Responses Of Nutrient Resorption Efficiency To Nitrogen And Phosphorus Addition And Stoichiometric Homeostasis Characteristics Of Herbaceous Plants

Nitrogen and phosphorus are the major limiting nutrients for plant growth on terrestrial plant ecosystoms.The resorption of nutrients from senescing leaves(organs)is a key component of the nutrient conservation strategy of plants.Stoichiometric homeostasis is the ability that plants maintain a relatively stable composition of elements regardless of changes in nutrient availability,via various physiological mechanisms.For a long time,reports about the relationship of resorption efficiency and plant nutrient availability have been inconsistent.Meanwhile,there are few studies on plant nutrient stoichiometric homeostasis.Field and greenhouse fertilization experiments could provide direct evidence for the relationship between nutrient resorption and nutrient availability.Studies on responses of nutrient resorption efficiency to nitrogen and phosphorus addition and stoichiometric homeostasis characteristics of herbaceous plants could help to understand nitrogen and phosphorus cycling,conservation and utilization strategies better.The studies could also improve the understanding of plant nutrition maintenance mechanisms during their growth,and help to optimize fertilization management of agricultural crops and natural vegetation.Fertilization experiments of herbaceous community and single species Amaranthus mangostanus were conducted in Inner Mongolia and greenhouse at China Agricultural University respectively.The main results were as follows:1.Plant N concentrations of aboveground part was positively correlated with increasing N supply in flourishing stage(Ngr),while that was independent in senescent stage(Nsen).P concentrations of aboveground in flourishing stage(Pgr)and senescent stage(Psen)were positively correlated with P supply.The relationship between N resorption efficiency(NRE)and Nsen,P resorption efficiency(PRE)and Psen were negatively correlated,respectively.Relative nutrient resorption efficiency(RR,RR=NRE-PRE)was negatively correlated with N:PSsen.The critical N and P co-limited threshold of N:Psen for the grassland community was 3.9?6.2 according to relative nutrient resorption efficiency hypothesis(RRH).The critical N and P co-limited thresholds for different species were quite different,which were 4.7?7.0,5.6?7.4,7.4?9.6,5.5?7.9 for Carex korshinskyi,Cleistogenes squarrosa,Poa sphondylodes,Artemisia desertorum,respectively.The nutrient resorption efficiency of A.mangostanus was studied.Nutrient resorption responses of plants to environmental nutrient availability were different across growth stages.NRE and PRE decreased with the increase of nutrient relative availability.NRE was higher and PRE was lower at flowering than seed-filling stage.RR was negative correlated with N:Pgr.Optimal/critical N:Pgr varies across growth stages,suggesting that the growth of A.mangostanus may switch from being more N-to P-limited from vegetative to reproductive stage.2.N addition accelerated NRE but P addition had no significant effects on PRE of herbaceous community.N addition lead to the biomass increment of the community which directly increased the nutrient accumulation.The increase of plant N-pool lead to the increase of nutrient resorption efficiency.Using A.mangostanus in two experiments over two successive years,we found that NRE and PRE increased with plant body-size but decreased with root:shoot ratio,suggesting that nutrient conservation could be modulated by the size of nutrient sink-pool and the trade-offs between taking up soil nutrients and remobilizing senesced-leaf nutrients.Nutrient supply could not only decrease the NuRE through direct effect on soil nutrient availability,but also promote the NuRE through indirect effect on plant-size and root:shoot ratio.3.The stoichiometric homeostasis of plants varied with species,growth stages and element types.The stoichiometric homeostasis coefficient(H)of N(HN)was consistently higher than P(HP)at the levels of community,family,and species.HN was higher at seedling stage than at flourishing stage,while Hp was lower at seedling stage than at flourishing stage.The H of N:P(HN:P)increased with plant growth stages.Concentrations,as well as HN and HP,varied among different families.The mean nutrient(N and P)concentration of graminoid was lowest,while N concentarions of compositae and rosaceae were highest,and P concentration of sedges was highest.Graminoid presented the highest P homeostasis and lowest N homeostasis in our research.The Hp in green leaf of A.mangostanus varied significantly,while HN in green-leaf maintained relatively stable across growth stages.Moreover,HN was higher at seedling stage but lower at flowering stage than that of P at corresponding stages.The dynamics of HN and HP suggested different physiological importance of N and P across growth stages.In summary,the nutrient resorption efficiency increased with the increment of plants' nutrient sink.Nutrient supply could not only decrease the NuRE through direct effect on soil nutrient availability,but also promote the NuRE through indirect effect on plant-size and root:shoot ratio.There was a negative correlation between RR and N:Psen.According to the RRH,the critical N and P co-limited thresholds could be calculated.The stoichiometric homeostasis of herbaceous plants varied within species,growth stages and elements.The HN was higher than HP.The HN was higher at seedling stage than flourishing stage,and HP was lower at seedling stage than flourishing stage.The HN:P increased with the growth of plants.Different species had different values of H.