The international 4 per 1000 initiative aims at supporting states and non-governmental stakeholders in their efforts towards a better management of soil carbon (C) stocks. These stocks depend on soil ...C inputs and outputs. They are the result of fine spatial scale interconnected mechanisms, which stabilise/destabilise organic matter-borne C. Since 2016, the CarboSMS consortium federates French researchers working on these mechanisms and their effects on C stocks in a local and global change setting (land use, agricultural practices, climatic and soil conditions, etc.). This article is a synthesis of this consortium’s first seminar. In the first part, we present recent advances in the understanding of soil C stabilisation mechanisms comprising biotic and abiotic processes, which occur concomitantly and interact. Soil organic C stocks are altered by biotic activities of plants (the main source of C through litter and root systems), microorganisms (fungi and bacteria) and ‘ecosystem engineers’ (earthworms, termites, ants). In the meantime, abiotic processes related to the soil-physical structure, porosity and mineral fraction also modify these stocks. In the second part, we show how agricultural practices affect soil C stocks. By acting on both biotic and abiotic mechanisms, land use and management practices (choice of plant species and density, plant residue exports, amendments, fertilisation, tillage, etc.) drive soil spatiotemporal organic inputs and organic matter sensitivity to mineralisation. Interaction between the different mechanisms and their effects on C stocks are revealed by meta-analyses and long-term field studies. The third part addresses upscaling issues. This is a cause for major concern since soil organic C stabilisation mechanisms are most often studied at fine spatial scales (mm–μm) under controlled conditions, while agricultural practices are implemented at the plot scale. We discuss some proxies and models describing specific mechanisms and their action in different soil and climatic contexts and show how they should be taken into account in large scale models, to improve change predictions in soil C stocks. Finally, this literature review highlights some future research prospects geared towards preserving or even increasing C stocks, our focus being put on the mechanisms, the effects of agricultural practices on them and C stock prediction models.
There is currently an intense debate about the potential for additional organic carbon storage in soil, the strategies by which it may be accomplished and what the actual benefits might be for ...agriculture and the climate. Controversy forms an essential part of the scientific process, but on the topic of soil carbon storage, it may confuse the agricultural community and the general public and may delay actions to fight climate change. In an attempt to shed light on this topic, the originality of this article lies in its intention to provide a balanced description of contradictory scientific opinions on soil carbon storage and to examine how the scientific community can support decision-making despite the controversy. In the first part, we review and attempt to reconcile conflicting views on the mechanisms controlling organic carbon dynamics in soil. We discuss the divergent opinions about chemical recalcitrance, the microbial or plant origin of persistent soil organic matter, the contribution of particulate organic matter to additional organic carbon storage in soil, and the spatial and energetic inaccessibility of soil organic matter to decomposers. In the second part, we examine the advantages and limitations of big data management and modeling, which are essential tools to link the latest scientific theories with the actions taken by stakeholders. Finally, we show how the analysis and discussion of controversies can guide scientists in supporting stakeholders for the design of (i) appropriate trade-offs for biomass use in agriculture and forestry and (ii) climate-smart management practices, keeping in mind their still unresolved effects on soil carbon storage.
Abstract
Numerical models are crucial to understand and/or predict past and future soil organic carbon dynamics. For those models aiming at prediction, validation is a critical step to gain ...confidence in projections. With a comprehensive review of ~250 models, we assess how models are validated depending on their objectives and features, discuss how validation of predictive models can be improved. We find a critical lack of independent validation using observed time series. Conducting such validations should be a priority to improve the model reliability. Approximately 60% of the models we analysed are not designed for predictions, but rather for conceptual understanding of soil processes. These models provide important insights by identifying key processes and alternative formalisms that can be relevant for predictive models. We argue that combining independent validation based on observed time series and improved information flow between predictive and conceptual models will increase reliability in predictions.
