| Terrestrial ecosystem carbon cycle plays an important role in global climate regulation.Soil carbon pool is about three times that of the atmospheric carbon pool,and each year soil returned 80%of the gross carbon uptake to the atmosphere through soil respiration,thus soil is a key factor to positive or negative feedback to global warming.Although there have been many long-term and short-term monitoring studies on soil respiration,the inter-annual fluctuations and spatial variability of the China's temperate forest soil temperate(Rs)and its components—rhizospheric(RR)and heterotrophic(RH)respirations,the annual scale especially component partitioning of the non-growing season,the interrelations of canopy leaf phenology and belowground phenology(fine root and microbial seasonal dynamics)is still not well understood.This paper used several years of continuous field observations to study these key scientific issues and found the following important findings:(1)There are few studies in the mid-high latitude forest ecosystem that distinguish RS from RR and RH on a full-year scale.A trenching experiment was conducted to quantify RR and RH using chamber methods in a temperate deciduous forest in Northeast China for two years.Temperature sensitivities(Q10)for Rs and for RH were both much higher in non-growing season(November to April)than those in growing season.The Q10 for Rs was higher than Q10 for RH in both seasons,indicating a higher temperature sensitivity of roots versus microorganisms.Mean non-growing season Rs,RH and RR for the two years were 94,79 and 14 g C m-2,respectively,which contributed 10.8%,14.5%and 4.5%to the corresponding annual fluxes.The contributions of RH to Rs for the non-growing season,growing season and annual period were 84.8%,60.7%and 63.3%,respectively.Using the same contribution of non-growing season Rs to annual Rs to scale growing season measurements to the annual scale would introduce significant biases on annual RH(-34 g C m-2 yr-1 or-6%)and RR(16 g C m-2 yr-1 or 5%).In the context of global change,to accurately partitioning the soil respiration components of northern forests,it is necessary to determine the soil respiration components in non-growth seasons.(2)The temporal and spatial variability of Rs and RH between 2008 and 2011 was studied in eight plots randomly seted in a temperate broadleaved deciduous forests.The Rs and RH rates of the eight plots were positively correlated with temperature,but whether affected by the water content were plot-specific,the respiration rates were negatively correlated with soil volumetric water content in all water-affected plots.The 4-year average annual Rs and RH of the eight plots were 843.36 g C m-2 yr-1(Wang model)?830.89 g C m-2 yr-1(Gama model)and 531.31 g C m-2 yr-1,respectively.Wang model)?523.46 g C m-2 yr-1(Gama model),so it was thought that the model selection had little effect on the annual cumulative flux.The spatial heterogeneity analysis of soil respiration components indicated that the inter-annual variation was generally close to the inter-annual fluctuations(overall not exceeding 10%).Compared between the four years,the drought in 2008 caused a large inter-sample variation in the cumulative growth of the growing season,indicating that the spatial heterogeneity of soil moisture caused by topography should be considered in the year with an anomaly climate.The variation in soil respiration among plots was mainly related to small root biomass,soil bulk densities of the organic and eluvial layer.The plots with higher small fine root biomass(more substrate),lower bulk density of organic layer(favorable for soil permeability)and of eluvial layer(facilitating soil permeable)were corresponded to higher Rs and its components.Therefore,it was believed that in a small scale,increasing the repeat of measurement points within the plot and reduce the number of the plot would accurately estimate Rs rate.(3)The fine root production of the forest was estimated using the belowground carbon balance method and the sequential root coring with decision matrix method in the deciduous broadleaved plots.The results showed that the total belowground carbon allocation(TBCA)of the eight plots was 614.13±68.40 g C m-2 yr-1,which was 2.83 times of the aboveground litter-fall production,while the fine root productivity estimated by the belowground carbon balance method was 306.70±46.06 g C m-2 yr-1,which was very close to the values estimated by the sequential root coring with decision matrix in three temporary plots(267.78-354.36 g C m-2 yr-1),indicating that the two ways converged.Fine root production accounted for approximately 50%of TBCA,indicating that a half of root gross primary production is approximately used for respiration and the other half for root biomass growth.Canopy phenology was quantified with simple vegetation index and leaf area index,and fine root biomass(FRB)and necromass(FRN)were measured the sequential root coring with decision matrix and soil microbial biomass carbon(MBC)and nitrogen(MBN)was measured in a temperate deciduous forest for two years.The FRB and FRN had roughly opposite bimodal patterns,but both seasonally asynchronous with the MBC and MBN.The FRB generally maximized before leaf expansion,followed by a peak of mortality during leaf expansion period,then reached the second small peak in early summer and declined to the minimum after leaf senescence.Seasonal changes of FRB,FRN and fine root mortality were not directly related to soil temperature and water content,but significantly correlated to microbial biomass in the following month.The dynamics of fine roots were asynchronous with leaves and soil microbes in this temperate forest.The seasonal alternation of microbial and fine root growth and mortality indicated that the co-evolutionary relationship between plants and microorganisms formed a nutrient retention mechanism for temperate forest ecosystems.(4)The small-scale spatial variation in Rs and its influencing factors were studied by using 121 mechanical arranged measuring points in a 2-m interval within a 20 m × 20 m plot,the measurement frequency was approximately monthly.Semi-variance analysis of Rs rate and its corresponding soil temperature and humidity as well as soil organic carbon and total nitrogen showed that the range in RS rate was significantly less than those of the temperatures measured at the two depth and of the moisture content at 0-10 cm.This reflected that the spatial variation in Rs rate was greater than those in soil temperature and soil moisture.The spatial heterogeneity of each parameter was greater than 90%(except soil moisture content).The variance decomposition showed that the soil respiration variation in the plot was mainly controlled by the joint effect of substrate factors(accumulation of litter,topsoil organic carbon content and total nitrogen content)and environmental factors(surface soil moisture content and soil bulk density)and the unique effect of substrate factors.The optimal model explained 45%of the total spatial variation in Rs.The design of soil respiration measurement locations in the plot should focus on the spatial location of individual trees and the spatial heterogeneity of soil organic matter and litter stocks caused by microtopography.This paper innovatively:(1)Partitioning the soil respiration components and quantified the temperature sensitivity of the temperate forests in Northeast China.(2)Compared the spatial variability between the eight plots of a natural secondary forest and its inter-annual fluctuations.(3)Illustrated that the spring peak growth of fine roots of temperate deciduous broad-leaved forests are generally neglected,suggesting a pattern of plant-microbe interactions controlled by substrate supply and beneficial cooperation.(4)Geostatistical method was used to quantify the spatial variability of soil respiration and its influencing factors in different seasons.It was pointed out that the key considerations for soil respiration measurement location selection were the spatial location of individual trees and the heterogeneity of micro-topography.These findings not only expanded the dataset of field monitoring soil respiration in typical temperate forests in Northeast China,but also will provide a reference for future long-term field monitoring of soil respiration and its response to climate change. |
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