Activation Energy Is a Useful Proxy for Intrinsic Stability of Soil Organic Matter.

Both temperature and nutrient availability have essential roles in regulating the decomposition of soil carbon (C) and nitrogen (N), the main controls on organic matter accumulation in forest ecosystems. However, there is a lack of information about how N deposition and phosphorus (P) additions might impact soil C and N decomposition rates in subtropical forests under climate warming. We measured soil organic C and N mineralisation rates and corresponding exoenzyme activities in a subtropical forest soil that had received N and/or P additions for six years in experimental conditions at a range of temperatures between 10 and 40°C. Our results showed that soil organic C and N decomposition rates were positively correlated with the activities of their corresponding enzymes, which suggests that the extracellular enzyme activities could be the main influence on soil organic matter (SOM) decomposition rates. N additions had a significant positive effect on soil organic C mineralisation and enhanced oxidase and hydrolase activities. P additions had little effect on soil organic C and N decomposition rates. These results challenge the assumptions that soil microorganisms are N‐rich and P availability restricts organic matter decomposition, and provides additional evidence that N, not P, regulates organic matter decomposition in subtropical forests. While N additions significantly influenced the soil C and N decomposition rates, they had little effect on their sensitivity to temperature. In contrast, P additions had a significant effect on the temperature sensitivities of SOM decomposition and the βG and NAG Vmax. Overall, our results show that SOM decomposition is vulnerable to both N and P additions, and both should be considered when predicting how SOM decomposition and C cycling might change under warming.Highlights The effect of N and/or P additions on the temperature sensitivity of SOM decomposition was explored. N stimulated soil organic C decomposition rates and corresponding enzyme activities. P promoted the sensitivity of SOM decomposition to temperature changes. N and P additions could affect the stability of SOM in subtropical forests in the future.

There is increasing evidence that the accessibility of soil organic matter (SOM) to microbial decomposers is more important than chemical recalcitrance for regulating SOM stability. We show that the rapid reduction in SOM decomposition following the addition of sorptive mineral phases to soils in laboratory conditions leads to decreased accessibility of SOM to microbial decomposers due to the formation of organo‐mineral complexes. We manipulated SOM accessibility in a short‐term microcosm experiment by adding different proportions of a sorptive mineral material derived from an aluminium‐rich allophanic soil to a constant mass of soil to determine the effects on SOM decomposition after 1, 4 and 8 days. The decrease in SOM decomposition with increasing proportion of added sorptive mineral phase occurred within 1 day and did not change further at 4 and 8 days. In a second experiment, we added three proportions of the sorptive mineral phases (0%, 15% and 50%) to three soils with different carbon (C) concentrations and measured rates of SOM decomposition, changes in water extractable C, the formation of organo‐mineral complexes inferred from pyrophosphate‐extractable aluminium, and the natural abundance 13 C isotopic composition of CO 2 derived from SOM decomposition. We confirmed that the proportional decreases in SOM decomposition with increasing organo‐mineral complexes and decreasing microbial access to SOM was the same for the three soils, suggesting that the effects are independent of soil C concentration and pH. We also showed that the short‐term reductions in SOM accessibility led to microbial decomposition of more 13 C enriched substrates, suggesting preferential stabilisation of plant‐derived ( 13 C depleted) substrates. Our study demonstrated that SOM accessibility and decomposition could be reduced rapidly and proportionally to the amount of added sorptive mineral phases resulting from increased organo‐mineral interactions irrespective of the initial soil organic carbon concentration. Highlights Addition of sorptive mineral phases reduced short‐term soil organic matter (SOM) decomposition by the same proportion for three soils. Relatively 13 C depleted SOM was preferentially adsorbed onto the mineral phases. The reduction in SOM decomposition was attributed to reduced microbial access due to increased organo‐mineral interactions. The effects occurred rapidly and proportionally to the amount of added sorptive mineral phases.

The temperature sensitivity of soil organic matter decomposition is constrained by microbial access to substrates

