption is one of the leaf traits on efficient nutrient utilization in plants.Studies have shown that the resorption varies among genotypes,life forms and other genetic characteristics,and is regulated by nutrient status in soils and leaves,light,temperature and many other abiotic factors.However,it is still not sure how leaf nutrient resorption changes under soil nutrient limitation.Past studies have shown that the phosphatase was closely related to P retransfer in the plant;it is still unclear how the phosphatase regulates the resorption.In the background of global climate change,the precipitation and availabilities of soil P and K on the Loess Plateau of China have also changed accordingly,which will heavily influence the adaptation of key forages and the productivity of cultivated grasslands.In this study,lucerne(Medicago sativa L.)was used to reveal the changes of soil nutrient content and relative nutrient limitation and to explore the changes of leaf trait,nutrient resorption,soil and leaf phosphatase activity at different ages/cuts of lucerne under P and K fertilization with water supply.The relationships of soil nutrient content with leaf trait,and relative soil nutrient limitaion with leaf nutrient resoprtion,and phosphatase activity in the soil and leaf with leaf P resorption efficiency(PRE)were analyzed.The main results were as follows:1)Water supply significantly affected the modulation of fertilization on soil nutritional status of lucerne grassland.Under low water supply,K fertilization aggravated P limitation of lucerne grassland,while P fertilization,P+K co-fertilization alleviated P limitation.Under high water supply,P+K co-fertilization increased soil ammonium nitrogen(AN),nitrate nitrogen(NN),total potassium(TK)and available potassium(AK)contents,and decreased soil total nitrogen(TN)and available phosphorus(AP)contents in 2 and 3 year lucerne;P fertilization increased soil total phosphorus(TP)content in 2 and 3 year lucerne,and decreased soil TP content in the 3rd cut lucerne.The K or P fertilization alleviated P limitation of lucerne grassland.In addition,age and cut of lucerne had significant effects on soil nutrients.With increasing age of lucerne,soil TN,AN,TP,TK and AP contents showed a decreasing trend.The 1 year lucerne grassland was mainly N limited,while the 2 and 3 year grasslands were P limited.With increasing cut of lucerne,soil TN,TP and TK contents increased,while soil NN,AK and AP contents gradually declined.Compared with lucerne grassland of the 1st cut,P limitation aggravated in the 2nd and 3rd cut.2)The effect of fertilization on leaf trait of lucerne was regulated by water supply.Under low water supply,fertilization(P,K,P+K co-fertilization)decreased leaf PRE;P or K fertilization increased leaf length/width ratio,senesced leaf N and P concentrations of 1 and 2 year lucerne,and decreased leaf net photosynthetic rate,carboxylation rate and instantaneous water use efficiency.Under high water supply,fertilization increased leaf PRE,especially P+K co-fertilization;P fertilization promoted leaf net photosynthetic rate,carboxylation rate and instantaneous water use efficiency,and decreased leaf length/width ratio.With increasing water supply,K fertilization increased leaf area,decreased leaf dry matter content and leaf mass per unit area;while P+K co-fertilization firstly increased and then decreased green leaf N:K and increased leaf PRE.In addition,there were significant differences in leaf traits of lucerne at different ages and cuts.With increasing age of lucerne,leaf thickness,leaf mass per unit area,net photosynthetic rate,carboxylation rate,green leaf N:P,N:K and K:P,PRE and leaf potassium resorption efficiency(KRE)increased,and leaf nitrogen resorption efficiency(NRE)firstly increased and then decreased;the leaf length/width ratio,leaf area,green/senesced leaf N,P and K concentrations decreased.With increasing cut of lucerne,the leaf thickness,leaf mass per unit area,net photosynthetic rate,carboxylation rate,green leaf N:P and K:P,senesced leaf N and K concentrations increased,and the leaf length/width ratio,leaf area,green leaf N:K,NRE and KRE decreased.3)There were close relationships among leaf traits.The leaf mass per unit area of lucerne was significantly positively correlated with green leaf N and P concentrations,NRE and PRE,while it was significantly negatively correlated with net photosynthetic rate and leaf thickness,and the leaf thickness was significantly negatively correlated with green leaf N and P concentrations,instantaneous water use efficiency and NRE.Leaf functional traits were significantly positively correlated with green leaf N:P and N:K,NRE,PRE and KRE,and significantly negatively correlated with green/senesced P and K concentrations.There was a significant positive correlation among nutrient concentration or between green and senesced leaves.Most corrections among N,P and K stoichiometric ratios were also significantly positive.The relationships among leaf traits were regulated by fertilization and water supply,and changed with lucerne growth.The variations in water supply,fertilization,lucerne age and cut mainly resulted in changes of the significance degree of correlation among leaf traits,while had little effect on the correlation property(positive/negative).Compared with age and cut of lucerne,water supply and fertilization affected the correlation more significantly.4)Leaf