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  • The efficiency of organic C sequestration in deep soils is enhanced by drier climates

    Accurate assessment of organic C sequestration in deep soils is crucial to C management and understand the role of deep-rooted vegetation in the C cycle. Trees in drylands usually develop roots to access deep water resources. Deep soils typically contain large stores of sequestrated C because the microbial activities that decompose C are limited and C turnover time is long. However, we know little about whether root water uptake can benefit organic C sequestration in deep soils and the effect of precipitation on organic C sequestration. To address this, we selected five sites along a precipitation gradient from 422 mm to 606 mm on China’s Loess Plateau, and collected soil samples down to 1000 cm to measure soil organic C (SOC) content and soil water content (SWC) in both apple orchards and arable lands. We found that SOC storage (SOCS) and soil water storage (SWS) of two vegetation types in 0–800 cm soil layers increased significantly with increasing mean annual precipitation (MAP). Apple orchards showed greater SOC sequestration, particularly in deep soils (200–1000 cm), across each precipitation gradient relative to the corresponding arable lands. The ΔSOCS (difference in SOCS between apple orchards and the corresponding arable lands) in deep soils gradually decreased as MAP increased, and ΔSOCS for MAP = 422 mm was almost twice as great as that for MAP = 606 mm. Moreover, the ratio of ΔSOCS/ΔSWS in deep soils significantly increased as MAP decreased in the interval 400–610 mm. This indicates that the efficiency of SOC sequestration in deep soils is enhanced in a drier climate. The findings here indicate that deep soils may contribute greatly to organic C sequestration, and may provide insights into the water-C relationships in deep soils.
  • Is it possible to attain the same soil organic matter content in arable agricultural soils as under natural vegetation?

    Clearing natural vegetation to establish arable agriculture (cropland) almost invariably causes a loss of soil organic carbon (SOC). Is it possible to restore soil that continues in arable agriculture to the pre-clearance SOC level through modified management practices? To address this question we reviewed evidence from long-term experiments at Rothamsted Research, UK, Bad Lauchstädt, Germany, Sanborn Field, USA and Brazil and both experiments and surveys of farmers’ fields in Ethiopia, Australia, Zimbabwe, UK and Chile. In most cases SOC content in soil under arable cropping was in the range 38–67% of pre-clearance values. Returning crop residues, adding manures or including periods of pasture within arable rotations increased this, often to 60–70% of initial values. Under tropical climatic conditions SOC loss after clearance was particularly rapid, e.g. a loss of >50% in less than 10 years in smallholder farmers’ fields in Zimbabwe. If larger yielding crops were grown, using fertilizers, and maize stover returned instead of being grazed by cattle, the loss was reduced. An important exception to the general trend of SOC loss after clearance was clearing Cerrado vegetation on highly weathered acidic soils in Brazil and conversion to cropping with maize and soybean. Other exceptions were unrealistically large annual applications of manure and including long periods of pasture in a highly SOC-retentive volcanic soil. Also, introducing irrigated agriculture in a low rainfall region can increase SOC beyond the natural value due to increased plant biomass production. For reasons of sustainability and soil health it is important to maintain SOC as high as practically possible in arable soils, but we conclude that in the vast majority of situations it is unrealistic to expect to maintain pre-clearance values. To maintain global SOC stocks at we consider it is more important to reduce current rates of land clearance and sustainably produce necessary food on existing agricultural land.
  • Advancing the mechanistic understanding of the priming effect on soil organic matter mineralisation

    The priming effect (PE) is a key mechanism contributing to the carbon balance of the soil ecosystem. Almost 100 years of research since its discovery in 1926 have led to a rich body of scientific publications to identify the drivers and mechanisms involved. A few review articles have summarised the acquired knowledge; the last major one was published in 2010. Since then, knowledge on the soil microbial communities involved in PE and in PE + C sequestration mechanisms has been considerably renewed. This article reviews current knowledge on soil PE to state to what extent new insights may improve our ability to understand and predict the evolution of soil C stocks. We propose a framework to unify the different concepts and terms that have emerged from the international scientific community on this topic, report recent discoveries and identify key research needs. Seventy per cent of the studies on the soil PE were published in the last 10 years, illustrating a renewed interest for PE, probably linked to the increased concern about the importance of soil carbon for climate change and food security issues. Among all the drivers and mechanisms proposed along with the different studies to explain PE, some are named differently but actually refer to the same object. This overall introduces ‘artificial’ complexity for the mechanistic understanding of PE, and we propose a common, shared terminology. Despite the remaining knowledge gaps, consistent progress has been achieved to decipher the abiotic mechanisms underlying PE, together with the role of enzymes and the identity of the microbial actors involved. However, including PE into mechanistic models of SOM dynamics remains challenging as long as the mechanisms are not fully understood. In the meantime, empirical alternatives are available that reproduce observations accurately when calibration is robust. Based on the current state of knowledge, we propose different scenarios depicting to what extent PE may impact ecosystem services under climate change conditions. Read the free Plain Language Summary for this article on the Journal blog.
  • Do agrosystems change soil carbon and nutrient stocks in a semiarid environment?

