Land‐use change to bioenergy: grassland to short rotation coppice willow has an improved carbon balance

The effect of a transition from grassland to second‐generation (2G) bioenergy on soil carbon and greenhouse gas (GHG) balance is uncertain, with limited empirical data on which to validate landscape‐scale models, sustainability criteria and energy policies. Here, we quantified soil carbon, soil GHG emissions and whole ecosystem carbon balance for short rotation coppice (SRC) bioenergy willow and a paired grassland site, both planted at commercial scale. We quantified the carbon balance for a 2‐year period and captured the effects of a commercial harvest in the SRC willow at the end of the first cycle. Soil fluxes of nitrous oxide (N₂O) and methane (CH₄) did not contribute significantly to the GHG balance of these land uses. Soil respiration was lower in SRC willow (912 ± 42 g C m^?2 yr^?1) than in grassland (1522 ± 39 g C m^?2 yr^?1). Net ecosystem exchange (NEE) reflected this with the grassland a net source of carbon with mean NEE of 119 ± 10 g C m^?2 yr^?1 and SRC willow a net sink, ?620 ± 18 g C m^?2 yr^?1. When carbon removed from the ecosystem in harvested products was considered (Net Biome Productivity), SRC willow remained a net sink (221 ± 66 g C m^?2 yr^?1). Despite the SRC willow site being a net sink for carbon, soil carbon stocks (0–30 cm) were higher under the grassland. There was a larger NEE and increase in ecosystem respiration in the SRC willow after harvest; however, the site still remained a carbon sink. Our results indicate that once established, significant carbon savings are likely in SRC willow compared with the minimally managed grassland at this site. Although these observed impacts may be site and management dependent, they provide evidence that land‐use transition to 2G bioenergy has potential to provide a significant improvement on the ecosystem service of climate regulation relative to grassland systems.     

Modeling to discern nitrogen fertilization impacts on carbon sequestration in a Pacific Northwest Douglas‐fir forest in the first‐postfertilization year

This study investigated how nitrogen (N) fertilization with 200 kg N ha^?1 of urea affected ecosystem carbon (C) sequestration in the first‐postfertilization year in a Pacific Northwest Douglas‐fir (Pseudotsuga menziesii) stand on the basis of multiyear eddy‐covariance (EC) and soil‐chamber measurements before and after fertilization in combination with ecosystem modeling. The approach uses a data‐model fusion technique which encompasses both model parameter optimization and data assimilation and minimizes the effects of interannual climatic perturbations and focuses on the biotic and abiotic factors controlling seasonal C fluxes using a prefertilization 9‐year‐long time series of EC data (1998–2006). A process‐based ecosystem model was optimized using the half‐hourly data measured during 1998–2005, and the optimized model was validated using measurements made in 2006 and further applied to predict C fluxes for 2007 assuming the stand was not fertilized. The N fertilization effects on C sequestration were then obtained as differences between modeled (unfertilized stand) and EC or soil‐chamber measured (fertilized stand) C component fluxes. Results indicate that annual net ecosystem productivity in the first‐post‐N fertilization year increased by～83%, from 302 ± 19 to 552 ± 36 g m^?2 yr^?1, which resulted primarily from an increase in annual gross primary productivity of～8%, from 1938 ± 22 to 2095 ± 29 g m^?2 yr^?1 concurrent with a decrease in annual ecosystem respiration (R_e) of～5.7%, from 1636 ± 17 to 1543 ± 31 g m^?2 yr^?1. Moreover, with respect to respiration, model results showed that the fertilizer‐induced reduction in R_e (～93 g m^?2 yr^?1) principally resulted from the decrease in soil respiration R_s (～62 g m^?2 yr^?1).     

