Structure of a unique inland mangrove forest assemblage in fossil lagoons on the Caribbean Coast of Mexico

Scrub mangrove wetlands colonize the intertidal zone of fossil lagoons located in carbonate continental margins along the Yucatan Peninsula of Mexico. These unique ecological types were investigated in October, 1994, by locating transects in several mangrove forests along the Caribbean coast of the peninsula. Four species of mangrove occurred at these sites including Rhizophora mangle, Avicennia germinans, Laguncularia racemosa, Conocarpus erecta. This is one of the first examples of a species rich scrub forest. The mangroves fell into three height categories: short scrub less than 1.5 m, tall scrub to 3.0 m, and basin forests between 4.5 and 6 m. Average height, diameter (dbh), basal area, and complexity index generally increased from short scrub to basin forests. Basal area, ranged from 0.16 m² ha^–1 in a short scrub forest intermixed with Cladium jamaicense to 12.9 m² ha^–1 in a basin forest. Density ranged from 1520 trees ha^–1 to over 25,000 trees ha^–1 in a short scrub forest dominated by R. mangle. The complexity index ranged from 0.01 to 8.3. Height, dbh, basal area, and complexity index were positively related. A number of trees were growing as sprouts from larger downed trunks, suggesting that hurricanes, such as Gilbert that occurred in 1988, are important in controlling the structure of these forests. These forests appear isolated from the sea, but are influenced by groundwater exchange occurring at the land-margin zone.     

杉木林年龄序列地下碳分配变化

  森林地下碳分配在森林碳平衡和碳吸存中具有重要作用, 而揭示人工林生长过程中地下碳分配变化对于人工林碳汇估算和碳汇管理等有重要意义。通过采用年龄序列方法研究了杉木(Cunninghamia lanceolata)林生长过程中地下碳分配变化特点。年龄序列为福建省南平7 a生(幼龄林)、16 a生(中龄林)、21 a生(近熟林)、41 a生(成熟林)和88 a生(老龄林)的杉木林。细根净生产力测定采用连续土芯法, 根系呼吸测定采用壕沟法, 生物量增量测定采用异速生长方程, 地上年凋落物量采用凋落物收集框测定。结果表明: 杉木林细根净生产力在中龄林前没有显著差异, 维持在较高水平; 但此后则显著下降。细根净生产力/地上凋落物量比值随林龄增加而显著下降。老龄林的根系呼吸显著低于其它林龄林分, 根系呼吸与细根生物量间呈显著线性相关。中龄林和近成熟林的地下碳分配(Total belouground carbon allocation, TBCA)显著高于幼龄林和成熟林, 而老龄林的则最低。中龄林、近成熟林和成熟林的地上部分净生产力/TBCA比值显著高于幼龄林和老龄林, 而杉木林的根系碳利用效率(RCUE)则呈现出随林龄增加而降低的趋势。     

Variable effects of nutrient enrichment on soil respiration in mangrove forests

Background and Aims

Mangrove forests are globally important sites of carbon burial that are increasingly exposed to nutrient pollution. Here we assessed the response of soil respiration, an important component of forest carbon budgets, to nutrient enrichment over a wide range of mangrove forests.

Methods

We assessed the response of soil respiration to nutrient enrichment using fertilization experiments within 22 mangrove forests over ten sites. We used boosted regression tree (BRT) models to determine the importance of environmental and plant factors for soil respiration and its responsiveness to fertilizer treatments.

Results

Leaf area index explained the largest proportion of variation in soil respiration rates (LAI, 45.9 %) followed by those of site, which had a relative influence of 39.9 % in the BRT model. Nutrient enrichment enhanced soil respiration only in nine out of 22 forests. Soil respiration in scrub forests showed a positive response to nutrient addition more frequently than taller fringing forests. The response of soil respiration to nutrient enrichment varied with changes in specific leaf area (SLA) and stem extension, with relative influences of 14.4 %, 13.6 % in the BRT model respectively.

Conclusions

Soil respiration in mangroves varied with LAI, but other site specific factors also influenced soil respiration and its response to nutrient enrichment. Strong enhancements in aboveground growth but moderate increases in soil respiration with nutrient enrichment indicated that nutrient enrichment of mangrove forests has likely increased net ecosystem production.     

