The biological controls of soil carbon accumulation following wildfire and harvest in boreal forests: A review.

Boreal forests are frequently subjected to disturbances, including wildfire and clear-cutting. While these disturbances can cause soil carbon (C) losses, the long-term accumulation dynamics of soil C stocks during subsequent stand development is controlled by biological processes related to the balance of net primary production (NPP) and outputs via heterotrophic respiration and leaching, many of which remain poorly understood. We review the biological processes suggested to influence soil C accumulation in boreal forests. Our review indicates that median C accumulation rates following wildfire and clear-cutting are similar (0.15 and 0.20 Mg ha-1 year-1, respectively), however, variation between studies is extremely high. Further, while many individual studies show linear increases in soil C stocks through time after disturbance, there are indications that C stock recovery is fastest early to mid-succession (e.g. 15-80 years) and then slows as forests mature (e.g. >100 years). We indicate that the rapid build-up of soil C in younger stands appears not only driven by higher plant production, but also by a high rate of mycorrhizal hyphal production, and mycorrhizal suppression of saprotrophs. As stands mature, the balance between reductions in plant and mycorrhizal production, increasing plant litter recalcitrance, and ectomycorrhizal decomposers and saprotrophs have been highlighted as key controls on soil C accumulation rates. While some of these controls appear well understood (e.g. temporal patterns in NPP, changes in aboveground litter quality), many others remain research frontiers. Notably, very little data exists describing and comparing successional patterns of root production, mycorrhizal functional traits, mycorrhizal-saprotroph interactions, or C outputs via heterotrophic respiration and dissolved organic C following different disturbances. We argue that these less frequently described controls require attention, as they will be key not only for understanding ecosystem C balances, but also for representing these dynamics more accurately in soil organic C and Earth system models.

Variability of ecosystem carbon source from microbial respiration is controlled by rainfall dynamics.

Soil heterotrophic respiration (Rh) represents an important component of the terrestrial carbon cycle that affects whether ecosystems function as carbon sources or sinks. Due to the complex interactions between biological and physical factors controlling microbial growth, Rh is uncertain and difficult to predict, limiting our ability to anticipate future climate trajectories. Here we analyze the global FLUXNET 2015 database aided by a probabilistic model of microbial growth to examine the ecosystem-scale dynamics of Rh and identify primary predictors of its variability. We find that the temporal variability in Rh is consistently distributed according to a Gamma distribution, with shape and scale parameters controlled only by rainfall characteristics and vegetation productivity. This distribution originates from the propagation of fast hydrologic fluctuations on the slower biological dynamics of microbial growth and is independent of biome, soil type, and microbial physiology. This finding allows us to readily provide accurate estimates of the mean Rh and its variance, as confirmed by a comparison with an independent global dataset. Our results suggest that future changes in rainfall regime and net primary productivity will significantly alter the dynamics of Rh and the global carbon budget. In regions that are becoming wetter, Rh may increase faster than net primary productivity, thereby reducing the carbon storage capacity of terrestrial ecosystems.

Meta-analysis shows forest soil CO2 effluxes are dependent on the disturbance regime and biome type.

Forest soil CO2 efflux (FCO2 ) is a crucial process in global carbon cycling; however, how FCO2 responds to disturbance regimes in different forest biomes is poorly understood. We quantified the effects of disturbance regimes on FCO2 across boreal, temperate, tropical and Mediterranean forests based on 1240 observations from 380 studies. Globally, climatic perturbations such as elevated CO2 concentration, warming and increased precipitation increase FCO2 by 13% to 25%. FCO2 is increased by forest conversion to grassland and elevated carbon input by forest management practices but reduced by decreased carbon input, fire and acid rain. Disturbance also changes soil temperature and water content, which in turn affect the direction and magnitude of disturbance influences on FCO2 . FCO2 is disturbance- and biome-type dependent and such effects should be incorporated into earth system models to improve the projection of the feedback between the terrestrial C cycle and climate change.

Apparent thermal acclimation of soil heterotrophic respiration mainly mediated by substrate availability.

