Moisture-driven divergence in mineral-associated soil carbon persistence.

Mineral stabilization of soil organic matter is an important regulator of the global carbon (C) cycle. However, the vulnerability of mineral-stabilized organic matter (OM) to climate change is currently unknown. We examined soil profiles from 34 sites across the conterminous USA to investigate how the abundance and persistence of mineral-associated organic C varied with climate at the continental scale. Using a novel combination of radiocarbon and molecular composition measurements, we show that the relationship between the abundance and persistence of mineral-associated organic matter (MAOM) appears to be driven by moisture availability. In wetter climates where precipitation exceeds evapotranspiration, excess moisture leads to deeper and more prolonged periods of wetness, creating conditions which favor greater root abundance and also allow for greater diffusion and interaction of inputs with MAOM. In these humid soils, mineral-associated soil organic C concentration and persistence are strongly linked, whereas this relationship is absent in drier climates. In arid soils, root abundance is lower, and interaction of inputs with mineral surfaces is limited by shallower and briefer periods of moisture, resulting in a disconnect between concentration and persistence. Data suggest a tipping point in the cycling of mineral-associated C at a climate threshold where precipitation equals evaporation. As climate patterns shift, our findings emphasize that divergence in the mechanisms of OM persistence associated with historical climate legacies need to be considered in process-based models.

Roots selectively decompose litter to mine nitrogen and build new soil carbon.

Plant-microbe interactions in the rhizosphere shape carbon and nitrogen cycling in soil organic matter (SOM). However, there is conflicting evidence on whether these interactions lead to a net loss or increase of SOM. In part, this conflict is driven by uncertainty in how living roots and microbes alter SOM formation or loss in the field. To address these uncertainties, we traced the fate of isotopically labelled litter into SOM using root and fungal ingrowth cores incubated in a Miscanthus x giganteus field. Roots stimulated litter decomposition, but balanced this loss by transferring carbon into aggregate associated SOM. Further, roots selectively mobilized nitrogen from litter without additional carbon release. Overall, our findings suggest that roots mine litter nitrogen and protect soil carbon.

Arbuscular mycorrhiza convey significant plant carbon to a diverse hyphosphere microbial food web and mineral-associated organic matter.

Arbuscular mycorrhizal fungi (AMF) transport substantial plant carbon (C) that serves as a substrate for soil organisms, a precursor of soil organic matter (SOM), and a driver of soil microbial dynamics. Using two-chamber microcosms where an air gap isolated AMF from roots, we 13CO2-labeled Avena barbata for 6 wk and measured the C Rhizophagus intraradices transferred to SOM and hyphosphere microorganisms. NanoSIMS imaging revealed hyphae and roots had similar 13C enrichment. SOM density fractionation, 13C NMR, and IRMS showed AMF transferred 0.77 mg C g-1 of soil (increasing total C by 2% relative to non-mycorrhizal controls); 33% was found in occluded or mineral-associated pools. In the AMF hyphosphere, there was no overall change in community diversity but 36 bacterial ASVs significantly changed in relative abundance. With stable isotope probing (SIP)-enabled shotgun sequencing, we found taxa from the Solibacterales, Sphingobacteriales, Myxococcales, and Nitrososphaerales (ammonium oxidizing archaea) were highly enriched in AMF-imported 13C (> 20 atom%). Mapping sequences from 13C-SIP metagenomes to total ASVs showed at least 92 bacteria and archaea were significantly 13C-enriched. Our results illustrate the quantitative and ecological impact of hyphal C transport on the formation of potentially protective SOM pools and microbial roles in the AMF hyphosphere soil food web.

Carbon sequestration in soils and climate change mitigation-Definitions and pitfalls.