Carbon cycling and storage in world forests: biome patterns related to forest age

Forest age, which is affected by stand‐replacing ecosystem disturbances (such as forest fires, harvesting, or insects), plays a distinguishing role in determining the distribution of carbon (C) pools and fluxes in different forested ecosystems. In this synthesis, net primary productivity (NPP), net ecosystem productivity (NEP), and five pools of C (living biomass, coarse woody debris, organic soil horizons, soil, and total ecosystem) are summarized by age class for tropical, temperate, and boreal forest biomes. Estimates of variability in NPP, NEP, and C pools are provided for each biome‐age class combination and the sources of variability are discussed. Aggregated biome‐level estimates of NPP and NEP were higher in intermediate‐aged forests (e.g., 30–120 years), while older forests (e.g., >120 years) were generally less productive. The mean NEP in the youngest forests (0–10 years) was negative (source to the atmosphere) in both boreal and temperate biomes (?0.1 and –1.9 Mg C ha^?1 yr^?1, respectively). Forest age is a highly significant source of variability in NEP at the biome scale; for example, mean temperate forest NEP was ?1.9, 4.5, 2.4, 1.9 and 1.7 Mg C ha^?1 yr^?1 across five age classes (0–10, 11–30, 31–70, 71–120, 121–200 years, respectively). In general, median NPP and NEP are strongly correlated (R²=0.83) across all biomes and age classes, with the exception of the youngest temperate forests. Using the information gained from calculating the summary statistics for NPP and NEP, we calculated heterotrophic soil respiration (R_h) for each age class in each biome. The mean R_h was high in the youngest temperate age class (9.7 Mg C ha^?1 yr^?1) and declined with age, implying that forest ecosystem respiration peaks when forests are young, not old. With notable exceptions, carbon pool sizes increased with age in all biomes, including soil C. Age trends in C cycling and storage are very apparent in all three biomes and it is clear that a better understanding of how forest age and disturbance history interact will greatly improve our fundamental knowledge of the terrestrial C cycle.     

Soil carbon balance in a clonal Eucalyptus plantation in Congo: effects of logging on carbon inputs and soil CO2 efflux

Soil CO₂ efflux was measured in clear‐cut and intact plots in order to quantify the impact of harvest on soil respiration in an intensively managed Eucalyptus plantation, and to evaluate the increase in heterotrophic component of soil respiration because of the decomposition of harvest residues. Soil CO₂ effluxes showed a pronounced seasonal trend, which was well related to the pattern of precipitation and soil water content and were always significantly lower in the clear‐cut plots than in the intact plots. On an annual basis, soil respiration represented 1.57 and 0.91 kg_C m^?2 yr^?1 in intact and clear‐cut plots, respectively. During the first year following harvest, residues have lost 0.79 kg_C m^?2 yr^?1. Our estimate of heterotrophic respiration was calculated assuming that it was similar to soil respiration in the clear‐cut area except that the decomposition of residues did not occur, and it was further corrected for differences in soil water content between intact and clear‐cut plots and for the cessation of leaf and fine root turnover in clear cut. Heterotrophic respiration in clear‐cut plots was estimated at 1.18 kg_C m^?2 yr^?1 whereas it was only 0.65 kg_C m^?2 yr^?1 in intact plots (41% of soil respiration). Assumptions and uncertainties with these calculations are discussed.     

Measurement and modelling of CO2 flux from a drained fen peatland cultivated with reed canary grass and spring barley

Cultivation of bioenergy crops has been suggested as a promising option for reduction of greenhouse gas (GHG) emissions from arable organic soils (Histosols). Here, we report the annual net ecosystem exchange (NEE) fluxes of CO₂ as measured with a dynamic closed chamber method at a drained fen peatland grown with reed canary grass (RCG) and spring barley (SB) in a plot experiment (n = 3 for each cropping system). The CO₂ flux was partitioned into gross photosynthesis (GP) and ecosystem respiration (R_E). For the data analysis, simple yet useful GP and R_E models were developed which introduce plot‐scale ratio vegetation index as an active vegetation proxy. The GP model captures the effect of temperature and vegetation status, and the R_E model estimates the proportion of foliar biomass dependent respiration (R_fb) in the total R_E. Annual R_E was 1887 ± 7 (mean ± standard error, n = 3) and 1288 ± 19 g CO₂‐C m^?2 in RCG and SB plots, respectively, with R_fb accounting for 32 and 22% respectively. Total estimated annual GP was ?1818 ± 42 and ?1329 ± 66 g CO₂‐C m^?2 in RCG and SB plots leading to a NEE of 69 ± 36 g CO₂‐C m^?2 yr^?1 in RCG plots (i.e., a weak net source) and ?41 ± 47 g CO₂‐C m^?2 yr^?1 in SB plots (i.e., a weak net sink). Standard errors related to spatial variation were small (as shown above), but more significant uncertainties were related to the modelling approach for establishment of annual budgets. In conclusion, the bioenergy cropping system was not more favourable than the food cropping system when looking at the atmospheric CO₂ emissions during cultivation. However, in a broader GHG life‐cycle perspective, the lower fertilizer N input and the higher biomass yield in bioenergy cropping systems could be beneficial.     

