Storage And Key Soil Cycling Processes Of Carbon And Nitrogen Among Typical Forest Ecosystems In The Subalpine Area Of Western Sichuan

Improving carbon and nitrogen pools in terrestrial forest ecosystems through the practices of afforestation and forest restoration has been globally considered as an effective way to reduce atmospheric greenhouse gases concentration and to mitigate global climate change.The different forest types,such as primary forest and restored forests with different regeneration patterns after primary forest harvesting,may have different carbon and nitrogen transformation own to the possible alteration of tree species and strcutre,fine root dynamics,soil properties and microbial community,etc.Currently,the comprenhensive effects of forest type,such as primary forest and restored forests differing in regeneration patterns on storage,key soil cycling processes of carbon and nitrogen and their influence factors remain largely unclear.Historically,large areas of primary forests were cut down in the subalpine area of Western Sichuan,and the areas were subsequently restored with different methods,i.e.natural,aritificial and their combined processes.However,previous studies on storage and key soil cycling processes of carbon and nitrogen of forest ecosystems in this region have either foucesd on one aspect or specific forest types.There is still a lack of comprehensive analysis of ecosystem carbon and nitrogen storage and key soil carbon and nitrogen cycling processes of the four typical forest types.In this study,we selected Abies faxoniana primary forest?AF?and three restored forest ecosystems differing in regeneration patterns,i.e.,Picea asperata broadleaved mixed forest?natural regeneration after planting,MF?,Abies-Betula secondary forest?natural without assisted regeneration,SF?and Picea asperata plantation forest?PF?in a subalpine region of Western Sichuan as the objects.The methods of sequential soil coring,barometric process separation?Ba PS?,the close-top tube incubation method and the static chamber and gas chromatography techniques were adopted.We wanted to examine the carbon and nitrogen storage and spatial distribution in dfferent forest ecosystems,to evaluate how forest types influence fine root dynamics,and to explore the responses of soil carbon and nitrogen transformation to forest types and their influence factors.We aimed to reveal the influence of forest types on the ecosystem stocks and soil transformation of carbon and nitrogen and the relative mechanism,thus providing scientific basis for forest restoration and protection in this region,and evaluating the comprehensive benefits of regional major forestry eco-engineering.The main results were as follows:?1?Ecosystem carbon and nitrogen storages were dfferent among forest types.Ecosystem carbon storage of AF,MF,SF,and PF was 611.18,252.31,363.07,and 239.06 t C hm^-2,whereas nitrogen storage was 16.44,12.11,15.48,and 8.92 t N hm^-2,respectively.Distribution patterns of carbon storage between soil and plants,not nitrogen storage,had changed between primary and restored forests.Carbon storage of primary and restored forests was dominated by vegetation?67.98%?and soil?69.87%for MF,76.20%for SF,and 72.12%for PF?,respectively,whereas nitrogen storage was dominated by soil?76.80%–92.58%?.The proportion of carbon and nitrogen storage of woody debris and litter were 4.40–9.83%and 2.94–7.08%,respectively,which were higher than that of the understory vegetation.AF had the highest ecosystem carbon storage.It was further indicated that three kinds of restored forests had high carbon sequestration potential,especially for aboveground own to the low ratio of above-to under-ground carbon storage.SF was beneficial to carbon accumulation and PF had higher tree carbon storage.?2?Forest type altered fine root biomass,production and turnover rate.At the soil depth of0–30 cm,fine root biomass was the highest in PF and fine root necromass was the highest in AF,both were the lowest in MF.Fine root production of SF and PF was significantly higher than that of other two forests,and MF and SF had a higher fine root turnover rate than other two forests.Fine root dynamics of the four forest types tended to decrease with soil depth.Fine root biomass and production were the highest in PF in the soil layer of 0–10 cm,but were not significantly different among forest types in the lower soil layers.There were positive correlations between these parameters and aboveground biomass across three restored forest types in soil layer of 0–10 cm,but not in the lower soil layers.Fine root turnover rate was generally higher in mixed forests than in monocultures at all soil depths.?3?Soil net ammonification and nitrification rate had contrasting responses to forest type.Forest type had a significant effect on net ammonification rate,but not net nitrification rate.The cumulative ammonified N value was significantly higher in SF(11.40 g kg^-1)and lower in PF(4.79 g kg^-1),and the cumulative nitrified N value ranged from 17.75 to 27.98 g kg^-1without significant difference among forest types.However,the average net N mineralization rate was not significantly different among forest types with generally negative values of the average net ammonification rate in MF and SF.Net ammonification and nitrification rates exhibited different monthly patterns.Net ammonification rate had the higher values at the end of the period,whereas net nitrification rate had the higher values at the middle of the period.The dynamic of net ammonification rate was greatly affected by soil NH4⁺,total N,temperature and microbial PLFAs,whereas the dynamic of net nitrification rate was primarily regulated by temperature and microbial community structure.?4?The average heterotropic respiration and gross nitrification rate during the growing season were highest in MF and PF,respectively,and were all lowest in AF.Soil heterotropic respiration and gross nitrification rate varied obviously during the study period with peaking in July.Moreover,they were significantly correlated with soil temperature,but were not significantly correlated with soil moisture,indicating that soil temperature,rather than soil moisture was a controlling factor in the regulation of seasonal dynamic of soil heterotropic respiration and gross nitrification.The temperature sensitivity of heterotropic respiration varied from 2.59 to 4.71 with the highest in AF,suggesting that the primary forest in higher altitude might be more vulnerable to climate change.Soil heterotrophic respiration and gross nitrification rate among forest types were mainly influenced by litter mass,p H and soil organic matter.Soil heterotrophic respiration rate was positively correlated with gross nitrification rate in different forest types,implying a coupling relationship between heterotropic respiration and gross nitrification rate.?5?The soils of the four forest types acted as CH4 sinks,CO2 and N2O sources.Forest type altered soil greenhouse gases fluxes.The CO2 flux increased,while N2O flux decreased in three restored forests compared with AF.The average vaule of CO2 flux in AF was 230.69 mg m^-2 h^-1,which was 67.66?77.11 and 53.76 mg m^-2 h^-1 lower than that of MF,SF and PF,respectively.The average vaule of N₂O flux in AF was 2.67?g m^-2 h^-1,which was 0.07,0.16and 0.55?g m^-2 h^-1 higher than that of MF,SF and PF,respectively.SF had the highest CH₄uptake rate,and PF had the lowest CH4 uptake rate among the four forest types.Soil greenhouse gases had the obviously seasonal patterns,with higher rates of CO2 and N2O emission and lower rate of CH4 uptake in the middle of the study period.The seasonal trends were mainly regulated by soil temperature.CO2 flux was significantly correlated with SOC,NH₄⁺,G+and G+/G-.CH₄ flux was significantly correlated with NO₃^-and soil microbial community?except bacterial and G+PLFAs?.However,N2O flux was not significantly correlated with soil properties and microbial community.In conclusion,our results showed that the three restored forest types had lower ecosystem carbon and nitrogen storage compared with AF,indicating a higher carbon sequestration potential of these three forest types.SF had the highest ecosystem carbon and nitrogen storages,revealing a faster carbon and nitrogen sequestration rate.In addition,SF had a higher fine root production and turnover rate and the fundamental nutrient cycling,suggesting that SF might be conducive to the accumulation of soil carbon and nitrogen.Therefore,from these perspectives,natural regeneration might be a better way of forest restoration in this area.