Measured Soil CO2 Efflux Research Articles

Effect of temperature, water availability, and soil properties on soil CO2 efflux in beech-fir forests along the Carpathian Mts

Carpathian Mountain beech-fir forests are exposed to severe pressures related to climate change. Alterations in soil respiration, the main source of CO2 emissions from forest ecosystems into the atmosphere, could potentially impact the future carbon balance of these ecosystems. We performed soil CO2 efflux measurements along the whole Carpathian arc on eight selected locations during two separate campaigns in 2022 and 2023. The correlations between soil CO2 efflux and various climatic, micrometeorological, and soil conditions were evaluated as well as the impact of canopy gaps at each site.Soil CO2 efflux varied between the two campaigns primarily due to fluctuations in soil water content, while differences among the sites were more influenced by soil carbon and nitrogen content. The presence of a canopy gap did not consistently affect soil CO2 efflux; in some sites, it was lower in the gap than under a closed canopy, while in others, the opposite trend was observed. In conclusion, soil CO2 efflux measured at eight sites was closely associated with soil properties rather than with climate or micrometeorological parameters. Therefore, the potential influence of climate on the input of new organic matter through forest productivity and species composition will play a significant role in the decomposition and storage of soil organic matter.

Biodegradable plastic mulch films increase yield and promote nitrogen use efficiency in organic horticulture

Plastic film mulches (PFM) are used extensively due to their ability to increase yield and suppress weed emergence. Their effects on plant-soil-microbial interactions, however, are less well understood. Organic systems rely on the supply of nutrients from organic sources (e.g., manures and fertility-building leys) and where poor N availability often limits yield. The issue is compounded by horticultural crops (e.g., lettuce) with a high N demand, but which are inefficient at recovering N from soil. The effect of PFM on the mineralisation of organic fertilisers and its interaction with other agronomic factors such as planting density is also less well-researched. We hypothesised that biodegradable PFM would be a useful tool to increase the efficiency of N management in organic horticulture by increasing the supply of available N leading to increased crop N uptake and crop yield, and simultaneously reducing N losses during the growing season. We conducted two field experiments under a temperate maritime climate with lettuces grown with either conventional (LDPE) PFM or a polylactic acid-based biodegradable PFM alongside un-mulched controls. The first experiment involved black or white coloured PFMs and two planting densities while the second experiment involved treatments with and without addition of poultry manure. Overall, yields were increased by 39% in both experiments with PFM, while soil mineral N concentrations were up to 5 times higher with PFM than without. Measurements of soil organic matter (SOM) turnover (Tea Bag Index) and soil CO2 efflux indicated a more rapid decay of SOM in the presence of the PFM. The use of PFM also promoted N use efficiency (NUE) by 300% in the presence of poultry manure. Denser planting with PFM resulted in proportionately higher yields. Higher yield and higher N concentrations (5-10%) in crop tissue in mulched plots resulted in higher total N uptake, however, total N uptake was low compared to soil concentrations: mulching with biodegradable PFM resulted in higher residual mineral N than un-mulched plots (77-147 mg kg-1 vs. 19 -70 mg kg-1). Our results are consistent with reduced N losses to the environment during the growing season and increased mineralisation under PFM. In conclusion, our findings support the adoption of PFM for organic horticulture and show that biodegradable PFM perform similarly to LDPE-based PFMs.

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.

Spatio-temporal variability in soil CO2 efflux and regulatory physicochemical parameters from the tropical urban natural and anthropogenic land use classes