Carbon cycling in temperate grassland under elevated temperature

An increase in mean soil surface temperature has been observed over the last century, and it is predicted to further increase in the future. The effect of increased temperature on ecosystem carbon fluxes in a permanent temperate grassland was studied in a long‐term (6 years) field experiment, using multiple temperature increments induced by IR lamps. Ecosystem respiration (R‐eco) and net ecosystem exchange (NEE) were measured and modeled by a modified Lloyd and Taylor model including a soil moisture component for R‐eco (average R² of 0.78) and inclusion of a photosynthetic component based on temperature and radiation for NEE (R² = 0.65). Modeled NEE values ranged between 2.3 and 5.3 kg CO₂ m^?2 year^?1, depending on treatment. An increase of 2 or 3°C led to increased carbon losses, lowering the carbon storage potential by around 4 tonnes of C ha^?1 year^?1. The majority of significant NEE differences were found during night‐time compared to daytime. This suggests that during daytime the increased respiration could be offset by an increase in photosynthetic uptake. This was also supported by differences in δ¹³C and δ¹⁸O, indicating prolonged increased photosynthetic activity associated with the higher temperature treatments. However, this increase in photosynthesis was insufficient to counteract the 24 h increase in respiration, explaining the higher CO₂ emissions due to elevated temperature.     

Ecosystem fluxes of hydrogen in a mid‐latitude forest driven by soil microorganisms and plants

Molecular hydrogen (H₂) is an atmospheric trace gas with a large microbe‐mediated soil sink, yet cycling of this compound throughout ecosystems is poorly understood. Measurements of the sources and sinks of H₂ in various ecosystems are sparse, resulting in large uncertainties in the global H₂ budget. Constraining the H₂ cycle is critical to understanding its role in atmospheric chemistry and climate. We measured H₂ fluxes at high frequency in a temperate mixed deciduous forest for 15 months using a tower‐based flux‐gradient approach to determine both the soil‐atmosphere and the net ecosystem flux of H₂. We found that Harvard Forest is a net H₂ sink (?1.4 ± 1.1 kg H₂ ha^?1) with soils as the dominant H₂ sink (?2.0 ± 1.0 kg H₂ ha^?1) and aboveground canopy emissions as the dominant H₂ source (+0.6 ± 0.8 kg H₂ ha^?1). Aboveground emissions of H₂ were an unexpected and substantial component of the ecosystem H₂ flux, reducing net ecosystem uptake by 30% of that calculated from soil uptake alone. Soil uptake was highly seasonal (July maximum, February minimum), positively correlated with soil temperature and negatively correlated with environmental variables relevant to diffusion into soils (i.e., soil moisture, snow depth, snow density). Soil microbial H₂ uptake was correlated with rhizosphere respiration rates (r = 0.8, P < 0.001), and H₂ metabolism yielded up to 2% of the energy gleaned by microbes from carbon substrate respiration. Here, we elucidate key processes controlling the biosphere–atmosphere exchange of H₂ and raise new questions regarding the role of aboveground biomass as a source of atmospheric H₂ and mechanisms linking soil H₂ and carbon cycling. Results from this study should be incorporated into modeling efforts to predict the response of the H₂ soil sink to changes in anthropogenic H₂ emissions and shifting soil conditions with climate and land‐use change.     

Simulating the effects of harvest and biofuel production on the forest system carbon balance of the Midwest,USA

