Fine root dynamics and trace gas fluxes in two lowland tropical forest soils

Fine root dynamics have the potential to contribute significantly to ecosystem‐scale biogeochemical cycling, including the production and emission of greenhouse gases. This is particularly true in tropical forests which are often characterized as having large fine root biomass and rapid rates of root production and decomposition. We examined patterns in fine root dynamics on two soil types in a lowland moist Amazonian forest, and determined the effect of root decay on rates of C and N trace gas fluxes. Root production averaged 229 (±35) and 153 (±27) g m^?2 yr^?1 for years 1 and 2 of the study, respectively, and did not vary significantly with soil texture. Root decay was sensitive to soil texture with faster rates in the clay soil (k=?0.96 year^?1) than in the sandy loam soil (k=?0.61 year^?1), leading to greater standing stocks of dead roots in the sandy loam. Rates of nitrous oxide (N₂O) emissions were significantly greater in the clay soil (13±1 ng N cm^?2 h^?1) than in the sandy loam (1.4±0.2 ng N cm^?2 h^?1). Root mortality and decay following trenching doubled rates of N₂O emissions in the clay and tripled them in sandy loam over a 1‐year period. Trenching also increased nitric oxide fluxes, which were greater in the sandy loam than in the clay. We used trenching (clay only) and a mass balance approach to estimate the root contribution to soil respiration. In clay soil root respiration was 264–380 g C m^?2 yr^?1, accounting for 24% to 35% of the total soil CO₂ efflux. Estimates were similar using both approaches. In sandy loam, root respiration rates were slightly higher and more variable (521±206 g C m² yr^?1) and contributed 35% of the total soil respiration. Our results show that soil heterotrophs strongly dominate soil respiration in this forest, regardless of soil texture. Our results also suggest that fine root mortality and decomposition associated with disturbance and land‐use change can contribute significantly to increased rates of nitrogen trace gas emissions.     

Contribution of aboveground litter,belowground litter,and rhizosphere respiration to total soil CO<Subscript>2</Subscript> efflux in an old growth coniferous forest

In an old growth coniferous forest located in the central Cascade Mountains, Oregon, we added or removed aboveground litter and terminated live root activity by trenching to determine sources of soil respiration. Annual soil efflux from control plots ranged from 727 g C m⁻² year⁻¹ in 2002 to 841 g C m⁻² year⁻¹ in 2003. We used aboveground litter inputs (149.6 g C m⁻² year⁻¹) and differences in soil CO₂ effluxes among treatment plots to calculate contributions to total soil efflux by roots and associated rhizosphere organisms and by heterotrophic decomposition of organic matter derived from aboveground and belowground litter. On average, root and rhizospheric respiration (R_r) contributed 23%, aboveground litter decomposition contributed 19%, and belowground litter decomposition contributed 58% to total soil CO₂ efflux, respectively. These values fall within the range of values reported elsewhere, although our estimate of belowground litter contribution is higher than many published estimates, which we argue is a reflection of the high degree of mycorrhizal association and low nutrient status of this ecosystem. Additionally, we found that measured fluxes from plots with doubled needle litter led to an additional 186 g C m⁻² year⁻¹ beyond that expected based on the amount of additional carbon added; this represents a priming effect of 187%, or a 34% increase in the total carbon flux from the plots. This finding has strong implications for soil C storage, showing that it is inaccurate to assume that increases in net primary productivity will translate simply and directly into additional belowground storage.     

Nitrogen addition reduces soil respiration in a mature tropical forest in southern China

