Modeling Soil Carbon Dynamics in Tropical Ecosystems

AimTropical ecosystems have grown increasingly prone to fire over the last century. However, no consensus has yet emerged regarding the effects of fire disturbances on tropical biogeochemical cycles.LocationTropics.Time period1960–2018.Major taxa studiedTropical ecosystems: Above‐ and below‐ground carbon (C) and nitrogen (N) dynamics.MethodsWe analysed the impacts of fire on C and N dynamics in tropical ecosystems through a meta‐analysis of 1,420 observations from 87 studies.ResultsFire reduced both above‐ and below‐ground C and N pools, with greater reductions above‐ than below‐ground. Fire decreased soil total carbon (TC), total nitrogen (TN) and nitrate nitrogen () and increased ammonium nitrogen () in surface mineral soil layers but did not affect those in deep layers. Fire decreased TC and TN in savanna but did not affect those in tropical dry and moist forests. Fire did not affect and in savanna because of non‐significant responses of N mineralization rate (Nmin) to fire. Conversely, fire increased and decreased in tropical dry forest, but did not affect and increased in tropical moist forest owing to thermal decomposition of soil organic N and increased soil nitrification, respectively. Moreover, declined and increased initially and then decreased with time after fire. Above‐ and below‐ground response variables to prescribed fire were mediated largely by fire frequency and experimental duration, respectively.Main conclusionsOur results suggest a high vulnerability of the above‐ground C and N pools to fire, whereas the biogeochemical cycles below‐ground are of high complexity. Fire effects on below‐ground C and N pools, which are highly uncertain and vegetation specific, should be investigated further.

Given the magnitude of soil carbon stocks in northern ecosystems, and the vulnerability of these stocks to climate warming, land surface models must accurately represent soil carbon dynamics in these regions. We evaluate soil carbon stocks and turnover rates, and the relationship between soil carbon loss with soil temperature and moisture, from an ensemble of eleven global land surface models. We focus on the region of NASA’s Arctic-Boreal vulnerability experiment (ABoVE) in North America to inform data collection and model development efforts. Models exhibit an order of magnitude difference in estimates of current total soil carbon stocks, generally under- or overestimating the size of current soil carbon stocks by greater than 50 PgC. We find that a model’s soil carbon stock at steady-state in 1901 is the prime driver of its soil carbon stock a hundred years later—overwhelming the effect of environmental forcing factors like climate. The greatest divergence between modeled and observed soil carbon stocks is in regions dominated by peat and permafrost soils, suggesting that models are failing to capture the frozen soil carbon dynamics of permafrost regions. Using a set of functional benchmarks to test the simulated relationship of soil respiration to both soil temperature and moisture, we find that although models capture the observed shape of the soil moisture response of respiration, almost half of the models examined show temperature sensitivities, or Q10 values, that are half of observed. Significantly, models that perform better against observational constraints of respiration or carbon stock size do not necessarily perform well in terms of their functional response to key climatic factors like changing temperature. This suggests that models may be arriving at the right result, but for the wrong reason. The results of this work can help to bridge the gap between data and models by both pointing to the need to constrain initial carbon pool sizes, as well as highlighting the importance of incorporating functional benchmarks into ongoing, mechanistic modeling activities such as those included in ABoVE.

Influence of military land uses on soil carbon dynamics in forest ecosystems of Georgia, USA

Abstract. The diversity effect on decomposition, through the litter-mixing effects plays a central role in determining the nutrient and carbon dynamics in ecosystems. However, the litter-mixing effects are centered on a leaf litter perspective. Important aspects related to intraspecific interaction and biomass concentration are rarely evaluated, even though they could be essential to determine the litter decomposition dynamics. To our knowledge, we introduced a new perspective to evaluate whether and how the interaction between flower and leaf litter affects the occurrence, direction, and magnitude of litter-mixing effects in terrestrial and aquatic ecosystems. We performed laboratory experiments using flower and leaf litter from the yellow trumpet tree Tabebuia aurea (Silva Manso) Benth. and Hook. f. ex. S. Moore as a model. To obtain realistic results, we manipulated various scenarios of flower : leaf litter biomass proportion and measured 13 functional traits. Litter-mixing effects were consistent in both aquatic and terrestrial environments, with faster decomposition of both litter types in mixtures compared to their monocultures (synergistic effects). Litter-mixing effects were stronger in the terrestrial environment and at higher flower : leaf litter biomass proportions. Our results indicate that synergistic outcomes are mainly associated with complementary effects. Flower litter had a higher concentration of labile C compounds, N, P, and K and lower lignin concentrations, representing a labile litter, while leaf litter had a higher concentration of lignin, Ca, Mg, and Na, representing a refractory litter. Our results demonstrate the importance of litter-mixing effects between flower and leaf litter via complementary effects. These results shed light on the secondary consequences of flower litter on decomposition, suggesting that species with high reproductive investment in flower biomass may play an important role in the nutrient and carbon recycling of diverse plant communities, exerting a pivotal role in biogeochemical dynamics.