The term carbon (C) sequestration has not just become a buzzword but is something of a siren's call to scientific communicators and media outlets. Carbon sequestration is the removal of C from the atmosphere and the storage, for example, in soil. It has the potential to partially compensate for anthropogenic greenhouse gas emissions and is, therefore, an important piece in the global climate change mitigation puzzle. However, the term C sequestration is often used misleadingly and, while likely unintentional, can lead to the perpetuation of biased conclusions and exaggerated expectations about its contribution to climate change mitigation efforts. Soils have considerable potential to take up C but many are also in a state of continuous loss. In such soils, measures to build up soil C may only lead to a reduction in C losses (C loss mitigation) rather than result in real C sequestration and negative emissions. In an examination of 100 recent peer-reviewed papers on topics surrounding soil C, only 4% were found to have used the term C sequestration correctly. Furthermore, 13% of the papers equated C sequestration with C stocks. The review, further, revealed that measures leading to C sequestration will not always result in climate change mitigation when non-CO2 greenhouse gases and leakage are taken into consideration. This paper highlights potential pitfalls when using the term C sequestration incorrectly and calls for accurate usage of this term going forward. Revised and new terms are suggested to distinguish clearly between C sequestration in soils, SOC loss mitigation, negative emissions, climate change mitigation, SOC storage, and SOC accrual to avoid miscommunication among scientists and stakeholder groups in future.

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

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

Depth-dependent responses of soil organic carbon under nitrogen deposition.

Emerging evidence points out that the responses of soil organic carbon (SOC) to nitrogen (N) addition differ along the soil profile, highlighting the importance of synthesizing results from different soil layers. Here, using a global meta-analysis, we found that N addition significantly enhanced topsoil (0-30 cm) SOC by 3.7% (±1.4%) in forests and grasslands. In contrast, SOC in the subsoil (30-100 cm) initially increased with N addition but decreased over time. The model selection analysis revealed that experimental duration and vegetation type are among the most important predictors across a wide range of climatic, environmental, and edaphic variables. The contrasting responses of SOC to N addition indicate the importance of considering deep soil layers, particularly for long-term continuous N deposition. Finally, the lack of depth-dependent SOC responses to N addition in experimental and modeling frameworks has likely resulted in the overestimation of changes in SOC storage under enhanced N deposition.

Do Tasmanian devil declines impact ecosystem function?

Tasmanian eucalypt forests are among the most carbon-dense in the world, but projected climate change could destabilize this critical carbon sink. While the impact of abiotic factors on forest ecosystem carbon dynamics have received considerable attention, biotic factors such as the input of animal scat are less understood. Tasmanian devils (Sarcophilus harrisii)-an osteophageous scavenger that can ingest and solubilize nutrients locked in bone material-may subsidize plant and microbial productivity by concentrating bioavailable nutrients (e.g., nitrogen and phosphorus) in scat latrines. However, dramatic declines in devil population densities, driven by the spread of a transmissible cancer, may have underappreciated consequences for soil organic carbon (SOC) storage and forest productivity by altering nutrient cycling. Here, we fuse experimental data and modeling to quantify and predict future changes to forest productivity and SOC under various climate and scat-quality futures. We find that devil scat significantly increases concentrations of nitrogen, ammonium, phosphorus, and phosphate in the soil and shifts soil microbial communities toward those dominated by r-selected (e.g., fast-growing) phyla. Further, under expected increases in temperature and changes in precipitation, devil scat inputs are projected to increase above- and below-ground net primary productivity and microbial biomass carbon through 2100. In contrast, when devil scat is replaced by lower-quality scat (e.g., from non-osteophageous scavengers and herbivores), forest carbon pools are likely to increase more slowly, or in some cases, decline. Together, our results suggest often overlooked biotic factors will interact with climate change to drive current and future carbon pool dynamics in Tasmanian forests.

Mycorrhizal associations of temperate forest seedlings mediate rhizodeposition, but not soil carbon storage, under elevated nitrogen availability.