Response of soil respiration (CO₂ emission) to simulated nitrogen (N) deposition in a mature tropical forest in southern China was studied from October 2005 to September 2006. The objective was to test the hypothesis that N addition would reduce soil respiration in N saturated tropical forests. Static chamber and gas chromatography techniques were used to quantify the soil respiration, following four‐levels of N treatments (Control, no N addition; Low‐N, 5 g N m^?2 yr^?1; Medium‐N, 10 g N m^?2 yr^?1; and High‐N, 15 g N m^?2 yr^?1 experimental inputs), which had been applied for 26 months before and continued throughout the respiration measurement period. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates found in the warm and wet growing season (April–September) and the lowest rates in the dry dormant season (December–February). Soil respiration rates showed a significant positive exponential relationship with soil temperature, whereas soil moisture only affect soil respiration at dry conditions in the dormant season. Annual accumulative soil respiration was 601±30 g CO₂‐C m^?2 yr^?1 in the Controls. Annual mean soil respiration rate in the Control, Low‐N and Medium‐N treatments (69±3, 72±3 and 63±1 mg CO₂‐C m^?2 h^?1, respectively) did not differ significantly, whereas it was 14% lower in the High‐N treatment (58±3 mg CO₂‐C m^?2 h^?1) compared with the Control treatment, also the temperature sensitivity of respiration, Q₁₀ was reduced from 2.6 in the Control with 2.2 in the High‐N treatment. The decrease in soil respiration occurred in the warm and wet growing season and were correlated with a decrease in soil microbial activities and in fine root biomass in the N‐treated plots. Our results suggest that response of soil respiration to atmospheric N deposition in tropical forests is a decline, but it may vary depending on the rate of N deposition.     

Effects of experimental drought on soil respiration and radiocarbon efflux from a temperate forest soil

Soil moisture affects microbial decay of SOM and rhizosphere respiration (RR) in temperate forest soils, but isolating the response of soil respiration (SR) to summer drought and subsequent wetting is difficult because moisture changes are often confounded with temperature variation. We distinguished between temperature and moisture effects by simulation of prolonged soil droughts in a mixed deciduous forest at the Harvard Forest, Massachusetts. Roofs constructed over triplicate 5 × 5 m² plots excluded throughfall water during the summers of 2001 (168 mm) and 2002 (344 mm), while adjacent control plots received ambient throughfall and the same natural temperature regime. In 2003, throughfall was not excluded to assess the response of SR under natural weather conditions after two prolonged summer droughts. Throughfall exclusion significantly decreased mean SR rate by 53 mg C m^?2 h^?1 over 84 days in 2001, and by 68 mg C m^?2 h^?1 over 126 days in 2002, representing 10–30% of annual SR in this forest and 35–75% of annual net ecosystem exchange (NEE) of C. The differences in SR were best explained by differences in gravimetric water content in the Oi horizon (r²=0.69) and the Oe/Oa horizon (r²=0.60). Volumetric water content of the A horizon was not significantly affected by throughfall exclusion. The radiocarbon signature of soil CO₂ efflux and of CO₂ respired during incubations of O horizon, A horizon and living roots allowed partitioning of SR into contributions from young C substrate (including RR) and from decomposition of older SOM. RR (root respiration and microbial respiration of young substrates in the rhizosphere) made up 43–71% of the total C respired in the control plots and 41–80% in the exclusion plots, and tended to increase with drought. An exception to this trend was an interesting increase in CO₂ efflux of radiocarbon‐rich substrates during a period of abundant growth of mushrooms. Our results suggest that prolonged summer droughts decrease primarily heterotrophic respiration in the O horizon, which could cause increases in the storage of soil organic carbon in this forest. However, the C stored during two summers of simulated drought was only partly released as increased respiration during the following summer of natural throughfall. We do not know if this soil C sink during drought is transient or long lasting. In any case, differential decomposition of the O horizon caused by interannual variation of precipitation probably contributes significantly to observed interannual variation of NEE in temperate forests.     

Respiration and annual fungal production associated with decomposing leaf litter in two streams