Urban ecosystems, the heterogeneous and rapidly changing landscape, showed a considerable impact on the global C cycle. However, studies encompassing the spatial differences in urban land uses on soil C dynamics are limited in tropical ecosystems. In this study, seasonal and temporal variability in soil CO2 efflux (SCE) and its regulatory physicochemical variables under five urban land use classes viz., Bare (BAR), Agriculture (AGR), Plantation (PLT), Grassland (GRA) and Lawns (LAW) were assessed from 2014 to 2016. Bare land use was considered as the reference for observing the variation for different land uses. Seasonal measurements of SCE, soil temperature, moisture content, pH, ammonium-N, nitrate-N and microbial biomass C (MBC) were performed whereas soil organic C (SOC), soil N, and soil physical properties were measured annually. Our results showed a significant (P < 0.01) increase in SCE by 89%, 117%, 132% and 166% for land use types from BAR to AGR, PLT, GRA and LAW, respectively. The results revealed a two-fold increase in SCE from anthropogenically managed urban lawns as compared to bare soil. PLT and LAW land use classes showed higher SOC and N contents. SCE was found positively correlated with temperature, moisture, SOC, soil N and MBC whereas negatively correlated with ammonium-N and nitrate-N (at P < 0.05) for the overall dataset. Soil moisture, temperature, SOC, porosity and pH were identified as the major determinant of urban SCE by explaining 63% of the variability in overall SCE. Further, temperature for BAR and LAW; moisture for PLT; ammonium-N for GRA; and nitrate-N for AGR were identified as the major regulators of SCE for different land use classes. The findings revealed that the interaction of soil temperature and moisture with nutrient availability regulates overall and seasonal variability in SCE in an urban ecosystem. Since these variables are highly affected by climate change, thus, the soil C source-sink relationships in tropical urban ecosystems may further change and induce a positive global warming potential from urban ecosystems.

Soil CO2 emissions from a rubber plantation on tropical peat during a strong El Niño year

El Niño event potentially enhances the soil CO2 emissions due to the prolonged of the dry season which lowers the groundwater level more. However, field information on the effect of the El Niño years on the soil respiration is limited in Indonesia’s peatlands. The aim of this study is to compare the soil respiration between normal and El Niño years. We measured soil CO2 efflux using a closed chamber system monthly, in near and far positions from rubber trees, on the dry season from July to October, in normal year of 2014 and strong El Niño year of 2015, on a rubber plantation established on peat soil. No significant relationship was found between soil respiration with groundwater level or soil temperature. We found that soil respiration in the strong El Niño year of 2015 of 9.56 μ mol m−2 s−1 was significantly larger than that in the normal year of 2014 at 5.11 μ mol m−2 s−1. We expected this finding can be used as the first baseline information regarding soil CO2 emissions under different climatic conditions and challenging how to manage the peatland for mitigating high soil CO2 emissions during El Niño events.

Impact of climate and land degradation on soil carbon fluxes in dry semiarid grasslands in SE Spain

This study investigates how precipitation, temperature and seasonality (as a proxy of plant productivity) affect the temporal and spatial variability of soil CO2 efflux in two dry semiarid grasslands with different degrees of land degradation. We measured soil CO2 efflux over four years under plant, biological soil crust and bare soil patches and estimated annual soil carbon losses in both, a natural and a degraded grassland, by means of generalised additive mixed models considering temporal autocorrelation in the data. Soil CO2 efflux ranged from 0.08 to 3.70 and from 0.10 to 3.01 μmol CΟ2 m−2 s−1 in the natural and degraded grasslands, respectively. Daily soil CO2 efflux was mostly affected by moisture in the degraded grassland (25.4%), while in the natural grassland was affected jointly by seasonality, temperature and moisture (27.5%). Overall, the highest soil carbon fluxes were measured in soils covered by biological soil crusts (1.24 ± 0.02 and 1.10 ± 0.02) and the lowest in bare soils (1.11 ± 0.02 and 0.82 ± 0.02 μmol CΟ2 −2 s−1) in the natural and degraded sites, respectively. Cumulative soil carbon fluxes were mainly driven by temperature and previous precipitation (over three months). The highest soil carbon losses were estimated in the driest year (2009) and the lowest in the wettest (2010) with almost twice the amount of rainfall. The main difference between these years was the timing of the events that mostly occurred in the moments of maximum plant activity with optimum temperatures in spring in the dry year. Changes in precipitation patterns will affect soil carbon fluxes more than rainfall amount, particularly in degraded grasslands. Therefore, considering all climate drivers together with plant activity is essential to predict how climate change will affect soil biological processes in drylands.

LATEST CARBON BALANCE SURVEYS AND MONITORING AT THE KURSK BIOSPHERE STATION OF THE INSTITUTE OF GEOGRAPHY, RAS, IN THE FRAMEWORK OF THE NOVEL CONCEPT OF LAND DEGRADATION NEUTRALITY