Soil organic matter (SOM) transformation processes are regulated by the activities of plants, microbes, and fauna. Compared with plants and microbes, effects of soil fauna are less understood because of their high taxonomic and functional diversity, and mix of direct and indirect effect mechanisms. Trait‐based approaches offer a generic perspective to quantify mechanistic relationships between soil fauna and SOM transformations, including decomposition, translocation, and stabilisation of organic carbon. Yet, at present, we lack a consensus concerning relevant key effect traits of soil fauna (i.e. those affecting ecosystem functioning). Here, we address this knowledge gap by focusing on relationships between soil fauna effect traits and SOM transformations. Based on existing literature, we identify key processes linked to SOM transformations, and fauna effect traits universally applicable across taxa and soil types, and discuss the process‐trait links. We define eight SOM transformation processes that are directly affected by soil fauna: (i) litter mass loss, (ii) litter fragmentation, (iii) SOM aggregation in faeces, (iv) SOM aggregation in soil mineral particles, (v) decomposition of faeces, (vi) SOM and mineral translocation, (vii) pore space creation and maintenance and (viii) SOM stabilisation. We link these processes to general effect traits classified into four categories: (a) food selection and ingestion, (b), digestion and excretion, (c) mobility, and (d) body mass and metabolic rate. We also propose proxies when effect trait measurements are laborious. The proposed links between effect traits and SOM transformation processes need to be validated in targeted experiments. We urge researchers to obtain quantitative experimental data, together with metabolic approaches, to integratively quantify soil fauna contributions to soil functioning. Read the free Plain Language Summary for this article on the Journal blog.

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.

Planting trees on non-forested land has the potential to sequester atmospheric CO2 in biomass and soil. While afforestation on former agricultural land often results in an increased soil organic carbon sequestration, the outcomes of afforestation on pastures vary from carbon sink to source. Alpine soils are characterized by a higher proportion of labile carbon compounds compared to soils in temperate ecosystems, which makes alpine ecosystems more sensitive to environmental changes. The conversion of subalpine pasture to forests thereby might have a substantial effect on the SOC dynamics and&amp;#160; on soil organic matter (SOM) stabilization. In addition, the alteration in the proportion of aboveground biomass- and root-derived organic matter and the associated alterations in the soil microbial community following afforestation on subalpine pastures are not yet fully understood. In this study, the alteration in SOC stocks as well as in the SOM composition following &amp;#160;afforestation (0 to 130 years) with Norway spruce (Picea abies) on a subalpine pasture is investigated in the Swiss Alps. To determine the alteration of potential sources and decomposition of SOM, a multi-proxy molecular marker approach was applied. Specifically, the combination of n-fatty acids, n-alkanes, and n-alcohols was applied to identify possible sources of plant-derived SOM. For the identification of microorganism-derived SOM, a combination of phospholipid fatty acids and glycerol dialkyl glycerol tetraethers was used. Afforestation with Norway spruce on a subalpine pasture did not result in any significant change in SOC stocks (Pasture: 11.5 &amp;#177; 0.5 kg m-2; 130-year-old forest 11.0 &amp;#177; 0.3 kg m-2) after 130-years. The organic matter input, however, changed from grass leaves to spruce needles with increasing forest stand age. Surprisingly, root-derived organic matter seems to play a minor role in the pasture soil as well as in forest soils of all stand ages as one of the predominant sources of SOM. With increasing forest age an increased abundance of Gram+ bacteria as well as arbuscular mycorrhizal fungi was observed. In the pasture soil, a clearly higher abundance of archaea was observed compared with the forest. This shift in the soil microbial community shows its adaptation to the changes in the vegetation cover. &amp;#160;Furthermore, the difference in the soil microbial community structure implies a use of different carbon substrates of the microorganisms between the pasture and forests, which can have substantial effects on soil organic matter stabilization. Conclusively SOC stocks did not change after 130 years of afforestation on a subalpine pasture, but the SOM dynamics has changed due to the changes in the vegetation cover. For a better understanding of the connection between organic matter input and its decomposition, the analysis of plant polymers such as cutin and suberin polymers can help to unravel the difference in shoot- vs. root-derived organic matter and their contribution to the stable SOM pool in subalpine ecosystems.

Decomposition temperature sensitivity of isolated soil organic matter fractions

Substrate quality and the temperature sensitivity of soil organic matter decomposition

Ectomycorrhizal (EcM) fungi are one of the important functional groups of soil fungi, playing a crucial role in the formation, stabilization, and decomposition of soil organic matter (SOM). We summarized the main processes and mechanisms by which EcM fungi contribute to SOM formation, stabilization, and decomposition in forests. Plants allocate a portion of photosynthetic products to symbiotic EcM fungi, which participate in SOM formation by importing them into the soil in the form of mycorrhizal exudates or necromass, whose activities promote the formation of soil aggregate structure and SOM stabilization. EcM fungi decompose SOM directly by secreting extracellular enzymes or by driving the Fenton reaction to generate hydroxyl radicals. They also influence SOM decomposition indirectly by enhancing the activity of saprotrophic fungi (priming effect) or inhibiting their activity (Gadgil effect). The precise quantification of EcM fungi's role in SOM formation remains unclear. Most available studies are concentrated in Europe and North America, but the difference in methodologies makes it difficult to integrate data across regions. Future research should adopt standardized techniques and promote cross-regional collaborative studies. Current understanding of EcM fungi's role in SOM decomposition is mainly based on a few laboratory-cultured species. Future studies should include a broader range of EcM fungal species and investigate their roles in natural environments, particularly in different soil types and forest communities. In addition, the interactions between EcM fungi and saprotrophic fungi have significant impacts on SOM dynamics. Future research should explore the responses of EcM fungi to climate, soil and vegetation in depth to better understand their role in soil carbon cycling.