Crop residues are important inputs of carbon (C) and nitrogen (N) to soils and thus directly and indirectly affect nitrous oxide (N2O) emissions. As the current inventory methodology considers N ...inputs by crop residues as the sole determining factor for N2O emissions, it fails to consider other underlying factors and processes. There is compelling evidence that emissions vary greatly between residues with different biochemical and physical characteristics, with the concentrations of mineralizable N and decomposable C in the residue biomass both enhancing the soil N2O production potential. High concentrations of these components are associated with immature residues (e.g., cover crops, grass, legumes, and vegetables) as opposed to mature residues (e.g., straw). A more accurate estimation of the short‐term (months) effects of the crop residues on N2O could involve distinguishing mature and immature crop residues with distinctly different emission factors. The medium‐term (years) and long‐term (decades) effects relate to the effects of residue management on soil N fertility and soil physical and chemical properties, considering that these are affected by local climatic and soil conditions as well as land use and management. More targeted mitigation efforts for N2O emissions, after addition of crop residues to the soil, are urgently needed and require an improved methodology for emission accounting. This work needs to be underpinned by research to (1) develop and validate N2O emission factors for mature and immature crop residues, (2) assess emissions from belowground residues of terminated crops, (3) improve activity data on management of different residue types, in particular immature residues, and (4) evaluate long‐term effects of residue addition on N2O emissions.
Current inventories of nitrous oxide from agricultural crop residues do not differentiate between quality of residues. There is compelling evidence that emissions vary greatly between residues with different biochemical and physical characteristics, with immature residues having greater emission potential than mature residues.
We developed an Extracellular EnZYme model (EEZY) of decomposition that produces two separate pools of C- and N-acquiring enzymes, that in turn hydrolyze two qualitatively different substrates, one ...containing only C (e.g., cellulose) and the other containing both C and N (e.g., chitin or protein). Hence, this model approximates the actions of commonly measured indicator enzymes ß-1,4-glucosidase and ß-1,4-N-acetylglucosaminidase (or leucine aminopeptidase) as they hydrolyze cellulose and chitin (or protein), respectively. EEZY provides an analytical solution to the allocation of these two enzymes, which in turn release C and N from the two substrates to maximize microbial growth. Model behaviors were both qualitatively and quantitatively consistent with patterns of litter decay generated by other decomposition models. However, EEZY demonstrated greater sensitivity to the C:N of individual substrate pools in addition to responding to factors directly affecting enzyme activity. Output approximated field observations of extracellular enzyme activities from studies of terrestrial soils, aquatic sediments, freshwater biofilm and plankton communities. Although EEZY is largely a theoretical model, simulated C- and N-acquiring enzyme activities approximated a 1:1 ratio, consistent with the bulk of these field observations, only when the N-containing substrate had a C:N ratio similar to commonly occurring substrates (e.g., proteins or chitin). This result supported the emerging view of the stoichiometry of extracellular enzyme activities from an environmental context, which suggests that a relatively narrow range of microbial C:N, carbon use efficiency and soil/sediment organic matter C:N across ecosystems explains the tendency towards this 1:1 ratio of enzyme activities associated with C- and N-acquisition. Sensitivity analyses indicated that simulated extracellular enzyme activity was most responsive to variations in carbon use efficiency of microorganisms, although kinetic characteristics of enzymes also had significant impacts. Thus EEZY provides a quantitative framework in which to interpret mechanisms underlying empirical patterns of extracellular enzyme activity.
► A two enzyme-two substrate mechanistic model of decomposition. ► Decomposition balanced by C and N demands of microorganisms. ► Optimal enzyme allocation alters enzyme:biomass ratios. ► Biomass, respiration and N-mineralization driven by enzyme allocation. ► Simulated patterns of enzyme activities match observations.
Increasing the accumulation of organic carbon (C) in soils is a crucial challenge both for soil fertility and for climate change mitigation. Heterotrophic microbial communities are key drivers of C ...cycling in the soil and are influenced by cultural practices, among other factors. However, whether changes in microbial communities in turn affect their C degradation functions is not well understood. Here, we studied the effects of prior soil management on the microbial taxonomic composition and activity of soils amended with wheat litter. Prior soil management was either conventional (CONV) (i.e., full inversion ploughing) or reduced tillage (RT) during a 5-year period in the same loamy soil in northern France. Soil samples taken from the top 5 cm of field plots were incubated with 13C-labelled litter of either flowering wheat or mature wheat for 29 days at 15 °C. We measured the C-CO2 and 13C-CO2, microbial biomass C (MBC) and 13C, and hydrolytic enzyme activities during decomposition. The initial bacterial and fungal community diversity was studied via high-throughput sequencing of ribosomal genes.