1. We compared fungal biomass, production and microbial respiration associated with decomposing leaves in one softwater stream (Payne Creek) and one hardwater stream (Lindsey Spring Branch). 2. Both streams received similar annual leaf litter fall (478–492 g m^?2), but Lindsey Spring Branch had higher average monthly standing crop of leaf litter (69 ± 24 g m^?2; mean ± SE) than Payne Creek (39 ± 9 g m^?2). 3. Leaves sampled from Lindsey Spring Branch contained a higher mean concentration of fungal biomass (71 ± 11 mg g^?1) than those from Payne Creek (54 ± 8 mg g^?1). Maximum spore concentrations in the water of Lindsay Spring Branch were also higher than those in Payne Creek. These results agreed with litterbag studies of red maple (Acer rubrum) leaves, which decomposed faster (decay rate of 0.014 versus 0.004 day^?1), exhibited higher maximum fungal biomass and had higher rates of fungal sporulation in Lindsey Spring Branch than in Payne Creek. 4. Rates of fungal production and respiration per g leaf were similar in the two streams, although rates of fungal production and respiration per square metre were higher in Lindsey Spring Branch than in Payne Creek because of the differences in leaf litter standing crop. 5. Annual fungal production was 16 ± 6 g m^?2 (mean ± 95% CI) in Payne Creek and 46 ± 25 g m^?2 in Lindsey Spring Branch. Measurements were taken through the autumn of 2 years to obtain an indication of inter‐year variability. Fungal production during October to January of the 2 years varied between 3 and 6 g m^?2 in Payne Creek and 7–27 g m^?2 in Lindsey Spring Branch. 6. Partial organic matter budgets constructed for both streams indicated that 3 ± 1% of leaf litter fall went into fungal production and 7 ± 2% was lost as respiration in Payne Creek. In Lindsey Spring Branch, fungal production accounted for 10 ± 5% of leaf litter fall and microbial respiration for 13 ± 9%.     

高寒矮嵩草草甸冬季CO2释放特征

冬季碳排放在高寒草地年内碳平衡中占有重要位置。为探讨高寒草地冬季碳排放特征及温度敏感性,于2003-2005年在中国科学院海北高寒草甸生态系统研究站,利用密闭箱-气相色谱法连续观测了高寒矮嵩草草甸2个冬季的生态系统、土壤呼吸通量特征。结果表明:1)高寒矮嵩草草甸冬季生态系统呼吸、土壤呼吸均具有明显的日变化和季节变化规律,温度是其主要的控制因子,能够解释44%以上的呼吸速率变异。2)冬季生态系统呼吸与土壤呼吸速率在统计上没有显著差异,土壤呼吸占生态系统呼吸的比例高达85%以上。3)2003-2004年冬季生态系统呼吸、土壤呼吸的Q₁₀值分别为1.53,1.38;2004-2005年冬季生态系统呼吸与土壤呼吸的Q₁₀值为1.86,1.68,2个冬季生态系统呼吸的Q₁₀值均高于土壤呼吸。4)未发现高寒矮嵩草草甸冷冬年份的Q₁₀值高于暖冬年份以及冬季的Q₁₀值高于生长季。     

Linking Belowground Carbon Allocation to Anaerobic CH4 and CO2 Production in a Forested Peatland,New York State

Heterotrophic soil microorganisms rely on carbon (C) allocated belowground in plant production, but belowground C allocation (BCA) by plants is a poorly quantified part of ecosystem C cycling, especially, in peat soil. We applied a C balance approach to quantify BCA in a mixed conifer-red maple (Acer rubrum) forest on deep peat soil. Direct measurements of CH₄ and CO₂ fluxes across the soil surface (soil respiration), production of fine and small plant roots, and aboveground litterfall were used to estimate respiration by roots, by mycorrhizae and by free-living soil microorganisms. Measurements occurred in two consecutive years. Soil respiration rates averaged 1.2 bm μmol m^{? 2} s^{? 1} for CO₂ and 0.58 nmol m^{? 2} s^{? 1} for CH₄ (371 to 403 g C m^{? 2} year^{? 1}). Carbon in aboveground litter (144 g C m^{? 2} year^{? 1}) was 84% greater than C in root production (78 g C m^{? 2} year^{? 1}). Complementary in vitro assays located high rates of anaerobic microbial activity, including methanogenesis, in a dense layer of roots overlying the peat soil and in large-sized fragments within the peat matrix. Large-sized fragments were decomposing roots and aboveground leaf and twig litter, indicating that relatively fresh plant production supported most of the anaerobic microbial activity. Respiration by free-living soil microorganisms in deep peat accounted for, at most, 29 to 38 g C m^{? 2} year^{? 1}. These data emphasize the close coupling between plant production, ecosystem-level C cycling and soil microbial ecology, which BCA can help reveal.     