Forests of the Midwestern United States are an important source of fiber for the wood and paper products industries. Scientists, land managers, and policy makers are interested in using woody biomass and/or harvest residue for biofuel feedstocks. However, the effects of increased biomass removal for biofuel production on forest production and forest system carbon balance remain uncertain. We modeled the carbon (C) cycle of the forest system by dividing it into two distinct components: (1) biological (net ecosystem production, net primary production, autotrophic and heterotrophic respiration, vegetation, and soil C content) and (2) industrial (harvest operations and transportation, production, use, and disposal of major wood products including biofuel and associated C emissions). We modeled available woody biomass feedstock and whole‐system carbon balance of 220 000 km² of temperate forests in the Upper Midwest, USA by coupling an ecosystem process model to a collection of greenhouse gas life‐cycle inventory models and simulating seven forest harvest scenarios in the biological ecosystem and three biofuel production scenarios in the industrial system for 50 years. The forest system was a carbon sink (118 g C m^?2 yr^?1) under current management practices and forest product production rates. However, the system became a C source when harvest area was doubled and biofuel production replaced traditional forest products. Total carbon stores in the vegetation and soil increased by 5–10% under low‐intensity management scenarios and current management, but decreased up to 3% under high‐intensity harvest regimes. Increasing harvest residue removal during harvest had more modest effects on forest system C balance and total biomass removal than increasing the rate of clear‐cut harvests or area harvested. Net forest system C balance was significantly, and negatively correlated (R² = 0.67) with biomass harvested, illustrating the trade‐offs between increased C uptake by forests and utilization of woody biomass for biofuel feedstock.     

Soil carbon and belowground carbon balance of a short‐rotation coppice: assessments from three different approaches

Uncertainty in soil carbon (C) fluxes across different land‐use transitions is an issue that needs to be addressed for the further deployment of perennial bioenergy crops. A large‐scale short‐rotation coppice (SRC) site with poplar (Populus) and willow (Salix) was established to examine the land‐use transitions of arable and pasture to bioenergy. Soil C pools, output fluxes of soil CO₂, CH₄, dissolved organic carbon (DOC) and volatile organic compounds, as well as input fluxes from litter fall and from roots, were measured over a 4‐year period, along with environmental parameters. Three approaches were used to estimate changes in the soil C. The largest C pool in the soil was the soil organic carbon (SOC) pool and increased after four years of SRC from 10.9 to 13.9 kg C m^?2. The belowground woody biomass (coarse roots) represented the second largest C pool, followed by the fine roots (Fr). The annual leaf fall represented the largest C input to the soil, followed by weeds and Fr. After the first harvest, we observed a very large C input into the soil from high Fr mortality. The weed inputs decreased as trees grew older and bigger. Soil respiration averaged 568.9 g C m^?2 yr^?1. Leaching of DOC increased over the three years from 7.9 to 14.5 g C m^?2. The pool‐based approach indicated an increase of 3360 g C m^?2 in the SOC pool over the 4‐year period, which was high when compared with the ?27 g C m^?2 estimated by the flux‐based approach and the ?956 g C m^?2 of the combined eddy‐covariance + biometric approach. High uncertainties were associated to the pool‐based approach. Our results suggest using the C flux approach for the assessment of the short‐/medium‐term SOC balance at our site, while SOC pool changes can only be used for long‐term C balance assessments.     

Effects of logging on carbon dynamics of a jack pine forest in Saskatchewan,Canada

We calculated carbon budgets for a chronosequence of harvested jack pine (Pinus banksiana Lamb.) stands (0‐, 5‐, 10‐, and～29‐year‐old) and a～79‐year‐old stand that originated after wildfire. We measured total ecosystem C content (TEC), above‐, and belowground net primary productivity (NPP) for each stand. All values are reported in order for the 0‐, 5‐, 10‐, 29‐, and 79‐year‐old stands, respectively, for May 1999 through April 2000. Total annual NPP (NPP_T) for the stands (Mg C ha^?1 yr^?1±1 SD) was 0.9±0.3, 1.3±0.1, 2.7±0.6, 3.5±0.3, and 1.7±0.4. We correlated periodic soil surface CO₂ fluxes (R_S) with soil temperature to model annual R_S for the stands (Mg C ha^?1 yr^?1±1 SD) as 4.4±0.1, 2.4±0.0, 3.3±0.1, 5.7±0.3, and 3.2±0.2. We estimated net ecosystem productivity (NEP) as NPP_T minus R_H (where R_H was calculated using a Monte Carlo approach as coarse woody debris respiration plus 30–70% of total annual R_S). Excluding C losses during wood processing, NEP (Mg C ha^?1 yr^?1±1 SD) for the stands was estimated to be ?1.9±0.7, ?0.4±0.6, 0.4±0.9, 0.4±1.0, and ?0.2±0.7 (negative values indicate net sources to the atmosphere.) We also calculated NEP values from the changes in TEC among stands. Only the 0‐year‐old stand showed significantly different NEP between the two methods, suggesting a possible mismatch for the chronosequence. The spatial and methodological uncertainties allow us to say little for certain except that the stand becomes a source of C to the atmosphere following logging.     