Allometry, biomass, and productivity of mangrove forests: A review

We review 72 published articles to elucidate characteristics of biomass allocation and productivity of mangrove forests and also introduce recent progress on the study of mangrove allometry to solve the site- and species-specific problems. This includes the testing of a common allometric equation, which may be applicable to mangroves worldwide. The biomass of mangrove forests varies with age, dominant species, and locality. In primary mangrove forests, the above-ground biomass tends to be relatively low near the sea and increases inland. On a global scale, mangrove forests in the tropics have much higher above-ground biomass than those in temperate areas. Mangroves often accumulate large amounts of biomass in their roots, and the above-ground biomass to below-ground biomass ratio of mangrove forests is significantly low compared to that of upland forests (ANCOVA, P < 0.01). Several studies have reported on the growth increment of biomass and litter production in mangrove forests. We introduce some recent studies using the so-called “summation method” and investigate the trends in net primary production (NPP). For crown heights below 10 m, the above-ground NPP of mangrove forests is significantly higher (ANOVA, P < 0.01) than in those of tropical upland forests. The above-ground litter production is generally high in mangrove forests. Moreover, in many mangrove forests, the rate of soil respiration is low, possibly because of anaerobic soil conditions. These trends in biomass allocation, NPP, and soil respiration will result in high net ecosystem production, making mangrove forests highly efficient carbon sinks in the tropics.     

武夷山不同林龄甜槠林土壤呼吸特征及影响因素

为揭示中亚热带常绿阔叶林群落优势种一甜槠天然林不同林龄林下土壤呼吸(Soil respiration,RS)差异及影响因素,采用LI-8100开路式土壤碳通量系统对武夷山自然保护区不同林龄(18、36、54、72 a)天然甜槠林进行了1年的野外原位测定。结果表明:(1)不同林龄甜槠林RS季节动态呈现明显的单峰趋势,林龄对冬季RS影响并不显著(P>0.05),秋季18 a甜槠林RS与其他3种林龄差异显著(P<0.05),林龄对土壤含水率的季节变化没有显著影响(P>0.05);(2)不同林龄甜槠林5 cm深土壤温度与RS拟合R2明显高于土壤含水率与RS拟合R2,随着林龄增大,RS温度敏感性指数Q10值呈上升趋势,依次为1.551、1.589、1.640、1.664,且54、72 a甜槠林RS温度敏感性指数Q10值显著高于18、36 a(P<0.05);(3)土壤含水率与5 cm深土壤温度共同解释了RS变异的86%—90.3%;0—60 cm土层根系生物量与5 cm深土壤温度共同解释了RS变异的88.3%—91.8%,由此可见,生物因子与非生物因子双因素拟合可以更好地解释不同林龄RS差异。在对未来森林植被土壤呼吸及碳汇功能进行研究时,应在考虑林龄及季节差异的基础上,加强对生物因子的测定。     

Regeneration in fringe mangrove forests damaged by Hurricane Andrew

Mangrove forests along many tropical coastlines are frequently andseverely damaged by hurricanes. The ability of mangrove forests to regeneratefollowing hurricanes has been noted, but changes that occur in vegetationfollowing disturbance by hurricane winds and storm tides have not been studied.We measured changes in plant community structure and environmental variables intwo fringe mangrove forests in south Florida, USA that experienced high windvelocities and storm tides associated with Hurricane Andrew (August1992). Loss of the forest canopy stimulated regeneration via seedlinggrowth and recruitment, as well as resprouting of some trees that survived thehurricane. Initial regeneration differed among species in both forests:Rhizophora mangle L. regenerated primarily via growth ofseedlings present at the time of the hurricane (i.e., release of advancerecruits), but many trees of Avicennia germinans(L.) Stearn and Laguncularia racemosa Gaertn.f.resprouted profusely from dormant epicormic buds. In one forest, which wasformerly dominated by Laguncularia, high densities ofRhizophora seedlings survived the hurricane and grew toform dense stands of saplings and small trees ofRhizophora. In the other forest, there were lowerdensitiesof surviving Rhizophora seedlings (possibly due tohigher storm tide), and extensive bare areas that were colonized byAvicennia, Laguncularia, andherbaceous species. This forest, predominantly Rhizophoraat the time of the hurricane, now contains stands of saplings and small treesofall three species, interspersed with patches dominated by herbaceous plants.These findings indicate that moderately damaged fringe forests may regenerateprimarily via release of Rhizophora advance recruits,leading to single-species stands. In severely damaged forests, seedlingrecruitment may be more important and lead to mixed-species stands.Regeneration of mangrove forests following hurricanes can involve differentpathways produced by complex interactions between resprouting capability,seedling survival, post-hurricane seedling recruitment, and colonizationby herbaceous vegetation. These differences in relative importance ofregeneration pathways, which may result in post-hurricane forestsdifferent from their pre-hurricane structure, suggest that models forregeneration of mangrove forests will be more complex than directregeneration models proposed for other tropical forests whereregeneration after hurricanes is dominated by resprouting.     