Multiple lines of existing evidence suggest that increasing CO2 emission from soils in response to rising temperature could accelerate global warming. However, in experimental studies, the initial positive response of soil heterotrophic respiration (RH ) to warming often weakens over time (referred to apparent thermal acclimation). If the decreased RH is driven by thermal adaptation of soil microbial community, the potential for soil carbon (C) losses would be reduced substantially. In the meanwhile, the response could equally be caused by substrate depletion, and would then reflect the gradual loss of soil C. To address uncertainties regarding the causes of apparent thermal acclimation, we carried out sterilization and inoculation experiments using the soil samples from an alpine meadow with 6 years of warming and nitrogen (N) addition. We demonstrate that substrate depletion, rather than microbial adaptation, determined the response of RH to long-term warming. Furthermore, N addition appeared to alleviate the apparent acclimation of RH to warming. Our study provides strong empirical support for substrate availability being the cause of the apparent acclimation of soil microbial respiration to temperature. Thus, these mechanistic insights could facilitate efforts of biogeochemical modeling to accurately project soil C stocks in the future climate.

Landscape-variability of the carbon balance across managed boreal forests.

Boreal forests are important global carbon (C) sinks and, therefore, considered as a key element in climate change mitigation policies. However, their actual C sink strength is uncertain and under debate, particularly for the actively managed forests in the boreal regions of Fennoscandia. In this study, we use an extensive set of biometric- and chamber-based C flux data collected in 50 forest stands (ranging from 5 to 211 years) over 3 years (2016-2018) with the aim to explore the variations of the annual net ecosystem production (NEP; i.e., the ecosystem C balance) across a 68 km2 managed boreal forest landscape in northern Sweden. Our results demonstrate that net primary production rather than heterotrophic respiration regulated the spatio-temporal variations of NEP across the heterogeneous mosaic of the managed boreal forest landscape. We further find divergent successional patterns of NEP in our managed forests relative to naturally regenerating boreal forests, including (i) a fast recovery of the C sink function within the first decade after harvest due to the rapid establishment of a productive understory layer and (ii) a sustained C sink in old stands (131-211 years). We estimate that the rotation period for optimum C sequestration extends to 138 years, which over multiple rotations results in a long-term C sequestration rate of 86.5 t C ha-1 per rotation. Our study highlights the potential of forest management to maximize C sequestration of boreal forest landscapes and associate climate change mitigation effects by developing strategies that optimize tree biomass production rather than heterotrophic soil C emissions.

Significant effects of precipitation frequency on soil respiration and its components-A global synthesis.

Global warming intensifies the hydrological cycle, which results in changes in precipitation regime (frequency and amount), and will likely have significant impacts on soil respiration (Rs ). Although the responses of Rs to changes in precipitation amount have been extensively studied, there is little consensus on how Rs will be affected by changes in precipitation frequency (PF) across the globe. Here, we synthesized the field observations from 296 published papers to quantify the effects of PF on Rs and its components using meta-analysis. Our results indicated that the effects of PF on Rs decreased with an increase in background mean annual precipitation. When the data were grouped by climate conditions, increased PF showed positive effects on Rs under the arid condition but not under the semi-humid or humid conditions, whereas decreased PF suppressed Rs across all the climate conditions. The positive effects of increased PF mainly resulted from the positive response of heterotrophic respiration under the arid condition while the negative effects of decreased PF were mainly attributed to the reductions in root biomass and respiration. Overall, our global synthesis provided for the first time a comprehensive analysis of the divergent effects of PF on Rs and its components across climate regions. This study also provided a framework for understanding and modeling responses of ecosystem carbon cycling to global precipitation change.

Microbial communities in terrestrial surface soils are not widely limited by carbon.

