Soil Organic Carbon Pool Research Articles

Elevational pattern of soil organic carbon release in a Tibetan alpine grassland: Consequence of quality but not quantity of initial soil organic carbon

Soil organic carbon (SOC) dynamics along elevational gradients are highly uncertain due to our limited knowledge of the underlying mechanisms that determine SOC release. By combining field investigation and laboratory aerobic incubation with multiple soil analytical approaches, we explored the determinants of SOC release along an elevational gradient (3200–4200 m) in the Tibetan alpine grassland. We illustrated that the SOC quantity (i.e., the initial standing SOC stock) and SOC quality (i.e., chemical recalcitrance, physico-chemical protection and biological recalcitrant fraction) increased gradually with elevation, reaching a maximum value at the middle altitude (∼3600 m), while an opposite unimodal distribution pattern was observed in CO2–C release. Our results also showed that SOC quality explained more variance in CO2–C release than did SOC quantity and soil properties. The proportion of stable SOC fractions in larger initial SOC stocks is higher, especially the recalcitrant SOC pool and mineral-associated OC fractions, which could be the potential mechanism behind high initial SOC stocks leading to lower soil CO2–C release. Collectively, our results provide compelling evidence that SOC quality plays a primary role in the elevational pattern of CO2–C release in Tibetan alpine grassland. Our findings highlight the importance of SOC quality in predicting SOC dynamics in the Tibetan alpine grassland under climate change scenarios.

Effects of nitrogen fertilizer on soil microbial residues and their contribution to soil organic carbon and total nitrogen in a rice-wheat system

Soil microbial residues have an impact on the stability of the soil organic carbon (SOC) pool, since they are crucial and stable sources of SOC. However, it is not clear how soil microbial residues in aggregates affect the SOC and total nitrogen (TN) at different gradients of N fertilization. Therefore, we investigated the effect of N fertilizer on microbial residues and their contribution to SOC and TN in soil aggregates under a rice-wheat (Oryza sativa-Triticum aestivum) system with four rates of N fertilizer. The results indicated that the addition of N fertilizer clearly increased the phospholipid fatty acids of bacteria and enhanced the activities of soil enzymes (β-1,4-glucosidase, β-N-acetyl-glucosaminidase and phenol oxidase) in the macroaggregates. The content of bacterial residual carbon increased noticeably when N fertilizer was added, while the content of fungal residual carbon did not appear to change. The addition of N fertilizer increased the contribution of bacterial residual carbon to SOC and TN. There was no significant difference in the contribution of bacterial residues on SOC and TN at different N fertilizer rates. Nevertheless, their contribution gradually decreased with the decrease in aggregate size. Structural equation modeling showed that the contribution of soil microbial residue to TN was influenced by activities of soil enzymes, suggesting that N is an important factor that affects soil ecology and limits the availability of nitrogenous organic matter in agroecological systems. This finding is crucial to understanding how N fertilizer influences soil C and N cycle in rice-wheat systems.

Restoring soil carbon in marginal land of Indian Himalayas: Impact of crop intensification and conservation tillage.

Soil carbon (C) loss is the prime sign of land degradation, and C pools have a great impact on soil quality and climate change mitigation. Hence, a field experiment was conducted for three consecutive years to assess the impact of crop intensification and conservation tillage practices on changes in the C pool at different soil depths of marginal land of the Indian Himalayas. The experiment consisted of two intensified cropping systems viz., CS1-Summer maize (Zea mays L.) -rainy season maize-lentil (Lens esculenta L.) and CS2-Summer maize-rainy season maize-mustard (Brassica juncea (L.) Czern) and five tillage practices viz., No-till (NT); NT+live mulch of cowpea (NT+LMC); reduced tillage (RT); RT+LMC and conventional tillage (CT). Results revealed that CS2 produced significantly higher biomass, C retention efficiency (9.85%), and sequestrated greater C (0.42Mgha-1 yr-1) in the soil system than CS1. Of the various tillage practices, RT+LMC registered higher biomass and recycled greater biomass and C than those under other tillage practices. However, the highest soil organic carbon (SOC) content (7.03gkg-1) and pool (9.62Mgha-1) in 0-10cm depth were observed under NT+LMC. The non-labile C pool size under NT in 0-10cm and 10-20cm depths was significantly greater than those under CT. The NT+LMC sequestrated significantly higher SOC (0.57Mgha-1 yr-1) than other tillage practices. Thus, the study indicated that the adoption of an intensified maize-based system under RT+LMC or NT+LMC would increase SOC storage and C sequestration in marginal lands of the Indian Himalayas.