Uncoupled surface and below-ground soil respiration in mangroves: implications for estimates of dissolved inorganic carbon export

Potential disparities between rates of surface and below-ground respiration were examined in seven mangrove forests of different topographic height in Timor Leste. Differences in surface respiration between air-exposed and inundated soils were inconsistent, but surface respiration rates increased, with tidal elevation. Net primary production (NPP) on air-exposed soils declined with increasing forest cover indicating light limitation beneath the canopy. NPP and respiration were linearly related under both air-exposed and inundated conditions. Rates of DIC release from the soil surface varied among forests, correlating only with soil carbon (TOC) and nitrogen (TN) and their stoichiometric ratios. Sulfate reduction was detected to maximum depth of unconsolidated soil, correlating only with TOC and TN content at discrete depth intervals. DIC concentrations in drainage channels were equivalent to porewater concentrations. The rate of carbon mineralized by sulfate reducers (SRC) was equivalent to rates of total carbon oxidation (TCO) measured at the soil surface in forests at tidal heights?≤0.5?m above mean sea-level (MSL). However, SRC was increasingly greater than TCO in forests residing from 1.0 up to 2.5?m above MSL. Most carbon mineralized in subsurface deposits appears to seep out of the forest via groundwater. Rates of surface respiration therefore underestimate rates of total benthic carbon mineralization in forests at topographic heights?≥0.5?m above MSL, suggesting that the amount of respiratory carbon exported from many mangrove forests has also been underestimated.     

红树林湿地碳储量及碳汇研究进展

红树林是生长在热带和亚热带地区潮间带的特殊的湿地森林,在防风固田、促进淤泥沉积、抵御海啸和台风等自然灾害和保护海岸线方面起着重要的作用.全球约有红树林152000 km²,占陆地森林面积的0.4%,我国约有230 km².热带红树林湿地的碳储量平均高达1023 Mg C·hm^-2,全球红树林湿地的碳汇能力在0.18～0.228 Pg C·a^-1.影响红树林碳储量和碳汇能力的主要因子除了植物种类组成以外,气温、海水温度、海水盐度、土壤理化性质、大气CO₂浓度及人类干扰等均有着重要作用.红树林湿地碳储量、碳汇能力的研究方法以实测法为基础,包括异速方程、遥感反演和模型模拟等.研究红树林湿地碳储量及碳汇能力,有利于深入认识红树林湿地碳循环过程及其调控机制,对红树林湿地的保护和合理利用具有重要意义.     

Carbon fluxes in coniferous and deciduous forest soils

Aims

Our aims were to identify responsible factors for the site-to-site variability in soil CO₂ efflux and to assess the sources of soil CO₂ of different forest types on a regional scale.

Methods

Soil CO₂ effluxes were measured over 1–4 years in four coniferous and three deciduous forests of Bavaria, Germany, and related to climate, soil properties and forest productivity. Total belowground carbon allocation (TBCA) was assessed using soil CO₂ effluxes and aboveground litterfall. Additionally, CO₂ production of organic layers was examined over 10 months under constant conditions in an incubation experiment.