Microbial communities in soils are generally considered to be limited by carbon (C), which could be a crucial control for basic soil functions and responses of microbial heterotrophic metabolism to climate change. However, global soil microbial C limitation (MCL) has rarely been estimated and is poorly understood. Here, we predicted MCL, defined as limited availability of substrate C relative to nitrogen and/or phosphorus to meet microbial metabolic requirements, based on the thresholds of extracellular enzyme activity across 847 sites (2476 observations) representing global natural ecosystems. Results showed that only about 22% of global sites in terrestrial surface soils show relative C limitation in microbial community. This finding challenges the conventional hypothesis of ubiquitous C limitation for soil microbial metabolism. The limited geographic extent of C limitation in our study was mainly attributed to plant litter, rather than soil organic matter that has been processed by microbes, serving as the dominant C source for microbial acquisition. We also identified a significant latitudinal pattern of predicted MCL with larger C limitation at mid- to high latitudes, whereas this limitation was generally absent in the tropics. Moreover, MCL significantly constrained the rates of soil heterotrophic respiration, suggesting a potentially larger relative increase in respiration at mid- to high latitudes than low latitudes, if climate change increases primary productivity that alleviates MCL at higher latitudes. Our study provides the first global estimates of MCL, advancing our understanding of terrestrial C cycling and microbial metabolic feedback under global climate change.

Nonlinear responses of ecosystem carbon fluxes to nitrogen deposition in an old-growth boreal forest.

Nitrogen (N) deposition is known to increase carbon (C) sequestration in N-limited boreal forests. However, the long-term effects of N deposition on ecosystem carbon fluxes have been rarely investigated in old-growth boreal forests. Here we show that decade-long experimental N additions significantly stimulated net primary production (NPP) but the effect decreased with increasing N loads. The effect on soil heterotrophic respiration (Rh) shifted from a stimulation at low-level N additions to an inhibition at higher levels of N additions. Consequently, low-level N additions resulted in a neutral effect on net ecosystem productivity (NEP), due to a comparable stimulating effect on NPP and Rh, while NEP was increased by high-level N additions. Moreover, we found nonlinear temporal responses of NPP, Rh and NEP to low-level N additions. Our findings imply that actual N deposition in boreal forests likely exerts a minor contribution to their soil C storage.

Observation-based global soil heterotrophic respiration indicates underestimated turnover and sequestration of soil carbon by terrestrial ecosystem models.

Soil heterotrophic respiration (Rh ) refers to the flux of CO2 released from soil to atmosphere as a result of organic matter decomposition by soil microbes and fauna. As one of the major fluxes in the global carbon cycle, large uncertainties still exist in the estimation of global Rh , which further limits our current understanding of carbon accumulation in soils. Here, we applied a Random Forest algorithm to create a global data set of soil Rh , by linking 761 field observations with both abiotic and biotic predictors. We estimated that global Rh was 48.8 ± 0.9 Pg C year-1 for 1982-2018, which was 16% less than the ensemble mean (58.6 ± 9.9 Pg C year-1 ) of 16 terrestrial ecosystem models. By integrating our observational Rh with independent soil carbon stock data sets, we obtained a global mean soil carbon turnover time of 38.3 ± 11 year. Using observation-based turnover times as a constraint, we found that terrestrial ecosystem models simulated faster carbon turnovers, leading to a 30% (74 Pg C) underestimation of terrestrial ecosystem carbon accumulation for the past century, which was especially pronounced at high latitudes. This underestimation is equivalent to 45% of the total carbon emissions (164 Pg C) caused by global land-use change at the same time. Our analyses highlight the need to constrain ecosystem models using observation-based and locally adapted Rh values to obtain reliable projections of the carbon sink capacity of terrestrial ecosystems.

Appraisal of different land use systems for heterotrophic respiration in a Karst landscape.

Soil respiration, particularly heterotrophic respiration (RH), is a potent source of carbon dioxide (CO2) in the atmosphere. The current research focuses on the evaluation of RH for six land use systems including sloping cropland (SC), shrub land (SD), grassland (GD), shrub & grassland (SGD), newly abandoned cropland (NC) and afforested forest (AF). Heterotrophic respiration showed a diverse seasonal pattern over a year long period that was affected by various soil properties and climatic variables across the six land use systems in a subtropical Karst landscape. The lowest RH scores were found in the SD site (annual cumulative soil CO2 flux: 2447 kg C ha-1), whereas the maximum heterotrophic respiration occurred in the SF site (annual cumulative soil CO2 13597 kg C ha-1). The values of RH were: SC site: 3.8-191.5 mg C m-2 h-1, NC site: 1.04-129 mg C m-2 h-1, GD site: 3.6-100.7 mg C m-2 h-1, SGD site: 0.3-393.5 mg C m-2 h-1, SD site: 3-116 mg C m-2 h-1, and SF site: 10.6-398.2 mg C m-2 h-1. Highly significant (p ≤ 0.01) and positive correlations between RH rate and soil temperature were found for the studied land use types (correlation coefficients as follows; SC: 0.77, NC: 0.61, GD: 0.283, SGD: 0.535, SD: 0.230, SF: 0.85). However, water filled pore space (WFPS), NH4+, NO3-, dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations showed varied (positive and negative) correlations with RH. The overall results show that soil temperature can be considered as the most limiting factor for RH among all the sites studied in the present research. In these environments, soil heterotrophic respiration significantly correlated with soil temperature, highlighting the significance of climate on heterotrophic respiration.

