Dissolved Organic Carbon Dynamics and Controls of Planted Slash Pine Forest Soil in Subtropical Region in Southern China

Soil dissolved organic carbon （DOC） is an active fraction of the soil organic carbon （SOC） pool and links terrestrial and aquatic systems. The degradation of DOC can affect carbon cycling, nutrient dynamics and energy supply to microorganism, and consequently change biogeochemical processes. This study investigated the vertical and seasonal variability of soil DOC concentrations and its controls in a 23-year-old planted slash pine （Pinus elliottii） forest at Qianyanzhou Forest Experimental Station （QFES） in Southern China. Soil solutions were collected at bimonthly intervals at depths of 10, 20 and 30 cm by a mechanical-vacuum extractor from November 2007 to March 2009, and at monthly intervals at depths of 10, 30 and 50cm from April 2009 to October 2010. The DOC concentrations were determined with a total organic carbon （TOC） analyzer. Mean （±standard deviation） DOC concentrations at depths of 10cm, 20cm, 30cm and 50cm were 12.4±4.4, 10.6±6.3, 8.7±2.6, and 8.0±5.9 mg L^-1, respectively. Both seasonal and spring means of DOC concentration showed a decreasing trend with increasing depth, while there was no clear trend for the summer, autumn, or winter seasons. DOC concentrations during spring, summer, autumn and winter ranged from 4.8 to 21.5, 4.9 to 26.2, 5.4 to 17.1, 4.9 to 14.6 mg L-1, respectively, their mean DOC concentrations were 10.2, 10.5, 10.8 and 8.3 mg L 1, respectively. No consistent pattern of seasonal variability of DOC concentrations at different depths was observed. No obvious relationship between organic carbon content of forest litter and DOC concentration was found. There was a positive linear relationship between SOC and DOC concentration （R2=0.19, p〈0.01）, which showed that SOC was one of the main controls of DOC. A positive exponential relationship existed between soil temperature at 5 cm and DOC concentrations at 10 cm depth in slash pine, masson pine （Pinus massoniana） and Chinese fir （Cunninghamia lanceolata） planted forests （R2=0.12, p〈0.01）. DOC concentrations showed a negative linear relationship with soil moisture at all depths in slash pine forest （R2=0.15, p〈0.001）, and DOC concentrations at depth of 10 cm demonstrated a negative exponential relationship with soil moisture at 5 cm depth in three planted forests （R2=0.13,/）〈0.001）. Precipitation in sampling months and mean seasonal DOC concentration were not correlated. However, a more detailed analysis of precipitation events at different times before sampling and seasonal DOC concentration showed that the timing of precipitation events prior to sampling had different effects on seasonal DOC concentrations at different depths. Our study highlights the importance of DOC dynamics for the carbon cycle in planted slash pine forest and it provides evidence for evaluating the effects of ecological restoration in subtropical red soil region.

Divergent apparent temperature sensitivity of terrestrial ecosystem respiration

Aims Recent studies revealed convergent temperature sensitivity of ecosys-tem respiration(Re)within aquatic ecosystems and between terrestrial and aquatic ecosystems.We do not know yet whether various terres-trial ecosystems have consistent or divergent temperature sensitivity.Here,we synthesized 163 eddy covariance flux sites across the world and examined the global variation of the apparent activation energy(Ea),which characterizes the apparent temperature sensitivity of and its interannual variability(IAV)as well as their controlling factors.Methods We used carbon fluxes and meteorological data across FLUXNET sites to calculate mean annual temperature,tempera-ture range,precipitation,global radiation,potential radiation,gross primary productivity and Re by averaging the daily values over the years in each site.Furthermore,we analyzed the sites with>8 years data to examine the IAV of Ea and calculated the standard deviation of Ea across years at each site to character-ize IAV.Important Findings The results showed a widely global variation of Ea,with significantly lower values in the tropical and subtropical areas than in temperate and boreal areas,and significantly higher values in grasslands and wetlands than that in deciduous broadleaf forests and evergreen for-ests.Globally,spatial variations of Ea were explained by changes in temperature and an index of water availability with differing contribution of each explaining variable among climate zones and biomes.IAV and the corresponding coefficient of variation of Ea decreased with increasing latitude,but increased with radiation and corresponding mean annual temperature.The revealed patterns in the spatial and temporal variations of Ea and its controlling factors indicate divergent temperature sensitivity of Re,which could help to improve our predictive understanding of Re in response to climate change.