Results

Annual soil CO₂ effluxes were not different among the forest sites, but predicted effluxes at a given temperature of 10°C revealed some significant differences and correlated with the phosphorous stock of the organic layers. The incubation study indicated 50% faster decomposition of organic layers from deciduous than from coniferous forests. TBCA related to soil CO₂ efflux was smaller in the deciduous than in the coniferous forests. The ratio of TBCA to soil CO₂ efflux was positively correlated with the C stock of organic layers.

Conclusions

Our results suggest that marked differences in site characteristics have little impact on soil CO₂ effluxes at the regional scale, but the contribution of soil CO₂ sources varies among the forest types.     

Impact of environmental factors on the dependency of litter biomass in carbon cycling of Hooghly estuary,India

Litterfall of the mangroves and its subsequent decomposition is an important mechanism in terms of productivity and nutrient cycle of that ecosystem. Present study emphasizes on the significance of litter biomass and role of environmental factors impacting this process. Mangrove litter undergoes degradation and decomposition and serves as the main source of carbon in different forms within the system, mangrove forests adjacent to the creeks at Sagar Island of the Hooghly–Matla estuarine ecosystem. This system receives a major load of carbon from adjacent mangrove forest in the form of litterfall throughout the year. Keeping in view the effect of environmental factors on litterfall and dynamics of carbon, machine learning method has been applied for this study. Different forms of carbon and environmental factors like temperature, salinity, pH, dissolved oxygen are estimated following standard procedure. Correlation, redundancy analysis and LASSO (Least Absolute Shrinkage and Selection Operator) regression are done in order to know the impact of environmental variables on carbon pool dynamics and effect of litterfall on the carbon pools in soil and water. The results reflect a strong correlation among the studied environmental factors and carbon pool dynamics. It has been revealed from the LASSO prediction results that each carbon pool is sensitive to a separate set of environmental factors.     

Mangrove Forest and Soil Development on a Rapidly Accreting Shore in New Zealand

Mangrove forests are rapidly expanding their distribution in New Zealand, which is at the southern limit of their range. We investigated how these expanding mangrove forests develop through time. We assessed patterns in forest structure and function at the Firth of Thames, which is a rapidly accreting mangrove site in New Zealand where 1 km of mangrove of Avicennia marina has established seaward since the 1950s. Across the intertidal region, mangrove forest structure was highly variable. We used bomb-pulse radiocarbon dating to age the forest. Two major forest establishment events were identified; one in 1978–1981 and another in 1991–1995. These events coincided with sustained El Niño activity and are likely the result of reduced wind and wave energy at the site during these periods. We used the two forests of different ages to assess whether mangroves in New Zealand mature at similar rates as other mangroves and whether they conform to classic models of succession. The timing of forest maturation is similar in New Zealand as in more tropical locations with trees exhibiting features of mature forests as they age from about 10 to about 30 years. In older forest (~30 years old) trees become larger and stands more homogenous than in the younger forest (~10 years old). Carbon and nutrient concentrations in soils increased and soils become more aerobic in older forest compared to younger forest. Additionally, using fertilization experiments, we established that despite reduced growth rates in older forests, nitrogen remained limiting to growth in both older and young forests. However, in contrast to classic successional models leaf tissue nutrient concentrations and nutrient conservation (nutrient resorption from senescence leaf tissue) were similar in forests of differing ages and did not vary with fertilization. We conclude that mangrove forest expansion in New Zealand is influenced by climatic factors. Mangrove forests mature rapidly, even at the limits of their range and they satisfy many of the successional patterns predicted by Odum (1969) for the early stages of forest succession.     

Methane emissions from an Amazon floodplain lake: Enhanced release during episodic mixing and during falling water