ASSESSING CARBON SEQUESTRATION AND POSSIBLE GREENHOUSE GAS EMISSION WITHIN THE DANUBE DELTA SOILS – PAST AND CURRENT ENVIRONMENTAL CONSIDERATIONS

Soil organic carbon (SOC) sequestration generally occurs in wet ecosystems such as river flood plains and deltas. This paper deals with the carbon sequestration stock in the Danube Delta soils for various depths as based on the existing soil maps and updated materials and discusses about greenhouse gas emissions in order to enable evaluation of future evolution and possible scenarios in the light of global warming. Histosols represent about 28% of the Delta area and contribute with over 55% to the total SOC pool of this ecosystem. The histic subtypes of the Subaquatic Fluvisols, Gleysols and Arenosols also contribute much more to the total SOC pool than the non-histic subtypes. The large and significant SOC differences between mineral and organic soils is a strong reason for preservation of Histosolsʼ area and for renaturation of some less fertile soils from the lowest parts of Danube Delta in order to increase SOC and decrease atmospheric C. Only about 14.5% from the total Danube Delta area was taken for farming, mainly in its western part, where mineral soils or subtypes of organic soils occur. Histosols are especially situated in the maritime, eastern parts of the Delta ecosystem. In cropland areas the soil depth that is mobilized by plowing, disking or other works and from where the plants uptake water and nutrients is at least 0.5 m, and for some crops even from 1.0 m or below. The present paper deals with various soil depths for SOC referenced values, facilitating their use in specific estimation models. Policy makers, decision makers and opinion-formers should promote preservation of the natural landscape of the Delta under the best possible conditions to contribute to an increase in SOC stock. Maintaining the natural SOC stock at the present-day level and enhancing new organic C deposition in the renatured parts of Delta soils could contribute to global warming mitigation in the future. If global warming continues at the present rate or higher rates, the soil water regime will change reflecting the dynamics of sea level rising. This event will most probably accelerate peat formation and increase Histosol area in the lowest landforms across the Delta. Future research is needed for characteristic stationary sites specifically in the cropland area of the Danube Delta to deepen our knowledge regarding the dynamics of SOC.

Effects of nitrogen-enriched biochar on subtropical paddy soil organic carbon pool dynamics

Agronomic management practices present an opportunity to improve the sustainability of crop production, including reductions of greenhouse gas emissions through impacts on soil organic carbon (SOC) dynamics. We investigated the impacts of contrasting application rates of nitrogen (N)-enriched biochar (4 and 8 t ha−1) on the concentrations of total and active SOC, microbial biomass carbon (MBC), soil aggregates, and the carbon (C) pool management index (CPMI) as an indicator of soil quality in tillering and mature subtropical early and late rice in China. Soil salinity and soil bulk density increased, and soil water content generally decreased under the application of N-enriched biochar at 4 t ha−1. Following the application of the biochar, there were greater soil concentrations of SOC and lower concentrations of dissolved organic-C and active labile organic‑carbon, indicating reduced mineralization and enhanced stocks of stable-C. Biochar application (4 and 8 t ha−1) led to lower soil Ca-SOC concentrations and greater soil Fe(Al)-SOC concentrations. Concentrations of Fe(Al)-SOC were greater under the application of N-enriched biochar at 4 t ha−1, indicating the bonding capacity of iron‑aluminum oxide and organic carbon provided by biochar improved levels of SOC fixation. The composition of soil aggregates under each treatment was mainly micro-aggregates (<0.25 mm). The greater soil content of macro-aggregates (>0.25 mm) increased under amendment with 4 t of biochar ha −1 and the greater SOC content led to greater soil aggregate stability. Levels of C pool activity, C pool index, and CPMI reduced following application of the biochar, while C pool activity index increased slightly, indicating an increase in soil quality. These results indicate that the application of N-enriched biochar during rice cultivation may lead to reductions in SOC mineralization and C emissions and increases in soil C sink capacity, due to greater SOC pool stability, thus improving the sustainability of paddy rice production.