Tree-mycorrhizal associations are associated with patterns in nitrogen (N) availability and soil organic matter storage; however, we still lack a mechanistic understanding of what tree and fungal traits drive these patterns and how they will respond to global changes in soil N availability. To address this knowledge gap, we investigated how arbuscular mycorrhizal (AM)- and ectomycorrhizal (EcM)-associated seedlings alter rhizodeposition in response to increased seedling inorganic N acquisition. We grew four species each of EcM and AM seedlings from forests of the eastern United States in a continuously 13C-labeled atmosphere within an environmentally controlled chamber and subjected to three levels of 15N-labeled fertilizer. We traced seedling 15N uptake from, and 13C-labeled inputs (net rhizodeposition) into, root-excluded or -included soil over a 5-month growing season. N uptake by seedlings was positively related to rhizodeposition for EcM- but not AM-associated seedlings in root-included soils. Despite this contrast in rhizodeposition, there was no difference in soil C storage between mycorrhizal types over the course of the experiment. Instead root-inclusive soils lost C, while root-exclusive soils gained C. Our findings suggest that mycorrhizal associations mediate tree belowground C investment in response to inorganic N availability, but these differences do not affect C storage. Continued soil warming and N deposition under global change will increase soil inorganic N availability and our seedling results indicate this could lead to greater belowground C investment by EcM-associated trees. This potential for less efficient N uptake by EcM-trees could contribute to AM-tree success and a shift toward more AM-dominated temperate forests.

Divergent rhizosphere and non-rhizosphere soil microbial structure and function in long-term warmed steppe due to altered root exudation.

While there is an extensive body of research on the influence of climate warming on total soil microbial communities, our understanding of how rhizosphere and non-rhizosphere soil microorganisms respond to warming remains limited. To address this knowledge gap, we investigated the impact of 4 years of soil warming on the diversity and composition of microbial communities in the rhizosphere and non-rhizosphere soil of a temperate steppe, focusing on changes in root exudation rates and exudate compositions. We used open top chambers to simulate warming conditions, resulting in an average soil temperature increase of 1.1°C over a span of 4 years. Our results showed that, in the non-rhizosphere soil, warming had no significant impact on dissolved organic carbon concentrations, compositions, or the abundance of soil microbial functional genes related to carbon and nitrogen cycling. Moreover, soil microbial diversity and community composition remained largely unaffected, although warming resulted in increased complexity of soil bacteria and fungi in the non-rhizosphere soil. In contrast, warming resulted in a substantial decrease in root exudate carbon (by 19%) and nitrogen (by 12%) concentrations and induced changes in root exudate compositions, primarily characterized by a reduction in the abundance in alcohols, coenzymes and vitamins, and phenylpropanoids and polyketides. These changes in root exudation rates and exudate compositions resulted in significant shifts in rhizosphere soil microbial diversity and community composition, ultimately leading to a reduction in the complexity of rhizosphere bacterial and fungal community networks. Altered root exudation and rhizosphere microbial community composition therefore decreased the expression of functional genes related to soil carbon and nitrogen cycling. Interestingly, we found that changes in soil carbon-related genes were primarily driven by the fungal communities and their responses to warming, both in the rhizosphere and non-rhizosphere soil. The study of soil microbial structure and function in rhizosphere and non-rhizosphere soil provides an ideal setting for understanding mechanisms for governing rhizosphere and non-rhizosphere soil carbon and nitrogen cycles. Our results highlight the distinctly varied responses of soil microorganisms in the rhizosphere and non-rhizosphere soil to climate warming. This suggests the need for models to address these processes individually, enabling more accurate predictions of the impacts of climate change on terrestrial carbon cycling.

Elevated atmospheric CO2 drives decreases in stable soil organic carbon in arid ecosystems: Evidence from a physical fractionation and organic compound analysis.