Effects of Elevated Atmospheric Carbon Dioxide and Temperature on Soil Respiration in a Boreal Forest Using δ13C as a Labeling Tool

This study examines the effect of elevated atmospheric carbon dioxide [CO₂] (+340 ppm, ¹³C-depleted) and/or elevated air temperature (2.8–3.5°C) on the rate and δ¹³C of soil respiration. The study was conducted in a boreal Norway spruce forest using temperature-controlled whole-tree chambers and ¹³C as a marker for root respiration. The δ¹³C of needle carbohydrates was followed after the onset of the CO₂ treatment in August 2001 and during a 2.5-week period in the summer of 2002. Averaged over the growing seasons of 2002 and 2003, we observed a 48% and 62% increase, respectively, in soil respiration in response to elevated [CO₂], but no response to elevated air temperature. The percentage increase in response to elevated [CO₂] varied seasonally (between 10% and 190% relative to the control), but the absolute increase varied less (39 ± 11 mg C m⁻² h⁻¹; mean ± SD). Data on δ¹³C of soil respiration indicate that this increase in soil respiration rate resulted from increased root/rhizosphere respiration of recently fixed carbon. Our results support the hypothesis that root/rhizosphere respiration is sensitive to variation in substrate availability.     

A paired study of prairie carbon stocks, fluxes, and phenology: comparing the world's oldest prairie restoration with an adjacent remnant

We measured carbon (C) stocks and fluxes and vegetation phenology in the world's oldest prairie restoration (∼65 years) and an adjacent prairie remnant in southern Wisconsin from 2001–2004 to quantify structural and functional differences. While the species distributions and frequency differed, the number of species measured per 1 m² quadrat were not significantly different (15.8±4.4 and 14.1±2.1 for remnant and planted [order for all reported values in abstract]; P=0.29), and the annual average aboveground net primary productivity (271±51 and 330±55 g C m⁻²) and peak leaf area index (2.9–4.9 m² m⁻²) were comparable under similar fire management. Total root biomass was not significantly different in 2002 (1736±1062 and 1690±459 g dry matter m⁻²) or 2003 (3029±2081 and 2146±898 g m⁻²), but annual average soil respiration (1229±77 and 1428±24 g C m⁻² yr⁻¹) was significantly higher in the restoration (P<0.0001). However, the prairie remnant contained 37% greater soil C (P<0.0001) in the top 25 cm. Soil respiration response to 10 cm soil temperature (Q₁₀) varied with respect to prairie and soil moisture conditions as annual Q₁₀ values ranged from 2.5 to 3.6. We calculated a range of net ecosystem production (NEP) values using estimated heterotrophic respiration and three root turnover values. Average NEP varied from −1.4 to 1.9 and −2.3 to 1.3 Mg C ha⁻¹ yr⁻¹ for the remnant and planted prairies, respectively. While these two prairies share similar structural components and functional attributes, the large uncertainty in NEP casts doubt as to whether we can verify these prairies as C sources or sinks without direct measures of heterotrophic respiration and root turnover. We argue that quantitative studies of C exchange in prairies, which differ in restoration methodology, management intensity, and fire frequency, are needed to solidify the relationship between prairie structure and potentially desired functions such as C sequestration.     

Simulated chronic NO3− deposition reduces soil respiration in northern hardwood forests

Chronic N additions to forest ecosystems can enhance soil N availability, potentially leading to reduced C allocation to root systems. This in turn could decrease soil CO₂ efflux. We measured soil respiration during the first, fifth, sixth and eighth years of simulated atmospheric NO₃^? deposition (3 g N m^?2 yr^?1) to four sugar maple‐dominated northern hardwood forests in Michigan to assess these possibilities. During the first year, soil respiration rates were slightly, but not significantly, higher in the NO₃^?‐amended plots. In all subsequent measurement years, soil respiration rates from NO₃^?‐amended soils were significantly depressed. Soil temperature and soil matric potential were measured concurrently with soil respiration and used to develop regression relationships for predicting soil respiration rates. Estimates of growing season and annual soil CO₂ efflux made using these relationships indicate that these C fluxes were depressed by 15% in the eighth year of chronic NO₃^? additions. The decrease in soil respiration was not due to reduced C allocation to roots, as root respiration rates, root biomass, and root turnover were not significantly affected by N additions. Aboveground litter also was unchanged by the 8 years of treatment. Of the remaining potential causes for the decline in soil CO₂ efflux, reduced microbial respiration appears to be the most likely possibility. Documented reductions in microbial biomass and the activities of extracellular enzymes used for litter degradation on the NO₃^?‐amended plots are consistent with this explanation.     

