Soil CO2 Efflux Research Articles

The Contribution of Roots, Mycorrhizal Hyphae, and Soil Free-Living Microbes to Soil Respiration and Its Temperature Sensitivity in a Larch Forest

Soil respiration plays a critical role in driving soil carbon (C) cycling in terrestrial forest ecosystems. However, evidence to demonstrate the response of roots, mycorrhizal hyphae, and soil free-living microbes of soil respiration and their temperature sensitivity (Q10) remains lacking. Here, we used a root exclusion method to assess the contribution and response of root respiration (Rroot), mycorrhizal respiration (Rmyc), and (soil organic matter) SOM respiration (Rsom) to soil temperature in a larch forest. During the growing period, the contributions of Rroot, Rmyc, and Rsom to soil respiration were 42%, 6%, and 52%, respectively. The respiration rates of all components increased exponentially with increasing temperature. Based on these constitutive respiration rates with soil temperature, the Q10 values for Rroot, Rmyc, and Rsom were 3.84, 5.18, and 1.86, respectively. The results showed that the response to temperature change was different among roots, mycorrhizal hyphae, and microbes in the soil, while the temperature sensitivity of autotrophic respiration was higher than that of heterotrophic respiration. Importantly, the Rmyc at this site was extremely sensitive to temperature, although its overall emission was small. Mycorrhizal associations were identified as the key drivers of soil respiration and temperature sensitivity. A good understanding of the different soil CO2 efflux components will provide useful information for determining soil C fluxes and predicting soil C dynamics under changing environments.

Effects of aromatic crops based cropping systems and residue returning on soil CO2 emission compared to fallow soil

Two years field experiment was done to investigate the impact of aromatic crops based cropping systems and crop residue returning on soil CO2 efflux and its relationship with soil temperature and moisture. Eight diversified cropping systems were compared to fallow soil as control, total nine treatments: T1-(B-W-C) (basil-wheat-cowpea), T2-(B-W-M) (basil-wheat-menthol mint), T3-(B-P-C) (basil-potato-cowpea), T4-(B-P-M) (basil-potato-menthol mint), T5-(B-L-C) (basil-lentil-cowpea), T6 -(B-L-M) (basil-lentil-menthol mint), T7-(B-Pe-C) (basil-pea-cowpea), T8-(B-Pe-M) (basil-pea-menthol mint) and T9-(F-F-F) (fallow soil). Surface soil CO2 efflux, temperature and moisture of each crop were measured in each season at 15 days interval. Cumulative soil CO2-C emission was calculated on basis of soil CO2 efflux values obtained. T9-(F-F-F) emitted maximum CO2-C in both cropping years, 21.57 Mg C ha−1 and 24.28 Mg C ha−1 in first and second year, respectively. Within cropping systems, maximum was emitted by T1-(B-W-C) and T7-(B-Pe-C). T1 (13.64 Mg C ha−1) and T7 (13.48 Mg C ha−1) in first cropping year while in second cropping year both these cropping systems (T1 and T7) emitted 15.82 Mg C ha−1. Minimum soil CO2-C in both the cropping years was emitted from cropping system T4-(B-P-M) (10.66 Mg C ha−1 and 12.74 Mg C ha−1). Crop residue of the previous crop returned back to soil had significant effect on soil CO2 efflux. Soil CO2-C emission of all treatments (T1-T9) was found to increase with subsequent cropping year. Simple linear regression analysis was performed to elucidate the correlation between soil CO2 efflux, temperature and moisture in all the seasons (rainy, winter and summer). In each season, the soil CO2 efflux was positively correlated to soil temperature and negatively correlated with soil moisture in both the cropping years. Aromatic crops based diversified cropping systems along with residue retention did not decrease the soil CO2-C emission but showed less emission compared to fallow soil.