The increasing concentration of CO2 in the atmosphere is perturbing the global carbon (C) cycle, altering stocks of organic C, including soil organic matter (SOM). The effect of this disturbance on soils in arid ecosystems may differ from other ecosystems due to water limitation. In this study, we conducted a density fractionation on soils previously harvested from the Nevada Desert FACE Facility (NDFF) to understand how elevated atmospheric CO2 (eCO2 ) affects SOM stability. Soils from beneath the perennial shrub, Larrea tridentata, and from unvegetated interspace were subjected to a sodium polytungstate density fractionation to separate light, particulate organic matter (POM, <1.85 g/cm3 ) from heavier, mineral associated organic matter (MAOM, >1.85 g/cm3 ). These fractions were analyzed for organic C, total N, δ13 C and δ15 N, to understand the mechanisms behind changes. The heavy fraction was further analyzed by pyrolysis GC/MS to assess changes in organic compound composition. Elevated CO2 decreased POM-C and MAOM-C in soils beneath L. tridentata while interspace soils exhibited only a small increase in MAOM-N. Analysis of δ13 C revealed incorporation of new C into both POM and MAOM pools indicating eCO2 stimulated rapid turnover of both POM and MAOM. The largest losses of POM-C and MAOM-C observed under eCO2 occurred in soils 20-40 cm in depth, highlighting that belowground C inputs may be a significant driver of SOM decomposition in this ecosystem. Pyrolysis GC/MS analysis revealed a decrease in organic compound diversity in the MAOM fraction of L. tridentata soils, becoming more similar to interspace soils under eCO2 . These results provide further evidence that MAOM stability may be compromised under disturbance and that SOC stocks in arid ecosystems are vulnerable under continued climate change.

The effect of repeated hurricanes on the age of organic carbon in humid tropical forest soil.

Increasing hurricane frequency and intensity with climate change is likely to affect soil organic carbon (C) stocks in tropical forests. We examined the cycling of C between soil pools and with depth at the Luquillo Experimental Forest in Puerto Rico in soils over a 30-year period that spanned repeated hurricanes. We used a nonlinear matrix model of soil C pools and fluxes ("soilR") and constrained the parameters with soil and litter survey data. Soil chemistry and stable and radiocarbon isotopes were measured from three soil depths across a topographic gradient in 1988 and 2018. Our results suggest that pulses and subsequent reduction of inputs caused by severe hurricanes in 1989, 1998, and two in 2017 led to faster mean transit times of soil C in 0-10 cm and 35-60 cm depths relative to a modeled control soil with constant inputs over the 30-year period. Between 1988 and 2018, the occluded C stock increased and δ13C in all pools decreased, while changes in particulate and mineral-associated C were undetectable. The differences between 1988 and 2018 suggest that hurricane disturbance results in a dilution of the occluded light C pool with an influx of young, debris-deposited C, and possible microbial scavenging of old and young C in the particulate and mineral-associated pools. These effects led to a younger total soil C pool with faster mean transit times. Our results suggest that the increasing frequency of intense hurricanes will speed up rates of C cycling in tropical forests, making soil C more sensitive to future tropical forest stressors.

A stoichiometric approach to estimate sources of mineral-associated soil organic matter.

Mineral-associated soil organic matter (MAOM) is the largest, slowest cycling pool of carbon (C) in the terrestrial biosphere. MAOM is primarily derived from plant and microbial sources, yet the relative contributions of these two sources to MAOM remain unresolved. Resolving this issue is essential for managing and modeling soil carbon responses to environmental change. Microbial biomarkers, particularly amino sugars, are the primary method used to estimate microbial versus plant contributions to MAOM, despite systematic biases associated with these estimates. There is a clear need for independent lines of evidence to help determine the relative importance of plant versus microbial contributions to MAOM. Here, we synthesized 288 datasets of C/N ratios for MAOM, particulate organic matter (POM), and microbial biomass across the soils of forests, grasslands, and croplands. Microbial biomass is the source of microbial residues that form MAOM, whereas the POM pool is the direct precursor of plant residues that form MAOM. We then used a stoichiometric approach-based on two-pool, isotope-mixing models-to estimate the proportional contribution of plant residue (POM) versus microbial sources to the MAOM pool. Depending on the assumptions underlying our approach, microbial inputs accounted for between 34% and 47% of the MAOM pool, whereas plant residues contributed 53%-66%. Our results therefore challenge the existing hypothesis that microbial contributions are the dominant constituents of MAOM. We conclude that biogeochemical theory and models should account for multiple pathways of MAOM formation, and that multiple independent lines of evidence are required to resolve where and when plant versus microbial contributions are dominant in MAOM formation.