    Ecological processes, such as net primary production, root system development, organic matter mineralization, nutrient removal and fertilizer application interfere in gains and losses of C and nutrients (N, P, K, Ca and Mg) in soils. Herein, we studied how five rainfed livestock and four irrigated agricultural systems affected soil C and nutrient stocks in a semi-arid environment. Soil concentrations, stocks, gains and losses of the nine land-uses were compared to those of the preserved native deciduous forest (Caatinga) along the top 1 m soil layer. Open Caatinga used as pasture, gliricidia and leucaena fields maintained the stocks of most nutrients. The shallower roots of buffel grass and prickly pear led to C (7 and 18%) and N (7 and 20%, respectively) losses, and P, Ca and Mg accumulations in the deeper layers. Irrigated crops reduced soil C and N stocks. C losses in irrigated maize and beans fields (23%) were lower than in rainfed fields in the region, while those in mango fields were large (70 and 66%). Fertilization in beans and grapes increased soil P and K stocks. Knowledge of stock changes allows proper system management to reduce the negative impacts of land-use change and promote sustainable production.
  • COMIFER JT MOS 2021 7 Avril_S Recous_Couplage des cycles du carbone et de l'azote

    La Journée Technique organisée par le COMIFER le 7 avril 2021 a pour ambition de présenter un état des connaissances et des exemples concrets sur les différentes thématiques concernant les matières organiques dans les sols agricoles. Pour cela, l’animation et les interventions seront portées par des représentants de la Recherche francophone (AgroParisTech, ENS, FIBL, HEPIA, INRAE, UCL), des instituts techniques (ARVALIS, IFV, Terres Inovia), des organismes de transfert (Agro-Transfert, RITTMO), des laboratoires d’analyses (Auréa AgroSciences, CAPINOV, Celesta-Lab, LDAR, GEMAS), des conseillers terrain (Chambres d’Agriculture d’Alsace et du Rhône, SATEGE du Nord Pas-de-Calais), ainsi que l’ADEME et la DGAL/MAA. La matinée posera les bases d’une bonne compréhension des matières organiques dans les sols agricoles. La séquence introductive définira les matières organiques, leurs rôles et leur évolution, ainsi que le lien étroit avec le cycle de l’azote.
  • COMIFER JT MOS 2021 7 Avril_A Duparque_H Clivot_F Ferchaud: Modélisation de l'évolution du C organiq

    La Journée Technique organisée par le COMIFER le 7 avril 2021 a pour ambition de présenter un état des connaissances et des exemples concrets sur les différentes thématiques concernant les matières organiques dans les sols agricoles. Pour cela, l’animation et les interventions seront portées par des représentants de la Recherche francophone (AgroParisTech, ENS, FIBL, HEPIA, INRAE, UCL), des instituts techniques (ARVALIS, IFV, Terres Inovia), des organismes de transfert (Agro-Transfert, RITTMO), des laboratoires d’analyses (Auréa AgroSciences, CAPINOV, Celesta-Lab, LDAR, GEMAS), des conseillers terrain (Chambres d’Agriculture d’Alsace et du Rhône, SATEGE du Nord Pas-de-Calais), ainsi que l’ADEME et la DGAL/MAA. La matinée posera les bases d’une bonne compréhension des matières organiques dans les sols agricoles. La séquence introductive définira les matières organiques, leurs rôles et leur évolution, ainsi que le lien étroit avec le cycle de l’azote. La deuxième séquence abordera les méthodes de mesure de ces matières organiques des sols (MOS), l’état des lieux du statut organique des sols français et luxembourgeois, et les interprétations possibles de ces niveaux de MOS, en discutant de la pertinence d’informations complémentaires issues de la caractérisation biologique des sols. La fin de la matinée portera sur la dynamique des MOS à moyen et long terme, avec présentation d’outils disponibles pour prédire cette évolution et illustration de l’impact des pratiques agricoles, à l’échelle de la parcelle mais également du territoire français.
  • A well-established fact: rapid mineralization of organic inputs is an important factor for soil carbon sequestration

