| Pinus massoniana is one of the important timber species in southern China. Thedistribution areas of its natural forests and plantations are forefront of the southern plantationspecies, and it widely used in the pulp and paper, construction, rosin ect. However, forest soilin southern China are acidic red soil, mostly severe scarcity of phosphorus(P) and phosphorushighly heterogeneous distribution. The limited availability of P in forest soils is thus one of themost important factors causing a decline in growth and productivity of Pinus massonianaplantations. The current study had revealed Pinus massoniana biological mechanisms ofadapting to different types of low P stress. Atmospheric nitrogen(N) deposition dramaticallyraised in recent decades, resulting in increases of soil N availability and N/P ratio, whichwould impact Pinus massoniana P efficiencyand biological mechanisms of adapting to low Pstress. Taking different excellent families of Pinus massoniana as test materials, a potexperiment was conducted to simulate two P conditions, i.e., homogeneous low P vs.heterogeneous low P among soil layers, in combination with two N deposition levels on rootmorphology, root exudates, photosynthetic, mycorrhizal symbiosis and P efficiency. The studyillustrated the differentes perception and response of Pinus massonianaã€Cunninghamialanceolata and Schima superba to soil N content increased. The objectives of this study wereto provide theoretical foundation and scientific guidance for genetic improvement of treesphosphorus efficiency in the context of atmospheric N deposition. The main results obtainedfrom experiments are as follows:1. Effects of simulated N deposition on root and phosphorus efficiency of Pinusmassoniana under low P conditionUnder the homogeneous low phosphorus condition, growth traits of seedling height,diameter, biomass etc and P efficiency of Pinus massoniana were not significantly improvedby simulated N deposition, but significant N×family interaction effect was detected. Under the heterogeneous low P condition, significant N effects on the seedling height, biomass and Pabsorption efficiency were observed, due to promoted root length and root distribution ratio oftopsoil. In addition, the effects of simulated N deposition on growth and P efficiency of Pinusmassoniana were relevant to the N/P ratio. Under the homogeneous low P condition, the N/Pratio of Pinus massoniana plant was13.8, plants exhibited a low sensitivity to simulated Ndeposition, root secreted APase activity was increased but the plant growth was not promoted.In comparison, the plant N/P ratio was9.7under the heterogeneous low P condition, and theplant growth and P efficiency were significantly promoted, while no obvious change occurredin root secreted APase activity.Under both the homogeneous and heterogeneous low P conditions, N/P stoichiometricratio in Pinus massoniana seedlings was significantly increased by simulated N deposition,which stimulated the amount of root acid phosphatase and organic acid secretion. The amountof root exudates was higher under the homogeneous low P condition than under theheterogeneous low P condition. The level of root secreted organic acids was significantlycorrelated with the growth in Pinus massoniana seedlings. Under the homogeneous low Pcondition, P acquisition efficiency and biomass in Pinus massoniana seedlings were notaffected by simulated N deposition, mainly due to the high N/P ratios and low sensitivity to Naddition. Significant variations among families in root exudates to the simulated N depositionwere observed.Simulated N deposition significantly increased the N content and N/P ratio of Pinusmassoniana needles under the homogeneous low P condition, meanwhile net photosyntheticrate improveded. Under the heterogeneous low P treatment, P content of Pinus massoniananeedles increased but N/P decreased. The increasing degree of net photosynthetic rate wassmaller than simulated N deposition. Under homogeneous low P, simulated N depositiondespite increased in the plant net photosynthetic rate, but did not significantly improve plantbiomass.The reasons was that simulated N deposition increased the N content of soil, yetCausing the P content of soil and plant was relative scarcity, and reduced the P absorption efficiency ofPinus massonianaplant. In addition, the simulated N deposition inhibited rootgrowth, reducing the absorption of nutrients, and photosynthesis increases, further aggravatingthe plant nutrient deficiency. Under heterogeneous low P conditions, the root-shoot ratioofPinus massoniana was increased, meanwhile N, P uptake and biomass were increased, whichprovided necessary nutrients for the photosynthesis.2. Effects of simulated N deposition on mycorrhizal symbiosis and P efficiency underlow P stressUnder different P contitions, the effects of simulated N deposition on nutrient content inmycorrhizal soil of Pinus massoniana is different. Simulated N deposition increased thecontent of available N, available phosphorus and potassium in mycorrhizal soil underhomogeneous low P treatment, and reduced the effective aluminum content, but soilacidification did not cause obvious. The results showed that inoculation mycorrhiza fungicould improve the effects of simulated N deposition on nutrient content in mycorrhizal soil ofPinus massoniana and adjust N/P ratio and soil physicochemical properties. Underheterogeneous low P treatment, Simulated N deposition increased the content of available N,available phosphorus, K and effective aluminum, but N/P ratio and pH significantly decreased.Under different P conditions, the effects of simulated N deposition on mycorrhizalinfection of Pinus massoniana was different.The colonization ratio and infection degree ofmycorrhizal fungi under homogeneous low P was larger than under heterogeneous low P.Under homogeneous low P condition, simulated N deposition increased the effectiveness ofsoil N, but reduced the1-year and2-year-old seedlings mycorrhizal infection rate andinfection degree. The number of mycorrhizal hairs was decreased, but hyphae of mycorrhizaewere thick and dense. Under heterogeneous low P treatment,simulated N deposition increasedthe1-year and2-year-old seedlings mycorrhizal infection rate. The number of mycorrhizalhairs were increased, and surface hyphae of