Carbon sequestration in the subsoil and the time required to stabilize carbon for climate change mitigation.

Soils store large quantities of carbon in the subsoil (below 0.2 m depth) that is generally old and believed to be stabilized over centuries to millennia, which suggests that subsoil carbon sequestration (CS) can be used as a strategy for climate change mitigation. In this article, we review the main biophysical processes that contribute to carbon storage in subsoil and the main mathematical models used to represent these processes. Our guiding objective is to review whether a process understanding of soil carbon movement in the vertical profile can help us to assess carbon storage and persistence at timescales relevant for climate change mitigation. Bioturbation, liquid phase transport, belowground carbon inputs, mineral association, and microbial activity are the main processes contributing to the formation of soil carbon profiles, and these processes are represented in models using the diffusion-advection-reaction paradigm. Based on simulation examples and measurements from carbon and radiocarbon profiles across biomes, we found that advective and diffusive transport may only play a secondary role in the formation of soil carbon profiles. The difference between vertical root inputs and decomposition seems to play a primary role in determining the shape of carbon change with depth. Using the transit time of carbon to assess the timescales of carbon storage of new inputs, we show that only small quantities of new carbon inputs travel through the profile and can be stabilized for time horizons longer than 50 years, implying that activities that promote CS in the subsoil must take into consideration the very small quantities that can be stabilized in the long term.

Stabilisation of soil organic matter with rock dust partially counteracted by plants.

Soil application of Ca- and Mg-rich silicates can capture and store atmospheric carbon dioxide as inorganic carbon but could also have the potential to stabilise soil organic matter (SOM). Synergies between these two processes have not been investigated. Here, we apply finely ground silicate rock mining residues (basalt and granite blend) to a loamy sand in a pot trial at a rate of 4% (equivalent to 50 t ha-1 ) and investigate the effects of a wheat plant and two watering regimes on soil carbon sequestration over the course of 6 months. Rock dust addition increased soil pH, electric conductivity, inorganic carbon content and soil-exchangeable Ca and Mg contents, as expected for weathering. However, it decreased exchangeable levels of micronutrients Mn and Zn, likely related to the elevated soil pH. Importantly, it increased mineral-associated organic matter by 22% due to the supply of secondary minerals and associated sites for SOM sorption. Additionally, in the nonplanted treatments, rock supply of Ca and Mg increased soil microaggregation that subsequently stabilised labile particulate organic matter as organic matter occluded in aggregates by 46%. Plants, however, reduced soil-exchangeable Mg and Ca contents and hence counteracted the silicate rock effect on microaggregates and carbon within. We suggest this cation loss might be attributed to plant exudates released to solubilise micronutrients and hence neutralise plant deficiencies. The effect of enhanced silicate rock weathering on SOM stabilisation could substantially boost its carbon sequestration potential.

Straw application improved soil biological properties and the growth of rice plant under low water irrigation.

The application of organic amendments is one way to manage low water irrigation in paddy soils. In this 60-day greenhouse pot experiment involving paddy soil undergoing drying-rewetting cycles, we examined the effects of two organic amendments: azo-compost with a low carbon to phosphorus ratio (C:P) of 40 and rice straw with a high C:P ratio of 202. Both were applied at rates of 1.5% of soil weight (w/w). The investigation focused on changes in certain soil biochemical characteristics related to C and P in the rice rhizosphere, as well as rice plant characteristics. The irrigation regimes applied in this study included constant soil moisture in a waterlogged state (130% water holding capacity (WHC)), mild drying-rewetting (from 130 to 100% WHC), and severe drying-rewetting (from 130 to 70% WHC). The results indicated that the application of amendments was effective in severe drying-rewetting irrigation regimes on soil characteristics. Drying-rewetting decreased soil respiration rate (by 60%), microbial biomass carbon (by 70%), C:P ratio (by 12%), soil organic P (by 16%), shoot P concentration (by 7%), and rice shoot biomass (by 30%). However, organic amendments increased soil respiration rate (by 8 times), soil microbial biomass C (51%), total C (TC) (53%), dissolved organic carbon (3 times), soil available P (AP) (100%), soil organic P (63%), microbial biomass P (4.5 times), and shoot P concentration (21%). The highest significant correlation was observed between dissolved organic carbon and total C (r= 0.89**). Organic amendments also increased P uptake by the rice plant in the order: azo-compost > rice straw > control treatments, respectively, and eliminated the undesirable effect of mild drying-rewetting irrigation regime on rice plant biomass. Overall, using suitable organic amendments proves promising for enhancing soil properties and rice growth under drying-rewetting conditions, highlighting the interdependence of P and C biochemical changes in the rhizosphere during the rice vegetative stage.