Leveraging observed soil heterotrophic respiration fluxes as a novel constraint on global-scale models.

Microbially explicit models may improve understanding and projections of carbon dynamics in response to future climate change, but their fidelity in simulating global-scale soil heterotrophic respiration (RH ), a stringent test for soil biogeochemical models, has never been evaluated. We used statistical global RH products, as well as 7821 daily site-scale RH measurements, to evaluate the spatiotemporal performance of one first-order decay model (CASA-CNP) and two microbially explicit biogeochemical models (CORPSE and MIMICS) that were forced by two different input datasets. CORPSE and MIMICS did not provide any measurable performance improvement; instead, the models were highly sensitive to the input data used to drive them. Spatial variability in RH fluxes was generally well simulated except in the northern middle latitudes (~50°N) and arid regions; models captured the seasonal variability of RH well, but showed more divergence in tropic and arctic regions. Our results demonstrate that the next generation of biogeochemical models shows promise but also needs to be improved for realistic spatiotemporal variability of RH . Finally, we emphasize the importance of net primary production, soil moisture, and soil temperature inputs, and that jointly evaluating soil models for their spatial (global scale) and temporal (site scale) performance provides crucial benchmarks for improving biogeochemical models.

Soil carbon balance of afforested peatlands in the maritime temperate climatic zone.

Drainage and conversion of natural peatlands to forestry increases soil CO2 emissions through decomposition of peat and modifies the quantity and quality of litter inputs and therefore the soil carbon balance. In organic soils, CO2 net emissions and removals are reported using carbon emission factors (EF). The choice of specific default Tier 1 EF values from the IPCC 2013 Wetlands supplement depends on land-use categories and climate zones. However, Tier 1 EF for afforested peatlands in the temperate maritime climate zone are based on data from eight sites, mainly located in the hemiboreal zone, and the uncertainty associated with these default values is a concern. In addition, moving from Tier 1 to higher-Tier carbon reporting values is highly desirable when large areas are affected by land-use changes. In this study, we estimated site-specific soil carbon balance for the development of Tier 2 soil CO2 -C EFs for afforested peatlands. Soil heterotrophic respiration and aboveground tree litterfall were measured during two years at eight afforested peatland sites in Ireland. In addition, fine-root turnover rate and site-specific fine-root biomass were used to quantify belowground litter inputs. We found that drainage of peatlands and planting them with either Sitka spruce or lodgepole pine, resulted in soils being net carbon sources. The soil carbon balance at multi-year sites varied between 63 ± 92 and 309 ± 67 g C m-2  year-1 . Mean CO2 -C EF for afforested peatlands was 1.68 ± 0.33 t CO2 -C ha-1  year-1 . The improved CO2 -C EFs presented here for afforested peatlands are proposed as a basis to update national CO2 -C emissions from this land-use class in Ireland. Furthermore, new data from these sites will significantly contribute to the development of more reliable IPCC default Tier 1 CO2 -C EFs for afforested peatlands in the maritime temperate climate zone.

Spatial biases of information influence global estimates of soil respiration: How can we improve global predictions?