The paper focuses on the applicability and adaptation of the novel international concept of Land Degradation Neutrality (LDN) of the United Nations, as applied to the territory of Russian Federation, taking into account the large variety of its soil and climatic conditions and economic approaches, as well as the historical past. In particular, in this publication, a team of authors made an attempt to assess to what extent the estimates of the carbon balance and its main components for the study area can be used as one of the indicators of the LDN in relation to the most fertile chernozem agrolandscapes in the south of the European part of Russia (Kursk region). Field work and modelling have been carried out in natural ecosystems and agrolandscapes in the neighbourhood of the Kursk Biosphere Station (KBS) of the Institute of Geography, Russian Academy of Sciences. This valuable russian scientific research station has a long and rich history of geo(eco)system observations with LDN and net C-balance monitoring of age-old agrolandscapes, becoming one of its present focuses. Using simulation models DNDC, RothC, T&amp;P, C-calculators, as well as long-term field measurements of soil CO2 efflux and carbon net exchange, it was found that individual software tools are not able to adequately assess carbon exchange processes of the complex landscape, but correctly selected ensembles consisting of C-balance simulations and C-calculators supported by field data are able to meet such challenges more successfully. On this condition the area's carbon net balance can serve as an alternative to the recommended global LDN indicators.

Harvesting intensity and tree species affect soil respiration in uneven-aged Dinaric forest stands

Forest management, especially thinning and harvesting measures, has a significant impact on the forest carbon balance especially in the forests with long-term continuous cover history. We measured soil CO2 efflux (Rs) in three forest complexes of mixed, uneven-aged Dinaric forests with predominating silver fir (Abies alba Mill.), beech (Fagus sylvatica L.), and Norway spruce (Picea abies Karst.). Rs was measured after removal of mature forest stands with 50% and 100% intensity of living stock and compared with Rs on the control plots without any applied silvicultural measures. Rs was measured monthly in three consecutive 2012, 2013 and 2014 growing periods.Soil CO2 efflux increased after harvest of both intensities in all studied forest stands. The biggest increase was measured in beech stands and amounted up to 47 and 69% for 50% and 100% harvest intensities, respectively. The effect of harvest on Rs in spruce and fir stands was similar – up to 26% for 50% harvest intensity and 48% for 100% harvest intensity. Despite the biggest increase after harvest, Rs in beech stands returned the fastest to the level of the uncut forest and this levelling period (LP) took 14–17 months with a little delay of the stands with 100% harvest intensity. The LP for all fir stands, for spruce stands with 50% harvest intensity and for one spruce stand with 100% harvest intensity, was 26–29 months. At two spruce stands with 100% harvest intensity we did not record Rs levelling during our three-year study. This study involved forest stands of three predominating tree species growing under the same conditions, which allowed us to determine the species-specific sensitivity of soil CO2 efflux to the different harvesting intensities.

Hedgerow effects on CO2 emissions are regulated by soil type and season: Implications for carbon flux dynamics in livestock-grazed pasture

In this study we assess the potential for farmland hedgerows to provide climate mitigation via carbon (C) storage, using soil carbon dioxide (CO2) efflux to improve upscaling validity. Two contrasting sites, freely-draining (FD) versus seasonally-wet (SW), situated in mixed-livestock farms (Conwy, Wales, UK), were selected. We measured soil CO2 efflux associated with three field boundaries: hedgerow on SW soil; hedgerow on FD soil; stone wall (abiotic control) on FD soil, quantifying the influence of distance from field boundary and grazing occurrence (grazed pasture versus un-grazed zone adjacent to hedgerows) on annual C budgets based on soil CO2 flux and net primary productivity. For the FD site, the annual C budget showed that pasture was a net source of C emissions (11 ± 1.5 t CO2 ha−1 yr−1) and the un-grazed zone adjacent to the hedgerow a net sink (-0.9 ± 2.2 t CO2 ha−1 yr−1). For the SW site, pasture acted as a small net sink of C (-0.1 ± 1.3 t CO2 ha−1 yr−1) and the hedgerow zone a net source (5.8 ± 0.8 t CO2 ha−1 yr−1), due entirely to a spike in soil CO2 efflux associated with a relatively unusual summer drought. To investigate the effect of this observed summer drought on more typical (for the UK maritime climate) annual C source-sink dynamics, we modelled soil CO2 efflux for a summer-drought-excluded year for both FD and SW soils. With greater hedgerow cover (modelled prediction compared with a baseline of no hedgerows), annual CO2 flux became more negative (greater net sink) in fields on FD soil (by 1 t CO2 ha−1 yr−1 at 8% hedgerow cover), with drought limiting the effect size. In SW soils, greater hedgerow cover also led to a more negative annual CO2 flux (by 0.4 t CO2 ha−1 yr−1 at 8% hedgerow cover) when drought was excluded, but a more positive flux (net C source) with drought included (by 0.5 t CO2 ha−1 yr−1 at 8% hedgerow cover). This study illustrates the importance of the interaction between soil type and seasonal events such as drought on the ability of hedgerows to act as a net C sink.