Reduced basal and increased topdressing fertilizer rate combined with straw incorporation improves rice yield stability and soil organic carbon sequestration in a rice-wheat system.

Fertilizer management is vital for sustainable agriculture under climate change. Reduced basal and increased topdressing fertilizer rate (RBIT) has been reported to improve the yield of in–season rice or wheat. However, the effect of RBIT on rice and wheat yield stability and soil organic carbon (SOC) sequestration potential is unknown, especially when combined with straw incorporation. Here, we report the effect of RBIT with/without straw incorporation on crop yields, yield stability, SOC stock, and SOC fractions in the lower Yangtze River rice–wheat system region over nine years. RBIT with/without straw incorporation significantly increased nine–year average and annual rice yields but not wheat yields. Compared with conventional fertilization (CF), RBIT did not significantly affect wheat or rice yield stability, but combined with straw incorporation, it increased the sustainable yield index (SYI) of wheat and rice by 7.6 and 12.8%, respectively. RBIT produced a higher C sequestration rate (0.20 Mg C ha−1 year−1) than CF (0.06 Mg ha−1 year−1) in the 0–20 cm layer due to higher root C input and lower C mineralization rate, and RBIT in combination with straw incorporation produced the highest C sequestration rate (0.47 Mg ha−1 year−1). Long–term RBIT had a greater positive effect on silt+clay (0.053 mm)–associated C, microbial biomass C (MBC), dissolved organic C, and hot water organic C in the surface layer (0–10 cm) than in the subsurface layer (10–20 cm). In particular, the increases in SOC pools and mean weight diameter (MWD) of soil aggregates were greater when RBIT was combined with straw incorporation. Correlation analysis indicated that topsoil SOC fractions and MWD were positively correlated with the SYI of wheat and rice. Our findings suggest that the long–term application of RBIT combined with straw incorporation contributed to improving the sustainability of rice production and SOC sequestration in a rice–wheat system.

Lateral carbon export has low impact on the net ecosystem carbon balance of a polygonal tundra catchment

Abstract. Permafrost-affected soils contain large quantities of soil organic carbon (SOC). Changes in the SOC pool of a particular ecosystem can be related to its net ecosystem carbon balance (NECB) in which the balance of carbon (C) influxes and effluxes is expressed. For polygonal tundra landscapes, accounts of ecosystem carbon balances in the literature are often solely based on estimates of vertical carbon fluxes. To fill this gap, we present data regarding the lateral export rates of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) from a polygonal tundra site in the north Siberian Lena River delta, Russia. We use water discharge observations in combination with concentration measurements of waterborne carbon to derive the lateral carbon fluxes from one growing season (2 June–8 September 2014 for DOC, 8 June–8 September 2014 for DIC). To put the lateral C fluxes into context, we furthermore present the surface–atmosphere eddy covariance fluxes of carbon dioxide (CO2) and methane (CH4) from this study site. The results show cumulative lateral DIC and DOC fluxes of 0.31–0.38 and 0.06–0.08 g m−2, respectively, during the 93 d observation period (8 June–8 September 2014). Vertical turbulent fluxes of CO2-C and CH4-C accumulated to −19.0 ± 1.2 and 1.0 ± 0.02 g m−2 in the same period. Thus, the lateral C export represented about 2 % of the net ecosystem exchange of (NEE) CO2. However, the relationship between lateral and surface–atmosphere fluxes changed over the observation period. At the beginning of the growing season (early June), the lateral C flux outpaced the surface-directed net vertical turbulent CO2 flux, causing the polygonal tundra landscape to be a net carbon source during this time of the year. Later in the growing season, the vertical turbulent CO2 flux dominated the NECB.