    We have read with interest an opinion paper recently published in the European Journal of Soil Science (Berthelin et al., 2022). This paper presents some interesting considerations, at least one of which is already well known to soil scientists working on soil organic carbon (SOC), i.e. a large portion (80-90%) of fresh carbon inputs to soil is subject to rapid mineralization. The short-term mineralization kinetics of organic inputs are well-known and accounted for in soil organic matter models. Thus, clearly, the long-term predictions based on these models do not overlook short-term mineralization. We point out that many agronomic practices can significantly contribute to SOC sequestration. If conducted responsibly whilst fully recognizing the caveats, SOC sequestration can lead to a win-win situation where agriculture can both contribute to the mitigation of climate change and adapt to it, whilst at the same time delivering other co-benefits such as reduced soil erosion and enhanced biodiversity.
  • The Science and Semantics of “Soil Organic Matter Stabilization”

    It is a shared goal of the scientific community to improve model projections of soil organic carbon dynamics such that the carbon cycle is represented as accurately as possible in global land models. For several decades, those pursuing this objective have operated under the assumption that “the longer soil carbon persists, the more stable we regard it as being.” While this understanding was helpful in facilitating conversations on the topic, it has created the notion of the existence of a chemically and physically definable organic phase in soil with the ability to “resist” decomposition. Here we argue that “stability” is a conceptual and not a physically existing state of organic matter. To this end, we analyze the semantics of “persistence” and “stability” to point out that (i) they are independent concepts and not interchangeable terms and (ii) that “persistence” is not an automatic consequence of “stability.” Through extensive revision of the literature, we show that slow-cycling carbon does not do so because it has the material property of being “stable,” rather, it persists because it is not being decomposed. It follows that the notion of soil organic matter stabilization is a flawed concept that distracts from the actual causes for slow carbon cycling. We suggest that the question to be asked in future research should no longer be “Why is soil carbon stable?.” Rather, the question should be: What are the constraints that prevent the decomposer community from processing soil carbon to their full metabolic potential?
  • Soil carbon sequestration for climate change mitigation: Mineralization kinetics of organic inputs as an overlooked limitation

    Over the last few years, the question of whether soil carbon sequestration could contribute significantly to climate change mitigation has been the object of numerous debates. All of these debates so far appear to have entirely overlooked a crucial aspect of the question. It concerns the short-term mineralization kinetics of fresh organic matter added to soils, which is occasionally alluded to in the literature, but is almost always subsumed in a broader modelling context. In the present article, we first summarise what is currently known about the kinetics of mineralization of plant residues added to soils, and about its modelling in the long run. We then argue that in the short run, this microbially-mediated process has important practical consequences that cannot be ignored. Specifically, since at least 90% of plant residues added to soils to increase their carbon content over the long term are mineralized relatively rapidly and are released as CO2 to the atmosphere, farmers would have to apply to their fields 10 times more organic carbon annually than what they would eventually expect to sequester. Over time, because of a well-known sink saturation effect, the multiplier may even rise significantly above 10, up to a point when no net carbon sequestration takes place any longer. The requirement to add many times more carbon than what one aims to sequester makes it practically impossible to add sufficient amounts of crop residues to soils to have a lasting, non-negligible effect on climate change. Nevertheless, there is no doubt that raising the organic matter content of soils is desirable for other reasons, in particular guaranteeing that soils will be able to keep fulfilling essential functions and services in spite of fast-changing environmental conditions. Highlights Attempts to promote soil carbon sequestration to mitigate climate change have so far ignored the short-term effects of the mineralization of plant residues added to soils. Only about 10%, at most, of added plan residues remain in soils after mineralization by soil organisms. To have a significant effect on climate change, farmers would need to add impractically large amounts of plant residues, requiring unrealistic nitrogen inputs. Therefore, rather than as a mitigation strategy, farmers should aim to increase the carbon content of soils to make them resilient to climate change.
  • Conservation agriculture practices drive maize yield by regulating soil nutrient availability, arbuscular mycorrhizas, and plant nutrient uptake