Respiratory carbon use and carbon storage in mid‐rotation loblolly pine (Pinus taeda L.) plantations: the effect of site resources on the stand carbon balance

We used estimates of autotrophic respiration (R_A), net primary productivity (NPP) and soil CO₂ evolution (S_ff), to develop component carbon budgets for 12‐year‐old loblolly pine plantations during the fifth year of a fertilization and irrigation experiment. Annual carbon use in R_A was 7.5, 9.0, 15.0, and 15.1 Mg C ha^?1 in control (C), irrigated (I), fertilized (F) and irrigated and fertilized (IF) treatments, respectively. Foliage, fine root and perennial woody tissue (stem, branch, coarse and taproot) respiration accounted for, respectively, 37%, 24%, and 39% of R_A in C and I treatments and 38%, 12% and 50% of R_A in F and IF treatments. Annual gross primary production (GPP=NPP+R_A) ranged from 13.1 to 26.6 Mg C ha^?1. The I, F, and IF treatments resulted in a 21, 94, and 103% increase in GPP, respectively, compared to the C treatment. Despite large treatment differences in NPP, R_A, and carbon allocation, carbon use efficiency (CUE=NPP/GPP) averaged 0.42 and was unaffected by manipulating site resources. Ecosystem respiration (R_E), the sum of S_ff, and above ground R_A, ranged from 12.8 to 20.2 Mg C ha^?1 yr^?1. S_ff contributed the largest proportion of R_E, but the relative importance of S_ff decreased from 0.63 in C treatments to 0.47 in IF treatments because of increased aboveground R_A. Aboveground woody tissue R_A was 15% of R_E in C and I treatments compared to 25% of R_E in F and IF treatments. Net ecosystem productivity (NEP=GPP‐R_E) was roughly 0 in the C and I treatments and 6.4 Mg C ha^?1 yr^?1 in F and IF treatments, indicating that non‐fertilized treatments were neither a source nor a sink for atmospheric carbon while fertilized treatments were carbon sinks. In these young stands, NEP is tightly linked to NPP; increased ecosystem carbon storage results mainly from an increase in foliage and perennial woody biomass.     

Response of carbon fluxes to drought in a coastal plain loblolly pine forest

Full accounting of ecosystem carbon (C) pools and fluxes in coastal plain ecosystems remains less studied compared with upland systems, even though the C stocks in these systems may be up to an order of magnitude higher, making them a potentially important component in regional C cycle. Here, we report C pools and CO₂ exchange rates during three hydrologically contrasting years (i.e. 2005–2007) in a coastal plain loblolly pine plantation in North Carolina, USA. The daily temperatures were similar among the study years and to the long‐term (1971–2000) average, whereas the amount and timing of precipitation differed significantly. Precipitation was the largest in 2005 (147 mm above normal), intermediate in 2006 (48 mm below) and lowest in 2007 (486 mm below normal). The forest was a strong C sink during all years, sequestering 361 ± 67 (2005), 835 ± 55 (2006) and 724 ± 55 (2007) g C m^?2 yr^?1 according to eddy covariance measurements of net ecosystem CO₂ exchange (NEE). The interannual differences in NEE were traced to drought‐induced declines in canopy and whole tree hydraulic conductances, which declined with growing precipitation deficit and decreasing soil volumetric water content (VWC). In contrast, the interannual differences were small in gross ecosystem productivity (GEP) and ecosystem respiration (ER), both seemingly insensitive to drought. However, the drought sensitivity of GEP was masked by higher leaf area index and higher photosynthetically active radiation during the dry year. Normalizing GEP by these factors enhanced interannual differences, but there were no signs of suppressed GEP at low VWC during any given year. Although ER was very consistent across the 3 years, and not suppressed by low VWC, the total respiratory cost as a fraction of net primary production increased with annual precipitation and the contribution of heterotrophic respiration (R_h) was significantly higher during the wettest year, exceeding new litter inputs by 58%. Although the difference was smaller during the other 2 years (R_h : litterfall ratio was 1.05 in 2006 and 1.10 in 2007), the soils lost about 109 g C m^?2 yr^?1, outlining their potential vulnerability to decomposition, and pointing to potential management considerations to protect existing soil C stocks.     