Soil respiration rates vary significantly among major plant biomes, suggesting that vegetation type influences the rate of soil respiration. However, correlations among climatic factors, vegetation distributions, and soil respiration rates make cause-effect arguments difficult. Vegetation may affect soil respiration by influencing soil microclimate and structure, the quantity of detritus supplied to the soil, the quality of that detritus, and the overall rate of root respiration. At the global scale, soil respiration rates correlate positively with litterfall rates in forests, as previously reported, and with aboveground net primary productivity in grasslands, providing evidence of the importance of detritus supply. To determine the direction and magnitude of the effect of vegetation type on soil respiration, we collated data from published studies where soil respiration rates were measured simultaneously in two or more plant communities. We found no predictable differences in soil respiration between cropped and vegetation-free soils, between forested and cropped soils, or between grassland and cropped soils, possibly due to the diversity of crops and cropping systems included. Factors such as temperature, moisture availability, and substrate properties that simultaneously influence the production and consumption of organic matter are more important in controlling the overall rate of soil respiration than is vegetation type in most cases. However, coniferous forests had 10% lower rates of soil respiration than did adjacent broad-leaved forests growing on the same soil type, and grasslands had, on average, 20% higher soil respiration rates than did comparable forest stands, demonstrating that vegetation type does in some cases significantly affect rates of soil respiration.     

The resilience of Indian Western Himalayan forests to regime shift: Are they reaching towards no return point?

Forests across the latitudes are facing regime shift under the influence of changing climate where temperature and precipitation are recognized as prominent drivers. Regime shift of forests can be in the form of conversion of one forest type into another or the alteration of forests into degraded class such as “scrub”. The Indian Western Himalayan (IWH) region hosts valuable forests to support multiple ecosystem services which may be impacted under different thresholds of regime shift. We assessed the threshold of regime shift as transition of forests into scrub considering temperature and precipitation records of the recent decade (2000–2019). A logistic regression model was developed using the forest cover data of IWH as a dependent variable and climatic records (temperature and precipitation) obtained from ERA-5 data as independent variables. Probability values of two classes (forest and scrub) were computed and were used to define present resilience states. The majority of the forest of the IWH region may not withstand any significant rise in temperature or a reduced amount of precipitation as almost 88.68% of forests of the IWH are under the low to moderate resilience category. Forest resilience significantly decreases below 1500 mm of precipitation indicating its tipping point of regime shift into the scrub. Temperature below 6 °C is not favourable for forests whereas a temperature range of 10–20 °C was found as the conducive range for the existence of forests in the region. Such empirical study would support the formulation of management plans and policies for sustainable forest resources and to assess the impacts of climate change.     

A multiyear synthesis of soil respiration responses to elevated atmospheric CO2 from four forest FACE experiments

The rapidly rising concentration of atmospheric CO₂ has the potential to alter forest and global carbon cycles by altering important processes that occur in soil. Forest soils contain the largest and longest lived carbon pools in terrestrial ecosystems and are therefore extremely important to the land–atmosphere exchange of carbon and future climate. Soil respiration is a sensitive integrator of many soil processes that control carbon storage in soil, and is therefore a good metric of changes to soil carbon cycling. Here, we summarize soil respiration data from four forest free‐air carbon dioxide enrichment (FACE) experiments in developing and established forests that have been exposed to elevated atmospheric [CO₂] (168 μL L^?1 average enrichment) for 2–6 years. The sites have similar experimental design and use similar methodology (closed‐path infrared gas analyzers) to measure soil respiration, but differ in species composition of the respective forest communities. We found that elevated atmospheric [CO₂] stimulated soil respiration at all sites, and this response persisted for up to 6 years. Young developing stands experienced greater stimulation than did more established stands, increasing 39% and 16%, respectively, averaged over all years and communities. Further, at sites that had more than one community, we found that species composition of the dominant trees was a major controller of the absolute soil CO₂ efflux and the degree of stimulation from CO₂ enrichment. Interestingly, we found that the temperature sensitivity of bulk soil respiration appeared to be unaffected by elevated atmospheric CO₂. These findings suggest that stage of stand development and species composition should be explicitly accounted for when extrapolating results from elevated CO₂ experiments or modeling forest and global carbon cycles.     