Soil respiration (Rs), the efflux of CO2 from soils to the atmosphere, is a major component of the terrestrial carbon cycle, but is poorly constrained from regional to global scales. The global soil respiration database (SRDB) is a compilation of in situ Rs observations from around the globe that has been consistently updated with new measurements over the past decade. It is unclear whether the addition of data to new versions has produced better-constrained global Rs estimates. We compared two versions of the SRDB (v3.0 n = 5173 and v5.0 n = 10,366) to determine how additional data influenced global Rs annual sum, spatial patterns and associated uncertainty (1 km spatial resolution) using a machine learning approach. A quantile regression forest model parameterized using SRDBv3 yielded a global Rs sum of 88.6 Pg C year-1 , and associated uncertainty of 29.9 (mean absolute error) and 57.9 (standard deviation) Pg C year-1 , whereas parameterization using SRDBv5 yielded 96.5 Pg C year-1 and associated uncertainty of 30.2 (mean average error) and 73.4 (standard deviation) Pg C year-1 . Empirically estimated global heterotrophic respiration (Rh) from v3 and v5 were 49.9-50.2 (mean 50.1) and 53.3-53.5 (mean 53.4) Pg C year-1 , respectively. SRDBv5's inclusion of new data from underrepresented regions (e.g., Asia, Africa, South America) resulted in overall higher model uncertainty. The largest differences between models parameterized with different SRDVB versions were in arid/semi-arid regions. The SRDBv5 is still biased toward northern latitudes and temperate zones, so we tested an optimized global distribution of Rs measurements, which resulted in a global sum of 96.4 ± 21.4 Pg C year-1 with an overall lower model uncertainty. These results support current global estimates of Rs but highlight spatial biases that influence model parameterization and interpretation and provide insights for design of environmental networks to improve global-scale Rs estimates.

Major and persistent shifts in below-ground carbon dynamics and soil respiration following logging in tropical forests.

Soil respiration is the largest carbon efflux from the terrestrial ecosystem to the atmosphere, and selective logging influences soil respiration via changes in abiotic (temperature, moisture) and biotic (biomass, productivity, quantity and quality of necromass inputs) drivers. Logged forests are a predominant feature of the tropical forest landscape, their area exceeding that of intact forest. We quantified both total and component (root, mycorrhiza, litter, and soil organic matter, SOM) soil respiration in logged (n = 5) and old-growth (n = 6) forest plots in Malaysian Borneo, a region which is a global hotspot for emission from forest degradation. We constructed a detailed below-ground carbon budget including organic carbon inputs into the system via litterfall and root turnover. Total soil respiration was significantly higher in logged forests than in old-growth forests (14.3 ± 0.23 and 12.7 ± 0.60 Mg C ha-1  year-1 , respectively, p = 0.037). This was mainly due to the higher SOM respiration in logged forests (55 ± 3.1% of the total respiration in logged forests vs. 50 ± 3.0% in old-growth forests). In old-growth forests, annual SOM respiration was equal to the organic carbon inputs into the soil (difference between SOM respiration and inputs 0.18 Mg C ha-1  year-1 , with 90% confidence intervals of -0.41 and 0.74 Mg C ha-1  year-1 ), indicating that the system is in equilibrium, while in logged forests SOM respiration exceeded the inputs by 4.2 Mg C ha-1  year-1 (90% CI of 3.6 and 4.9 Mg C ha-1  year-1 ), indicating that the soil is losing carbon. These results contribute towards understanding the impact of logging on below-ground carbon dynamics, which is one of the key uncertainties in estimating emissions from forest degradation. This study demonstrates how significant perturbation of the below-ground carbon balance, and consequent net soil carbon emissions, can persist for decades after a logging event in tropical forests.

The response of soil respiration to precipitation change is asymmetric and differs between grasslands and forests.

Intensification of the Earth's hydrological cycle amplifies the interannual variability of precipitation, which will significantly impact the terrestrial carbon (C) cycle. However, it is still unknown whether previously observed relationship between soil respiration (Rs ) and precipitation remains applicable under extreme precipitation change. By analyzing the observations from a much larger dataset of field experiments (248 published papers including 151 grassland studies and 97 forest studies) across a wider range of precipitation manipulation than previous studies, we found that the relationship of Rs response with precipitation change was highly nonlinear or asymmetric, and differed significantly between grasslands and forests, between moderate and extreme precipitation changes. Response of Rs to precipitation change was negatively asymmetric (concave-down) in grasslands, and double-asymmetric in forests with a positive asymmetry (concave-up) under moderate precipitation changes and a negative asymmetry (concave-down) under extreme precipitation changes. In grasslands, the negative asymmetry in Rs response was attributed to the higher sensitivities of soil moisture, microbial and root activities to decreased precipitation (DPPT) than to increased precipitation (IPPT). In forests, the positive asymmetry was predominantly driven by the significant increase in microbial respiration under moderate IPPT, while the negative asymmetry was caused by the reductions in root biomass and respiration under extreme DPPT. The different asymmetric responses of Rs between grasslands and forests will greatly improve our ability to forecast the C cycle consequences of increased precipitation variability. Specifically, the negative asymmetry of Rs response under extreme precipitation change suggests that the soil C efflux will decrease across grasslands and forests under future precipitation regime with more wet and dry extremes.

