Publications

Plant roots are one of the major mediators that allocate carbon captured from the atmosphere to soils as rhizodeposits, including root exudates. Although rhizodeposition regulates both microbial activity and the biogeochemical cycling of nutrients, the effects of particular exudate species on soil carbon fluxes and key rhizosphere microorganisms remain unclear. By combining high-throughput sequencing, q-PCR, and NanoSIMS analyses, we characterized the bacterial community structure, quantified total bacteria depending on root exudate chemistry, and analyzed the consequences on the mobility of mineral-protected carbon. Using well-controlled incubation experiments, we showed that the three most abundant groups of root exudates (amino acids, carboxylic acids, and sugars) have contrasting effects on the release of dissolved organic carbon (DOC) and bioavailable Fe in an Ultisol through the disruption of organo-mineral associations and the alteration of bacterial communities, thus priming organic matter decomposition in the rhizosphere. High resolution (down to 50 nm) NanoSIMS images of mineral particles indicated that iron and silicon co-localized significantly more organic carbon following amino acid inputs than treatments without exudates or with carboxylic acids. The application of sugar strongly reduced microbial diversity without impacting soil carbon mobilization. Carboxylic acids increased the prevalence of

EDITORIAL article Front. Microbiol., 30 October 2023Sec. Terrestrial Microbiology Volume 14 - 2023 | https://doi.org/10.3389/fmicb.2023.1295624

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

The equilibrium between energy persistence and microbial accessibility of soil organic matter (SOM) determines its contribution to climate mitigation and soil health. Labile SOM maintains the activity of soil microorganisms, which provide nutrients for crops and increase yields. Stable SOM is crucial to long-term carbon (C) storage and sequestration in soils. No-till and subsoiling combined with straw return increase SOM content and stocks, yet their specific impacts on the thermodynamic equilibrium between SOM stability and availability remain unclear. We studied the effects of 22 years of no-till and subsoiling with straw return on SOM thermal stability in loamy Calcaric Cambisols. SOM thermal stability was assessed based on the balance between the energy density (ED) in labile, stable, persistent, and refractory SOM and their activation energy (E_a) for thermal oxidation. The energy properties of SOM were evaluated via differential scanning calorimetry and thermogravimetry of soils from six treatments (3 tillage practices (no-till, subsoiling, and conventional tillage as a control) with 2 straw management practices (straw return and straw removal) at 0-12 cm, 12-30 cm and 30-35 cm depths. Compared with conventional tillage, long-term no-till and subsoiling increased the content and thermal stability of SOM, with these effects being amplified by straw return. Conservation tillage with straw return contrastingly affects SOM pools: 1) E_a of labile SOM decreased by 5 %, increasing energy availability for microorganisms; and 2) E_a of recalcitrant SOM increased by 10 %, which was sixfold higher E_a than thermally labile SOM. Subsoiling preferentially raised the content of energy-rich SOM pools, whereas no-till demonstrated superior efficacy in thermodynamic stabilization of SOM compared to subsoiling. Conservation tillage with straw return increased both highly stable organic C and energy-rich available C. Concluding, conservation tillage (no-till and subsoiling) increased content of available energy in soil via contribution to labile SOM, and intensified C stabilization through increased energetic constraints of persistent SOM. This makes conservation tillage with straw return a promizing strategy for climate-smart soil management.

Terrestrial ecosystem respiration (Re) is a crucial component of the carbon cycle and is expected to increase with anthropogenic warming. The temperature response of Re is typically parameterized using temperature sensitivity Q10, which describes the increase in respiration with a 10oC rise in temperature. The respiration increase largely determines the future direction of the terrestrial-atmosphere carbon balance. However, our current understanding of the mechanisms driving Q10 variation across latitudes and biomes is still insufficient. As a result, it remains difficult to constrain predictions of future Re dynamics. The Michaelis–Menten (MM) kinetics , developed to describe enzyme-catalyzed reactions, is a cornerstone to understand biochemical processes at the cellular and molecular levels. This model effectively captures the relationship between substrate concentration and reaction rates, simplifying complex biochemical interactions into manageable mathematical expressions using the key parameters Vmax (maximum reaction rate) and Km (substrate concentration at half the maximum rate). However, the applicability of this microscopic model to large-scale ecosystem processes can be questioned . Most Earth system models incorporate the Farquhar–von Caemmerer–Berry (FvCB) biochemical model, which is grounded in Michaelis–Menten kinetics, to simulate photosynthesis at ecosystem or larger scales. However, the description of respiration processes over large ecosystem scale still predominantly relies on more empirical models, projecting an exponential temperature response with Arrhenius or Q10 types of functions.

Abstract Understanding drivers of soil organic carbon (SOC) dynamics and stocks is critical for carbon (C) neutrality in Earth’s extensive dryland croplands. Here, we analyzed 721 soil samples across northern China and revealed divergent regional dynamics of SOC stocks in topsoil (0–20 cm) over three decades. Agricultural intensification in northern China, marked by tripled productivity via strategic N-fertilization and policy-driven increases in straw return, created a major C sink (+3.1 Mg C ha −1 ), offsetting ~1.5% of China’s average annual emissions. Conversely, Northeast China experienced SOC depletion (−3.9 Mg C ha −1 ; ~12% loss since 1980s) driven by warming-enhanced decomposition of organic matter. Encouragingly, an annual straw return in excess of 270 kg C ha −1 could offset SOC losses in the face of future warming. Therefore, it is imperative to implement effective soil management practices and adaptation strategies to bolster soil resilience and health, as continued climate warming may undermine soil carbon sequestration.