    Conservation agriculture (CA) can sustainably increase crop productivity through improved soil chemical, physical, and biological properties, among others. However, the implementation of all its three main components (i.e., no-tillage, organic soil cover/mulch, and crop diversification) in southern Africa is often challenging, resulting in variable yield responses. Disentangling the contributions of CA practices is necessary to understand the drivers of maize grain yield within the region. Here we analysed two 6-year long component omission experiments, one at a sandy soil location and the other at a clay soil location. In these two experiments, soil chemical parameters, total plant nutrient uptake, rate of crop residue decomposition, and arbuscular mycorrhizal fungi (AMF) colonization of maize roots were assessed. Soil chemical properties only differed across systems at the sandy soil location with the mulched systems under no-tillage (NT) resulting in increased soil organic carbon levels, total nitrogen, and soil available phosphorus as compared to conventional tillage with no mulch or rotation (CT). Conventional tillage-based systems resulted in fastest decomposition of maize residues, while systems with NT and rotation resulted in highest AM fungal root colonization rate of maize at the clay soil location. Total plant N uptake was almost 2-fold higher in tilled and no-tilled systems with both mulch (M) and rotations (R) (i.e., NT+M+R and CT+M+R) as compared to CT. Structural equation modeling was used to disentangle the links between cropping systems, soil chemical and biological properties, plant nutrient uptake, and maize grain yield. Cropping systems had direct and indirect influences on yield at both locations. At both locations, cropping systems influenced yield via plant N uptake, with the NT+M+R and CT+M+R systems having more beneficial effects compared to other systems, as shown by their higher path coefficients. In conclusion, we recommend a more holistic approach to cropping system assessment that includes a higher number of abiotic and biotic determinants. This would allow for a more rigorous evaluation of the drivers of yield and increase our understanding of the effects and performance of practices under the prevailing agro-ecological conditions.
  • Root litter decomposition in a sub-Sahelian agroforestry parkland dominated by Faidherbia albida

    In agroforestry systems, fine roots grow at several depths due to the mixture of trees and annual crops. The decomposition of fine roots contributes to soil organic carbon stocks and may impact soil fertility, particularly in poor soils, such as those encountered in sub-Sahelian regions. The aim of our study was to measure the decomposition rate of root litter from annual and perennial species according to soil depth and location under and far from trees in a sub-Sahelian agroforestry parkland. Soil characteristics under and far from the trees were analysed from topsoil to 200 cm depth. Faidherbia tree, pearl millet and cowpea root litter samples were buried in litterbags for 15 months at 20, 40, 90 and 180 cm depths. Root litter decomposition was mainly impacted by soil moisture and soil depth. Faidherbia decomposed more slowly (36 ± 12% remaining mass after 15 months) than cowpea and pearl millet roots (23 ± 7% and 29 ± 11% respectively). Pearl millet aboveground biomass, at harvesting time, was twice as high under (992 g m−2) than far (433 g m−2) from the tree, and belowground biomass (0–200 cm of depth) was 30.9 g m−2 and 19.6 g m−2 under and far from the tree, respectively. Faidherbia fine roots contributed slightly (p-value < 0.1) to higher stocks of C under the tree (7761 ± 346 g m−2) than far from it (5425 ± 558 g m−2) and from 0 cm down to 200 cm depth.
  • Carbon sequestration capacity in no-till soil decreases in the long-term due to saturation of fine silt plus clay-size fraction

    The capacity of soils to stabilize carbon (C) may decrease over time, limiting the potential of no-till soil to act as a C sink in the long-term. Our objectives were to evaluate the effects of long-term no-till cropping systems on (i) C storage in soil, (ii) C stabilization in the fine silt plus clay-size (<20 μm) fraction and its relationship with the decrease of C saturation deficit (CSD) in this fraction, and (iii) on C accumulation in labile fractions of soil organic matter (SOM) in 0–2.5, 2.5–5, 5–10 and 10–20 cm layers of a subtropical Acrisol. The study was based on a long-term (36 years) no-till experiment where five cropping systems, with variable annual C inputs, were assessed: [i] bare-soil, [ii] black oat/maize, [iii] black oat + vetch/maize + cowpea, [iv] lablab + maize and [v] pigeon pea + maize. Cropping systems including maize and tropical legumes (lablab, pigeon pea and cowpea) with high C input led to the highest C storage in the top layers (up to 10 cm depth) of this no-till soil. Also, a decrease of CSD in fine silt plus clay-size fraction was observed in all soil layers to 20 cm depth, but the most expressive impact on CSD occurred in the topsoil (0–2.5 cm), where the capacity to further stabilize more carbon decreased by 90–97% when compared to bare soil. Considering the full C saturation level of the silt plus clay-size fraction and the current C contents in the soil, the remaining capacity of C sequestration up to 20 cm was estimated as ranging from 22.5 to 32.8 Mg C ha−1, and much of it (58–75%) was in the 10–20 cm layer. Our results highlight the importance of diversified cropping systems with high input (quantity and quality) crop residues to C sequestration in soil. Moreover, although the mineral-associated SOM of the top layer reached a C stabilization limit, C accumulation continues in non-saturated labile fractions, and in non-saturated fine silt plus clay-size fraction in deeper layers of subtropical no-till soils.
  • Will fungi solve the carbon dilemma?