Improving current understanding of the factors that control soil carbon (C) dynamics in forest ecosystems remains an important topic of research as it plays an integral role in the fertility of forest soils and the global C cycle. Invasive earthworms have the potential to alter soil C dynamics, though mechanisms and effects remain poorly understood. To investigate potential effects of invasive earthworms on forest C, the forest floor, mineral soil, fine root biomass, litterfall and microbial litter decay rates, and total soil respiration (TSR) over a full year were measured at an invaded and uninvaded deciduous forest site in southern Ontario. The uninvaded site was approximately 300 m from the invaded site and a distinct invasion front between sites was present. Along the invasion front, the biomass of the forest floor was negatively correlated with earthworm abundance and biomass. There was no significant difference between litterfall, microbial litter decay, and TSR between the invaded and uninvaded sites, but fine root biomass was approximately 30% lower at the invaded site. There was no significant difference in total soil C pools (0–30 cm) between the invaded and uninvaded sites. Despite profound impacts on forest floor soil C pools, earthworm invasion does not significantly increase TSR, most likely because increased heterotrophic respiration associated with earthworms is largely offset by a decrease in autotrophic respiration caused by lower fine root biomass.

Warming and precipitation are key global change factors driving soil carbon (C) dynamics in terrestrial ecosystems. However, the effects of warming and altered precipitation on soil microbial diversity and functional genes involved in soil C cycling remain largely unknown. We investigated the effects of warming and increased precipitation on soil C cycling in a temperate desert steppe of Inner Mongolia using metagenomic sequencing. We found that warming reduced plant richness, Shannon–Wiener and Simpson index. In contrast, increased precipitation significantly influenced Shannon–Wiener and Simpson index. Warming reduced soil microbial species by 5.4% while increased precipitation and warming combined with increased precipitation led to increases in soil microbial species by 23.3% and 2.7%, respectively. The relative abundance of Proteobacteria, which involve C cycling genes, was significantly increased by warming and increased precipitation. Warming significantly reduced the abundance of GAPDH (Calvin cycle) and celF (cellulose degradation) while it enhanced the abundance of glxR (lignin degradation). Increased precipitation significantly enhanced the abundance of pgk (Calvin cycle), coxL (carbon monoxide oxidation), malZ (starch degradation), and mttB (methane production). Moreover, a wide range of correlations among soil properties and C cycling functional genes was detected, suggesting the synergistic and/or antagonistic relationships under scenario of global change. These results may suggest that warming is beneficial to soil C storage while increased precipitation negatively affects soil C sequestration. These findings provide a new perspective for understanding the response of microbial communities to warming and increased precipitation in the temperate desert steppe.

Shrub encroachment has significant impacts on carbon (C) and nitrogen (N) dynamics of grassland ecosystems, but, how it influences soil organic C (SOC) and soil total N (STN) stocks of grasslands is not well documented, especially in deep soils. Here, we examined these effects by investigating the SOC density (SOCD) and the STN density (STND) at depths of up to 5 m in shrub patches and grassy matrix at 18 grassland sites in Inner Mongolia, China. Our results revealed that 44.0–54.1% of SOC and 65.5–74.8% of STN were stored in soil layers of 100 cm to 500 cm, and that shrub encroachment resulted in a significant accumulation of SOC and STN in shrub patches. The mean SOCD at 0–500 cm soil layers in shrub patches and grassy matrix was 11.34 and 8.39 kg C m−2, respectively, and the mean STND was 2.45 and 2.13 kg N m−2, respectively. These differences between the shrub patches and grassy matrix were equivalent to an increase in the mean SOC and STN stocks of 2.95 kg C m−2 and 0.32 kg N m−2, respectively. Increases in SOCD and STND mainly occurred in 30–300 cm soil layers, and varied between typical grassland and desert grassland. The changes in SOCD and STND exhibited significant negative correlations with temperature and pH and positive correlations with precipitation and plant biomass. Our findings suggest that shrub encroachment can significantly accumulate SOC and STN in deep soils in temperate grasslands, which helps to understand soil C and N dynamics in shrub‐encroached grasslands.