Forests have a higher soil C sequestration benefit due to lower C mineralization efficiency: Evidence from the central loess plateau case

Soil carbon (C) mineralization varies with vegetation restoration and plays an essential role in the soil C cycle. However, the response of soil C mineralization to different vegetation restoration types remains unclear. In this study, three vegetation restoration types for the restoration of cropland, namely forestland (Robinia pseudoacacia), shrubland (Caragana korshinskii), and grassland (Artemisia sacrorum and Stipa bungeana), were selected to determine how vegetation restoration types affect soil C mineralization. The vegetation in the assessed areas had been restored 20 years before this study. The results showed that vegetation restoration significantly increased soil CO2 efflux, whereas soil C mineralization efficiency (CME) decreased after cropland conversion. The lowest soil CO2 efflux and CME were observed in forestland, with the highest stable soil organic carbon (SOC) pool. Vegetation restoration significantly increased SOC and its C fraction stocks, and the variation in soil C fractions was regulated by vegetation restoration type. Soil microbial biomass carbon (MBC) and dissolved organic carbon (DOC) effectively reflected the variation in soil C mineralization and stability of the SOC pool, respectively. Soil CO2 efflux was controlled by litter biomass, macroaggregates, particulate organic C, MBC, and DOC, whereas CME was mainly influenced by aboveground biomass, pH, silt and clay content, macroaggregates and DOC. The study suggested that forestland should be selected as the preferred vegetation restoration type in the central Loess Plateau owing to its lower CME with similar C stocks.

Fertilizer quality and labile soil organic matter fractions are vital for organic carbon sequestration in temperate arable soils within a long-term trial in Switzerland

Agricultural management of soils has led to severe losses of soil organic matter (SOM), accompanied by an increased release of CO2 into the atmosphere and a reduction of soil fertility. Especially under the aspect of global warming and the increasing demand for food, there is a need for sustainable management options increasing soil organic carbon (SOC) storage in agricultural soils, but knowledge gaps exist regarding C persistence in, and its transfer between functional SOC pools, within different farming systems. Here we report on impacts of different farming systems on the temporal dynamics of SOM fractions within the DOK long-term trial (Switzerland), from 1982 to 2017. A purely minerally (CONMIN), a minerally and organically (CONFYM), and a purely organically fertilized farming system (BIODYN) were compared with an unfertilized control (NOFERT). We separated archived soils from the Haplic Luvisol (0–20 cm depth) into particulate (POM) and mineral-associated OM (MAOM) fractions, via physical fractionation, and analyzed the chemical composition of selected fractions via solid-state 13C CPMAS-NMR spectroscopy. We demonstrate that under none of the analyzed farming systems, additional SOC was sequestered in the clay-sized MAOM fraction (<6.3 µm) over a period of 36 years. In all fertilized systems, the amount of SOC in this pool did not change, but strongly decreased in NOFERT (-27%). Bulk SOC increased in BIODYN (+13%) and CONFYM (+5%), but decreased in CONMIN (-8%) and NOFERT (-20%). As no additional SOC accumulated in the clay-sized MAOM fraction, this implies that bulk SOC increases were solely stored within labile POM fractions. NMR spectra showed comparable POM chemical compositions between different systems. Differences in fertilizer quality (BIODYN = composted farmyard manure vs CONFYM = stacked farmyard manure + mineral fertilizer) and the omission of pesticides resulted in better conditions for POM stabilization and consequently significantly higher C contents of occluded POM (oPOM) within aggregates, in BIODYN. However, this labile fraction is at high risk of being lost within a few days, as illustrated by the strong annual oPOM-C content fluctuations depending on the timing of soil sampling after harvest. The highest post-harvest oPOM-C losses in BIODYN indicate the higher dynamics compared to CONFYM. It is anticipated that only sustainable fertilization methods with continuous application of solely organic fertilizers in the long-run can maintain SOC in the labile POM fractions at elevated levels, thereby ensuring soil fertility. It also illustrates the need for prevention of major losses by careful management of the labile POM fractions, as this OM could associate with fine mineral particles at a later stage and thus contribute to OC sequestration in the stable SOC pool. Overall, the potential of arable soils to accumulate stable OC for long-term sequestration is questioned.

Soil Inorganic Carbon as a Potential Sink in Carbon Storage in Dryland Soils—A Review

Soil organic carbon (SOC) pool has been extensively studied in the carbon (C) cycling of terrestrial ecosystems. In dryland regions, however, soil inorganic carbon (SIC) has received increasing attention due to the high accumulation of SIC in arid soils contributed by its high temperature, low soil moisture, less vegetation, high salinity, and poor microbial activities. SIC storage in dryland soils is a complex process comprising multiple interactions of several factors such as climate, land use types, farm management practices, irrigation, inherent soil properties, soil biotic factors, etc. In addition, soil C studies in deeper layers of drylands have opened-up several study aspects on SIC storage. This review explains the mechanisms of SIC formation in dryland soils and critically discusses the SIC content in arid and semi-arid soils as compared to SOC. It also addresses the complex relationship between SIC and SOC in dryland soils. This review gives an overview of how climate change and anthropogenic management of soil might affect the SIC storage in dryland soils. Dryland soils could be an efficient sink in C sequestration through the formation of secondary carbonates. The review highlights the importance of an in-depth understanding of the C cycle in arid soils and emphasizes that SIC dynamics must be looked into broader perspective vis-à-vis C sequestration and climate change mitigation.