No abstract is provided for this article.

Although phosphorus (P) is a limiting nutrient for plant growth in subtropical forests, the effects of forest succession on soil P dynamics, which in turn influences P availability, are unclear. The objective was to access the impacts of forest succession on P fractions of different availability (Hedley sequential fractionation) in highly weathered subtropical soils. We compared the P dynamics and availability under the chronosequence of forest succession from four stages: i) Cunninghamia lanceolata plantation, ii) through mixed broadleaf-conifer, iii) deciduous broadleaved, and finally iv) evergreen broadleaved forest. The soil P was dominated by stable P fractions (69–76%) in all successional stages. Forest succession increased total P content from plantation (199 mg kg−1) to evergreen broadleaved forest (253 mg kg−1), whereas P reached the peak in deciduous broadleaved forest and then remains stable due to balance between input with litter and litter decomposition and tree uptake. Stable P (non-available P) increased for 31–39% with forest succession because soil acidification led to more Fe and Al (oxyhydr)oxides strongly bounding P. Moderately labile P (moderately available P) contents under deciduous and evergreen broadleaved forests were higher than under plantation and mixed forest due to organic matter accumulation. However, labile (easily available) P content was reduced 35–50% by succession because of P removal by plant uptake. Available P content reached the peak under deciduous broadleaved forest (64–81 mg kg−1) and decreased again, indicating that forest ecosystem transit from P acquiring to P recycling system (litter input and plant uptake of mineralized P). Fine root biomass was the primary driver that controlled total, moderately labile and stable P contents during forest succession. Available P increased with soil organic carbon (SOC), suggesting that organic matter is crucial to maintain P availability. The C/P ratio of litter was the primary driver decreasing available P because litter decomposition released P is the main source determining P availability in soil. The increase of moderately labile P following forest succession played a crucial role for accumulation of available P. These results suggest that forest succession increases soil P availability until deciduous broadleaved forest. Therefore, strong measures to facilitate succession to the deciduous broadleaved forest stage should be a key approach to increase long-term soil P availability in subtropics.

Intensive clear cutting of natural forests and conversion to monoculture plantations are ongoing worldwide, leading to degradation of soil quality and microbial functions. Here, we compared soil quality index (SQI) and fungal community in a natural forest (Forest) and four 5-year-old monoculture plantations, including Camellia oleifera (Oil), Amygdalus persica (Peach), Myrica rubra (Berry) and Cunninghamia lanceolata (Fir), in a subtropical region of China. After conversion, soil pH rose up to 0.31, but organic carbon, total nitrogen, sucrase, acid protease, glutaminase and phosphatase activities decreased by 83%, 59%, 43%, 31%, 64%, 66% and 77%, respectively, in the plantations. Correspondingly, the SQI dropped by 65%. High-throughput sequencing of the ITS1 region demonstrated an increase in α-diversity and a striking difference in β-diversity following conversion. Changes in the dominant fungal taxa following forest conversion to plantations was interpreted by Grime’s C-S-R life history framework. Conversion increased the fungal groups with stress-tolerant (S) and ruderal (R) strategies - mainly copiotrophic saprophytes, such as Ascomycota and Zygomycota, but decreased the fungal groups with competitor (C) strategies - mainly oligotrophic saprophytes and mycorrhizal fungi, such as Basidiomycota. Genera affiliated to those phyla including Pseudophialophora, Rhytisma increased, but Russula decreased. Redundancy analysis and structural equation modeling indicated that the diversity and composition of fungal communities changed with soil quality degradation, which were mainly driven by increased soil pH, decreased available carbon and nutrients (N, P), and related enzymes activities.

Postagrarische Veranderungen der Vegetation, Boden, C-Sequestrierung und C-Pools wurden anhand von drei Luvisol-Chronosequenzen entlang des Nord – Mitte – Suden Gradienten der Laubwaldregion Russlands untersucht. Boden und Vegetation entwickelten sich nach der Auflassung in Richtung naturlicher Auspragung: Luvisol unter Eichenwald mit Krautschicht. Dabei bekam der ehemaligen Ap-Horizont eine neue morphologische und chemische Stratifizierung ahnlich wie in naturlichen Luvisolen. Der Ap-Horizont war nach 120 Jahren durch Farbe, Gefuge und Lagerungsdichte noch erkennbar. C-Vorrate stiegen im Oberboden (5 cm) von 0.8 bis 1.7 kg m-2 (Norden), von 1.3 bis 1.9 kg m-2 (Mitte) und von 1.4 bis 2.3 kg m-2 (Suden). Die C-Akkumulation wurde mit zunehmender Bodentiefe geringer. Die Zunahme des Gesamt-C verlief in signifikant positiver Korrelation mit der C-Akkumulation in aktiven (freie partikulare organisches Material (POM); POM in Aggregaten) und passiven (C in Tonfraktion) C-Pools (g kg-1). Durch den Einbau des neuen C nahm das 14C-Alter des C in der Tonfraktion und in Aggregaten ab. Das deutet darauf hin, dass beide C-Pools (aktiv und passiv) auf die Umnutzung sensitiv reagieren.