Plant diversity loss reduces soil respiration across terrestrial ecosystems.

The rapid global biodiversity loss has led to the decline in ecosystem function. Despite the critical importance of soil respiration (Rs) in the global carbon and nutrient cycles, how plant diversity loss affects Rs remains uncertain. Here we present a meta-analysis using 446 paired observations from 95 published studies to evaluate the effects of plant and litter mixtures on Rs and its components. We found that total Rs and heterotrophic respiration (Rh) were, on average, greater in plant mixtures than expected from those of monocultures. These mixture effects increased with increasing species richness (SR) in both plant and litter mixtures. While the positive effects of species mixtures remained similar over time for total Rs, they increased over time for Rh in plant mixtures but decreased in litter mixtures. Despite the wide range of variations in mean annual temperature, annual aridity index, and ecosystem types, the plant mixture effects on total Rs and Rh did not change geographically, except for a more pronounced increase of total Rs in species mixtures with reduced water availability. Our structural equation models suggested that the positive effects of SR and stand age on total and Rh were driven by increased plant inputs and soil microbial biomass. Our results suggest that plant diversity loss has ubiquitous negative impacts on Rs, one of the fundamental carbon-cycle processes sustaining terrestrial element cycling and ecosystem function.

Microbial community responses reduce soil carbon loss in Tibetan alpine grasslands under short-term warming.

Changes in labile carbon (LC) pools and microbial communities are the primary factors controlling soil heterotrophic respiration (Rh ) in warming experiments. Warming is expected to initially increase Rh but studies show this increase may not be continuous or sustained. Specifically, LC and soil microbiome have been shown to contribute to the effect of extended warming on Rh . However, their relative contribution is unclear and this gap in knowledge causes considerable uncertainty in the prediction of carbon cycle feedbacks to climate change. In this study, we used a two-step incubation approach to reveal the relative contribution of LC limitation and soil microbial community responses in attenuating the effect that extended warming has on Rh . Soil samples from three Tibetan ecosystems-an alpine meadow (AM), alpine steppe (AS), and desert steppe (DS)-were exposed to a temperature gradient of 5-25°C. After an initial incubation period, soils were processed in one of two methods: (a) soils were sterilized then inoculated with parent soil microbes to assess the LC limitation effects, while controlling for microbial community responses; or (b) soil microbes from the incubations were used to inoculate sterilized parent soils to assess the microbial community effects, while controlling for LC limitation. We found both LC limitation and microbial community responses led to significant declines in Rh by 37% and 30%, respectively, but their relative contributions were ecosystem specific. LC limitation alone caused a greater Rh decrease for DS soils than AMs or ASs. Our study demonstrates that soil carbon loss due to Rh in Tibetan alpine soils-especially in copiotrophic soils-will be weakened by microbial community responses under short-term warming.

Soil organic carbon stability in forests: Distinct effects of tree species identity and traits.