    Soils are hotspots of diversity and sustain many globally important functions. Here we focus on the most burning issue: how to keep soils as carbon sinks while maintaining their productivity. Evidence shows that life in soils plays a crucial role in improving soil health yet soil ecological processes are often ignored in soil sciences. In this review, we highlight the potential of fungi to increase soil carbon sequestration while maintaining crop yield, functions needed to sustain human population on Earth and at same time keep the Earth livable. We propose management strategies that steer towards more fungal activity but also high functional diversity of fungi which will lead to more stable carbon sources in soil but also affects the structure of the soil food web up to ecosystem level. We list knowledge gaps that limit our ability to steer soil fungal communities such that stabilising carbon in top soils becomes more effective. Using the natural capacity of a biodiverse soil community to sequester carbon delivers double benefit: reduction of atmospheric carbon dioxide by storing photosynthesized carbon in soil and increasing agricultural yields by restoring organic matter content of degraded soils.
  • Estimates of carbon stocks in sandy soils cultivated under local management practices in Senegal’s groundnut basin

    Soil organic carbon (SOC) is essential for the productivity of agroecosystems and for mitigating climate change. Because the SOC contents of sandy soils are usually small, the effects of agricultural management upon SOC stocks in such soils have been insufficiently studied. In West sub-arid Africa, the coarse-textured soils (mostly Arenosols) are diversely managed by smallholders. In this study, we aimed to quantify SOC stocks in cultivated soils of that region, in a context where agricultural practices rely mainly upon organic inputs derived from various integrated crop-livestock systems. SOC stocks were estimated for the 0–30 cm depth in 1,813 plots in Senegal’s groundnut basin. We found that SOC stocks in farmers’ fields varied between 2.3 and 59.8 Mg C ha-1 (mean ± standard deviation, 14.6 ± 0.14 Mg C ha-1). SOC stocks were influenced slightly by soil type, but were only weakly correlated to soils’ clay and silt contents. SOC stocks differed significantly among the three studied village territories due to contrasting livestock-raising systems. Average stocks were significantly higher in plots close to housings (home-fields), which receive larger amounts of organic inputs, than in plots farther from the village (out-fields). Thus, the organic inputs to home-fields improves soil C stocks of these sandy soils in the short term. Innovative agricultural practices in the studied area probably need to target options for managing all fields optimally. Those options will require continuous application of organic products—a measure that will in turn require solutions for improving availability or management of local organic resources.
  • Simulating soil organic carbon in maize-based systems under improved agronomic management in Western Kenya

    Improved management practices should be implemented in croplands in sub-Saharan Africa to enhance soil organic carbon (SOC) storage and/or reduce losses associated with land-use change, thereby addressing the challenge of ongoing soil degradation. DayCent, a process-based biogeochemical model, provides a useful tool for evaluating which management practices are most effective for SOC sequestration. Here, we used the DayCent model to simulate SOC using experimental data from two long-term field sites in western Kenya comprising of two widely promoted sustainable agricultural management practices: integrated nutrient management (i.e. mineral fertilizer and crop residues/farmyard manure incorporation) and conservation agriculture (i.e. minimum tillage and crop residue retention). At both sites, correlations between measured and simulated SOC were low to moderate (R2 of 0.25−0.55), and in most cases, the model produced fairly accurate prediction of the SOC trends with a low relative root mean squared error (RRMSE < 7%). Consistent with field measurements, simulated SOC declined under all improved management practices. The model projected annual SOC loss rates of between 0.32 to 0.35 Mg C ha-1 yr-1 in continuously tilled maize (Zea mays) systems without fertilizer or organic matter application over the period 2003–2050. The most effective practices in reducing the losses were the combined application of 4 Mg ha-1 of farmyard manure and 2 Mg ha-1 of maize residue retention (reducing losses up to 0.22 Mg C ha-1 yr-1), minimum tillage in combination with maize residue retention (0.21 Mg C ha-1 yr-1), and rotation of maize with soybean (Glycine max) under minimum tillage (0.17 Mg C ha-1 yr-1). Model results suggest that response of the passive SOC pool to the different management practices is a key driver of the long-term SOC trends at the two study sites. This study demonstrates the strength of the DayCent model in simulating SOC in maize systems under different agronomic management practices that are typical for western Kenya.
  • Reducing losses but failing to sequester carbon in soils – the case of Conservation Agriculture and Integrated Soil Fertility Management in the humid tropical agro-ecosystem of Western Kenya