腾格里沙漠东南缘不同类型生物土壤结皮对土壤有机碳矿化的影响

生物土壤结皮(BSCs)是荒漠生态系统的重要组成部分,是该区土壤碳循环及碳平衡的关键影响因素。研究了腾格里沙漠东南缘不同类型生物土壤结皮覆盖下土壤碳矿化过程及其对温度(10℃、25℃和35℃)和水分(土壤含水量10%和25%)变化响应特征,分析了土壤碳矿化过程与土壤理化性质的关系。结果表明:(1)结皮的形成和发育显著影响土壤有机碳矿化过程,藻类、地衣和藓类结皮覆盖的土壤碳矿化速率和CO_2-C累积释放量均显著高于去除结皮的土壤,不同类型BSCs覆盖土壤和去除结皮土壤之间均表现为藓类结皮土壤地衣结皮土壤藻类结皮。(2)含结皮层土壤的平均和最大矿化速率均随温度升高和水分增加而逐渐增大,有结皮覆盖的土壤和去除结皮的土壤对温度和水分变化的响应规律相同。(3)有结皮土壤和去除结皮土壤碳矿化速率的温度敏感性(Q_(10))与结皮类型密切相关,均表现为藓类结皮地衣结皮藻类结皮。结果表明生物土壤结皮由以藻类为主向以藓类为主的演变进一步促进了土壤碳矿化过程,结皮对土壤碳循环的调控作用受水热等环境因子的共同影响。     

集水区尺度下东北东部森林土壤呼吸的模拟

IBIS模型是陆地碳循环模拟的有利工具,土壤呼吸是陆地碳循环的关键生态学过程,利用IBIS模型模拟估算土壤呼吸对陆地碳循环和全球变化研究具有重要意义.在地形数据、植被参数、土壤质地参数和气象数据支持下,利用改造后的IBIS模型模拟2004年张家沟集水区5种森林类型的土壤呼吸,以实测数据对模拟结果进行验证,并分析土壤呼吸时空格局及其与土壤温湿度的关系.结果表明:(1)改造后的IBIS模型模拟的土壤呼吸值与实测值相关性显著,可较好地用于集水区尺度的森林土壤呼吸模拟估算.(2)土壤呼吸年均值为571 gCm-2 a-1,年土壤呼吸空间格局与生长季土壤呼吸空间格局相似,均表现为高值区主要分布在北部、西南和东南区域,低值区主要分布在沟谷附近,该格局与集水区的地形、植被及其组合等因素有关.(3)生长季内,5种森林类型土壤呼吸的季节性变化均呈单峰曲线形式,土壤呼吸峰值均出现在7月,其中落叶松林峰值最低,为85.5gC/m2,杂木林峰值最高,为146.3 gC/m2.(4)5种森林类型的土壤呼吸值与5 cm深土壤温度存在极显著的指数关系,与土壤湿度的相关性较低,土壤温度的变化可以解释土壤呼吸约70％的季节变化.     

The interplay between mangroves and saltmarshes at the transition between temperate and subtropical climate in Florida

The interplay between mangroves and saltmarshes at the temperate to subtropical transition in Florida results in dramatic changes to the appearance of the coastal landscape. In the 1980s, freezes killed entire mangrove forests dominated by black mangroves, Avicennia germinans (L.). Following the freezes, saltmarshes dominated by smooth cordgrass, Spartina alterniflora Loisel, revegetated the intertidal zone. After a decade of mild winters, however, mangroves are beginning to reclaim the area. The rate of mangrove expansion was determined by comparing aerial photography (change from 1995 to 1999), and from monitoring transects (over a 3 year period) on three of the Cedar Keys, Florida (Lat. 29°08′). The rate of mangrove expansion varied among islands, and the mechanism of expansion ranged from propagule-trapping by saltmarshes along the edges of mangrove clumps to widespread dispersal and growth of existing or newly imported propagules. A freeze occurred during the study, which may have set back mangrove expansion by defoliating mangrove trees and resetting mangrove reproduction. Mangrove expansion was projected to take 20–30 years for complete seedling cover. Given the possibility of global climate change and its potential influence on the distribution of coastal vegetation, the timeframes and implications to coastal wetland ecosystems involved in this regular interplay will provide valuable baseline information for future studies.     