Nitrogen deposition accelerates soil carbon sequestration in tropical forests.

Terrestrial ecosystem carbon (C) sequestration plays an important role in ameliorating global climate change. While tropical forests exert a disproportionately large influence on global C cycling, there remains an open question on changes in below-ground soil C stocks with global increases in nitrogen (N) deposition, because N supply often does not constrain the growth of tropical forests. We quantified soil C sequestration through more than a decade of continuous N addition experiment in an N-rich primary tropical forest. Results showed that long-term N additions increased soil C stocks by 7 to 21%, mainly arising from decreased C output fluxes and physical protection mechanisms without changes in the chemical composition of organic matter. A meta-analysis further verified that soil C sequestration induced by excess N inputs is a general phenomenon in tropical forests. Notably, soil N sequestration can keep pace with soil C, based on consistent C/N ratios under N additions. These findings provide empirical evidence that below-ground C sequestration can be stimulated in mature tropical forests under excess N deposition, which has important implications for predicting future terrestrial sinks for both elevated anthropogenic CO2 and N deposition. We further developed a conceptual model hypothesis depicting how soil C sequestration happens under chronic N deposition in N-limited and N-rich ecosystems, suggesting a direction to incorporate N deposition and N cycling into terrestrial C cycle models to improve the predictability on C sink strength as enhanced N deposition spreads from temperate into tropical systems.

Reduced microbial stability in the active layer is associated with carbon loss under alpine permafrost degradation.

Permafrost degradation may induce soil carbon (C) loss, critical for global C cycling, and be mediated by microbes. Despite larger C stored within the active layer of permafrost regions, which are more affected by warming, and the critical roles of Qinghai-Tibet Plateau in C cycling, most previous studies focused on the permafrost layer and in high-latitude areas. We demonstrate in situ that permafrost degradation alters the diversity and potentially decreases the stability of active layer microbial communities. These changes are associated with soil C loss and potentially a positive C feedback. This study provides insights into microbial-mediated mechanisms responsible for C loss within the active layer in degraded permafrost, aiding in the modeling of C emission under future scenarios.

Estimation of annual soil CO2 efflux under the erosion and deposition conditions by measuring and modeling its respiration rate in southern China.

Soil respiration (Rs) is a crucial ecological process of carbon (C) cycling in the terrestrial ecosystems, and soil erosion has a significant impact on its C budget and balance. However, the variations of Rs rate and their CO2 efflux induced by erosion are currently poorly understood. To this end, four landscape positions (top, up, middle and toe) with different erosional and depositional characteristics were selected on a typical eroded slope in southern China to conduct field experiments, aiming to explore the effects of erosion and deposition on Rs among various sites. From March 2021 to February 2022, the in-situ Rs were measured using an automated soil respiration system, together with soil temperature at 5 cm depth (Ts5) and water content at 10 cm depth (SWC10). We initially constructed various Rs models across a one-year period, based on its relationships with Ts5 and SWC10. Subsequently, the seasonal changes of Rs at different erosional sites were simulated by the optimum models, and their annual CO2 fluxes were further estimated. The results showed that Rs rates at all sites displayed a bimodal seasonal pattern, with the highest values in May and August. And the measured Rs of the eroding and depositional sites were 0.05-7.71 and 1.47-13.03 µmol m-2 s-1, respectively. Also, remarkably higher Ts5 and SWC10 were observed in depositional sites versus the eroding sites (P < 0.05). Additionally, Rs rates at all sites were positively correlated with SOC and Ts5, but negatively correlated with SWC10. Herein, Rs models to single- and double-variable were established at different positions, and we found that the fitted R2 and AIC differed on various sites, primarily in erosional and depositional sites. Furthermore, through the best-fitting models (higher R2 and lowest AIC) we screened, the average Rs values of 3.03 and 4.46 µmol m-2 s-1 were quantitatively estimated for the eroding and depositional sites, respectively. Finally, it could be further assessed that the mean annual soil CO2-C efflux of eroded site (1104.14 g m-2) was significantly lower than that of depositional site (1629.46 g m-2). These findings highlighted the effect of erosion and deposition on Rs, which will facilitate a better understanding of C cycling in terrestrial ecosystems.