    Agriculture is a global contributor to greenhouse gas emissions, causing climate change. Soil organic carbon (SOC) sequestration is seen as a pathway to climate change mitigation. But, long-term data on the actual contribution of tropical soils to SOC sequestration are largely absent. To contribute to filling this knowledge gap, we measured SOC in the top 15cm over 12 years in two agronomic long-term trials in Western Kenya. These trials include various levels – from absence to full adoption – of two widely promoted sustainable agricultural management practices: Integrated Soil Fertility Management (ISFM; i.e. improved varieties, mineral fertilizer and organic matter/manure incorporation) and Conservation Agriculture (CA; improved varieties, mineral fertilizer, zero-tillage and crop residues retention). None of the tested ISFM and CA treatments turned out successful in sequestering SOC long-term. Instead, SOC decreased significantly over time in the vast majority of treatments. Expressed as annual averages, losses ranged between 0.11 and 0.37tCha−1 yr−1 in the CA long-term trial and 0.21 and 0.96tCha−1 yr−1 in the ISFM long-term trial. Long-term application of mineral N and P fertilizer did not mitigate SOC losses in both trials. Adopting zero-tillage and residue retention alone (as part of CA) could avoid SOC losses of on average 0.13tCha−1 yr−1, while this was 0.26tCha−1 yr−1 in response to mere inclusion of manure as part of ISFM. However, cross-site comparison disclosed that initial SOC levels of the two trials were different, probably as a result of varying land use history. Such initial soil status was responsible for the bulk of the SOC losses and less so the various tested agronomic management practices. This means, while ISFM and CA in the humid tropical agro-ecosystem of Western Kenya contribute to climate change mitigation by reducing SOC losses, they do not help offsetting anthropogenic greenhouse gas emissions elsewhere.
  • Particulate organic matter as a functional soil component for persistent soil organic carbon

    The largest terrestrial organic carbon pool, carbon in soils, is regulated by an intricate connection between plant carbon inputs, microbial activity, and the soil matrix. This is manifested by how microorganisms, the key players in transforming plant-derived carbon into soil organic carbon, are controlled by the physical arrangement of organic and inorganic soil particles. Here we conduct an incubation of isotopically labelled litter to study effects of soil structure on the fate of litter-derived organic matter. While microbial activity and fungal growth is enhanced in the coarser-textured soil, we show that occlusion of organic matter into aggregates and formation of organo-mineral associations occur concurrently on fresh litter surfaces regardless of soil structure. These two mechanisms—the two most prominent processes contributing to the persistence of organic matter—occur directly at plant–soil interfaces, where surfaces of litter constitute a nucleus in the build-up of soil carbon persistence. We extend the notion of plant litter, i.e., particulate organic matter, from solely an easily available and labile carbon substrate, to a functional component at which persistence of soil carbon is directly determined.
  • Particulate organic matter as a functional soil component for persistent soil organic carbon

    The largest terrestrial organic carbon pool, carbon in soils, is regulated by an intricate connection between plant carbon inputs, microbial activity, and the soil matrix. This is manifested by how microorganisms, the key players in transforming plant-derived carbon into soil organic carbon, are controlled by the physical arrangement of organic and inorganic soil particles. Here we conduct an incubation of isotopically labelled litter to study effects of soil structure on the fate of litter-derived organic matter. While microbial activity and fungal growth is enhanced in the coarser-textured soil, we show that occlusion of organic matter into aggregates and formation of organo-mineral associations occur concurrently on fresh litter surfaces regardless of soil structure. These two mechanisms—the two most prominent processes contributing to the persistence of organic matter—occur directly at plant–soil interfaces, where surfaces of litter constitute a nucleus in the build-up of soil carbon persistence. We extend the notion of plant litter, i.e., particulate organic matter, from solely an easily available and labile carbon substrate, to a functional component at which persistence of soil carbon is directly determined.
  • Roots are key to increasing the mean residence time of organic carbon entering temperate agricultural soils