Response of soil and vegetation in a warm-temperate Pine forest to intensive biomass harvests, phosphorus fertilisation, and wood ash application

The objective of this study was to assess the effects of different intensities of biomass harvesting, and the possible effects of compensation methods, on forest functioning. To do so, we carried out a split-plot experiment (SW France) crossing four different intensities of biomass harvesting (Stem-Only Harvest [SOH], Aboveground Additional Harvest [AAH], Belowground Additional Harvest [BAH], and Whole-Tree Harvest [WTH]) and three compensation methods (control [C], wood ash application [A] and phosphorus fertilisation [P]). The experimental treatments were followed by the plantation of pines (Pinus pinaster). The environmental consequences of treatments on soil and vegetation were evaluated 11 years after the tree plantation.Despite their low additional biomass exports (+10 % for AAH to +34 % for WTH), the non-conventional harvest practices exported much higher quantities of nutrients than the conventional SOH technique (+145 % of exported N in WTH). Additional biomass harvests impacted the soil organic matter content, with negative effects on P-organic, soil cation exchange capacity, exchangeable Ca, and most extractible nutrients. However, tree nutritional status was improved by P-fertiliser or wood ash. We observed a positive effect of wood ash application on soil pH and nutrient content but, like additional harvests, wood ash application decreased the pool of soil organic carbon (~10 %).Overall, the experiment showed that exporting more forest biomass due to the additional harvesting of biomass had negative consequences on the ecosystem biogeochemistry. Additional harvests have impoverished the soil, and decreased the soil organic carbon content. Importantly, applying nutrients as fertiliser or wood ash did not compensate for all the negative impacts of biomass exports and the method of wood ash recycling in forests could even decrease the soil organic carbon.

Soil organic carbon pools and soil quality indicators 3 and 24 years after a one-time compost application in organic dryland wheat systems

Quantifying changes in soil organic carbon (SOC) is essential for understanding the effect of managements on carbon (C) sequestration, especially in cultivated marginal lands. The objectives of this study were to determine the short- (<3 yr) and long-term (>24 yr) effects of a one-time compost application on soil organic matter (SOM) pools and stocks: coarse particulate organic matter (cPOM), fine particulate organic matter (fPOM), mineral-associated organic matter (MAOM), and soil physical quality (SOC:clay ratio) in surface- and sub-soils. Manure compost (0, 25, and 50 Mg dry weight ha–1) was applied to three organic dryland winter wheat-fallow systems (WW-F): Historic, (HT, >24 yr), Snowville (SN, < 3 yr), and Blue Creek (BC, <3 yr), in northern Utah. At HT, cPOM-C concentrations in amended plots were higher than under the control plots at the 0–10- (84.6 vs 54.4 g OC kg-1 fraction) and 10–30-cm (34.4 vs 19.4 g OC kg-1 fraction) depth. Compared to control plots, compost increased MAOM-C concentration from 1.8 to 3.4 g OC kg-1 fraction at the 30–60-cm at SN and from 2.35 to 3.99 g OC kg-1 fraction at the 60–90-cm depth at BC. Similarly, cPOM-C concentration was also increased at SN (5.5–23.6 g OC kg-1 fraction, 30–60-cm) and BC (8.3–31.2 g OC kg-1 fraction, 60–90-cm). The SOC:clay ratio was significantly higher at the 30–60-cm depth at BC (1:50–1:40) and at the 60–90-cm depth at SN (1:70–1:30), which suggests that compost application can impact subsoil physical quality. Thus, one-time compost application at 16.5 Mg C ha-1 every two decades might constitute a reasonable management option for managing C in dryland WW-F agroecosystems.