Long‐term (64 years) annual burning lessened soil organic carbon and nitrogen content in a humid subtropical grassland

Burning has commonly been used to increase forage production and nutrients cycling in grasslands. However, its long-term effects on soil organic carbon (SOC) and nitrogen (N) pools within the aggregates and the relation between aggregates-associated SOC and soil CO2 emissions need further appraisal. This study evaluated the effects of 64years of annual burning on SOC and N dynamics compared to annual mowing and undisturbed treatments in a grassland experiment established in 1950. Soils were sampled from four depths representing the upper 30cm layer and fractionated into macroaggregates, microaggregates and silt+clay fractions. The macroaggregates were further fractionated into three occluded fractions. The SOC in the bulk soil and aggregates were correlated to soil CO2 effluxes measured under field conditions. Compared to the undisturbed treatment, annual burning decreased aggregates stability, SOC and N in the upper 30cm layer by 8%, 5% and 12%, respectively. Grassland mowing induced greater aggregates stability than burning only in the upper 5cm. Burning also decreased SOC in the large macroaggregates (e.g., 0-5cm) compared to mowing and the undisturbed grasslands but proportionally increased the microaggregates and their associated SOC. Soil N associated with aggregates decreased largely following grassland burning, for example, by 8.8-fold in the microaggregates within the large macroaggregates at 20-30cm compared to the undisturbed grassland. Burning also increased soil CO2 emissions by 33 and 16% compared to undisturbed and mowing, respectively. The combustion of fresh C and soil organic matter by fire is likely responsible for the low soil aggregation, high SOC and N losses under burned grassland. These results suggested a direct link between grass burning and SOC losses, a key component for escalating climate change severity. Therefore, less frequent burning or a rotation of burning and mowing should be investigated for sustainable grasslands management.

Contribution Analysis Of Permanent And Sporadic Controls Of Co2 Efflux From Chernozems Over Four Seasons

We analyzed four years field observations (2017–2020) of soil CO2 efflux from Chernozems of arable and foreststeppe ecosystems of Kursk region (Russia), which correspond to the period of the maximal current warming. Three wellknown simulation models of different structure and variable sets (DNDC, RothC, T&P) and nonparametric regression analysis were used to estimate annual CO2 emission from soil and contributions of constant and sporadic controls. The applied models satisfactorily predict both the rate of annual soil CO2 emission and its seasonal dynamics on arable Chernozems. However, while RothC is suitable for the whole set of crops considered, DNDC is most suitable for cereals and T&R for bare soils only. A comparison of the contributions of permanent and sporadic factors to soil respiration showed that on an inter-annual scale soil temperature and moisture are less important than yearly crop rotation in Chernozem plowlands, making the latter the most important predictor apart from general land-use type. Although the combination of significant permanent and sporadic factors is able to explain 41% of the soil CO2 emission variance, the leading involvement of spatial controls prevents the construction of quantitative regression models that are able to make forecasts, requiring the use of more sophisticated simulation models (i.e. RothC) in this case. However, the use of the latter does not yet solve the problem of predicting soil CO2 emission and its net balance in forest-covered or steppe areas of Chernozem forest-steppe landscape.

Effects of solar park construction and solar panels on soil quality, microclimate, CO2 effluxes, and vegetation under a Mediterranean climate

AbstractSolar energy is increasingly used to produce electricity in Europe, but the environmental impact of constructing and running solar parks (SP) is not yet well studied. Solar park construction requires partial vegetation removal and soil leveling. Additionally, solar panels may alter soil microclimate and functioning. In our study of three French Mediterranean solar parks, we analyzed:(1) effects of solar park construction on soil quality by comparing solar park soils with those of semi‐natural land cover types (pinewood and shrubland) and abandoned croplands (former vineyards); and (2) the effect of solar panels on soil microclimate, CO2 effluxes, and vegetation. We measured 21 soil properties of physical, chemical, and microbiological soil quality in one solar park and its surroundings to calculate integrated indicators of soil quality. We surveyed soil temperature and moisture, CO2 effluxes, and vegetation below and outside solar panels of three solar parks. Soil aggregate stability was reduced by SP construction resulting in a degradation of soil physical quality. Soil chemical quality and a general indicator of soil quality were lower in anthropogenic (SP and abandoned vineyards) than in semi‐natural (pinewood and shrubland) land cover types. However, differences between abandoned vineyards representing the preconstruction land cover type and solar parks were not significant. Solar panels reduced the soil temperature by 10% and soil CO2 effluxes by 50% but did not affect early successional plant communities. Long‐term monitoring is needed to evaluate the effects of solar panels on vegetation.