Grassland to shrubland state transitions enhance carbon sequestration in the northern Chihuahuan Desert

The replacement of native C₄‐dominated grassland by C₃‐dominated shrubland is considered an ecological state transition where different ecological communities can exist under similar environmental conditions. These state transitions are occurring globally, and may be exacerbated by climate change. One consequence of the global increase in woody vegetation may be enhanced ecosystem carbon sequestration, although the responses of arid and semiarid ecosystems may be highly variable. During a drier than average period from 2007 to 2011 in the northern Chihuahuan Desert, we found established shrubland to sequester 49 g C m^?2 yr^?1 on average, while nearby native C₄ grassland was a net source of 31 g C m^?2 yr^?1 over this same period. Differences in C exchange between these ecosystems were pronounced – grassland had similar productivity compared to shrubland but experienced higher C efflux via ecosystem respiration, while shrubland was a consistent C sink because of a longer growing season and lower ecosystem respiration. At daily timescales, rates of carbon exchange were more sensitive to soil moisture variation in grassland than shrubland, such that grassland had a net uptake of C when wet but lost C when dry. Thus, even under unfavorable, drier than average climate conditions, the state transition from grassland to shrubland resulted in a substantial increase in terrestrial C sequestration. These results illustrate the inherent tradeoffs in quantifying ecosystem services that result from ecological state transitions, such as shrub encroachment. In this case, the deleterious changes to ecosystem services often linked to grassland to shrubland state transitions may at least be partially offset by increased ecosystem carbon sequestration.     

Anthropogenic nitrogen deposition enhances carbon sequestration in boreal soils

It is proposed that carbon (C) sequestration in response to reactive nitrogen (N_r) deposition in boreal forests accounts for a large portion of the terrestrial sink for anthropogenic CO₂ emissions. While studies have helped clarify the magnitude by which N_r deposition enhances C sequestration by forest vegetation, there remains a paucity of long‐term experimental studies evaluating how soil C pools respond. We conducted a long‐term experiment, maintained since 1996, consisting of three N addition levels (0, 12.5, and 50 kg N ha^?1 yr^?1) in the boreal zone of northern Sweden to understand how atmospheric N_r deposition affects soil C accumulation, soil microbial communities, and soil respiration. We hypothesized that soil C sequestration will increase, and soil microbial biomass and soil respiration will decrease, with disproportionately large changes expected compared to low levels of N addition. Our data showed that the low N addition treatment caused a non‐significant increase in the organic horizon C pool of ~15% and a significant increase of ~30% in response to the high N treatment relative to the control. The relationship between C sequestration and N addition in the organic horizon was linear, with a slope of 10 kg C kg^?1 N. We also found a concomitant decrease in total microbial and fungal biomasses and a ~11% reduction in soil respiration in response to the high N treatment. Our data complement previous data from the same study system describing aboveground C sequestration, indicating a total ecosystem sequestration rate of 26 kg C kg^?1 N. These estimates are far lower than suggested by some previous modeling studies, and thus will help improve and validate current modeling efforts aimed at separating the effect of multiple global change factors on the C balance of the boreal region.     

Estimates of soil respiration and net primary production of three forests at different succession stages in South China

Soil respiration (heterotropic and autotropic respiration, R_g) and aboveground litter fall carbon were measured at three forests at different succession (early, middle and advanced) stages in Dinghushan Biosphere Reserve, Southern China. It was found that the soil respiration increases exponentially with soil temperature at 5 cm depth (T_s) according to the relation R_g=a exp(bT_s), and the more advanced forest community during succession has a higher value of a because of higher litter carbon input than the forests at early or middle succession stages. It was also found that the monthly soil respiration is linearly correlated with the aboveground litter carbon input of the previous month. Using measurements of aboveground litter and soil respiration, the net primary productions (NPPs) of three forests were estimated using nonlinear inversion. They are 475, 678 and 1148 g C m^?2 yr^?1 for the Masson pine forest (MPF), coniferous and broad‐leaf mixed forest (MF) and subtropical monsoon evergreen broad‐leaf forest (MEBF), respectively, in year 2003/2004, of which 54%, 37% and 62% are belowground NPP for those three respective forests if no change in live plant biomass is assumed. After taking account of the decrease in live plant biomass, we estimated the NPP of the subtropical MEBF is 970 g C m^?2 yr^?1 in year 2003/2004. Total amount of carbon allocated below ground for plant roots is 388 g C m^?2 yr^?1 for the MPF, 504 g C m^?2 yr^?1 for the coniferous and broad‐leaf MF and 1254 g C m^?2 yr^?1 for the subtropical MEBF in 2003/2004. Our results support the hypothesis that the amount of carbon allocation belowground increases during forest succession.     