Effects of nutrient additions on ecosystem carbon cycle in a Puerto Rican tropical wet forest

Wet tropical forests play a critical role in global ecosystem carbon (C) cycle, but C allocation and the response of different C pools to nutrient addition in these forests remain poorly understood. We measured soil organic carbon (SOC), litterfall, root biomass, microbial biomass and soil physical and chemical properties in a wet tropical forest from May 1996 to July 1997 following a 7‐year continuous fertilization. We found that although there was no significant difference in total SOC in the top 0–10 cm of the soils between the fertilization plots (5.42±0.18 kg m^?2) and the control plots (5.27±0.22 kg m^?2), the proportion of the heavy‐fraction organic C in the total SOC was significantly higher in the fertilized plots (59%) than in the control plots (46%) (P<0.05). The annual decomposition rate of fertilized leaf litter was 13% higher than that of the control leaf litter. We also found that fertilization significantly increased microbial biomass (fungi+bacteria) with 952±48 mg kg^?1soil in the fertilized plots and 755±37 mg kg^?1soil in the control plots. Our results suggest that fertilization in tropical forests may enhance long‐term C sequestration in the soils of tropical wet forests.     

Response of soil respiration to simulated N deposition in a disturbed and a rehabilitated tropical forest in southern China

Responses of soil respiration (CO₂ emission) to simulated N deposition were studied in a disturbed (reforested forest with previous understory and litter harvesting) and a rehabilitated (reforested forest with no understory and litter harvesting) tropical forest in southern China from October 2005 to September 2006. The objectives of the study were to test the following hypotheses: (1) soil respiration is higher in rehabilitated forest than in disturbed forest; (2) soil respiration in both rehabilitated and disturbed tropical forests is stimulated by N additions; and (3) soil respiration is more sensitive to N addition in disturbed forest than in rehabilitated forest due to relatively low soil nutrient status in the former, resulting from different previous human disturbance. Static chamber and gas chromatography techniques were employed to quantify the soil respiration, following different N treatments (Control, no N addition; Low-N, 5 g N m⁻² year⁻¹; Medium-N, 10 g N m⁻² year⁻¹), which had been applied continuously for 26 months before the respiration measurement. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates observed in the hot and wet growing season (April–September) and the lowest rates in winter (December–February) in both rehabilitated and disturbed forests. Soil respiration rates exhibited significant positive exponential relationship with soil temperature and significant positive linear relationship with soil moisture. Soil respiration was also significantly higher in the rehabilitated forest than in the disturbed forest. Annual mean soil respiration rate in the rehabilitated forest was 20% lower in low-N plots (71 ± 4 mg CO₂-C m⁻² h⁻¹) and 10% lower in medium-N plots (80 ± 4 mg CO₂-C m⁻² h⁻¹) than in the control plots (89 ± 5 mg CO₂-C m⁻² h⁻¹), and the differences between the control and low-N or medium-N treatments were statistically significant. In disturbed forest, annual mean soil respiration rate was 5% lower in low-N plots (63 ± 3 mg CO₂-C m⁻² h⁻¹) and 8% lower in medium-N plots (61 ± 3 mg CO₂-C m⁻² h⁻¹) than in the control plots (66 ± 4 mg CO₂-C m⁻² h⁻¹), but the differences among treatments were not significant. The depressed effects of experimental N deposition occurred mostly in the hot and wet growing season. Our results suggest that response of soil respiration to elevated N deposition in the reforested tropical forests may vary depending on the status of human disturbance. Responsible Editor: Hans Lambers.     