The assessment of carbon dynamics in terrestrial ecosystems is of great importance in global climate change research. Caatinga is an exclusively Brazilian biome, and quite lacking in studies, particularly of carbon sequestered by biome. A major difficulty for studies is a function of no observation data in micrometeorological station. The hyperspectral and multispectral remote sensing become increasingly feasible to evaluate the dynamics of carbon in ecosystems, in particular in this Caatinga study area. The objective of this study was to apply correlation analysis and multiple regression to find a relationship between gross primary production (GPP) and vegetation cover index in the Caatinga biome, located in vicinities of Petrolina city, Pernambuco State, Brazil. It was used a HandHeldFieldSpec of ASD to obtain the reflectance data of plant canopy and the vegetation indices. The GPP data were obtained in micrometeorological tower installed into a preserved area of Caatinga biome. The results showed that those vegetation indexes which take water into consideration presented higher correlation with GPP, r2=0.95. We conclude that it is possible to obtain GPP using only data of vegetation indexes estimated with remote sensing.

Interactions between soil nitrogen (N) availability, fungal community composition, and soil organic matter (SOM) regulate soil carbon (C) dynamics in many forest ecosystems, but context dependency in these relationships has precluded general predictive theory. We found that ectomycorrhizal (ECM) fungi with peroxidases decreased with increasing inorganic N availability across a natural inorganic N gradient in northern temperate forests, whereas ligninolytic fungal saprotrophs exhibited no response. Lignin-derived SOM and soil C were negatively correlated with ECM fungi with peroxidases and were positively correlated with inorganic N availability, suggesting decay of lignin-derived SOM by these ECM fungi reduced soil C storage. The correlations we observed link SOM decay in temperate forests to tradeoffs in tree N nutrition and ECM composition, and we propose SOM varies along a single continuum across temperate and boreal ecosystems depending upon how tree allocation to functionally distinct ECM taxa and environmental stress covary with soil N availability.

SummaryHerbivory and litter decomposition are key controllers of ecosystem carbon and nutrient cycling. We hypothesized that foliar defences of plant species against vertebrate herbivores would reduce leaf digestibility and would subsequently, through ‘afterlife effects’, reduce litter decomposability.We tested this hypothesis by screening 32 subarctic plant species, belonging to eight types in terms of life form and nutrient economy strategy, for (1) leaf digestibility in cow rumen juice; (2) biochemical and structural traits that might explain variation in digestibility; and (3) litter mass loss during simultaneous incubation in an outdoor subarctic litter bed.Interspecific variation in green‐leaf digestibility corresponded significantly with that in litter decomposability; this relationship was strongly driven by overall variation among the eight plant types (r = 0·92). The same relationship was not detectable within plant types in taxonomic relatedness tests.Several biochemical and structural parameters (phenol‐to‐N ratio, lignin‐to‐N ratio) explained a significant part of the variation in leaf digestibility, but again only between and not within plant types.Our results provide further support for the role played by foliar defence in the link between plant and soil via the decomposition pathway. They are also a new example of the potential control of plant functional types over carbon and nutrient dynamics in ecosystems.

We evaluated plant species effects on soil carbon (C) and nitrogen (N) dynamics in a steppe ecosystem of northern China. In two subsequent years, we measured soil properties in the top 10 cm of the soil under replicated mono-dominant plant patches in two sites that differed in land-use history: a cultivated site (old field) and an uncultivated site (steppe). Both in the cultivated site and the uncultivated site, we selected patches of three of the dominant plant species. Contrast analyses in ANOVA showed that soil organic carbon (SOC) and total N content (g per m2) was on average lower in the cultivated site than in the uncultivated site. On average, soil respiration was also lower in the cultivated site than in the uncultivated site. However, overall differences in soil C and N dynamics between the cultivated site and the uncultivated site (if existing) were generally small compared to the effects of individual plant species. Soil respiration differed among plant species in the cultivated site, but not in the uncultivated site. In contrast, SOC content, total N, and N mineralization rate differed among plant species in the uncultivated site, but not in the cultivated site. Mineralization and nitrification rates strongly varied among the dominant plant species, particularly in the uncultivated site. Variation in both C and N pools and fluxes could be best explained by a combination of plant biomass, litter, and soil microbial and micro-climatic parameters. Cultivation can directly affect soil C and N dynamics. However, importantly, our data suggest that indirect effects through changes in plant species composition are also important, and probably strongly interact with direct effects in affecting soil C and N dynamics after land-use change. Hence, evaluation of land-use history on soil C and N dynamics requires integral analyses of changes in plant community composition.