Factors regulating carbon sinks in mangrove ecosystems

Mangroves are recognized as one of the richest carbon storage systems. However, the factors regulating carbon sinks in mangrove ecosystems are still unclear, particularly in the subtropical mangroves. The biomass, production, litterfall, detrital export and decomposition of the dominant mangrove vegetation in subtropical (Kandelia obovata) and tropical (Avicennia marina) Taiwan were quantified from October 2011 to July 2014 to construct the carbon budgets. Despite the different tree species, a principal component analysis revealed the site or environmental conditions had a greater influence than the tree species on the carbon processes. For both species, the net production (NP) rates ranged from 10.86 to 27.64 Mg C ha^?1 year^?1 and were higher than the global average rate due to the high tree density. While most of the litterfall remained on the ground, a high percentage (72%–91%) of the ground litter decomposed within 1 year and fluxed out of the mangroves. However, human activities might cause a carbon flux into the mangroves and a lower NP rate. The rates of the organic carbon export and soil heterotrophic respiration were greater than the global mean values and those at other locations. Only a small percentage (3%–12%) of the NP was stored in the sediment. The carbon burial rates were much lower than the global average rate due to their faster decomposition, indicating that decomposition played a critical role in determining the burial rate in the sediment. The summation of the organic and inorganic carbon fluxes and soil heterotrophic respiration well exceeded the amount of litter decomposition, indicating an additional source of organic carbon that was unaccounted for by decomposition in the sediment. Sediment‐stable isotope analyses further suggest that the trapping of organic matter from upstream rivers or adjacent waters contributed more to the mangrove carbon sinks than the actual production of the mangrove trees.     

CO2 flux from soil in pastures and forests in southwestern Amazonia

Stocks of carbon in Amazonian forest biomass and soils have received considerable research attention because of their potential as sources and sinks of atmospheric CO₂. Fluxes of CO₂ from soil to the atmosphere, on the other hand, have not been addressed comprehensively in regard to temporal and spatial variations and to land cover change, and have been measured directly only in a few locations in Amazonia. Considerable variation exists across the Amazon Basin in soil properties, climate, and management practices in forests and cattle pastures that might affect soil CO₂ fluxes. Here we report soil CO₂ fluxes from an area of rapid deforestation in the southwestern Amazonian state of Acre. Specifically we addressed (1) the seasonal variation of soil CO₂ fluxes, soil moisture, and soil temperature; (2) the effects of land cover (pastures, mature, and secondary forests) on these fluxes; (3) annual estimates of soil respiration; and (4) the relative contributions of grass‐derived and forest‐derived C as indicated by δ¹³CO₂. Fluxes were greatest during the wet season and declined during the dry season in all land covers. Soil respiration was significantly correlated with soil water‐filled pore space but not correlated with temperature. Annual fluxes were higher in pastures compared with mature and secondary forests, and some of the pastures also had higher soil C stocks. The δ¹³C of CO₂ respired in pasture soils showed that high respiration rates in pastures were derived almost entirely from grass root respiration and decomposition of grass residues. These results indicate that the pastures are very productive and that the larger flux of C cycling through pasture soils compared with forest soils is probably due to greater allocation of C belowground. Secondary forests had soil respiration rates similar to mature forests, and there was no correlation between soil respiration and either forest age or forest biomass. Hence, belowground allocation of C does not appear to be directly related to the stature of vegetation in this region. Variation in seasonal and annual rates of soil respiration of these forests and pastures is more indicative of flux of C through the soil rather than major net changes in ecosystem C stocks.     

Partitioning of belowground C in young sugar maple forest

Background and aims

Trees allocate a high proportion of assimilated carbon belowground, but the partitioning of that C among ecosystem components is poorly understood thereby limiting our ability to predict responses of forest C dynamics to global change drivers.

Methods

We labeled sugar maple saplings in natural forest with a pulse of photosynthetic ¹³C in late summer and traced the pulse over the following 3 years. We quantified the fate of belowground carbon by measuring ¹³C enrichment of roots, rhizosphere soil, soil respiration, soil aggregates and microbial biomass.

Results

The pulse of ¹³C contributed strongly to root and rhizosphere respiration for over a year, and respiration comprised about 75 % of total belowground C allocation (TBCA) in the first year. We estimate that rhizosphere carbon flux (RCF) during the dormant season comprises at least 6 % of TBCA. After 3 years, 3.8 % of the C allocated belowground was recovered in soil organic matter, mostly in water-stable aggregates.

Conclusions

A pulse of carbon allocated belowground in temperate forest supplies root respiration, root growth and RCF throughout the following year and a small proportion becomes stabilized in soil aggregates.