Ecophysiology of an urban citrus orchard

Characterizing the carbon and water economy of non-commercial urban citrus orchards can help determine their value in urban settings. This includes provisions of urban ecology and ecosystem services, such as shade, conservation of biodiversity, and carbon sequestration, under space and water limitations that may be particularly suitable in the semi-arid Mediterranean regions. We carried out canopy-scale eddy covariance (EC) measurements of net ecosystem CO2 exchange (NEE) and evapotranspiration (ET), partitioned these fluxes to their components using chamber-based soil fluxes, and combined them with carbon stock to provide a first approximation of the apparent ecosystem carbon turnover rate (τeco). The urban orchard switched from a carbon sink of 0.6 μmol m−2 s−1 to a carbon source of 0.2 μmol m−2 s−1 between winter and summer, with a first approximation of the annual carbon storage capacity of ∼75.3 g m−2 and a total carbon accumulation over its 40 year life span of ∼3014 g m−2. Carbon accumulated predominantly below ground (67 % of total), and soil CO2 effluxes showed low sensitivity to temperature (Q10 ∼1.6) and therefore also to climate warming, but also a fast C turnover rate (∼5.4 y), and therefore sensitivity to disturbances. The rates of ET increased from 0.40 to 1.25 mmol m−2 s−1 between the wet and dry seasons, and was ∼50 % of a similar nearby commercial orchard. Partitioning of the ecosystem carbon and water fluxes indicated high canopy water use efficiency (11.7 μmol CO2/mmol H2O; during the peak activity period). We demonstrate the potential of urban citrus orchards with low supplemental irrigation (50 % compared with commercial orchards in the area) to store significant amounts of carbon with high transpiration efficiencies. The results will help decision making in regard to urban tree planting and the ecological management of urban green spaces and community use in water-limited environments.

Responses of grassland soil CO2 production and fluxes to drought are shifted in a warmer climate under elevated CO2

As the climate warms, drought events are expected to increase in intensity and frequency, with consequences for the carbon cycle. Soil respiration (Rs) accounts for the largest flux of CO2 from terrestrial ecosystems to the atmosphere. While the drought responses of Rs have been well studied, it is uncertain how they will be modified in a future world, when higher temperatures will occur in combination with elevated atmospheric CO2 concentrations. In a global change experiment in a managed temperate grassland, we studied drought and post-drought responses of Rs dynamics under current versus likely future conditions (+3°, +300 ppm CO2). Furthermore, to understand the soil CO2 production (Ps) and transport dynamics underlying Rs fluxes we continuously monitored in-situ soil CO2 concentrations across the soil profile. Our results show that Rs was higher and that drought-induced reductions in Rs were delayed under future compared to current conditions. Peak drought reductions and post-drought pulses of Rs were more pronounced in the future scenario. Annual Rs was reduced by drought only under current but not under future conditions. An in-depth analysis of soil CO2 gradients and fluxes across the soil profile showed that elevated CO2 stimulated Ps primarily in the main rooting horizon and that warming affected Ps also in deeper soil layers. We found that both in the current and the future scenario drought led to the strongest reductions of Ps in the most productive soil layers, which also exhibited the largest depletion of soil moisture. We conclude that a future warmer climate under elevated CO2 amplifies soil CO2 production and efflux and their peak drought and post-drought responses, but delays the onset of the drought responses and thereby eliminates the overall drought effect on annual soil CO2 emissions.

Diurnal and seasonal soil CO2 efflux variation in Scots pine (Pinus sylvestris L.) forests in the European hemi-boreal zone, Lithuania

Diurnal and seasonal soil CO2 efflux variation in Scots pine (Pinus sylvestris L.) forests in the European hemi-boreal zone, Lithuania

Temporal dynamics of dryland soil CO2 efflux using high-frequency measurements: Patterns and dominant drivers among biocrust types, vegetation and bare soil