trait of lucerne was related to soil nutritional status,and leaf nutrient resorption changed with the shift of relative soil nutrient limitation.Lucerne leaf thickness was significantly positively correlated with soil TP and TK;green leaf N concentration was significantly positively correlated with soil TN,AN and AP;senesced leaf N,green/senesced P and K concentrations were significantly positively correlated with soil nutrient contents.There were significant negative correlations of green leaf N:P,carboxylation rate,instantaneous water use efficiency,stomatal limitation,NRE and PRE with soil TN,AN,TP,AP and TK,of leaf length/width ratio and green leaf N:K with soil TN,TP,AP,TK and AK,of leaf mass per unit area with soil TN,TP,TK and AK,of green leaf K:P with soil AN,of net photosynthetic rate with soil AP and of KRE with soil NN and AP.Leaf NRE of lucerne increased with green leaf N:P increase when grassland soil was in N limitation and P or P+N co-limitation.Leaf NRE and PRE firstly increased and then decreased with green leaf N:K increase when grassland soil was in N limitation and K or K+N co-limitation.Moreover,leaf NRE was greater than PRE in soil N limitation and K or K+N co-limitation.Leaf NRE and PRE firstly decreased and then increased with green leaf K:P increase when grassland soil was in P or P+N co-limitation.And when 4.87 < green leaf K:P or green leaf K:P > 13.77,PRE > NRE;when 4.87 < green leaf K:P < 13.77,NRE > PRE.5)Water supply affected the regulation of fertilization on phosphatase activity in soils and lucerne.With increasing water supply,soil phosphatase activity and acid phosphatase activities in green and senesced leaves of lucerne generally tended to decrease.Under low and normal water supply,fertilization decreased soil acid phosphatase activity.Under high water supply,K fertilization promoted acid phosphatase activity in the soil,and P fertilization promoted alkaline phosphatase activity in the soil.In addition,soil acid phosphatase activity and green and senesced leaf acid phosphatase activity of lucerne under 3 year grassland were higher than those under 2 year grassland.The activity of soil phosphatase was significantly positively correlated with soil AP and TP.There was no significant correlation between soil phosphatase activity with green/senesced leaf P concentration,green/senesced leaf N:P and K:P,leaf PRE of lucerne.Plant acid phosphatase activity significantly differed between green and senesced leaf,and the acid phosphatase activities in green and senesced leaves of lucerne were significantly positively correlated with soil TP and green leaf K:P,and significantly negatively correlated with green leaf P concentration,senesced leaf K:P and leaf PRE of lucerne.Compared with the acid phosphatase activities in green leaf,the acid phosphatase activities in senesced leaf had a stronger correlation with other parameters.6)Water supply and fertilization interactively affected lucerne biomass and stem/leaf ratio.Under low water supply,P or K fertilization increased the biomass of 2 and 3 year lucerne and the stem/leaf ratio of 1 and 2 year lucerne.Under high water supply,fertilization increased the biomass of 1 and 3 year lucerne,and decreased the stem/leaf ratio.Leaf biomass was significantly positively correlated with green/senesced leaf K concentrations,green leaf K:P,senesced leaf N and P concentrations,and negatively correlated with leaf length/width ratio,leaf mass per unit area,green leaf N:K,leaf NRE and PRE.The biomass of lucerne was significantly positively correlated with the stem/leaf ratio,leaf thickness,net photosynthetic rate,carboxylation rate,green leaf K:P,leaf KRE,acid phosphatase activities in green/senesced leaf,and negatively correlated with leaf length/width ratio,leaf area,green/senesced leaf N and P concentrations and green leaf N:K.Conclusively,this study showed that water supply and fertilization significantly affected soil nutritional status,leaf trait and leaf nutrient resorption,which showed temporal variability.The effect of fertilization on leaf PRE was regulated by water supply.Fertilization increased leaf PRE at high water supply,while decreased the PRE at low water supply.In nutrient–constrained environments,lucerne may have developed strategies mainly through nutrient resorption but not minimizing leaf nutrient loss to construct leaf structure and nutrient balance.And relative soil nutrient limitation in lucerne grassland had obvious regulation on leaf nutrient resorption.Compared with that in soil N limiation,leaf NRE was higher when the soil was in P or P+N co-limiation.When the soil was in N limitation and K or K+N co-limiation,leaf NRE was greater than PRE.In addition,soil phosphatase activity had no significant effect on leaf PRE of lucerne,while leaf acid phosphatase reduced leaf PRE,and the acid phosphatase activities in senesced leaf had more significant effect than that in green leaf.In the management of lucerne grassland,low water supply + P fertilization(including P+K co-fertilization),or high water supply + K fertilization could effectively alleviate P limitation of lucerne grassland.In the production of lucerne on the Loess Plateau of eastern Gansu province,China,rational fertilization would effectively promote leaf proportion in the biomass and improve forage quality,and could increase biomass yield if fertilization is performed in consideration of natural rainfall.