    The ratio of soil organic carbon stock (SOC) to annual carbon input gives an estimate of the mean residence time of organic carbon that enters the soil (MRTOC). It indicates how efficiently biomass can be transformed into SOC, which is of particular relevance for mitigating climate change by means of SOC storage. There have been few comprehensive studies of MRTOC and their drivers, and these have mainly been restricted to the global scale, on which climatic drivers dominate. This study used the unique combination of regional-scale cropland and grassland topsoil (0–30 cm) SOC stock data and average site-specific OC input data derived from the German Agricultural Soil Inventory to elucidate the main drivers of MRTOC. Explanatory variables related to OC input composition and other soil-forming factors were used to explain the variability in MRTOC by means of a machine-learning approach. On average, OC entering German agricultural topsoils had an MRT of 21.5 ± 11.6 years, with grasslands (29.0 ± 11.2 years, n = 465) having significantly higher MRTOC than croplands (19.4 ± 10.7, n = 1635). This was explained by the higher proportion of root-derived OC inputs in grassland soils, which was the most important variable for explaining MRTOC variability at a regional scale. Soil properties such as clay content, soil group, C:N ratio and groundwater level were also important, indicating that MRTOC is driven by a combination of site properties and OC input composition. However, the great importance of root-derived OC inputs indicated that MRTOC can be actively managed, with maximization of root biomass input to the soil being a straightforward means to extend the time that assimilated C remains in the soil and consequently also increase SOC stocks.
  • Carbon allocation to the rhizosphere is affected by drought and nitrogen addition

    Photosynthetic carbon (C) allocated below-ground can be shared with mycorrhizal fungi in exchange for nutrients, but also added into soil as rhizodeposits that potentially increases plant nutrient supply by supporting microbial nutrient mineralization from organic matter. How water and nitrogen (N) availability affects plant C allocation to the rhizosphere, including both arbuscular mycorrhizal fungi (AMF) symbionts and rhizodeposits, remains largely unknown. We used a 13CO2 pulse labelling experiment to assess the effects of drought and N addition on below-ground allocation of C to soils and roots (quantified as excess 13C) and tested their relationships with AMF colonization in an Australian grassland. We also examined relationships between AMF and previously reported root respiration and decomposition of rhizodeposits in this study. We found that drought decreased the absolute amount of excess 13C allocated to both soils and roots, likely due to less photosynthetic C fixation. In contrast, proportionally more excess 13C was allocated to soils but less to root biomass with drought, suggesting that relatively more C was allocated to rhizodeposits and to AMF hyphal growth and extension. However, N addition reversed drought effects on below-ground C allocation by retaining proportionally more excess 13C in roots and less in soils, congruent with higher soil N and phosphorus availability, root biomass and number of root tips compared to drought without N addition. This suggests that the alleviation of nutrient limitation promoted plants to expend relatively more C on root growth and root trait adjustment, but less C on rhizodeposition and mycorrhizal symbiosis. Synthesis. Mycorrhizal colonization related negatively to rhizodeposit decomposition rate but positively to both excess 13C in root biomass and root respiration, suggesting a possible trade-off in C allocation between mycorrhizal symbiosis and rhizodeposition. We conclude that below-ground C allocation in this grassland can be mediated by mycorrhizal colonization and is strongly affected by water and nutrient availability.
  • Chromolaena odorata (L.) K&R (Asteraceae) invasion effects on soil microbial biomass and activities in a forest-savanna mosaic

    Plant invasion may have significant ecological and socio-economic impacts across agroecologies. Chromolaena odorata (Asteraceae) is one of the world’s most invasive plants albeit it is considered a suitable fallow plant in West Africa. However, its impacts on soil biological processes are poorly understood. This study was conducted in intermingled forest and savanna sites invaded by C. odorata in Central Côte d’Ivoire (West Africa) to bridge this knowledge gap. Invaded forest sites (COFOR) were compared to adjacent natural forest fragments (FOR) while invaded savanna sites (COSAV) were compared to adjacent natural savanna fragments (SAV). Soil (0–10 cm depth) physico-chemical variables, including soil organic C (SOC), total soil N and available N and P concentrations were measured. Additionally, soil microbial biomass (MBC), carbon mineralization (Cmin), acid phosphatase, β-glucosidase, and fluorescein diacetate were measured. Further, the MBC/SOC ratio and the metabolic quotient (qCO2) were calculated. An index of invasion effect (IE) computed as the cumulative percent change in the microbial and enzyme activities was determined for each ecosystem context. Results showed that soil MBC and MBC/SOC ratio declined in COFOR relative to FOR. In general, Cmin, enzymatic activities, qCO2 and available N and P significantly increased in the C. odorata sites relative to the respective reference ecosystems, particularly savanna, potentially due to a larger gap in the litters’ quality. As a result, the invasion effect was twice as high in savanna (IE = 292.8%) as in forest (IE = 147.5%). However, a Principal Component Analysis showed that the COSAV were close to COFOR stands without mixing, probably due to contrasting initial soil organic matter and clay contents. These results improved our knowledge on the changes in soil microbial attributes and the mechanisms of soil fertility restoration or improvement in response to C. odorata invasion in natural forests and savannas of West Africa.
  • Soil organic carbon sequestration in temperate agroforestry systems – A meta-analysis

