Temperature and moisture are minor drivers of regional-scale soil organic carbon dynamics

Radiocarbon signatures of carbon phases exported by Swiss rivers in the Anthropocene

Lateral carbon transport through the land-to-ocean-aquatic-continuum (LOAC) represents a key component of the global carbon cycle. This LOAC involves complex processes, many of which are prone to anthropogenic perturbation, yet the influence of natural and human-induced drivers remains poorly constrained. This study examines the radiocarbon (14C) signatures of particulate and dissolved organic carbon (POC, DOC) and dissolved inorganic carbon (DIC) transported by Swiss rivers to assess controls on sources and cycling of carbon within their watersheds. Twenty-one rivers were selected and sampled during high-flow conditions in summer 2021, a year of exceptionally high rainfall. Δ14C values of POC range from -446‰ to -158‰, while corresponding ranges of Δ14C values for DOC and DIC are -377‰ to -43‰ and -301‰ to -40‰, respectively, indicating the prevalence of pre-aged carbon. Region-specific agricultural practices seem to have an influential effect on all three carbon phases in rivers draining the Swiss Plateau. Based on Multivariate Regression Analysis, mean basin elevation correlated negatively with Δ14C values of all three carbon phases. These contrasts between alpine terrain and the lowlands reflect the importance of overriding ecoregional controls on riverine carbon dynamics within Switzerland, despite high spatial variability in catchment properties. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Evaluation of carbon mineralization and its temperature sensitivity in different soil aggregates and moisture regimes: A 21-year tillage experiment

Characterizing soil organic carbon (SOC) mineralization and its temperature sensitivity (Q₁₀) under different soil moisture in tillage systems is crucial for determining global carbon balance under climate warming and increasing precipitation. Aggregate protection can potentially govern SOC mineralization and its Q₁₀. However, how tillage and aggregate sizes affect SOC mineralization and its Q₁₀, especially under varying soil moisture, remains unclear. Soil samples (0-10 cm and 10-20 cm) were collected from a 21-year field study with four tillage treatments: conventional tillage (CT), reduced tillage (RT), no-tillage (NT), and subsoiling (SS). Bulk soil and dry-sieved aggregates were incubated at 15°C and 25°C at low, medium, and high moistures (i.e., 40%, 70%, and 100% water-holding capacity, respectively). Macro-aggregates (> 0.25 mm) had lower SOC mineralization relative to micro-aggregates (< 0.25 mm) across all soil temperatures, moistures, and depths (P < 0.01), which was attributed to their lower SOC quality (i.e., higher ratio of SOC to total nitrogen and lower ratio of dissolved organic carbon to SOC). Moreover, NT and SS promoted macro-aggregation relative to CT and RT, and thereby decreased mineralization (P < 0.001). However, Q₁₀ was higher in macro-aggregates than in micro-aggregates at low and medium moistures. The Q₁₀ was negatively correlated with the SOC quality in macro-aggregates (P < 0.05). The macroaggregate-associated SOC quality was lower under NT and SS than under CT and RT, which resulted in a greater Q₁₀ under NT and SS at low and medium moistures, suggesting that NT and SS may accelerate SOC losses under global warming. Furthermore, increased soil moisture could lower Q₁₀, and no differences among tillage practices were observed at high moisture levels (P > 0.05). Overall, our findings indicated that NT and SS decreased SOC mineralization but increased Q₁₀ because of their large amounts of macro-aggregates with low SOC quality, and the improvement of Q₁₀ was constrained by increasing soil moisture.

Profile Soil Carbon and Nitrogen Dynamics in Typical Chernozem under Long-Term Tillage Use

For the first time in research literature, this report presents the seasonal changes of total organic carbon (TOC), total nitrogen (TN), and TOC:TN ratio in Chernozem solum (0–100 cm) as effected by 14 years of application of conventional tillage (CTu), deep reduced tillage (DRTu), and reduced tillage (RTu) under barley growing. During the season, TOC content drastically declined in the spring, increased in the summer, decreased in the middle of August, and recovered in October. TN content was gradually decreased during a crop growing season and renewed in the autumn. A trend of TOC:TN changes (vertical peak curve) in 0–30 cm soil layer varied from TOC (S-shaped curve) and TN (unsymmetrical decayed curve). The amplitude of seasonal TOC and TN changes in deeper layers was far fewer related to the upper horizons. The highest amplitude in 0–30, 30–60 and 60–100 cm layers was under: DRTu, CTu, DRTu—for TOC and DRTu, CTu, RTu—for TN correspondently. Tillage practices differently stratified the content of organic carbon and nitrogen in Chernozem profile. Minimum tillage benefited TOC sequestration in 0–5 and 5–10 cm layers: 24.83 ± 0.64- and 24.65 ± 0.57 g kg−1—under RTu, 24.49 ± 0.62- and 24.71 ± 0.47 g kg−1—under DRTu, while CT—deeper than 20 cm: 22.49–15.03 g kg−1. The vertical distribution of TN content repeated TOC trend. TOC:TN ratio upraised from 12.60 in 0–5 to 14.33 in 80–100 cm layer and was the highest in summertime. A total (0–100 cm) profile was much greater under RTu and DRTu—for TN, and CTu, DRTu—for TOC. The correlation coefficient (r) was almost negligible between TOC and: T (air temperature), P (precipitation) and W (soil moisture). The strong and very strong r was found for TN—W, and P—W pairs. The negative r was between: TOC–P, TN–P, TOC:TN-W, TOC:TN–T and P–W pairs.

