Phosphorus Bioavailability Mechanism And Cycling Characteristics In Chinese Fir Plantations With Stand Age

The maintenance of productivity is an important guarantee for sustainable management of plantation,and nutrients play a vital role in the maintenance of productivity.As stand age increased,the growth and development characteristics of trees changed,accordingly,the growth rate,nutrient requirement,nutrient uptake,nutrient-use strategy and nutrient cycle characteristic of trees changed,as a result,soil property and nutrient status also changed.Therefore,studying on the dynamic change of the relationship between soil nutrient supply and tree growth with stand age has important scientific significance for sustainable management of plantations.Phosphorus(P)is one of the essential elements for plant growth,which plays an important role in various processes of physiological,biochemical and components formation.However,soil P is often deficiency in subtropical area and soil P availability is very low,which usually can not meet the P requirement of plant growth,it has become one of the limiting nutrients for plant growth.In response to P deficient,plants could improve soil P bioavailability through rhizospheric biological processes,such as root exudation,which promote insoluble and recalcitrant P forms(i.e.exchangeable-P,hydrolysable-P and ligand-P)converting into readily uptake P form(soluble-P).Therefore,revealing the variations in bioavailable P along stand age and the mechanism of P bioavailability in plantations can provide effective nutrient management measures for the maintenance of plantation productivity.Chinese fir(Cunninghamia lanceolata(Lamb.)Hook.,Taxodiaceae)is a fast-growing native tree species that has been planted for timber production.The total area of Chinese fir plantation and their stock volume take the first place in China plantations.Understanding the shifts of the key drive factors of nutrient-use strategies,the mechanisms of soil P bioavailability processes,P allocation and P cycle characteristics in plantations along stand age will help to improve soil P utilization efficiency and maintain sustainable management of Chinese fir plantations.However,the nutrient-use strategy,the mechanism of soil P bioavailability and the ecosystem P cycle characteristics in Chinese fir plantations along stand age remain unclear.Therefore,we selected a chronosequence of second generation Chinese fir plantations with increasing stand ages(3,8-11,16,21,25,29 and 32 years),starting with the investigation of theshifts in nutrient limitation of plantations with stand age,to research the dynamics of nutrient-use strategy,reveal the regulation mechanisms and the drive factors of these dynamics;to research the variations of bioavailable P in rhizosphere and bulk soils with stand age,and explicit the regulation mechanisms of rhizospheric processes of P bioavailability;to research the changes of P stocks allocations and P cycle characteristics with stand age,explore sustainable management practices and measures of P nutrient management of plantations.The main results in this study are as follows.(1)As stand age increasing,fresh leaves P concentrations decreased,fresh leaves N:P ratios increased,and N:P ratios of fresh leaves and leaf litterfall negatively and significantly correlated with P concentrations,respectively,suggesting that P limitation increased with stand age in Chinese fir plantations.Leaf litterfall P concentrations significantly increased with stand age,while P resorption efficiency decreased,indicating that the nutrient-use strategy may shifted from “conservative consumption” to “resource spending” along stand age.This pattern was co-regulated by tree growth rate and soil nutrient status.In young plantations,high tree relative growth rate,high P requirement,and insufficient P supply results from mineral soil P depletion by roots uptake,leading to high P resorption efficiency;whereas with stand aging,tree relative growth rate decreased,P requirement decreased,and sufficient P supply resources from age-related increased in P in organic horizon soil,resulting in low P resorption efficiency.This indicated that if the supply of soil nutrients can not meet the demands of trees growth,the resorption of internal nutrients is an important strategy to ensure trees growth.Thus,the supply–demand relationship reflect the nutrient cycling status and could be adopted for sustainable nutrient management practices in tree plantations.