Interaction of abiotic factor on soil CO2 efflux in three forest communities in tropical deciduous forest from India.

Environmental factors along with soil physico-chemical properties play a significant role on the diurnal trend of soil CO2 efflux. Soil CO2 efflux in Indian tropical forests is poorly studied. We studied the soil CO2 efflux in a representative tropical deciduous forest at Katerniaghat Wildlife Sanctuary (KWLS), Uttar Pradesh. The three forest communities namely dry mixed (DMF), Sal mixed (SMF), and Teak plantation (TPF) were selected for measuring soil CO2 efflux in the summer season during April to May 2017 using automated LI-COR 8100 soil CO2 flux system. Soil physico-chemical parameters were also studied in the three abovementioned forest communities. We also measured the different microclimatic variables at forest understorey in all three communities during the summer season. Total day time soil CO2 efflux of 826.70, 1089.24, and 828.94 (μmolCO2 m-2d-1) was observed in TPF, SMF, and DMF respectively. Soil CO2 efflux observed significant differences (P < 0.01) among the three forest communities studied for the summer season in tropical deciduous forest of Terai Himalaya. Average soil CO2 efflux rate (μmol CO2 m-2 s-1) of 4.06 ± 0.36, 5.03 ± 0.45, and 4.37 ± 0.79 was observed in TPF, SMF, and DMF, respectively, which is positively correlated with total organic carbon (TOC) and water holding capacity (WHC) among soil physico-chemical variables. Among microclimatic variables, soil temperature (ST, °C) and air temperature (AT, °C) observed strong positive correlation with day time soil CO2 efflux in all three communities. Significant increase in soil CO2 flux was observed with increasing air and soil temperature (AT and ST) in DMF and SMF. Maximum TOC of 19.23 g Kg-1 was observed in SMF among all communities in the summer season. The result showed that soil CO2 efflux is closely associated with TOC, WHC, AT, and ST for Indian deciduous forest ecosystems.

Temperature sensitivity of decomposition decreases with increasing soil organic matter stability

Evaluation of the temperature sensitivity of soil organic matter (SOM) decomposition is critical for forecasting whether soils in a warming world will lose or gain carbon and, therefore, accelerate or mitigate climate warming. It is usually described, using Arrhenius kinetics, as increasing with the stability of the substrate in laboratory conditions, where substrate availability is non-limiting and where chemical recalcitrance, therefore, predominantly regulates stability. However, conditions of non-limiting subtrate availability are rare in the undisturbed soil, where physicochemical protection of substrates may control their stability. The aim of this study was to assess the temperature sensitivity of decomposition of SOM with contrasting stability in the field. Our conceptual approach was based on in situ measurements of soil CO2 efflux at a range of temperatures from root exclusion plots of increasing age (1 month and three decades) and, therefore, with SOM of increasing stability. From a set of short-term measurements in spring, using diurnal temperature variation, the relative temperature sensitivity of SOM decomposition decreased significantly (p < 0.0001) with increasing SOM stability, and was weak (Q10 < 1.3) in long-term root exclusion plots. This result was confirmed in a similar set of short-term measurements repeated later in the year, in summer, as well as from an analysis perfomed at the seasonal timscale. We provide direct field evidence that the temperature sensitivity of SOM decomposition decreases with increasing stability, in direct contrast with Arrhenius kinetics prediction, and therefore show that stability of SOM in the field cannot be the sole result of chemical recalcitrance. We conclude that the physicochemical protection of SOM, which controls SOM stability in the field, constrains the temperature sensitivity of SOM decomposition under field conditions.