Soil respiration is an important component of the carbon (C) cycle and a major contributor to total ecosystem C efflux. Knowledge of the factors that drive soil respiration in drylands is limited, despite these regions represent more than 40% of the Earth land’s surface. In these environments, biocrusts play an important role in CO2 exchange towards and from soils. However, the temporal dynamics and main drivers of CO2 efflux in biocrusts compared to other representative dryland covers such as bare soil and soil under perennial grasses, has not been fully investigated. In this study, we measured the soil CO2 molar fraction (χc) at 2 and 5 cm depths in representative surface covers (cyanobacteria and lichen dominated biocrusts, soil under the alpha grass Macrochloa tenacissima and bare soil) from a semiarid area in SE Spain (Tabernas desert, Almeria) using small solid-state CO2 sensors, during one hydrological year. We determined the CO2 efflux (Fs) from the 0 to 5 cm soil profile using the gradient method. Our results show that soil χc and Fs were low in all surface covers (on average, 464 ppm at 5 cm) during dry soil periods (soil moisture <0.05 m3 m−3). χc and subsequent Fs rapidly increased after rainfall, and showed the highest values in the soil under grass (M. tenacissima) and lichen biocrusts. Time series analysis of Fs allowed identifying periodic patterns in Fs strongly related to moisture and temperature periodicities. Moisture was the main driver for Fs on timescales of weeks and months, while temperature was the main driver on daily scales. Moisture exerted a greater influence on Fs in lichen and soil under grass, while temperature had a greater effect on Fs in cyanobacteria and bare soil. Estimated annual CO2 efflux was 633 g CO2 m−2 y-1 in the soil under M. tenacissima, 450 g CO2 m−2 y-1 in the lichen biocrust, 268 g CO2 m−2 y-1 in the cyanobacterial biocrust and 188 g CO2 m−2 y-1 in the bare soil. On the whole, we demonstrate the suitability of automated Fs measurements for characterising rapid changes in C efflux from dryland surfaces due to changing environmental conditions, which can help improve C predictions in drylands under current climate change.

Spatiotemporal Monitoring of Soil CO2 Efflux in a Subtropical Forest during the Dry Season Based on Field Observations and Remote Sensing Imagery

The CO2 efflux from forest soil (FCO2) is one of the largest components of the global carbon cycle. Accurate estimation of FCO2 can help us better understand the carbon cycle in forested areas and precisely predict future climate change. However, the scarcity of field-measured FCO2 data in the subtropical forested area greatly limits our understanding of FCO2 dynamics at regional and global scales. This study used an automatic cavity ring-down spectrophotometer (CRDS) analyzer to measure FCO2 in a typical subtropical forest of southern China in the dry season. We found that the measured FCO2 at two experimental areas experienced similar temporal trends in the dry season and reached the minima around December, whereas the mean FCO2 differed apparently across the two areas (9.05 vs. 5.03 g C m−2 day−1) during the dry season. Moreover, we found that both abiotic (soil temperature and moisture) and biotic (vegetation productivity) factors are significantly and positively correlated, respectively, with the FCO2 variation during the study period. Furthermore, a machine-learning random forest model (RF model) that incorporates remote sensing data is developed and used to predict the FCO2 pattern in the subtropical forest, and the topographic effects on spatiotemporal patterns of FCO2 were further investigated. The model evaluation indicated that the proposed model illustrated high prediction accuracy for the training and testing dataset. Based on the proposed model, the spatiotemporal patterns of FCO2 in the forested watershed that encloses the two monitoring sites were mapped. Results showed that the spatial distribution of FCO2 is obviously affected by topography: the high FCO2 values mainly occur in relatively high altitudinal areas, in slopes of 10–25°, and in sunny slopes. The results emphasized that future studies should consider topographical effects when simulating FCO2 in subtropical forests. Overall, our study unraveled the spatiotemporal variations of FCO2 and their driving factors in a subtropical forest of southern China in the dry season, and demonstrated that the proposed RF model in combination with remote sensing data can be a useful tool for predicting FCO2 in forested areas, particularly in subtropical and tropical forest ecosystems.