Effect of forest thinning on soil organic carbon stocks from the perspective of carbon-degrading enzymes

Forest thinning greatly affects the soil organic carbon (SOC) pool by changing the soil microenvironment, organic matter input, and microbial metabolism. However, the heterogeneity of thinning intensity, recovery stage, and microclimatic conditions increases uncertainty about the effect of thinning on SOC. The activities of C-degrading enzymes (ligninases and cellulases) secreted by soil microorganisms can be used to explore the effects of thinning on soil C storage. Our meta-analysis showed that forest thinning significantly increased soil temperature (ST), soil moisture (SM), and pH, but significantly decreased litter and fine root biomass. Moderate thinning and recovery time extension increased soil C storage, SOC, and recalcitrant organic C (ROC). An increase in the thinning intensity significantly improved ligninase activity, while decreasing cellulase activity and extending recovery time after thinning. Increased SOC, soil labile organic C (LOC), and ROC with thinning were associated with increased ligninase activity that was significantly positively correlated with soil pH, SM, and ST after thinning. The variation in soil ligninase: cellulase ratio induced by thinning was significantly positively correlated with the increases of soil ROC: LOC ratio and SOC after thinning. Therefore, forest thinning enhanced SOC pool stability through the effect of ligninases and cellulases on SOC sequestration. The global pattern of soil C-degrading enzymes in response to thinning has deepened our understanding of the mechanism by which thinning affects the SOC cycle.

The effect of dissolved char on microbial activity in an extract from the forest floor

Abstract Climate change is associated with an increased risk in the occurrence of wildfires. Forests store large amounts of carbon (C), which are threatened by these wildfires. Pyrogenic material produced after a wildfire constitutes an important part of the soil organic carbon pool in forest soils. Microorganisms play an important role in the cycling of C. This study investigated microbial activity in dissolved char from burned wood from two tree species in different stages of decay. The char from branches of beech and Norway spruce was produced under laboratory fire conditions and extracted in water after which microbial activity was measured for a 4-week period. Further stages of decay resulted in increased flammability with higher peak temperatures and combustion completeness. For the beech samples, further decay also resulted in a decrease of extractable C but a higher proportion of stable C. Further decay resulted in less respiration for beech and more for Norway spruce. With less C being respired, this points towards an increased C sequestration potential in the form of microbial C and microbial derived products for beech compared to Norway spruce. This study provided a workflow to assess the effects of dissolved char on microbial activity by mimicking natural fire conditions. It also indicated the need for future research to further elucidate the underlying mechanisms explaining why fire-originated dissolved char of wood in different decay stages influences microbial respiration with diverging effects per species.

The importance of incorporating soil in the life cycle assessment procedure to improve the sustainability of agricultural management

The formidable ability of soil to store carbon has attracted an increasing number of studies, but few of them included soil organic carbon (SOC) sequestration as part of a carbon balance assessment in the agroecosystem. This raises some interesting questions: 1) how orchards conversion increase soil capacity to mitigate the green–house gases (GHG) emissions by storing C? 2) can it be considered in life cycle assessment (LCA)? 3) can SOC pools and soil biochemical properties determination improve LCA interpretation? To answer these questions, this study selected a ten– and fifteen–years–old peach orchards, a twenty–years–old pear orchard, a thirty–years–old kiwi orchard in south-east part of Emilia–Romagna Region (Italy), and a cereals’ field as reference. Soil samples were collected from 0 to 15 and 15–30 cm depths, and the SOC pool amounts (i.e., labile and recalcitrant) determined. LCA was used to estimate the GHG emissions (CO2eq) from the orchards. Results showed that the conversion from cereals to orchard production increased OC stock (+82 % on average) suggesting that orchards cultivation systems have the capacity to enrich soil organic matter. Fertilization had the greatest impact on CO2eq emission accounting for at least 40 % of total CO2eq emissions. Kiwi cultivation had the highest impact on GHG emissions mainly due to the high water and nutrient demand (0.045 and 0.149 kg CO2eq kg−1 fruit yr−1, respectively). When taking into account the C–CO2eq loss by fruit cultivation and C storage in soils, results would indicate that peach and pear orchard agroecosystems promote C sequestration. Conversely, kiwi cultivation showed large CO2eq emissions only partly counterbalanced by SOC sequestration. This study highlights the importance of including soils in LCA: if made mandatory this would allow a wider, yet more detailed, picture of the impact of agricultural practices on C budget. This simple step could help optimise resource management and at the same time improve agroecosystem sustainability.