Temporal and spatial variability in soil CO2 efflux in the patagonian steppe

Soil respiration is a major flux of CO2 to the atmosphere. Despite its significance there is a limited understanding of its magnitude, controlling factors and how it varies over time and space in arid ecosystems. We evaluated the temporal pattern of soil CO2 efflux and their response to rain pulses in a patagonian steppe, taking into account the spatial heterogeneity (bare soil and vegetated patches). We measured soil CO2 efflux in bare soil and vegetated patches along the year. We also analyzed physical and chemical soil traits, root density and heterotrophic bacterial count. Soil water content and temperature exhibited seasonal variability and it was larger in bare soil patches than in vegetated patches. Root density, organic matter and phosphorus were higher in vegetated patches than in bare soil. CO2 efflux was 48% higher in vegetated patches than in bare soil patches. Soil CO2 efflux decreased from summer to winter, reaching its maximum value (about 0.6 μmol m2 s−1) in spring. In both patch types, soil CO2 efflux was explained by the interaction between soil temperature and soil water content. Soil CO2 efflux also was positively correlated with soil root density. Bare soil and vegetated patches exhibited distinct response to a rain pulses. Vegetated patches were highly sensitive to rainfall events, generating a large CO2 pulse, returning to previous values after three days. Bare soil CO2 efflux did not exhibit significant changes after a rain pulse. In patagonian arid ecosystems, the seasonal variation in soil respiration is explained mainly by the interaction between soil temperature and water content. Bare soil patches had higher water content but lower root density resulting in lower soil CO2 respiration than vegetated patches. However, at ecosystem level the contribution of bare soil to total soil CO2 efflux was similar to the contribution of vegetated patches because bare soil cover is 65% in the study area. Changes in the number of small rain events as well as changes in plant cover could have large consequences on soil ecology and biochemistry in dry and heterogeneous ecosystems.

Effects of aspect and altitude on carbon cycling processes in a temperate mountain forest catchment

ContextVarying altitudes and aspects within small distances are typically found in mountainous areas. Such a complex topography complicates the accurate quantification of forest C dynamics at larger scales.ObjectivesWe determined the effects of altitude and aspect on forest C cycling in a typical, mountainous catchment in the Northern Limestone Alps.MethodsForest C pools and fluxes were measured along two altitudinal gradients (650–900 m a.s.l.) at south-west (SW) and north-east (NE) facing slopes. Net ecosystem production (NEP) was estimated using a biometric approach combining field measurements of aboveground biomass and soil CO2 efflux (SR) with allometric functions, root:shoot ratios and empirical SR modeling.ResultsNEP was higher at the SW facing slope (6.60 ± 3.01 t C ha−1 year−1), when compared to the NE facing slope (4.36 ± 2.61 t C ha−1 year−1). SR was higher at the SW facing slope too, balancing out any difference in NEP between aspects (NE: 1.30 ± 3.23 t C ha−1 year−1, SW: 1.65 ± 3.34 t C ha−1 year−1). Soil organic C stocks significantly decreased with altitude. Forest NPP and NEP did not show clear altitudinal trends within the catchment.ConclusionsUnder current climate conditions, altitude and aspect adversely affect C sequestering and releasing processes, resulting in a relatively uniform forest NEP in the catchment. Hence, including detailed climatic and soil conditions, which are driven by altitude and aspect, will unlikely improve forest NEP estimates at the scale of the studied catchment. In a future climate, however, shifts in temperature and precipitation may disproportionally affect forest C cycling at the southward slopes through increased water limitation.

Soil CO2 efflux in uneven-aged and even-aged Norway spruce stands in southern Finland

Abstract: Even-aged forests usually act as carbon sinks during most of their rotation. However, after clearcut they become sources of carbon for a period of several years. Applying uneven-aged forest management with selective cuttings will maintain tree cover and reduce the environmental impact on forest floor. The aim of this study was to compare the soil CO2 efflux between uneven-aged and even-aged Norway spruce stands with similar site properties, to investigate the effect of management practices on soil CO2 efflux and its possible correlation with soil environmental and chemical properties. We measured soil CO2 efflux in even- and uneven-aged Norway spruce stands (Picea abies [L.] Karst) in southern Finland during the summer of 2013 using closed chamber method on fixed measuring points. The study included two uneven-aged stands and two even-aged stands (a clearcut site and a mature even-aged stand). Soil moisture and soil temperature were measured at the same time as soil CO2 efflux. Soil cores were collected from the topsoil of each study plot to determine soil carbon and nitrogen concentrations. Mean soil CO2 efflux through the summer was highest in the clearcut plot (0.367 mg m-2 s-1) followed by the uneven-aged stands (0.298 and 0.257 mg m-2 s-1, respectively) and the smallest fluxes were measured in the mature even-aged stand (0.224 mg m-2 s-1). There was no statistically significant difference in soil CO2 efflux between the even- and uneven-aged stands of the same site fertility. Even- and uneven-aged stands did not differ significantly in soil moisture or soil temperature. Soil CO2 efflux increased steadily with soil temperature, whereas increasing soil moisture considerably increased soil CO2 efflux at lower moisture levels but only moderately at higher soil moisture levels. Soil carbon and nitrogen concentration did not differ between the study plots of the same fertility. Uneven-aged structure forestry did not prevent the increase in soil CO2 efflux after cuttings. However, the large variation in soil CO2 efflux rates within the uneven-aged stands suggests that the stand level CO2 efflux can be controlled with the intensity of the cutting.