Drought legacies on soil respiration and microbial community in a Mediterranean forest soil under different soil moisture and carbon inputs

Soil moisture can strongly affect the fate of soil organic carbon (C) during microbial decomposition, but the characterisation and prediction of the effects remain challenging, especially the long-term effects of drought history and its interaction with current levels of soil moisture. We investigated the legacy effects of drought on soil activity and microbial community composition and its interaction with actual soil moisture and C addition. Soils from a long-term drought field experiment in a forest were incubated for 80 days under two levels of soil moisture after the addition of 13C-labelled glucose or cellulose. The drought legacy manifested as significantly higher soil CO2 efflux in wet soils from the long-term drought plots than in soils from all other treatments, including the historical control plots, as well as by a higher respiration in the dry treatment, but not in the wet one when cellulose was added. The supply of glucose primed the decomposition of SOM during the whole incubation whereas the supply of cellulose caused a negative priming at the very early stage but had an overall positive priming effect. The composition of the bacterial community varied with soil moisture, but the fungal community was more resistant to water stress and acquired labile C more efficiently under low moisture levels. Fungi dominated cellulose decomposition and bacteria dominated glucose decomposition. These results suggest a key role of fungi in SOM decomposition in the often water-stressed Mediterranean ecosystems. The legacy effects of long-term drought may increase soil respiration during the periods when soils are wet, although the effect may vary with the type of the C inputs.

Annual and seasonal variations in soil volatile organic compound concentrations in a Mediterranean shrubland and holm oak forest

Soil biogenic volatile organic compounds (VOCs) play an important role in soil ecology and function and may affect atmospheric chemistry. While previous studies of soil VOCs have predominantly measured surface flux exchange rates, VOC concentrations within the surface soil layer are largely unknown, especially in Mediterranean ecosystems. In this study, we measured seasonal and annual concentrations of soil VOCs in a Mediterranean shrubland and a holm oak forest over the period 2014–2016. Soil CO2 efflux, and soil enzyme and plant activities were measured as explanatory variables of soil VOC concentrations. There were greater total soil VOC concentrations in the shrubland (3.66 ± 1.01 ppb) than the holm oak forest (2.23 ± 0.51 ppb) across the study period. There were the greatest concentrations of monoterpenes (0.85 ± 0.43 ppb) and methanol (0.81 ± 0.20 ppb) in the shrubland and forest, respectively, and concentrations of methanol, acetic acid, formaldehyde, ethanol, and acetaldehyde were the dominant compounds in both ecosystems (>0.1 ppb). Although concentrations of some VOCs in both ecosystems were highest and lowest in spring and winter, respectively, the variability of other VOCs depended on compound and ecosystem. Soil temperature and water content, CO2 efflux, and enzyme activity were the best explanatory variables for variation in soil VOC concentrations in the two ecosystems: there was a stronger association between concentration of dominant compounds, except formaldehyde, with soil temperature and/or CO2 efflux than with soil water content. Activity of C- and N-degrading enzymes was positively associated with the concentration of VOCs, depending on ecosystem, and consistently correlated with high soil water content. In the holm oak forest soils, net photosynthetic rate (A) was positively correlated with soil concentration of monoterpenes. These results show that soil VOC concentrations in these Mediterranean ecosystems are driven by soil temperature and water content, and microbial activity, in combination with ecosystem plant activity. It is thus likely that projected climate change increases in temperature increase soil VOC concentrations and lead to increases in emissions to the atmosphere; however, microbial production and consumption of soil VOCs may be modulated by soil water content.

Diel and seasonal patterns of soil CO2 efflux in a temperate tidal marsh

Tidal marshes store large amounts of carbon; however, little is known about the patterns, magnitudes, and biophysical drivers that regulate CO2 efflux from these ecosystems. Due to harsh environmental conditions (e.g., flooding), it is difficult to measure continuous soil CO2 efflux in tidal marshes. These data are necessary to inform empirical and process-based models and to better quantify carbon budgets. We performed automated (30 min) and manual (bi-monthly) soil CO2 efflux measurements, for ~20 months, at two sites in a temperate tidal marsh: tall Spartina (TS; dominated by S. cynosuroides) and short Spartina (SS; dominated by S. alterniflora). These measurements were coupled with water quality, canopy spectral reflectance, and meteorological measurements. There were no consistent diel patterns of soil CO2 efflux, suggesting a decoupling of soil CO2 efflux with diel variations in temperature and tides (i.e., water level) showing a hysteresis effect. Mean soil CO2 efflux was significantly higher at SS (2.15 ± 1.60 μmol CO2 m−2 s−1) than at TS (0.55 ± 0.80 μmol CO2 m−2 s−1), highlighting distinct biogeochemical spatial variability. At the annual scale, air temperature explained >50% of the variability in soil CO2 efflux at both sites; and water level and salinity were secondary drivers of soil CO2 efflux at SS and TS, respectively. Annual soil CO2 efflux varied from 287-876 to 153-211 g C m−2 y−1 at SS and TS, respectively, but manual measurements underestimated the annual flux by <67% at SS and <23% at TS. These results suggest that measuring and modeling diel soil CO2 efflux variability in tidal marshes may be more challenging than previously expected and highlight large discrepancies between manual and automated soil CO2 efflux measurements. New technical approaches are needed to implement long-term automated measurements of soil CO2 efflux across wetlands to properly estimate the carbon balance of these ecosystems.