The results showed that the MBC in the RT soil was initially 1.5-fold greater than that in the CONV soil; contrasting taxonomic compositions were also recorded. The soil biotic legacy impacted the degradation functions when the soils were amended with wheat litter. Compared with that in the CONV soil, the enzymatic efficiency of microorganisms in the RT soil increased by 49% and 61% in the presence of mature and flowering wheat litter, respectively. Enzyme efficiency was positively correlated with microbial litter C use efficiency (CUE) (r = 0.92, P-Value < 0.001) but negatively associated with the priming effect (PE) (r = −0.85, P-value < 0.001) across all soils and litter treatments. These findings demonstrated that the RT soil benefited both from an increase in litter C incorporated in the microbial biomass and from a reduction in soil C loss due to the PE, regardless of the quality of the decomposed litter. Our study indicated that agricultural practices such as RT, which enriches the amount of soil organic C (SOC) in the topsoil layer, can lead to positive feedback against C stabilization functions.
•The contrasting 5-year tillage practices induced changes in soil microbial biomass and taxonomy.•Soil biotic legacy impacted microbial functions during litter degradation.•Soil microorganisms under reduced tillage had higher C use and enzyme efficiencies.•Conventional tillage led to greater priming effects than did reduced tillage.•High CUE and low priming effects promoted soil C stabilization under reduced tillage.
The emission of nitrous oxide (N2O), a strong greenhouse gas, during crop residue decomposition in the soil can offset the benefits of residue recycling. The IPCC inventory considers agricultural N2O ...emissions proportional to the amount of nitrogen (N) added by residues to soils. However, N2O involves several emission pathways driven directly by the form of N returned and indirectly by changes in the soil induced by decomposition. We investigated the decomposition factors related to N2O emissions under controlled conditions. Residues of sugar beet (SUB), wheat (WHT), rape seed (RAS), potato (POT), pea (PEA), mustard (MUS), red clover (RC), alfalfa (ALF), and miscanthus (MIS), varying by maturity at the time of collection, were incubated in two soils (GRI and SLU) at 15 °C with a water-filled pore space of 60%. The residues contained a wide proportion range of water-soluble components, components soluble in neutral detergent (SOL-NDS), hemicellulose, cellulose, and lignin. Their composition drastically influenced the dynamics of C mineralization and soil ammonium and nitrate and was correlated with N2O flux dynamics. The net cumulative N2O emitted after 60 days originated mostly from MUS (4828 ± 892 g N-N2O ha-1), SUB (2818 ± 314 g N-N2O ha-1) and RC (2567 ± 1245 g N-N2O ha-1); the other residue treatments had much lower emissions (<200 g N-N2O ha-1). For the first time N2O emissions could be explained only by the residue content in the SOL-NDS, according to an exponential relationship. Residues with a high SOL-NDS (>25% DM) were also non-senescent and promoted high N2O emissions (representing 1–5% of applied N), likely directly by nitrification and indirectly by denitrification in microbial hotspots. Crop residue quality appears to be valuable information for accurately predicting N2O emissions and objectively weighing their other potential benefits to agriculture and the environment.
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•N2O emissions were measured during laboratory incubations of soil and crop residues.•The coupling of N2O and CO2 emissions and soil mineral N dynamics was studied.•Nine crop residues whose compositions widely varied were incubated in two soils.•The residue neutral detergent soluble content explained the observed N2O emissions.•The IPCC could underestimate the N2O emissions of residues from nonsenescent crops.
Extracellular enzymes catalyze plant litter decomposition, including enzymes that degrade holocellulose (E2) and lignin (E3). To estimate relative enzyme activities associated with observed patterns ...of hollocellulose (C2) and lignin (C3) decay, we set observed decay rates equal to reverse Michaelis-Menten equations. Results were consistent with empirical studies, showing a negative relationship of E2/(E2+E3) to litter lignin content, C3/(C2+C3), above a minimum threshold at which lignin begins to decay. This threshold was previously reported to be 40% lignin content, but our results demonstrated substantial variability with litter type and environment. To our knowledge, this is the first mechanistic explanation of microbial allocation of cellulolytic and ligninolytic enzymes as a function of the lignin concentration of the lignocellulose complex but raises further questions about factors controlling the threshold for lignin decay, such as nitrogen availability.