Alder encroachment alters subsoil organic carbon pool and chemical structure in a boreal peatland of Northeast China

Boreal peatlands have been experiencing increased abundances of symbiotic dinitrogen-fixing woody plants (mainly alder species). However, how alder encroachment alters soil organic carbon (C) pool and stability is unclear. To examine the effects of alder encroachment on soil organic C, we measured soil organic C pool, phenol oxidase (POX) activity, organic C mineralization rate, and organic C chemical structure (alkyl C, O-alkyl C, aromatic C, and carbonyl C) using solid-state 13C nuclear magnetic resonance spectroscopy in the 0–10 cm, 10–20 cm, and 20–40 cm depths in the Alnus sibirica islands and adjacent open peatlands in the north of Da'xingan Mountain, Northeast China. A. sibirica islands had 28 %, 25 %, and 30 % greater POX activity and 36 %, 31 %, and 100 % higher organic C mineralization than open peatlands in the 0–10 cm, 10–20 cm, and 20–40 cm soil depths, respectively. Despite no significant changes in the 0–10 cm and 10–20 cm depths, alder encroachment reduced soil organic C pool in the 20–40 cm depth. Soil organic C pool in the 0–40 cm depth was lower in A. sibirica islands (298 Mg ha−1) than in the open peatlands (315 Mg ha−1). Moreover, alder encroachment increased alkyl (7 %) and carbonyl (57 %) C fractions but reduced O-alkyl C fraction (16 %) in the 20–40 cm depth, resulting in increased aliphaticity and recalcitrance indices. These findings suggest that alder encroachment will reduce soil organic C accumulation by accelerating microbial decomposition, and highlight that increased biochemical stabilization would attenuate soil organic C loss after alder expansion in boreal peatlands. Our results will help assess and project future C budgets in boreal peatlands.

A regional assessment of permanganate oxidizable carbon for potential use as a soil health indicator in managed pine plantations

Soil health assessments require the establishment of soil indicators that are easy to measure and sensitive to changes in management practices. Despite the global demand for wood products and the intensification of silvicultural practices, very few indicators are currently used to monitor changes in soil properties and processes in managed forest plantations. Labile pools of soil organic carbon (SOC) have been widely used in agriculture to detect early alterations in carbon (C) cycling processes but are largely untested in managed forests. Here, we investigate the potential use of permanganate oxidizable carbon (POXC) to monitor impacts of silvicultural practices on soil properties in loblolly pine plantations. Soil samples were collected from 77 managed forest sites and 297 experimental plots throughout the southeastern U.S. at four soil depths: 0–10, 10–20, 20–50, and 50–100 cm. Sites encompassed plantations with ages ranging from 2 to 29 yr that were distributed over five soil orders. Silvicultural practices included herbicide, fertilization at multiple rates, and thinning. Our study showed that POXC concentration decreases with soil depth, while POXC increased proportionally to SOC with depth. POXC positively correlates with SOC, total nitrogen, and mass of soil woody detritus other than live roots, and negatively correlates with soil pH across all soil depths. Impacts of silvicultural practices on POXC concentration were variable across sites. The mean relative response of POXC to silvicultural practices was negative in general but only significant (p < 0.10) in herbicide treatments; compared to control plots herbicide plots at 50–100 cm depth had on average 20% less POXC. The decade-long rotation lengths of southern pine plantations and the carbon cycling dynamics of forest soil O horizons may impact the sensitivity of POXC as an indicator in managed plantations. In contrast to this regional analysis, monitoring POXC in local assessments may be a more practical approach to investigate short-term impacts of silvicultural practices.