Influence of different tree species on autotrophic and heterotrophic soil respiration in a mined area under reclamation

AbstractPlanting trees is one of the most effective activities in recovering soil organic carbon (SOC) stocks of degraded areas, but we still lack information on how different tree species can influence soil respiration, one of the main sources of carbon dioxide (CO2) to the atmosphere. This study aimed to explore the influence of different tree species on the autotrophic and heterotrophic components of the total soil respiration in a bauxite mining area under reclamation. We analyzed the soil CO2 efflux under five treatments: (i) monoculture of clonal Eucalyptus; (ii) monoculture of Anadenanthera peregrina (L.); (iii) a mixed plantation of 16 native tree species; (iv) a mined area without vegetation cover as a control site; and (v) a natural forest cover as a reference site. This design allowed exploring the soil CO2 dynamics in a gradient of recovery, from a degraded area to natural vegetation. Additionally, we measured soil temperature, moisture, and soil attributes. Soil CO2 efflux increased with increasing tree species cover in the rainy months. There was no significant change in CO2 efflux among the tree species. However, heterotrophic soil respiration contributed to 64% of total soil CO2 efflux and was associated with litter decomposition. Among the abiotic variables, increases in soil moisture had the most influence on CO2 efflux. Therefore, these results help to understand the factors that underpin the loss of SOC and show that afforestation with different tree species can recovery the soil biological activity by improving litter deposition and is a promising way to restore soil quality in degraded areas.

Carbon and Nitrogen Dynamics, and CO2 Efflux in the Calcareous Sandy Loam Soil Treated with Chemically Modified Organic Amendments.

In Saudi Arabia, more than 335,000 tons of cow manure is produced every year from dairy farming. However, the produced cow manure is usually added to the agricultural soils as raw or composted manure; significant nitrogen losses occur during the storage, handling, and application of the raw manure. The recovery of ammonia from cow manure through thermochemical treatments is a promising technique to obtain concentrated nitrogen fertilizer and reducing nitrogen losses from raw manure. However, the byproduct effluents from the recovery process are characterized by different chemical properties from the original raw manure; thus, its impact as soil amendments on the soil carbon and nitrogen dynamics is unknown. Therefore, a 90-day incubation experiment was conducted to study the impact of these effluents on CO2 efflux, organic C, microbial biomass C, available NH4+, and NO3− when added to agricultural soil. In addition to the two types of effluents (produced at pH 9 and pH 12), raw cow manure (CM), composted cow manure (CMC), cow manure biochar (CMB), and control were used for comparison. The application of CM resulted in a considerable increase in soil available nitrogen and CO2 efflux, compared to other treatments. Cow manure biochar showed the lowest CO2 efflux. Cumulative CO2 effluxes of cow manure effluents were lower than CM; this is possibly due to the relatively high C:N ratio of manure effluent. The content of P, Fe, Cu, Zn, and Mn decreased as incubation time increased. Soil microbial biomass C for soil treated with cow manure effluents (pH 12 and 7) was significantly higher than the rest of the soil amendments and control.