Mechanisms of soil organic carbon stability and its response to no-till: A global synthesis and perspective

Mechanisms of soil organic carbon (SOC) stabilization have been widely studied due to their relevance in the global carbon cycle. No-till (NT) has been frequently adopted to sequester SOC; however, limited information is available regarding whether sequestered SOC will be stabilized for long term. Thus, we reviewed the mechanisms affecting SOC stability in NT systems, including the priming effects (PE), molecular structure of SOC, aggregate protection, association with soil minerals, microbial properties, and environmental effects. Although a more steady-state molecular structure of SOC is observed in NT compared with conventional tillage (CT), SOC stability may depend more on physical and chemical protection. On average, NT improves macro-aggregation by 32.7%, and lowers SOC mineralization in macro-aggregates compared with CT. Chemical protection is also important due to the direct adsorption of organic molecules and the enhancement of aggregation by soil minerals. Higher microbial activity in NT could also produce binding agents to promote aggregation and the formation of metal-oxidant organic complexes. Thus, microbial residues could be stabilized in soils over the long term through their attachment to mineral surfaces and entrapment of aggregates under NT. On average, NT reduces SOC mineralization by 18.8% and PE intensities after fresh carbon inputs by 21.0% compared with CT (p < .05). Although higher temperature sensitivity (Q₁₀ ) is observed in NT due to greater Q₁₀ in macro-aggregates, an increase of soil moisture regime in NT could potentially constrain the improvement of Q₁₀ . This review improves process-based understanding of the physical and chemical mechanism of protection that can act, independently or interactively, to enhance SOC preservation. It is concluded that SOC sequestered in NT systems is likely to be stabilized over the long term.

Temperature effects on carbon storage are controlled by soil stabilisation capacities

Physical and chemical stabilisation mechanisms are now known to play a critical role in controlling carbon (C) storage in mineral soils, leading to suggestions that climate warming-induced C losses may be lower than previously predicted. By analysing > 9,000 soil profiles, here we show that, overall, C storage declines strongly with mean annual temperature. However, the reduction in C storage with temperature was more than three times greater in coarse-textured soils, with limited capacities for stabilising organic matter, than in fine-textured soils with greater stabilisation capacities. This pattern was observed independently in cool and warm regions, and after accounting for potentially confounding factors (plant productivity, precipitation, aridity, cation exchange capacity, and pH). The results could not, however, be represented by an established Earth system model (ESM). We conclude that warming will promote substantial soil C losses, but ESMs may not be predicting these losses accurately or which stocks are most vulnerable.

Dynamic soil functions assessment employing land use and climate scenarios at regional scale

Soils as key component of terrestrial ecosystems are under increasing pressures. As an advance to current static assessments, we present a dynamic soil functions assessment (SFA) to evaluate the current and future state of soils regarding their nutrient storage, water regulation, productivity, habitat and carbon sequestration functions for the case-study region in the Lower Austrian Mostviertel. Carbon response functions simulating the development of regional soil organic carbon (SOC) stocks until 2100 are used to couple established indicator-based SFA methodology with two climate and three land use scenarios, i.e. land sparing (LSP), land sharing (LSH), and balanced land use (LBA). Results reveal a dominant impact of land use scenarios on soil functions compared to the impact from climate scenarios and highlight the close link between SOC development and the quality of investigated soil functions, i.e. soil functionality. The soil habitat and soil carbon sequestration functions on investigated agricultural land are positively affected by maintenance of grassland under LSH (20% of the case-study region), where SOC stocks show a steady and continuous increase. By 2100 however, total regional SOC stocks are higher under LSP compared to LSH or LBA, due to extensive afforestation. The presented approach may improve integrative decision-making in land use planning processes. It bridges superordinate goals of sustainable development, such as climate change mitigation, with land use actions taken at local or regional scales. The dynamic SFA broadens the debate on LSH and LSP and can reduce trade-offs between soil functions through land use planning processes.

Influence of Vegetation Coverage and Climate Environment on Soil Organic Carbon in the Qilian Mountains

Studying the spatial distribution pattern of soil organic carbon and its influencing factors is essential for understanding the carbon cycle in terrestrial ecosystems. Soil samples from four active layers of typical vegetation types (Populus, subalpine shrubs, Picea crassifolia Kom, and alpine meadow) in the upper reaches of Shiyang River basin in the Qilian Mountains were collected to determine the soil organic carbon content and physicochemical properties. The results show the following: (1) There are significant differences in the vertical distribution of Soil organic carbon in the watershed, and the Soil organic carbon content decreases significantly with increasing soil depth. (2) Mainly affected by biomass, the organic carbon content of different vegetation types in different soil layers is as follows: Alpine meadow > Picea crassifolia Kom > Populus > Subalpine shrub, and the soil organic carbon content increases with increasing altitude. Under different vegetation types, the Soil organic content is the highest in the 0-30 cm soil profile, and the maximum value often appears in the 0-10 cm layer, then gradually decreases downward. (3) When soil organic carbon is determined in different vegetation types in the study area, the change of hydrothermal factors has little effect on soil organic carbon content in the short term.