Impact of wildfire recurrence on soil properties and organic carbon fractions.

The recurrence and severity of wildfire is on the rise due to factors like global warming and human activities. Mediterranean regions are prone to significant wildfire events, which cause extensive damage to ecosystems and soil properties. This study focuses on the municipality of Allande in south-western Asturias (Spain), a region highly affected by recurrent wildfires. In this regard, we sought to examine how the recurrence of such fires influences soil organic carbon fractionation and other soil parameters, such as nitrogen fractionation, pH, and cation exchange capacity. The study involved six sampling plots with between varying fire recurrence levels, from 0 to 4 events between 2005 and 2022. The results revealed some significant effects of wildfires recurrence on soil texture, inorganic elemental composition and CEC, but not on pH and CE. In soil affected by recurrent fires, labile carbon fractions (cold-water extractable & hot-water extractable), and fulvic acid concentrations decreased by up to 36%, 5%, and 45%, respectively in comparison with undisturbed soil. In contrast, humic acid concentration remained stable or increased in soils damaged by fire. Additionally, nitrogen species in soil were observed to decrease significantly in high recurrence scenarios, especially nitrate. On the basis of our findings, we conclude that wildfires impact the distinct fractions of organic carbon and nitrogen in soils and that this effect is aggravated by increasing recurrence.

Multivariate and scale-dependent controls of deep soil carbon after afforestation in a typical loess-covered region.

Afforestation is beneficial to improving soil carbon pools. However, due to the lack of deep databases, the variations in soil carbon and the combined effects of multiple factors after afforestation have yet to be adequately explored in >1 m deep soils, especially in areas with deep-rooted plants and thick vadose zones. This study examined the multivariate controls of soil organic carbon (SOC) and inorganic carbon (SIC) in 0-18 m deep under farmland, grassland, willow, and poplar in loess deposits. The novelty of this study is that the factors concurrently affecting deep soil carbon were investigated by multiwavelet coherence and structural equation models. On average, the SOC density (53.1 ± 5.0 kg m-2) was only 12% of SIC density (425.4 ± 13.8 kg m-2), with depth-dependent variations under different land use types. In the soil profiles, the variations in SOC were more obvious in the 0-6 m layer, while SIC variations were mainly observed in the 6-12 m layer. Compared with farmland (SOC: 17.0 kg m-2; SIC: 122.9 kg m-2), the plantation of deciduous poplar (SOC: 28.5 kg m-2; SIC: 144.2 kg m-2) increased the SOC and SIC density within the 0-6 m layer (p < 0.05), but grassland and evergreen willow impacted SOC and SIC density insignificantly. The wavelet coherence analysis showed that, at the large scale (>4 m), SOC and SIC intensities were affected by total nitrogen-magnetic susceptibility and magnetic susceptibility-water content, respectively. The structural equation model further identified that SOC density was directly controlled by total nitrogen (path coefficient = 0.64) and indirectly affected by magnetic susceptibility (path coefficient = 0.36). Further, SOC stimulated the SIC deposition by improving water conservation and electrical conductivity. This study provides new insights into afforestation-induced deep carbon cycles, which have crucial implications for forest management and enhancing ecosystem sustainability in arid regions.