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.     

Elevated CO2 and temperature impacts on different components of soil CO2 efflux in Douglas-fir terracosms

Although numerous studies indicate that increasing atmospheric CO₂ or temperature stimulate soil CO₂ efflux, few data are available on the responses of three major components of soil respiration [i.e. rhizosphere respiration (root and root exudates), litter decomposition, and oxidation of soil organic matter] to different CO₂ and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO₂ and elevated temperature on these components of soil CO₂ efflux in Douglas-fir terracosms. We measured both soil CO₂ efflux rates and the ¹³C and ¹⁸O isotopic compositions of soil CO₂ efflux in 12 sun-lit and environmentally controlled terracosms with 4-year-old Douglas fir seedlings and reconstructed forest soils under two CO₂ concentrations (ambient and 200 ppmv above ambient) and two air temperature regimes (ambient and 4 °C above ambient). The stable isotope data were used to estimate the relative contributions of different components to the overall soil CO₂ efflux. In most cases, litter decomposition was the dominant component of soil CO₂ efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO₂ concentration and elevated temperature stimulated rhizosphere respiration and litter decomposition. The oxidation of soil organic matter was stimulated only by increasing temperature. Release of newly fixed carbon as root respiration was the most responsive to elevated CO₂, while soil organic matter decomposition was most responsive to increasing temperature. Although some assumptions associated with this new method need to be further validated, application of this dual-isotope approach can provide new insights into the responses of soil carbon dynamics in forest ecosystems to future climate changes.     

Ecosystem–atmosphere exchange of CH4 and N2O and ecosystem respiration in wetlands in the Sanjiang Plain, Northeastern China

Natural wetlands are critically important to global change because of their role in modulating atmospheric concentrations of CO₂, CH₄, and N₂O. One 4‐year continuous observation was conducted to examine the exchanges of CH₄ and N₂O between three wetland ecosystems and the atmosphere as well as the ecosystem respiration in the Sanjiang Plain in Northeastern China. From 2002 to 2005, the mean annual budgets of CH₄ and N₂O, and ecosystem respiration were 39.40 ± 6.99 g C m^?2 yr^?1, 0.124 ± 0.05 g N m^?2 yr^?1, and 513.55 ± 8.58 g C m^?2 yr^?1 for permanently inundated wetland; 4.36 ± 1.79 g C m^?2 yr^?1, 0.11 ± 0.12 g N m^?2 yr^?1, and 880.50 ± 71.72 g C m^?2 yr^?1 for seasonally inundated wetland; and 0.21 ± 0.1 g C m^?2 yr^?1, 0.28 ± 0.11 g N m^?2 yr^?1, and 1212.83 ± 191.98 g C m^?2 yr^?1 for shrub swamp. The substantial interannual variation of gas fluxes was due to the significant climatic variability which underscores the importance of long‐term continuous observations. The apparent seasonal pattern of gas emissions associated with a significant relationship of gas fluxes to air temperature implied the potential effect of global warming on greenhouse gas emissions from natural wetlands. The budgets of CH₄ and N₂O fluxes and ecosystem respiration were highly variable among three wetland types, which suggest the uncertainties in previous studies in which all kinds of natural wetlands were treated as one or two functional types. New classification of global natural wetlands in more detailed level is highly expected.     

Fate of leaf litter in restored Kandelia obovata (S. L.) mangrove forests with different ages in Jiulong River Estuary,China