•Lignocellulose index (LCI) controls microbial allocation of extracellular enzymes.•We provide an analytical solution to the LCI control on enzyme allocation.•The ratio of cellulose-to-lignin degrading enzyme activities declines with increasing LCI.•LCI threshold values at which ligninolytic enzymes are allocated varies among studies.
•Hemp harvest time impacts on the dynamics of stems dew retting.•Hemp stems harvested at hemp flowering and maturity differ in chemical composition.•Colorimetry and ATR-FTIR analyses show dynamic of ...biofilm formation on hemp stem surface.•Fiber bundles decohesion is faster for the flowering-harvested stems.
Dew retting, a selective microbial degradation of industrial hemp (Cannabis sativa L.) stems occurring after harvest under local climatic conditions, is an important field process for plant fibre uses. We investigated how the crop harvest time, which depends on the hemp valorization strategy envisaged by the farmer, affected the initial stem quality and the subsequent microbial degradation dynamics of retting. We used simulated rains and dews to carry out retting under laboratory conditions for 42 days at 15 °C with hemp stems harvested at different times (flowering and seed maturity stage). The microbial colonization and changes in the bast tissue architecture were tracked from day 0 to day 42 using colorimetry, surface infrared spectroscopy, chemical analysis, and scanning electron microscopy. The early changes in the microbial colonization and color of the stem surface and the degradation of the bast tissue parenchyma followed the same pattern during retting for the two stem qualities. However, the kinetic of these processes was 7–14 days faster for the flowering stems, which had higher initial soluble and lower lignin contents than the mature stems. The results suggest a promising potential use of colorimetry and surface infrared spectroscopy data as candidate indicators for the dew retting progress in the field. Besides the results of this study indicate that differences in hemp stem chemical composition due to different crop harvest time significantly impacted microbial colonization, biofilm formation and eventually dew retting duration.
Crop residues may serve as a significant source of soil emissions of N2O and other trace gases. According to the emission factors (EFs) set by the Intergovernmental Panel on Climate Change (IPCC), ...N2O emission is proportional to the amount of N added by residues to the soil. However, the effects of crop residues on the source and sink strength of agroecosystems for trace gases are regulated by their properties, such as the C and N content; C/N ratio; lignin, cellulose, and soluble fractions; and residue humidity. In the present study, an automated dynamic chamber method was used in combination with soil mesocosms to simultaneously measure the effects of nine different crop residues (oilseed rape, winter wheat, field pea, maize, potato, mustard, red clover, sugar beet, and ryegrass) on soil respiration (CO2) and reactive N fluxes (N2O, NO, and NH3) at a high temporal resolution. Specifically, crop residues were incorporated in the 0–4 cm topsoil layer and incubated for 60 days at a constant temperature (15 °C) and water-filled pore space (60% WFPS). Residue incorporation immediately and sharply increased soil N2O and CO2 emissions, but these were short-lived and returned to background levels within respectively 10 and 30 days. The magnitude of increase in soil NO flux following residue incorporation was lower than that in CO2 and N2O fluxes, with peak emissions observed around day 20. Overall, the N content or C/N ratio of the applied residue could not sufficiently explain the variation in soil N2O and NO emissions. The range of the calculated N2O EFs over a 60-day period was −0.17 to +4.5, being wider than that proposed by the IPCC (+0.01 to +1.1). Therefore, the residue maturity stage may be used as a simple proxy to estimate the N2O + NO emissions from incorporated residue.
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•Simultaneous quantification of N losses (N2O, NO, NH3) during residue decomposition.•Range of residue N emission factor for N2O (EFN2O) exceeds IPCC value.•Total residue N or residue C/N ratio does not explain EFN2O variability.•Residue properties (e.g., soluble, lignin) control potential N trace gas emissions.•N trace gas emissions decreased as physiological stage of residues increased.
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