A simulation model of the carbon cycle in land ecosystems (Sim-CYCLE): a description based on dry-matter production theory and plot-scale validation

We summarize the results of a recent interagency assessment of land carbon dynamics in Alaska, in which carbon dynamics were estimated for all major terrestrial and aquatic ecosystems for the historical period (1950-2009) and a projection period (2010-2099). Between 1950 and 2009, upland and wetland (i.e., terrestrial) ecosystems of the state gained 0.4TgC/yr (0.1% of net primary production, NPP), resulting in a cumulative greenhouse gas radiative forcing of 1.68×10-3 W/m2 . The change in carbon storage is spatially variable with the region of the Northwest Boreal Landscape Conservation Cooperative (LCC) losing carbon because of fire disturbance. The combined carbon transport via various pathways through inland aquatic ecosystems of Alaska was estimated to be 41.3TgC/yr (17% of terrestrial NPP). During the projection period (2010-2099), carbon storage of terrestrial ecosystems of Alaska was projected to increase (22.5-70.0TgC/yr), primarily because of NPP increases of 10-30% associated with responses to rising atmospheric CO2 , increased nitrogen cycling, and longer growing seasons. Although carbon emissions to the atmosphere from wildfire and wetland CH4 were projected to increase for all of the climate projections, the increases in NPP more than compensated for those losses at the statewide level. Carbon dynamics of terrestrial ecosystems continue to warm the climate for four of the six future projections and cool the climate for only one of the projections. The attribution analyses we conducted indicated that the response of NPP in terrestrial ecosystems to rising atmospheric CO2 (~5% per 100 ppmv CO2 ) saturates as CO2 increases (between approximately +150 and +450 ppmv among projections). This response, along with the expectation that permafrost thaw would be much greater and release large quantities of permafrost carbon after 2100, suggests that projected carbon gains in terrestrial ecosystems of Alaska may not be sustained. From a national perspective, inclusion of all of Alaska in greenhouse gas inventory reports would ensure better accounting of the overall greenhouse gas balance of the nation and provide a foundation for considering mitigation activities in areas that are accessible enough to support substantive deployment.

The Relationship between Atmospheric Change and Carbon Dynamics in Terrestrial Ecosystems : A Global Study Using a Mechanistic Model,Sim-CYCLE

To better understand the eco-physiological characteristics of dominant tree species in Korean pine broad-leaved forests, and to provide fundamental data for modelling and predicting carbon dynamics of forest ecosystems, we measured leaf CO2 assimilation rate versus intercellular CO2 concentration curves of four canopy dominant tree species in a Korean pine broad-leaved forest, in situ, for the first time, using a canopy crane in the Changbai Mountain Forest Ecosystem Research Station. Several important photosynthetic parameters were fitted with the FvCB model. Photosynthe-tic rate (A), maximum carboxylation rate (Vc max) and stomatal conductance (gs) were lowest in Pinus koraiensis (Pk), while stomatal limitation on photosynthesis (Ls) was highest in Pk. There were significant variations of photosynthetic characteristics among the three broad-leaved tree species [i.e., Fraxinus mandshurica (Fm), Quercus mongolica (Qm) and Tilia amurensis (Ta)]. The rank of tree species with respect to area-based Vc max was: Fm (83.2 μmol·m-2·s-1) and Qm (89.3 μmol·m-2·s-1) > Ta (68.4 μmol·m-2·s-1) and Pk (68.8 μmol·m-2·s-1) (P<0.05), and their rank with respect to mass-based Vc max was: Fm (1.36 μmol·g-1·s-1) > Qm (1.03 μmol·g-1·s-1) > Ta (0.90 μmol·g-1·s-1) > Pk (0.42 μmol·g-1·s-1) (P<0.05). From July to September, A value significantly declined in Fm and Qm, but remained stable in Ta and Pk. By contrast, Vc max significantly decreased in all tree species from July to September. Our results indicated that seasonal variation of Vcmax should be taken into consideration in the modelling and predicting of forest ecosystem carbon dynamics in northeastern China.