Ecological processing of leaf litter plays important roles in carbon dynamics of mangrove forests. Fate of leaf litter, that is, removal by crabs, microbial decomposition, and tidal export was quantified in two restored Kandelia obovata forests with ages of 24 years and 48 years, respectively, from December 2009 to November 2010. Crab abundance was also investigated to test the role of crabs in leaf litter processing. Daily leaf litter production was 1.064 ± 0.108 g C m^?2 day^?1 at the 24‐year forest and was 0.689 ± 0.040 g C m^?2 day^?1 at the 48‐year forest. Annual mean removal of leaf litter by crabs was lower at the 24‐year forest than at the 48‐year forest (0.177 ± 0.046 g C m^?2 day^?1 vs. 0.220 ± 0.050 g C m^?2 day^?1), due to a higher crab abundance at the older forest. Microbial decomposition and change in standing stock of leaf litter on the forest floor made a negligible contribution to the annual leaf litter production. Tidal exports of leaf litter were estimated as 0.875 ± 0.090 g C m^?2 day^?1 and 0.458 ± 0.086 g C m^?2 day^?1 at the 24‐year and 48‐year forests, respectively, accounting for 82.2% and 66.5% of their daily leaf litter production. Turnover rate of leaf litter was higher at the younger forest (1.7 ± 0.4 day^?1) than the older forest (1.2 ± 0.3 day^?1). Removal of leaf litter by crabs was higher in warm months while tidal export of leaf litter showed a much less apparent seasonal pattern. Spatial variations of crab removal and tidal export of leaf litter with forest zones were observed within each forest, while microbial decomposition of leaf litter was comparable among the different zones. These indicated that the ecosystem functions of restored mangrove forest could not reach a level equivalent to those of a mature forest even 24 years after restoration.     

Productivity overshadows temperature in determining soil and ecosystem respiration across European forests

This paper presents CO₂ flux data from 18 forest ecosystems, studied in the European Union funded EUROFLUX project. Overall, mean annual gross primary productivity (GPP, the total amount of carbon (C) fixed during photosynthesis) of these forests was 1380 ± 330 gC m^?2 y^?1 (mean ±SD). On average, 80% of GPP was respired by autotrophs and heterotrophs and released back into the atmosphere (total ecosystem respiration, TER = 1100 ± 260 gC m^?2 y^?1). Mean annual soil respiration (SR) was 760 ± 340 gC m^?2 y^?1 (55% of GPP and 69% of TER). Among the investigated forests, large differences were observed in annual SR and TER that were not correlated with mean annual temperature. However, a significant correlation was observed between annual SR and TER and GPP among the relatively undisturbed forests. On the assumption that (i) root respiration is constrained by the allocation of photosynthates to the roots, which is coupled to productivity, and that (ii) the largest fraction of heterotrophic soil respiration originates from decomposition of young organic matter (leaves, fine roots), whose availability also depends on primary productivity, it is hypothesized that differences in SR among forests are likely to depend more on productivity than on temperature. At sites where soil disturbance has occurred (e.g. ploughing, drainage), soil espiration was a larger component of the ecosystem C budget and deviated from the relationship between annual SR (and TER) and GPP observed among the less‐disturbed forests. At one particular forest, carbon losses from the soil were so large, that in some years the site became a net source of carbon to the atmosphere. Excluding the disturbed sites from the present analysis reduced mean SR to 660 ± 290 gC m^?2 y^?1, representing 49% of GPP and 63% of TER in the relatively undisturbed forest ecosystems.     

Partitioning of ecosystem respiration of CO2 released during land‐use transition from temperate agricultural grassland to Miscanthus × giganteus

Conversion of large areas of agricultural grassland is inevitable if European and UK domestic production of biomass is to play a significant role in meeting demand. Understanding the impact of these land‐use changes on soil carbon cycling and stocks depends on accurate predictions from well‐parameterized models. Key considerations are cultivation disturbance and the effect of autotrophic root input stimulation on soil carbon decomposition under novel biomass crops. This study presents partitioned parameters from the conversion of semi‐improved grassland to Miscanthus bioenergy production and compares the contribution of autotrophic and heterotrophic respiration to overall ecosystem respiration of CO₂ in the first and second years of establishment. Repeated measures of respiration from within and without root exclusion collars were used to produce time‐series model integrations separating live root inputs from decomposition of grass residues ploughed in with cultivation of the new crop. These parameters were then compared to total ecosystem respiration derived from eddy covariance sensors. Average soil surface respiration was 13.4% higher in the second growing season, increasing from 2.9 to 3.29 g CO₂‐C m^?2 day^?1. Total ecosystem respiration followed a similar trend, increasing from 4.07 to 5.4 g CO₂‐C m^?2 day^?1. Heterotrophic respiration from the root exclusion collars was 32.2% lower in the second growing season at 1.20 g CO₂‐C m^?2 day^?1 compared to the previous year at 1.77 g CO₂‐C m^?2 day^?1. Of the total respiration flux over the two‐year time period, aboveground autotrophic respiration plus litter decomposition contributed 38.46% to total ecosystem respiration while belowground autotrophic respiration and stimulation by live root inputs contributed 46.44% to soil surface respiration. This figure is notably higher than mean figures for nonforest soils derived from the literature and demonstrates the importance of crop‐specific parameterization of respiration models.     