Benchmarking carbon sequestration potentials in arable soils by on-farm research on innovative pioneer farms

PurposeTackling the global carbon deficit through soil organic carbon (SOC) sequestration in agricultural systems has been a focal point in recent years. However, we still lack a comprehensive understanding of actual on-farm SOC sequestration potentials in order to derive effective strategies.MethodsTherefore, we chose 21 study sites in North-Eastern Austria covering a wide range of relevant arable soil types and determined SOC pool sizes (0–35 cm soil depth) in pioneer versus conventional management systems in relation to permanently covered reference soils. We evaluated physico-chemical predictors of SOC stocks and SOC quality differences between systems using Fourier-transform infrared (FTIR) spectroscopy.ResultsCompared to conventional farming systems, SOC stocks were 14.3 Mg ha− 1 or 15.7% higher in pioneer farming systems, equaling a SOC sequestration rate of 0.56 Mg ha− 1 yr− 1. Reference soils however showed approximately 30 and 50% higher SOC stocks than pioneer and conventional farming systems, respectively. Nitrogen and dissolved organic carbon stocks showed similar patterns. While pioneer systems could close the SOC storage deficit in coarse-textured soils, SOC stocks in medium- and fine-textured soils were still 30–40% lower compared to the reference soils. SOC quality, as inferred by FTIR spectra, differed between land-use systems, yet to a lesser extent between cropping systems.ConclusionsInnovative pioneer management alleviates SOC storage. Actual realized on-farm storage potentials are rather similar to estimated SOC sequestration potentials derived from field experiments and models. The SOC sequestration potential is governed by soil physico-chemical parameters. More on-farm approaches are necessary to evaluate close-to-reality SOC sequestration potentials in pioneer agroecosystems.

Soil carbon consequences of historic hydrologic impairment and recent restoration in coastal wetlands

Coastal wetlands provide key ecosystem services, including substantial long-term storage of atmospheric CO2 in soil organic carbon pools. This accumulation of soil organic matter is a vital component of elevation gain in coastal wetlands responding to sea-level rise. Anthropogenic activities that alter coastal wetland function through disruption of tidal exchange and wetland water levels are ubiquitous. This study assesses soil vertical accretion and organic carbon accretion across five coastal wetlands that experienced over a century of impounded hydrology, followed by restoration of tidal exchange 5 to 14 years prior to sampling. Nearby marshes that never experienced tidal impoundment served as controls with natural hydrology to assess the impact of impoundment and restoration. Dated soil cores indicate that elevation gain and carbon storage were suppressed 30–70 % during impoundment, accounting for the majority of elevation deficit between impacted and natural sites. Only one site had substantial subsidence, likely due to oxidation of soil organic matter. Vertical and carbon accretion gains were achieved at all restored sites, with carbon burial increasing from 96 ± 33 to 197 ± 64 g C m−2 y−1. The site with subsidence was able to accrete at double the rate (13 ± 5.6 mm y−1) of the natural complement, due predominantly to organic matter accumulation rather than mineral deposition, indicating these ecosystems are capable of large dynamic responses to restoration when conditions are optimized for vegetation growth. Hydrologic restoration enhanced elevation resilience and climate benefits of these coastal wetlands.

Particulate and mineral-associated organic carbon turnover revealed by modelling their long-term dynamics

Particulate (POC) and mineral-associated organic carbon (MOC) are measurable carbon pools with distinct function. Their turnover properties have been rarely assessed because of their contrasting stabilization and destabilization processes and the difficulty of in situ monitoring. In this study we used two carbon models (a three-pool and a four-pool model) driven by measured POC, MOC and a more resistant charred organic carbon (COC) pool to obtain insights into the turnover of POC and MOC and to predict long-term soil organic carbon (SOC) dynamics. The models were constrained by measurements of POC, MOC and COC at 159 long-term trials (average trial duration is 22 years) across Australian agricultural regions. Results showed that POC-, MOC- and COC-constrained models, particularly the three-pool model, have less parameter collinearity and predict less uncertainties in most model parameters as well as in SOC dynamics and its vulnerability, compared with the models constrained by total SOC alone. The three-pool model estimated an average decay rate of 0.46 and 0.044 yr−1 for POC and MOC, respectively, across the trials. The four-pool model provided additional insights that on average a large fraction of MOC (28% on average across the trials) is physically protected against decomposition, and the remaining fraction is labile with an average decay rate of 0.068 yr−1. The existence of protected MOC results in prolonged overall residence time of MOC. Additionally, we found that the turnover of POC and MOC are distinctly influenced by climate and soil properties with involvement of non-linear relationships. These results shed new lights on the dynamics of measurable functional SOC pools and demonstrate that models constrained by these pools can provide more accurate predictions of model parameters thereby more accurate projections of SOC dynamics compared to models constrained by total SOC only.