Heat and drought impact on carbon exchange in an age-sequence of temperate pine forests

Background:Most North American temperate forests are plantation or regrowth forests,which are actively managed.These forests are in different stages of their growth cycles and their ability to sequester atmospheric carbon is affected by extreme weather events.In this study,the impact of heat and drought events on carbon sequestration in an age‑sequence(80,45,and 17 years as of 2019)of eastern white pine(Pinus strobus L.)forests in southern Ontario,Canada was examined using eddy covariance flux measurements from 2003 to 2019.Results:Over the 17‑year study period,the mean annual values of net ecosystem productivity(NEP)were 180±96,538±177 and 64±165 g C m^(–2)yr^(–1)in the 80‑,45‑and 17‑year‑old stands,respectively,with the highest annual carbon sequestration rate observed in the 45‑year‑old stand.We found that air temperature(Ta)was the dominant control on NEP in all three different‑aged stands and drought,which was a limiting factor for both gross ecosystem productivity(GEP)and ecosystems respiration(RE),had a smaller impact on NEP.However,the simultaneous occurrence of heat and drought events during the early growing seasons or over the consecutive years had a significant negative impact on annual NEP in all three forests.We observed a similar trend of NEP decline in all three stands over three consecutive years that experienced extreme weather events,with 2016 being a hot and dry,2017 being a dry,and 2018 being a hot year.The youngest stand became a net source of carbon for all three of these years and the oldest stand became a small source of carbon for the first time in 2018 since observations started in 2003.However,in 2019,all three stands reverted to annual net carbon sinks.Conclusions:Our study results indicate that the timing,frequency and concurrent or consecutive occurrence of extreme weather events may have significant implications for carbon sequestration in temperate conifer forests in Eastern North America.This study is one of few globally available to provide long‑term observational data on carbon exchanges in different‑aged temperate plantation forests.It highlights interannual variability in carbon fluxes and enhances our understanding of the responses of these forest ecosystems to extreme weather events.Study results will help in developing climate resilient and sustainable forestry practices to offset atmospheric greenhouse gas emissions and improving simulation of carbon exchange processes in terrestrial ecosystem models.

Changes in ecosystem carbon stocks in a grassland ash (Fraxinus excelsior) afforestation chronosequence in Ireland

Aims Government policy in Ireland is to increase the national forest cover from the current 10%to 18%of the total land area by 2020.This represents a major land use change that is expected to impact on the national carbon(C)stocks.While the C stocks of ecosystem bio-mass and soils of Irish grasslands and coniferous forests have been quantified,little work has been done to assess the impact of broad-leaf afforestation on C stocks.Methods In this study,we sampled a chronosequence of ash(Fraxinus excel-sior)forests aged 12,20,27,40 and 47 years on brown earth soils.A grassland site,representative of the pre-afforestation land use,was sampled as a control.Important Findings Our results show that there was a significant decline(P<0.05)in the carbon density of the soil(0-30 cm)following afforestation from the grassland(90.2 Mg C ha^(−1))to the 27-year-old forest(66.7 Mg C ha^(−1)).Subsequently,the forest soils switched from being a C source to a C sink and began to sequester C to 71.3 Mg C ha^(−1) at the 47-year-old forest.We found the amount of C stored in the above-and belowground biomass increased with age of the forest stands and offset the amount of C lost from the soil.The amount of C stored in the above-and belowground biomass increased on average by 1.83 Mg C ha^(−1) year^(−1).The increased storage of C in the biomass led to an increase in the total ecosystem C,from 90.2 Mg C ha^(−1) at the grassland site to 162.6 Mg C ha^(−1) at the 47-year-old forest.On a national scale,projected rates of ash afforestation to the year 2020 may cause a loss of 290752 Mg C from the soil compared to 2525936 Mg C sequestered into the tree biomass.The effects of harvesting and reforestation may further modify the development of ecosystem C stocks over an entire ash rotation.