Soil carbon dioxide effluxes from riparian areas of two hydrogeomorphic settings in the Ozark National Forest, USA

Riparian buffers serve as conduits through which carbon (C) cycles from terrestrial landscapes to aquatic systems and, ultimately, to oceanic or atmospheric sinks. Riparian areas have also been identified as zones of concentrated biogeochemical activity that often produce greater amounts of greenhouse gases than neighboring upland terrestrial and adjacent aquatic systems. Little work has explored how hydrologic classification of a stream may influence the magnitude of riparian soil processes. A three-way factorial field study examined soil carbon dioxide (CO2) effluxes across flow regime (Groundwater and Runoff streams), season (autumn, winter, spring, and summer), and distance from stream edge (0, 10, and 20 m). Soil CO2 efflux differed between flow regimes across seasons (P = 0.01), where Runoff sites had greater spring soil surface CO2 efflux (i.e., soil respiration mean ± 1 SE = 2.98 ± 0.29 μmol CO2 m−2 s−1) than Groundwater sites (1.40 ± 0.30 μmol CO2 m−2 s−1). Soil CO2 effluxes from Runoff sites were related to soil temperature (rs value range = 0.58 to 0.82, all P < 0.0001) and soil moisture (rs value range = −0.35 to 0.27, P = 0.02 to 0.03). However, effluxes were unrelated to soil moisture at Groundwater sites. Results of this study suggest that consideration of the riparian zone and demonstrated differences in flow regime are needed to understand forested stream C budgets. Anthropogenic landscape alteration and climate change can modify the natural flow regime which, in turn, may have consequences for aquatic C dynamics in streams draining altered riparian zones, particularly in forested settings.

Nutrient limitations regulate soil greenhouse gas fluxes from tropical forests: evidence from an ecosystem-scale nutrient manipulation experiment in Uganda

Abstract. Soil macronutrient availability is one of the abiotic controls that alters the exchange of greenhouse gases (GHGs) between the soil and the atmosphere in tropical forests. However, evidence on the macronutrient regulation of soil GHG fluxes from central African tropical forests is still lacking, limiting our understanding of how these biomes could respond to potential future increases in nitrogen (N) and phosphorus (P) deposition. The aim of this study was to disentangle the regulation effect of soil nutrients on soil GHG fluxes from a Ugandan tropical forest reserve in the context of increasing N and P deposition. Therefore, a large-scale nutrient manipulation experiment (NME), based on 40 m×40 m plots with different nutrient addition treatments (N, P, N + P, and control), was established in the Budongo Central Forest Reserve. Soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes were measured monthly, using permanently installed static chambers, for 14 months. Total soil CO2 fluxes were partitioned into autotrophic and heterotrophic components through a root trenching treatment. In addition, soil temperature, soil water content, and nitrates were measured in parallel to GHG fluxes. N addition (N and N + P) resulted in significantly higher N2O fluxes in the transitory phase (0–28 d after fertilization; p&lt;0.01) because N fertilization likely increased soil N beyond the microbial immobilization and plant nutritional demands, leaving the excess to be nitrified or denitrified. Prolonged N fertilization, however, did not elicit a significant response in background (measured more than 28 d after fertilization) N2O fluxes. P fertilization marginally and significantly increased transitory (p=0.05) and background (p=0.01) CH4 consumption, probably because it enhanced methanotrophic activity. The addition of N and P (N + P) resulted in larger CO2 fluxes in the transitory phase (p=0.01), suggesting a possible co-limitation of both N and P on soil respiration. Heterotrophic (microbial) CO2 effluxes were significantly higher than the autotrophic (root) CO2 effluxes (p&lt;0.01) across all treatment plots, with microbes contributing about two-thirds of the total soil CO2 effluxes. However, neither heterotrophic nor autotrophic respiration significantly differed between treatments. The results from this study suggest that the feedback of tropical forests to the global soil GHG budget could be disproportionately altered by increases in N and P availability over these biomes.