Seasonal and annual variation of carbon exchange in an evergreen Mediterranean forest in southern France

We present 9 years of eddy covariance measurements made over an evergreen Mediterranean forest in southern France. The goal of this study was to quantify the different components of the carbon (C) cycle, gross primary production (GPP) and ecosystem respiration (R_eco), and to assess the effects of climatic variables on these fluxes and on the net ecosystem exchange of carbon dioxide. The Puéchabon forest acted as a net C sink of ?254 g C m^?2 yr^?1, with a GPP of 1275 g C m^?2 yr^?1 and a R_eco of 1021 g C m^?2 yr^?1. On average, 83% of the net annual C sink occurred between March and June. The effects of exceptional events such the insect‐induced partial canopy defoliation that occurred in spring 2005, and the spring droughts of 2005 and 2006 are discussed. A high interannual variability of ecosystem C fluxes during summer and autumn was observed but the resulting effect on the annual net C budget was moderate. Increased severity and/or duration of summer drought under climate change do not appear to have the potential to negatively impact the average C budget of this ecosystem. On the contrary, factors affecting ecosystem functioning (drought and/or defoliation) during March–June period may reduce dramatically the annual C balance of evergreen Mediterranean forests.     

Contrasting effects of low and high nitrogen additions on soil CO2 flux components and ectomycorrhizal fungal sporocarp production in a boreal forest

Nitrogen (N) added through atmospheric deposition or as fertilizer to boreal and temperate forests reduces both soil decomposer activity (heterotrophic respiration) and the activity of roots and mycorrhizal fungi (autotrophic respiration). However, these negative effects have been found in studies that applied relatively high levels of N, whereas the responses to ambient atmospheric N deposition rates are still not clear. Here, we compared an unfertilized control boreal forest with a fertilized forest (100 kg N ha^?1 yr^?1) and a forest subject to N‐deposition rates comparable to those in Central Europe (20 kg N ha^?1 yr^?1) to investigate the effects of N addition rate on different components of forest floor respiration and the production of ectomycorrhizal fungal sporocarps. Soil collars were used to partition heterotrophic (R_h) and autotrophic (R_a) respiration, which was further separated into respiration by tree roots (R_tr) and mycorrhizal hyphae (R_m). Total forest floor respiration was twice as high in the low N plot compared to the control, whereas there were no differences between the control and high N plot. There were no differences in R_h respiration among plots. The enhanced forest floor respiration in the low N plot was, therefore, the result of increased R_a respiration, with an increase in R_tr respiration, and a doubling of R_m respiration. The latter was corroborated by a slightly greater ectomycorrhizal (EM) fungal sporocarp production in the low N plot as compared to the control plot. In contrast, EM fungal sporocarp production was nearly eliminated, and R_m respiration severely reduced, in the high N plot, which resulted in significantly lower R_a respiration. We thus found a nonlinear response of the R_a components to N addition rate, which calls for further studies of the quantitative relations among N addition rate, plant photosynthesis and carbon allocation, and the function of EM fungi.     

Effects of seasonality,transport pathway,and spatial structure on greenhouse gas fluxes in a restored wetland