Hot-Moments of Soil CO2 Efflux in a Water-Limited Grassland

The metabolic activity of water-limited ecosystems is strongly linked to the timing and magnitude of precipitation pulses that can trigger disproportionately high (i.e., hot-moments) ecosystem CO2 fluxes. We analyzed over 2-years of continuous measurements of soil CO2 efflux (Fs) under vegetation (Fsveg) and at bare soil (Fsbare) in a water-limited grassland. The continuous wavelet transform was used to: (a) describe the temporal variability of Fs; (b) test the performance of empirical models ranging in complexity; and (c) identify hot-moments of Fs. We used partial wavelet coherence (PWC) analysis to test the temporal correlation between Fs with temperature and soil moisture. The PWC analysis provided evidence that soil moisture overshadows the influence of soil temperature for Fs in this water limited ecosystem. Precipitation pulses triggered hot-moments that increased Fsveg (up to 9000%) and Fsbare (up to 17,000%) with respect to pre-pulse rates. Highly parameterized empirical models (using support vector machine (SVM) or an 8-day moving window) are good approaches for representing the daily temporal variability of Fs, but SVM is a promising approach to represent high temporal variability of Fs (i.e., hourly estimates). Our results have implications for the representation of hot-moments of ecosystem CO2 fluxes in these globally distributed ecosystems.

Soil CO2 Efflux Measurements by Alkali Absorption and Infrared Gas Analyzer in the Brazilian Semiarid Region

ABSTRACT The CO2 emission from the soil surface, commonly referred to as soil CO2 efflux (ECO2) or soil respiration, is the sum of processes that include root respiration and microbial activity. Measuring this evolution is important to establish sustainable land use models and to estimate global fluxes of carbon, which affect climate change. Despite its importance, few measurements have been made in areas of the semiarid Brazilian Northeast region, and most of them were made using the alkali absorption method [...]

Stored root carbohydrates can maintain root respiration for extended periods.

Tight coupling between below-ground autotrophic respiration and the availability of recently assimilated carbon (C) has become a paradigm in the ecophysiological literature. Here, we show that stored carbohydrates can decouple respiration from assimilation for prolonged periods by mobilizing reserves from transport roots to absorptive roots. We permanently disrupted the below-ground transfer of recently assimilated C using stem girdling and root trenching and measured soil CO2 efflux for over 1 yr in longleaf pine (Pinus palustris), a species that has large reserves of stored carbohydrates in roots. Soil CO2 efflux was not influenced by girdling or trenching through the 14-month observation period. Stored carbohydrate concentrations in absorptive roots were not affected by the disrupted supply of current photosynthate for over 1 yr; however, carbohydrate concentrations in transport roots decreased. Our results indicate that root respiration can be decoupled from recent canopy assimilation and that stored carbohydrates can be mobilized from transport roots to absorptive roots to maintain respiration for over 1 yr. This refines the current paradigm that canopy assimilation and below-ground respiration are tightly coupled and provides evidence of the mechanism and dynamics responsible for decoupling the above- and below-ground processes.

Soil carbon dioxide emissions due to oxidative peat decomposition in an oil palm plantation on tropical peat