mycorrhizae was dense.Under homogeneous low P condition, simulated N deposition treatment significantlyreduced the mycorrhizal growth of1-year-old Pinus massoniana seedlings, but increased the root APase activity and the secretion of organic acids, organic acid secretion particularlyincreased by nearly three times. Correlation analysis showed that root biomass and APaseactivity was significantly negatively under homogeneous low P, but it was significantlypositive with organic acid secretion. The contribution of organic acids secreted to the growthof mycorrhizal seedlings was significantly higher than APase. Under heterogeneous low Ptreatment, simulated N deposition treatment significantly reduced the surface root growth of1-year-old Pinus massoniana mycorrhizal seedlings, but increased the growth of deep roots.Meanwhile, root dry matter accumulation and organic acid secretion increased, while APaseactivity decreased.Under homogeneous low P condition, inoculation mycorrhiza fungi significantlyimproved the growth status and promoted the biomass accumulation of Pinus massonianaseedlings. Simulated N deposition treatment improved the growth and biomass accumulationof1-year-old Pinus massoniana mycorrhizal seedlings. Although increased the height growthof2-year-old Pinus massoniana mycorrhizal seedlings, but reduced its biomass accumulation.Under heterogeneous low P treatment, simulated N deposition improved the growth andbiomass accumulation of Pinus massoniana1-year and2-year-old mycorrhizal seedlings.Under different soil P condition, the simulated N deposition level of this study had allpromoted the growth of Pinus massoniana mycorrhizal seedlings, but the dry matteraccumulation of heterogeneous low P was larger than that of homogeneous low P.Correlation analysis showed that the absorption efficiency of N and P were significantlypositive correlation with the growth and biomass accumulation of Pinus massonianamycorrhizal seedlings. Under homogeneous low P condition, simulated N deposition increasedthe absorption efficiency of N and P of Pinus massoniana1-year-old mycorrhizal seedlings,and plant N/P increased to11.70. The N absorption of Pinus massoniana2-year-oldmycorrhizal seedlings was increased, but the P absorption was decreased. Plant N/Psignificantly increased to14.99. Under heterogeneous low P treatment, simulated N depositionincreased the absorption efficiency of N and P of Pinus massoniana1-year and2-year-old mycorrhizal seedlings. Plant N/P of1-year-old mycorrhizal seedlings increased to10.96, andthat of2-year-old mycorrhizal seedlings decreased to5.56.3. The differences of growth and P efficency responses ofPinus massoniana,Cunninghamia lanceolata and Schima superba to simulated N deposition under low P stressBecause of three species have different biological characteristics, significant differencesin nutrient content of potting soil to the simulated N deposition were observed. The order ofincreasing extent of potting soil hydrolysis N content was Pinus massoniana, Cunninghamialanceolata and Schima superba. The degree of surface soil acidification of Pinus massonianaand Schima superba was large than Cunninghamia lanceolata, due to Schima superba strongerabsorption and eluviation of surface soil N, and N eluviation of surface soil of Pinusmassoniana was larger than Cunninghamia lanceolata. In addition, the decreasing degree ofpotting soil P and K content of Pinus massoniana and Schima superba were larger thanCunninghamia lanceolata. The increasing of soil Al content of three species may be related tothe reducing of soil P content.The order of increasing degree of leaf N/P ratio was Cunninghamia lanceolata,Schimasuperba.and Pinus massoniana. Cunninghamia lanceolata was sensitive to N deposition, itsleaf N/P ratio largely increased. Pinus massoniana had stronge ability of regulation andadaptability to increased N. The study on photosynthesis of three species showed that theincreasing degree of net photosynthetic rate of Pinus massoniana was large than Schimasuperba,.leaf net photosynthetic rate of Cunninghamia lanceolata decreased. Leaf endogenousacid phosphatase activity of three species were decreased, and the order of reducing extentwas Cunninghamia lanceolata,Schima superba.and Pinus massoniana. The result may beassociated with changes of leaf enzymes metabolism.due to increased leaf N content. Underthe heterogeneous low P conditions, leaf N/P ratio of Pinus massoniana.and Schima superbawere lower, but that of Cunninghamia lanceolata was increased. The heterogeneous low Ptreatment can promoted the photosynthesis of three species, and the order of increasing extentof leaf N/P ratio was Cunninghamia lanceolata,Schima superba.and Pinus massoniana.. The increased activity of leaf endogenous acid phosphatase of Cunninghamia lanceolata, andPinus massonian promoted the recycling of phosphorus in leaves.Under the simulated N deposition treatment, the order of increasing extent of rootexudates was Schima superba, Pinus massoniana and Cunninghamia lanceolata. This may beone of reasons for the high biomass of Schima superba, Pinus massoniana compared toCunninghamia lanceolata. Under the heterogeneous low P condition, acid phosphataseactivity and total organic acids of three species were significantly reduced. The influences ofsimulated N deposition on root growth of Pinus massoniana was less than Schima superba,and Cunninghamia lanceolata. Aboveground growth of Schima superba increased, but rootgrowth was inhibited, while the biomass of whole plant decreased. Increased N depositionseverely inhibited root growth and biomass accumulation of Cunninghamia lanceolata.Correlation analysis showed that growth index of height, ground stems and biomass of threespecies had significant correlation with plant N, phosphorus efficiency. Simulated Ndeposition inhibited the P absorption of Pinus massoniana and Cunninghamia lanceolata, butpromoted the P absorption of Schima superba. Simultaneously, the N absorption of Pinusmassoniana and Schima superba were increased by simulated N deposition, but that ofCunninghamia lanceolata was reduced. The biomass and root-shoot ratio were increased ofthree species under the heterogeneous low P condition. The order of increasing extent ofbiomass was Schima superba, Cunninghamia lanceolata and Pinus massoniana. |