    Soil organic carbon (SOC) sequestration by improved agricultural practices is an acclaimed strategy to combat climate change. Nevertheless, the aim of increasing of SOC encounters limitations, e.g. with regards to permanence of carbon storage or leakage effects in food production. Agroforestry systems (AFS) are a promising land use option that is able to sequester substantial amounts of SOC while addressing these challenges. With a focus on temperate climate zones worldwide, available information on SOC in AFS was reviewed to determine their SOC sequestration potential and respective controlling factors. From a total of 61 observations, SOC sequestration rates in soils of AFS were derived for alley cropping systems (n = 25), hedgerows (n = 26) and silvopastoral systems (n = 10). The results showed that AFS have a potential for substantial SOC sequestration in temperate climates. SOC stocks were higher in the topsoil (0–20 cm) than in the control in more than 70% of the observations, and higher within the subsoil (20–40 cm) for 81% of all observations, albeit large variation in the data. The mean SOC sequestration rates were slightly higher at 0–20 cm (0.21 ± 0.79 t ha-1 yr-1) compared to 20–40 cm soil depth (0.15 ± 0.26 t ha-1 yr-1). Hedgerows revealed highest SOC sequestration rates in topsoils and subsoils (0.32 ± 0.26 and 0.28 ± 0.15 t ha-1 yr-1, respectively), followed by alley cropping systems (0.26 ± 1.15 and 0.23 ± 0.25 t ha-1 yr-1) and silvopastoral systems showing a slight mean SOC loss (−0.17 ± 0.50 and −0.03 ± 0.26 t ha-1 yr-1). Moreover, SOC sequestration rates tended to be higher for AFS with broadleaf tree species compared to coniferous species. We conclude that temperate AFS sequester significant amounts of SOC in topsoils and subsoils and represent one of the most promising agricultural measures for climate change mitigation and adaption.
  • Soil organic carbon sequestration in temperate agroforestry systems – A meta-analysis

    Soil organic carbon (SOC) sequestration by improved agricultural practices is an acclaimed strategy to combat climate change. Nevertheless, the aim of increasing of SOC encounters limitations, e.g. with regards to permanence of carbon storage or leakage effects in food production. Agroforestry systems (AFS) are a promising land use option that is able to sequester substantial amounts of SOC while addressing these challenges. With a focus on temperate climate zones worldwide, available information on SOC in AFS was reviewed to determine their SOC sequestration potential and respective controlling factors. From a total of 61 observations, SOC sequestration rates in soils of AFS were derived for alley cropping systems (n = 25), hedgerows (n = 26) and silvopastoral systems (n = 10). The results showed that AFS have a potential for substantial SOC sequestration in temperate climates. SOC stocks were higher in the topsoil (0–20 cm) than in the control in more than 70% of the observations, and higher within the subsoil (20–40 cm) for 81% of all observations, albeit large variation in the data. The mean SOC sequestration rates were slightly higher at 0–20 cm (0.21 ± 0.79 t ha-1 yr-1) compared to 20–40 cm soil depth (0.15 ± 0.26 t ha-1 yr-1). Hedgerows revealed highest SOC sequestration rates in topsoils and subsoils (0.32 ± 0.26 and 0.28 ± 0.15 t ha-1 yr-1, respectively), followed by alley cropping systems (0.26 ± 1.15 and 0.23 ± 0.25 t ha-1 yr-1) and silvopastoral systems showing a slight mean SOC loss (−0.17 ± 0.50 and −0.03 ± 0.26 t ha-1 yr-1). Moreover, SOC sequestration rates tended to be higher for AFS with broadleaf tree species compared to coniferous species. We conclude that temperate AFS sequester significant amounts of SOC in topsoils and subsoils and represent one of the most promising agricultural measures for climate change mitigation and adaption.