Tree Species Effects On Ecosystem Nitrogenretention And Associated Mechanisms

Atmospheric nitrogen deposition rate has been increasing due to the excessive use of fossil fuels and nitrogen fertilizers since the last century.The increased nitrogen has caused a series of consequences,such as soil acidification,water eutrophication and biodiversity loss.Meanwhile,the increased nitrogen input into terrestrial ecosystem has gradually opened up the previously closed nitrogen cycling process.Therefore,investigating ecosystem nitrogen retention under the background of nitrogen deposition has become a hot topic for ecologists.The mechanisms of nitrogen retention in forest ecosystem included uptake of nitrogen by increased plant biomass,microbial immobilization of nitrogen during decomposition,and low nitrification rates,which reduce nitrate leaching.Large-scale afforestation has been carried out in China since the 1980s,but most of the plantations are dominated by a single fast-growing coniferous tree species.In recent years,afforestation with broad-leaved tree species rather than coniferous tree species has been widely used.However,the nitrogen retention capacities of ecosystem after afforestation with broad-leaved tree species rather than coniferous tree species remain unknown.The aim of the study was to study the effects of tree species on ecosystem nitrogen retention in a coniferous tree species?Cunninghamia lanceolata?and three broad-leaved tree species?Schima superba,Mytilaria laosensis,Liquidambar formosana?at two forest farms in Fujian province.Isotope tracer technique was used to study the fate of exogenous nitrogen addition in those forest ecosystems.Microbial biomass,community composition and structure,as well as nitrogen cycling-related genes were determined.We hypothesized that afforestation with broad-leaved tree species rather than coniferous tree species would increase plant biomass and plant nitrogen pool,increase microbial biomass and lower nitrification rates,which together would increase the ecosystem nitrogen retention.The results will benefit for tree species selections when afforestation in areas with high nitrogen deposition rates.The main results are as follows:?1?Ecosystem nitrogen pools were significantly higher after reforestation of coniferous species with broad-leaved species.The biomass,productivity and carbon and nitrogen pools after reforestation of C.lanceolata with M.laosensis were significantly higher than those with C.lanceolata itself.The nitrogen pools of 19-years-old M.laosensis and C.lanceolata ecosystem were 6.9 Mg·ha^-1(0.8 Mg·ha^-1 for plant and 6.1 Mg·ha^-1 for soil)and5.5 Mg·ha^-1(0.4 Mg·ha^-1 for plant and 5.1 Mg·ha^-1 for soil),respectively.The biomass,productivity and carbon and nitrogen pools after reforestation of C.lanceolata with S.superba were significantly higher than those with C.lanceolata.The nitrogen pools of35-years-old S.superba and C.lanceolata ecosystem were 15.3 Mg·ha^-1(1.7 Mg·ha^-1 for plant and 13.6 Mg·ha^-1 for soil)and 7.8 Mg·ha^-1(1.1 Mg·ha^-1 for plant and 6.7 Mg·ha^-1 for soil),respectively.?2?A 625-day field ¹⁵N tracer experiment was carried out on plantations of C.lanceolata,S.superba and F.formosa.The ¹⁵N recovery rate of different components represent different nitrogen turnover rates.Results showed that soil is the largest ammonium nitrogen retention pool of all three plantations.The recovery of ¹⁵N in soil was F.formosa?61.01%?>S.superba?43.61%?>C.lanceolata?36.19%?,and the recovery of ¹⁵N in plant?above-and belowground biomass and litter?was S.superba?49.65%?>C.lanceolata?43.9%?>F.formosa?34.3%?,respectively.For example,the 20g added nitrogen was equivalent to 7.2,8.7 and 12.2g N retained in the 0-40cm soil layer respectively,and 8.8,8.9 and 6.9g N retained in the plant of C.lanceolata,S.superba and F.formosa,respectively.?3?There was a consistent temporal trend in the uptake of ammonium nitrogen in roots,branches and leaves of tree species.The ¹⁵N abundance in plant root of all tree species increased and peaked in the first year and then decreased in the second year.The ¹⁵N abundance in C.lanceolate root peaked later than that of S.superba and F.formosa,indicating a lower rate of nitrogen uptake.The ¹⁵N recovery rate in branches and leaves of all tree species peaked around 380 days after nitrogen addition.However,the recovery rates in branches of S.superba and F.formosa were significantly higher than that of C.lanceolate,indicating that the uptake and turnover of nitrogen were faster in S.superba and F.formosa.There were significant differences in the temporal trend of ¹⁵N in litter of the three species.The ¹⁵N abundance of the fallen leaves of F.formosa tended to decrease after increasing with time,while the ¹⁵N abundance of the fallen leaves of S.superba continued to increase during the whole experimental period,but the ¹⁵N abundance of the fallen leaves of C.lanceolate did not change during the experimental period.?4?There were significant differences in soil microbial biomass nitrogen and ¹⁵N recovery among different tree species.The microbial biomass nitrogen and ¹⁵N recovery rate of C.lanceolate soil were lower than those of S.superba and F.formosa soil.Moreover,the microbial nitrogen pool of S.superba soil was significantly higher than that of C.lanceolate and F.formosa.Differences in Soil temperature,moisture and soil micro-environment of different tree species lead to significant differences in soil ammonification,nitrification and mineralization rates,which further affect soil nitrogen retention.Soil mineral nitrogen in subtropical forests is dominated by ammonium nitrogen,and ammonification plays a dominant role in net nitrogen mineralization.However,the nitrification of coniferous soil is significantly higher than that of broad-leaved soil.Therefore,reforestation of coniferous species with broad-leaved species reduces soil nitrification,thereby reducing the loss of nitrate nitrogen and increasing inorganic nitrogen uptake in the form of ammonium nitrogen,thus enhancing ecosystem nitrogen retention.?5?Soil microbial community composition and structure,and nitrogen cycle-related genes were significantly changed after reforestation of C.lanceolata with M.laosensis.The abundance of AOA amoA gene of M.laosensis soil decreased significantly,while the abundance of nirK,nirS and nosZ genes increased significantly,indicating that tree species had an important influence on the nitrogen cycle process in which soil microorganism participated.In summary,our results confirmed the previous hypothesis.Reforestation of conifer species with broad-leaved species significantly increase plant biomass,plant nitrogen pool,and soil microbial biomass,but lowered the nitrification rates,which together enhanced the ecosystem nitrogen retention.