Effects of clear-fell harvesting on soil CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;, and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O fluxes in an upland Sitka spruce stand in England

Abstract. The effect of clear-fell harvesting on soil greenhouse gas (GHG) fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) was assessed in a Sitka spruce forest growing on a peaty gley organo-mineral soil in northern England. Fluxes from the soil and litter layer were measured monthly by the closed chamber method and gas chromatography over 4 years in two mature stands, with one area harvested after the first year. Concurrent measurements of soil temperature and moisture helped to elucidate reasons for the changes in fluxes. In the 3 years after felling, there was a significant increase in the soil temperature, particularly between June and November (3 to 5 ∘C higher), and in soil moisture, which was 62 % higher in the felled area, and these had pronounced effects on the GHG balance in addition to the removal of the trees and their carbon input to the soil. Annual soil CO2 effluxes reduced to almost a third in the first year after felling (a drop from 24.0 to 8.9 t CO2 ha−1 yr−1) and half in the second and third year (mean 11.8 t CO2 ha−1 yr−1) compared to before felling, while those from the unfelled area were little changed. Annual effluxes of N2O more than doubled in the first two years (from 1.0 to 2.3 and 2.5 t CO2e ha−1 yr−1, respectively), although by the third year they were only 20 % higher (1.2 t CO2e ha−1 yr−1). CH4 fluxes changed from a small net uptake of −0.03 t CO2e ha−1 yr−1 before felling to a small efflux increasing over the 3 years to 0.34 t CO2e ha−1 yr−1, presumably because of the wetter soil after felling. Soil CO2 effluxes dominated the annual net GHG emission when the three gases were compared using their global warming potential (GWP), but N2O contributed up to 20 % of this. This study showed fluxes of CO2, CH4, and N2O responded differently to clear-felling due to the significant changes in soil biotic and abiotic factors and showed large variations between years. This demonstrates the need for multi-year measurements of all GHGs to enable a robust estimate of the effect of the clear-fell phase on the GHG balance of managed forests. This is one of very few multi-year monitoring studies to assess the effect of clear-fell harvesting on soil GHG fluxes.

Controlled Grazing of Maize Residues Increased Carbon Sequestration in No-Tillage System: A Case of a Smallholder Farm in South Africa

Despite the positive impact of no-tillage (NT) on soil organic carbon (SOC), its potential to reduce soil CO2 emission still needs enhancing for climate change mitigation. Combining NT with controlled-grazing of crop residues is known to increase nutrient cycling; however, the impacts on soil CO2 effluxes require further exploration. This study compared soil CO2 effluxes and SOC stocks from conventional tillage with free grazing (CTFG), NT with free grazing (NTFG), NT without grazing (NTNG), NT without crop residues (NTNR) and NT with controlled-grazing (NTCG), in South Africa. Soil CO2 effluxes were measured 1512 times over two years using LI-COR 6400XT, once to thrice a month. Baseline SOCs data were compared against values obtained at the end of the trial. Overall, NTCG decreased soil CO2 fluxes by 55 and 29% compared to CTFG and NTNR, respectively. NTCG increased SOCs by 3.5-fold compared to NTFG, the other treatments resulted in SOC depletion. The increase in SOCs under NTCG was attributed to high C input and also low soil temperature, which reduce the SOC mineralization rate. Combining NT with postharvest controlled-grazing showed high potential to increase SOCs, which would help to mitigate climate change. However, it was associated with topsoil compaction. Therefore, long-term assessment under different environmental, crop, and soil conditions is still required.

Different mechanisms underlying divergent responses of autotrophic and heterotrophic respiration to long-term throughfall reduction in a warm-temperate oak forest

BackgroundThere are many studies on disentangling the responses of autotrophic (AR) and heterotrophic (HR) respiration components of soil respiration (SR) to long-term drought, but few studies have focused on the mechanisms underlying its responses.MethodsTo explore the impact of prolonged drought on AR and HR, we conducted the 2-year measurements on soil CO2 effluxes in the 7th and 8th year of manipulated throughfall reduction (TFR) in a warm-temperate oak forest.ResultsOur results showed long-term TFR decreased HR, which was positively related to bacterial richness. More importantly, some bacterial taxa such as Novosphingobium and norank Acidimicrobiia, and fungal Leptobacillium were identified as major drivers of HR. In contrast, long-term TFR increased AR due to the increased fine root biomass and production. The increased AR accompanied by decreased HR appeared to counteract each other, and subsequently resulted in the unchanged SR under the TFR.ConclusionsOur study shows that HR and AR respond in the opposite directions to long-term TFR. Soil microorganisms and fine roots account for the respective mechanisms underlying the divergent responses of HR and AR to long-term TFR. This highlights the contrasting responses of AR and HR to prolonged drought should be taken into account when predicting soil CO2 effluxes under future droughts.