Abstract. Decomposition of soil organic matter (SOM) is limited by both the available substrate and the active decomposer community. The understanding of this colimitation strongly affects the understanding of feedbacks of soil carbon to global warming and its consequences. This study compares different formulations of soil organic matter (SOM) decomposition. We compiled formulations from literature into groups according to the representation of decomposer biomass on the SOM decomposition rate a) non-explicit (substrate only), b) linear, and c) non-linear. By varying the SOM decomposition equation in a basic simplified decomposition model, we analyzed the following questions. Is the priming effect represented? Under which conditions is SOM accumulation limited? And, how does steady state SOM stocks scale with amount of fresh organic matter (FOM) litter inputs? While formulations (a) did not represent the priming effect, with formulations (b) steady state SOM stocks were independent of amount of litter input. Further, with several formulations (c) there was an offset of SOM that was not decomposed when no fresh OM was supplied. The finding that a part of the SOM is not decomposed on exhaust of FOM supply supports the hypothesis of carbon stabilization in deep soil by the absence of energy-rich fresh organic matter. Different representations of colimitation of decomposition by substrate and decomposers in SOM decomposition models resulted in qualitatively different long-term behaviour. A collaborative effort by modellers and experimentalists is required to identify formulations that are more or less suitable to represent the most important drivers of long term carbon storage.

Rising temperatures enhance microbial decomposition of soil organic matter (SOM) and thereby increase the soil CO2 efflux. Elevated decomposition rates might differently affect distinct SOM pools, depending on their stability and accessibility. Soil fractions derived from density fractionation have been suggested to represent SOM pools with different turnover times and stability against microbial decomposition.To investigate the effect of soil warming on functionally different soil organic matter pools, we here investigated the chemical and isotopic composition of bulk soil and three density fractions (free particulate organic matter, fPOM; occluded particulate organic matter, oPOM; and mineral associated organic matter, MaOM) of a C-rich soil from a long-term warming experiment in a spruce forest in the Austrian Alps. At the time of sampling, the soil in this experiment had been warmed during the snow-free period for seven consecutive years. During that time no thermal adaptation of the microbial community could be identified and CO2 release from the soil continued to be elevated by the warming treatment. Our results, which included organic carbon content, total nitrogen content, δ13C, Δ14C, δ15N and the chemical composition, identified by pyrolysis-GC/MS, showed no significant differences in bulk soil between warming treatment and control. Surprisingly, the differences in the three density fractions were mostly small and the direction of warming induced change was variable with fraction and soil depth. Warming led to reduced N content in topsoil oPOM and subsoil fPOM and to reduced relative abundance of N-bearing compounds in subsoil MaOM. Further, warming increased the δ13C of MaOM at both sampling depths, reduced the relative abundance of carbohydrates while it increased the relative abundance of lignins in subsoil oPOM. As the size of the functionally different SOM pools did not significantly change, we assume that the few and small modifications in SOM chemistry result from an interplay of enhanced microbial decomposition of SOM and increased root litter input in the warmed plots. Overall, stable functional SOM pool sizes indicate that soil warming had similarly affected easily decomposable and stabilized SOM of this C-rich forest soil.

The temperature sensitivity of soil organic matter (SOM) decomposition has been a crucial topic in global change research, yet remains highly uncertain. One of the contributing factors to this uncertainty is the lack of understanding about the role of rhizosphere priming effect (RPE) in shaping the temperature sensitivity. Using a novel continuous 13C-labeling method, we investigated the temperature sensitivity of RPE and its impact on the temperature sensitivity of SOM decomposition. We observed an overall positive RPE. The SOM decomposition rates in the planted treatments increased 17–163% above the unplanted treatments in three growth chamber experiments including two plant species, two growth stages, and two warming methods. More importantly, warming by 5 °C increased RPE up to threefold, hence, the overall temperature sensitivity of SOM decomposition was consistently enhanced (Q10 values increased 0.3–0.9) by the presence of active rhizosphere. In addition, the proportional contribution of SOM decomposition to total soil respiration was increased by soil warming, implying a higher temperature sensitivity of SOM decomposition than that of autotrophic respiration. Our results, for the first time, clearly demonstrated that root–soil interactions play a crucial role in shaping the temperature sensitivity of SOM decomposition. Caution is required for interpretation of any previously determined temperature sensitivity of SOM decomposition that omitted rhizosphere effects using either soil incubation or field root-exclusion. More attention should be paid to RPE in future experimental and modeling studies of SOM decomposition.