Rising atmospheric CO2 concentrations have increased interest in the potential for forest ecosystems and soils to act as carbon (C) sinks. While soil organic C contents often vary with tree species identity, little is known about if, and how, tree species influence the stability of C in soil. Using a 40 year old common garden experiment with replicated plots of eleven temperate tree species, we investigated relationships between soil organic matter (SOM) stability in mineral soils and 17 ecological factors (including tree tissue chemistry, magnitude of organic matter inputs to the soil and their turnover, microbial community descriptors, and soil physicochemical properties). We measured five SOM stability indices, including heterotrophic respiration, C in aggregate occluded particulate organic matter (POM) and mineral associated SOM, and bulk SOM δ15 N and ∆14 C. The stability of SOM varied substantially among tree species, and this variability was independent of the amount of organic C in soils. Thus, when considering forest soils as C sinks, the stability of C stocks must be considered in addition to their size. Further, our results suggest tree species regulate soil C stability via the composition of their tissues, especially roots. Stability of SOM appeared to be greater (as indicated by higher δ15 N and reduced respiration) beneath species with higher concentrations of nitrogen and lower amounts of acid insoluble compounds in their roots, while SOM stability appeared to be lower (as indicated by higher respiration and lower proportions of C in aggregate occluded POM) beneath species with higher tissue calcium contents. The proportion of C in mineral associated SOM and bulk soil ∆14 C, though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change.

CO2 emissions from an undrained tropical peatland: Interacting influences of temperature, shading and water table depth.

Emission of CO2 from tropical peatlands is an important component of the global carbon budget. Over days to months, these fluxes are largely controlled by water table depth. However, the diurnal cycle is less well understood, in part, because most measurements have been collected daily at midday. We used an automated chamber system to make hourly measurements of peat surface CO2 emissions from chambers root-cut to 30 cm. We then used these data to disentangle the relationship between temperature, water table and heterotrophic respiration (Rhet ). We made two central observations. First, we found strong diurnal cycles in CO2 flux and near-surface peat temperature (<10 cm depth), both peaking at midday. The magnitude of diurnal oscillations was strongly influenced by shading and water table depth, highlighting the limitations of relying on daytime measurements and/or a single correction factor to remove daytime bias in flux measurements. Second, we found mean daily Rhet had a strong linear relationship to the depth of the water table, and under flooded conditions, Rhet was small and constant. We used this relationship between Rhet and water table depth to estimate carbon export from both Rhet and dissolved organic carbon over the course of a year based on water table records. Rhet dominates annual carbon export, demonstrating the potential for peatland drainage to increase regional CO2 emissions. Finally, we discuss an apparent incompatibility between hourly and daily average observations of CO2 flux, water table and temperature: water table and daily average flux data suggest that CO2 is produced across the entire unsaturated peat profile, whereas temperature and hourly flux data appear to suggest that CO2 fluxes are controlled by very near surface peat. We explore how temperature-, moisture- and gas transport-related mechanisms could cause mean CO2 emissions to increase linearly with water table depth and also have a large diurnal cycle.

Hydraulic redistribution affects modeled carbon cycling via soil microbial activity and suppressed fire.

Hydraulic redistribution (HR) of water from moist to drier soils, through plant roots, occurs world-wide in seasonally dry ecosystems. Although the influence of HR on landscape hydrology and plant water use has been amply demonstrated, HR's effects on microbe-controlled processes sensitive to soil moisture, including carbon and nutrient cycling at ecosystem scales, remain difficult to observe in the field and have not been integrated into a predictive framework. We incorporated a representation of HR into the Community Land Model (CLM4.5) and found the new model improved predictions of water, energy, and system-scale carbon fluxes observed by eddy covariance at four seasonally dry yet ecologically diverse temperate and tropical AmeriFlux sites. Modeled plant productivity and microbial activities were differentially stimulated by upward HR, resulting at times in increased plant demand outstripping increased nutrient supply. Modeled plant productivity and microbial activities were diminished by downward HR. Overall, inclusion of HR tended to increase modeled annual ecosystem uptake of CO2 (or reduce annual CO2 release to the atmosphere). Moreover, engagement of CLM4.5's ground-truthed fire module indicated that though HR increased modeled fuel load at all four sites, upward HR also moistened surface soil and hydrated vegetation sufficiently to limit the modeled spread of dry season fire and concomitant very large CO2 emissions to the atmosphere. Historically, fire has been a dominant ecological force in many seasonally dry ecosystems, and intensification of soil drought and altered precipitation regimes are expected for seasonally dry ecosystems in the future. HR may play an increasingly important role mitigating development of extreme soil water potential gradients and associated limitations on plant and soil microbial activities, and may inhibit the spread of fire in seasonally dry ecosystems.