(2)Exchangeable-P and ligand-P in mineral soil decreased with stand age owing to increasing P uptake by trees.The soluble-P,exchangeable-P,and ligand-P pools increased with stand age in the O horizon because litter input and decomposition increased.The exchangeable-P and ligand-P pools increased with stand age in rhizosphere because of root exudates(such as citric acid concentration)and soil p H declined,which can promote the precipitation of the orthophosphate ions in rhizosphere by combining with Fe and Al hydroxides,revealing that P bioavailability increased.The positive correlation between hydrolysis-P and acid phosphatase activity showed that mineralization of hydrolysis-P to inorganic P forms(soluble-P,exchangeable-P,and ligand-P)always occurred at all standages,which drove biological P recycling between soil,microorganisms and vegetation.The progressive incorporation of P in the biological with increasing stand age,the depleted soil P gradually return to soil via litter decomposition,leading to soil P stock recovered,indicating that Chinese fir plantations shifted from P acquiring to P recycling systems with stand age.The time of soil P stock re-increased to the original level of first generation old Chinese fir plantations was over 34 years.Extending the rotation period would be a management strategy to maintain soil P pools for supporting the long-term productivity in Chinese fir plantations.(3)The P stocks of each organ(i.e.stem,branch,leaf,and root)in Chinese fir plantations increased with stand age,and stem P stocks accounted for less than 1/4 of total P stock,which showed that the wood harvesting was not the main component of P losses of Chinese fir plantation.This indicated that if remained the harvested residues in the site,more than 3/4 of the P stocks depleted due to uptake by tree roots can be returned,and then avoid P losing.In the 3-year-old plantation,the biomass of understory vegetation is close to that of Chinese fir,which seriously affects Chinese fir seedlings growth.The results show that the removal of understory vegetation in the first three years after Chinese fir plantation establishment can reduce competition,and then benefit to the rapid growth of Chinese fir seedlings.The removed understory vegetation should be kept in the site to avoid P loss.With stand age increasing,the P stocks of forest floor litter layer initially increased and then decreased,suggesting that proper extending the rotation period would help to return more P to soil through litterfall,and supply P for the next generation or multi-generation.The P stocks in mineral soil initially decresed and then increased with stand age,and the changes did not signicant in 30-60 cm mineral soil,revealing that soil P stocks in upper layer significantly affected by trees growth,but not in deeper layer.In the practical management of plantations,the change patterns of soil P stocks are helpful to guide the formulation of plantation nutrient management measures(such as fertilization).Thus,clarifying stand biomass and P stocks allocation characteristics in Chinese fir plantations with stand age,which will great significance for the management of Chinese fir plantation at different growth stages.(4)With stand age increasing,the annual P uptake,annual P retention,annual P return and annual P resorption initially increased and then keep stable,while annual P return by fine-root turnover did not change.This indicated that P biological cyclegradually enhanced,and trees growth shifted from depend on soil P supply to depend on P resorption and P recycle.Due to P stocks of vegetation(trees and understory vegetation)is far below than soil P stocks,to a certain extent,extending the rotation period may recover soil P stocks.The annual P output of surface runoff initially increased and then stabilized with stand age,which closely correlated with undergrowth vegetation coverage decreased,P concentration in organic horizon soil increased,and P solubility increased.Maintaining certain coverage of understory vegetation may be one of the important ways to reduce the P output of surface runoff,so as to ensure the sustainable management of Chinese fir plantation.A framework was concluded through comprehensive analyses of nutrient limitation,nutrient-use strategy,P bioavailability mechanism and P cycle characteristics in Chinese fir plantations at different stand ages.In young plantations,high tree relative growth rate,high P demand,rapid growth of biomass,rapid increased in tree P stocks,decreased in P stocks in mineral soil because depleted by roots uptake,and very low P return by litterfall resulted in decreased of soil P stocks.Therefore,in order to meet the P requirement,trees increased P resorption efficiency to increase the ultilization of internal P,promoted root exudates releasing to increase P bioavailability for P uptake by roots,and increased fine-root biomass to increase P uptake.At this growth stage,plantation belongs to P-acquiring ecosystem with an open P cycle.With stand age increasing,tree relative growth rate decreased,P demand decreased,soil P depletion decreased,P return via litterfall increased which become the major P source,meanwhile,P resorption efficiency decreased resulting in P return increased,root exudates release changed to decrease P bioavailability leading to P accumulation,and fine-root biomass decreased resulting in soil P depletion decreased,so soil P stocks increased and recovering.This indicated that Chinese fir plantation gradually shifted from P-acquiring ecosystem to P-recycling ecosystems,and P cycle gradually tends to close along stand age.