Estimating the contribution of leaf litter decomposition to soil CO2 efflux in a beech forest using 13C-depleted litter

The contribution of leaf litter decomposition to total soil CO₂ efflux (F_L/F) was evaluated in a beech (Fagus sylvatica L.) forest in eastern France. The Keeling‐plot approach was applied to estimate the isotopic composition of respired soil CO₂ from soil covered with either control (?30.32‰) or ¹³C‐depleted leaf litter (?49.96‰). The δ¹³C of respired soil CO₂ ranged from ?25.50‰ to ?22.60‰ and from ?24.95‰ to ?20.77‰, respectively, with depleted or control litter above the soil. The F_L/F ratio was calculated by a single isotope linear mixing model based on mass conservation equations. It showed seasonal variations, increasing from 2.8% in early spring to about 11.4% in mid summer, and decreasing to 4.2% just after leaf fall. Between December 2001 and December 2002, cumulated F and F_L reached 0.98 and 0.08 kg_C m^?2, respectively. On an annual basis, decomposition of fresh leaf litter accounted for 8% of soil respiration and 80% of total C loss from fresh leaf litter. The other fraction of carbon loss during leaf litter decomposition that is assumed to have entered the soil organic matter pool (i.e. 20%) represents only 0.02 kg_C m^?2.     

Annual variation in soil respiration and its components in a coppice oak forest in Central Italy

In order to investigate the annual variation of soil respiration and its components in relation to seasonal changes in soil temperature and soil moisture in a Mediterranean mixed oak forest ecosystem, we set up a series of experimental treatments in May 1999 where litter (no litter), roots (no roots, by trenching) or both were excluded from plots of 4 m². Subsequently, we measured soil respiration, soil temperature and soil moisture in each plot over a year after the forest was coppiced. The treatments did not significantly affect soil temperature or soil moisture measured over 0–10 cm depth. Soil respiration varied markedly during the year with high rates in spring and autumn and low rates in summer, coinciding with summer drought, and in winter, with the lowest temperatures. Very high respiration rates, however, were observed during the summer immediately after rainfall events. The mean annual rate of soil respiration was 2.9 µ mol m^?2 s^?1, ranging from 1.35 to 7.03 µmol m^?2 s^?1. Soil respiration was highly correlated with temperature during winter and during spring and autumn whenever volumetric soil water content was above 20%. Below this threshold value, there was no correlation between soil respiration and soil temperature, but soil moisture was a good predictor of soil respiration. A simple empirical model that predicted soil respiration during the year, using both soil temperature and soil moisture accounted for more than 91% of the observed annual variation in soil respiration. All the components of soil respiration followed a similar seasonal trend and were affected by summer drought. The Q₁₀ value for soil respiration was 2.32, which is in agreement with other studies in forest ecosystems. However, we found a Q₁₀ value for root respiration of 2.20, which is lower than recent values reported for forest sites. The fact that the seasonal variation in root growth with temperature in Mediterranean ecosystems differs from that in temperate regions may explain this difference. In temperate regions, increases in size of root populations during the growing season, coinciding with high temperatures, may yield higher apparent Q₁₀ values than in Mediterranean regions where root growth is suppressed by summer drought. The decomposition of organic matter and belowground litter were the major components of soil respiration, accounting for almost 55% of the total soil respiration flux. This proportion is higher than has been reported for mature boreal and temperate forest and is probably the result of a short‐term C loss following recent logging at the site. The relationship proposed for soil respiration with soil temperature and soil moisture is useful for understanding and predicting potential changes in Mediterranean forest ecosystems in response to forest management and climate change.