Soil carbon dioxide (CO2) efflux was measured continuously for two years using an automated chamber system in an oil palm plantation on tropical peat. This study investigated the factors controlling the CO2 efflux and quantified the annual cumulative CO2 emissions through soil respiration and heterotrophic respiration, which is equivalent to oxidative peat decomposition. Soil respiration was measured in close-to-tree (<2.5m, CT) and far-from-tree (>3m, FT) plots, and heterotrophic respiration was measured in root-cut (RC) plots by a trenching method. The daily mean CO2 efflux values (mean±1 standard deviation) were 2.80±2.18, 1.59±1.18, and 1.94±1.58μmolm−2s−1 in the CT, FT, and RC plots, respectively. Daily mean CO2 efflux increased exponentially as the groundwater level or water-filled pore space decreased, indicating that oxidative peat decomposition and gas diffusion in the soil increased due to enhanced aeration resulting from lower groundwater levels. Mean annual gap-filled CO2 emissions were 1.03±0.53, 0.59±0.26, and 0.69±0.21kgCm−2 yr−1 in the CT, FT, and RC plots, respectively. Soil CO2 emissions were significantly higher in the CT plots (P<0.05), but did not differ significantly between the FT and RC plots. This implies that root respiration was negligible in the FT plots. Heterotrophic respiration accounted for 66% of soil respiration. Annual CO2 emissions through both soil and heterotrophic respiration were smaller than those of other oil palm plantations on tropical peat, possibly due to the higher groundwater levels, land compaction, and continuous measurement of soil CO2 efflux in this study. Mean annual total subsidence was 1.55 to 1.62cmyr−1, of which oxidative peat decomposition accounted for 72 to 74%. In conclusion, water management to raise groundwater levels would mitigate soil CO2 emissions from oil palm plantations on tropical peatland.

Tree regeneration retards decomposition in a temperate mountain soil after forest gap disturbance

Disturbances significantly affect the carbon (C) cycle of forest ecosystems. Surviving trees from an understory layer are recognized to play an important role in post-disturbance C dynamics. However, their influence on decomposition of soil organic matter (SOM), an important ecosystem C pool, has yet to be rigorously investigated. Over four consecutive years, we investigated the effects of advance spruce regeneration on decomposition processes in a mountain soil after forest gap disturbance. Gap disturbance was accomplished by clear cut harvest and decomposition was assessed by combining measurements of soil CO2 efflux, heterotrophic respiration, soil enzymes activity, and mass loss from standardized litter bags. Soil CO2 efflux showed no response to gap formation, independent of whether regeneration was present or absent, indicating that reduced autotrophic respiration was offset by accelerated heterotrophic respiration from decomposing microbes. Incubation studies revealed no effects of gap disturbance on heterotrophic respiration and its temperature sensitivity under controlled lab conditions. Since potential enzyme activities, at a given temperature, did not respond to gap formation either, it appears that neither C nor other nutrient limitations of the decomposing microbes changed in this SOM-rich mountain forest soil after disturbance. Mass loss of standardized litter was ∼5% higher after gap formation in plots without regeneration, while under the regeneration it remained at control stand levels. Our findings indicate that canopy removal by gap disturbance lead to an increase in decomposition, primarily due to warmer soil conditions. However, an established regeneration retards decomposition due to its modulating effect on soil temperature. Our study therefore shows that facilitating regeneration pre-disturbance can reduce post-disturbance soil C losses from decomposition.

Collar insertion depth effects on soil respiration in afforested peatlands

Using collars for measuring soil respiration and its component fluxes in closed chamber systems relies on two main assumptions. Firstly, it is assumed that shallow collars prevent lateral soil gas leakage beneath the chamber’s walls and the underestimation of soil CO2 fluxes, and secondly, the insertion of deeper collars excises all living roots and the autotrophic flux is eliminated. It was hypothesised that previous findings on collar insertion impacts on autotrophic and total soil respiration also apply to afforested peatlands. In these ecosystems, a large fraction of fine roots grow close to the soil surface. Therefore, the use of shallow collars may sever some fine roots and hyphae of mycorrhizal fungi, and therefore, it may lead to underestimation of total soil respiration. It was also hypothesised that this underestimation may be greater than a possible CO2 leakage from lateral diffusion of soil gas as a result of not using collars. In this study, we measured soil CO2 efflux in a Sitka spruce and a lodgepole pine plantation on blanket peat in southern Ireland. A surface collar (not inserted into the ground) and six insertion depths (5, 10, 15, 25, 35 and 45 cm) were established to assess the effect of the collar insertion depth on autotrophic and total soil respiration. The insertion depth of 5 cm reduced total soil respiration by 47 and 32% in the spruce and pine stands, respectively. Using nonlinear equations, it was estimated that a frequently used shallow insertion of 1.5 cm would have reduced this efflux by 35 and 20% in each stand, respectively. Moreover, it was demonstrated that this reduction was greater than a possible lateral soil gas leakage. These results suggest that the insertion of shallow collars should be avoided and surface collars permanently anchored in the soil should be used instead.