Wetlands can influence global climate via greenhouse gas (GHG) exchange of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Few studies have quantified the full GHG budget of wetlands due to the high spatial and temporal variability of fluxes. We report annual open‐water diffusion and ebullition fluxes of CO₂, CH₄, and N₂O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy‐covariance measurements of whole‐ecosystem CO₂ and CH₄ exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open‐water and vegetated cover types. Annual open‐water CO₂, CH₄, and N₂O emissions were 915 ± 95 g C‐CO₂ m^?2 yr^?1, 2.9 ± 0.5 g C‐CH₄ m^?2 yr^?1, and 62 ± 17 mg N‐N₂O m^?2 yr^?1, respectively. Diffusion dominated open‐water GHG transport, accounting for >99% of CO₂ and N₂O emissions, and ~71% of CH₄ emissions. Seasonality was minor for CO₂ emissions, whereas CH₄ and N₂O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open‐water fluxes (3.5 ± 0.3 kg CO₂‐eq m^?2 yr^?1) exceeded that of vegetated zones (1.4 ± 0.4 kg CO₂‐eq m^?2 yr^?1) due to high ecosystem respiration. After scaling results to the entire wetland using object‐based cover classification of remote sensing imagery, net uptake of CO₂ (?1.4 ± 0.6 kt CO₂‐eq yr^?1) did not offset CH₄ emission (3.7 ± 0.03 kt CO₂‐eq yr^?1), producing an overall positive radiative forcing effect of 2.4 ± 0.3 kt CO₂‐eq yr^?1. These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.     

A large proportion of North American net ecosystem production is offset by emissions from harvested products,river/stream evasion,and biomass burning

Diagnostic carbon cycle models produce estimates of net ecosystem production (NEP, the balance of net primary production and heterotrophic respiration) by integrating information from (i) satellite‐based observations of land surface vegetation characteristics; (ii) distributed meteorological data; and (iii) eddy covariance flux tower observations of net ecosystem exchange (NEE) (used in model parameterization). However, a full bottom‐up accounting of NEE (the vertical carbon flux) that is suitable for integration with atmosphere‐based inversion modeling also includes emissions from decomposition/respiration of harvested forest and agricultural products, CO₂ evasion from streams and rivers, and biomass burning. Here, we produce a daily time step NEE for North America for the year 2004 that includes NEP as well as the additional emissions. This NEE product was run in the forward mode through the CarbonTracker inversion setup to evaluate its consistency with CO₂ concentration observations. The year 2004 was climatologically favorable for NEP over North America and the continental total was estimated at 1730 ± 370 TgC yr^?1 (a carbon sink). Harvested product emissions (316 ± 80 TgC yr^?1), river/stream evasion (158 ± 50 TgC yr^?1), and fire emissions (142 ± 45 TgC yr^?1) counteracted a large proportion (35%) of the NEP sink. Geographic areas with strong carbon sinks included Midwest US croplands, and forested regions of the Northeast, Southeast, and Pacific Northwest. The forward mode run with CarbonTracker produced good agreement between observed and simulated wintertime CO₂ concentrations aggregated over eight measurement sites around North America, but overestimates of summertime concentrations that suggested an underestimation of summertime carbon uptake. As terrestrial NEP is the dominant offset to fossil fuel emission over North America, a good understanding of its spatial and temporal variation – as well as the fate of the carbon it sequesters ─ is needed for a comprehensive view of the carbon cycle.     

Carbon footprint of rice production under biochar amendment – a case study in a Chinese rice cropping system

As a controversial strategy to mitigate global warming, biochar application into soil highlights the need for life cycle assessment before large‐scale practice. This study focused on the effect of biochar on carbon footprint of rice production. A field experiment was performed with three treatments: no residue amendment (Control), 6 t ha^?1 yr^?1 corn straw (CS) amendment, and 2.4 t ha^?1 yr^?1 corn straw‐derived biochar amendment (CBC). Carbon footprint was calculated by considering carbon source processes (pyrolysis energy cost, fertilizer and pesticide input, farmwork, and soil greenhouse gas emissions) and carbon sink processes (soil carbon increment and energy offset from pyrolytic gas). On average over three consecutive rice‐growing cycles from year 2011 to 2013, the CS treatment had a much higher carbon intensity of rice (0.68 kg CO₂‐C equivalent (CO₂‐C_e) kg^?1 grain) than that of Control (0.24 kg CO₂‐C_e kg^?1 grain), resulting from large soil CH₄ emissions. Biochar amendment significantly increased soil carbon pool and showed no significant effect on soil total N₂O and CH₄ emissions relative to Control; however, due to a variation in net electric energy input of biochar production based on different pyrolysis settings, carbon intensity of rice under CBC treatment ranged from 0.04 to 0.44 kg CO₂‐C_e kg^?1 grain. The results indicated that biochar strategy had the potential to significantly reduce the carbon footprint of crop production, but the energy‐efficient pyrolysis technique does matter.