The soil organic matter pool represents a major part of the terrestrial carbon reservoir. Decomposition of soil organic matter contributes to important fluxes of the global carbon cycle. In order to understand, assess, and predict the impact of land use or climatic changes in the tropics, quantitative knowledge of stabilization and decomposition processes of soil organic matter is necessary. This thesis elucidated how soil properties and land use influence the carbon storage in bulk soil and in physically and chemically defined soil fractions. Furthermore, the stability of older forest-derived carbon and the stability of recently incorporated pasture-derived carbon in these fractions were investigated. For this purpose, fractionation procedures were combined with 13C isotope analyses on soil samples from sites that had undergone a land use change (natural forest > pasture, natural forest > pasture > secondary forest) which was connected with a vegetation change from C3 to C4 or from C3 to C4 to C3 plants. Soil sampling was conducted in the humid tropics in northwest Ecuador. Soils developed from different parent material (marine Tertiary sediments and volcanic ashes) differing in key factors which influence storage and stabilisation of soil organic matter.Besides parent material, soil texture was the main factor which controlled total soil organic carbon (soil C) contents in these soils. The major part of soil C was bound to the mineral phase. Higher soil C contents in the volcanic ash soils emphasise the higher carbon sequestration of Al-humus complexes and non-crystalline hydroxides in comparison to sedimentary soils dominated by smectite. Results of the chemical fractionation point to an enhanced stability of the mineral-bound soil C against oxidation with NaOCl in the volcanic ash soils. In addition, more soil C could be extracted by Na4P2O7 in these soils indicating the importance of Al-humus complexes. In contrast, treatment with HCl isolated a fixed proportion of soil C storage in both soil types indicating a similar composition of organic matter in the soils. The higher carbon sequestration capacity in the volcanic ash soils could be also related to higher carbon storage in the light fractions.Independent of soil parent material, deforestation followed by pasture establishment reduced soil C storage. By afforesting the former pasture, the amount soil C increased, but the values of the natural forest were not reached at all sites. Changes in soil C stocks were more pronounced in the light fractions compared to bulk soil. Thus, light fractions can be used as an early indicator for soil C changes induced by land use changes. The correlation of aggregation and the amount of occluded light fraction and the slow turnover support the hypothesis, that organic matter is stabilised by aggregation in both soil types.The recently incorporated pasture-derived carbon showed a faster turnover in the volcanic ash soils than in the sedimentary soils. Mineral surfaces of sedimentary soils form stronger interactions with the organic matter. In contrast to sedimentary soils, no pasture-derived carbon could be detected in any fraction of the volcanic ash soils under secondary forest. Thus, recently incorporated carbon was not stabilised in volcanic ash soils. However, pasture-derived carbon could be detected in the chemical isolated fractions under pasture. This indicates that none of the used chemical fractionation techniques isolated a soil organic matter pool that is under field conditions stable against microbial degradation.

Effect of microplastics on organic matter decomposition in paddy soil amended with crop residues and labile C: A three-source-partitioning study

No-tillage continuously contribute organic matter and increase carbon sequestration in the soil. However, global warming may stimulate microbial activity and accelerate soil organic matter (SOM) decomposition, also especially in the topsoil. Understanding the responses of microbial utilization of SOM accumulated under no-tillage to higher temperatures is necessary to develop management strategies to sustain soil fertility. Determine the response of microbial extracellular enzyme activities and SOM decomposition to long-term warming by incorporation of 14C-labelled maize litter to in situ warmed soil from no-tillage and till systems. We compared decomposability of litter and SOM in soils from a 5-year in situ experiment with two tillage systems (Till and No-till) and under two temperature levels: ambient temperature and continuous soil warming (+1.6 °C at 5 cm soil depth). We hypothesized that decomposition of crop litter (14C-labeled maize) at increasing temperature (15, 21, and 27 °C for 59 days) will be more intensive in warmed soil vs. ambient and the effects should be stronger under No-till vs. Till system. As expected, soil from field warming had always higher total CO2 efflux (from 5.5 to 12%) than the non-warmed counterpart in Till and No-till systems. Five-year warming increased temperature sensitivity (Q10) of CO2 efflux for 2-folds under No-tillage vs. Till. Litter decomposition (measured as 14CO2) in No-till with warming was 7.9% greater than in No-tillage without warming. Three extracellular enzymes (β-glucosidase, chitinase, and sulfatase) had higher activities under warming in No-till but not under Till system. Our results demonstrated that the 5-year in situ warming accelerated preferential microbial utilization of labile SOM fractions in No-till notably, and from stable SOM in Till system. Consequently, warming will accelerate SOM decomposition in future and this acceleration will be especially pronounced on the labile SOM under No-tillage systems.