Microbial necromass in cropland soils: A global meta-analysis of management effects

Pathways of soil organic carbon accumulation are related to microbial life history strategies in fertilized agroecosystems

Although the formation, turnover, and accumulation of soil organic carbon (SOC) are driven by different fertilizer inputs and their subsequent microbial-mediated transformation, the relationship between changes in plant-derived and microbial-derived components and soil microbial life history strategies under different fertilization regimes has not been well explored. In this study, the changes in microbial necromass carbon (MNC), lignin phenols, and glomalin-related soil protein (GRSP), as well as soil microbial life history strategy were determined in a 16-year field experiment in response to different fertilization regimes, including a no-fertilizer control (C), conventional chemical NPK fertilization (NPK), and partial substitutions of the NPK in chemical fertilizers with a low (30 %) or high (60 %) level of straw (0.3S and 0.6S) or cattle manure (0.3M and 0.6M). The results showed that total lignin phenol content and its contribution to SOC were significantly increased by 88.7 % and 74.2 %, respectively, in high-level straw substitution treatment as compared to chemical fertilization. Both high-level straw and cattle manure substitution increased MNC and total GRSP contents, but did not alter their contributions to SOC compared to chemical fertilization. In fertilized treatments, the high-level cattle manure substitution had the lowest and highest bacterial and fungal K/r ratio, respectively. Bacterial K/r ratio was an important factor in predicting bacterial necromass carbon content and there was a significant negative correlation between them. The ratio of ectomycorrhizal to saprotrophic fungi and fungal diversity were important factors for predicting lignin phenol and GRSP contents, respectively. In addition, the SEMs modeling indicated that straw substitution directly affected lignin phenol and MNC accumulation, whereas cattle manure substitution indirectly affected MNC accumulation by affecting microbial life history strategies. In conclusions, agricultural residues inputs support the formation of a multiple carbon pool of SOC compared to chemical fertilization; and microbial life history strategy is an important driver of SOC formation and affects SOC accumulation and stability in agroecosystems.

Long-term residue returning increased subsoil carbon quality in a rice-wheat cropping system

Residue returning (RR) was widely implemented to increase soil organic carbon (SOC) in farmland. Extensive studies concentrated on the effects of RR on SOC quantity instead of SOC fractions at aggregate scales. This study investigated the effects of 20-year RR on the distribution of labile (e.g., dissolved, microbial biomass, and permanganate oxidizable organic) and stable (e.g., microbial necromass) carbon fractions at aggregate scales, as well as their contribution to SOC accumulation and mineralization. The findings indicated a synchronized variation in the carbon content of bacterial and fungal necromass. Residue retention (RR) notably elevated the concentration of bacterial and fungal necromass carbon, while it did not amplify the microbial necromass carbon (MNC) contribution to SOC when compared to residue removal (R0) in the topsoil (0-5 cm). In the subsoil (5-15 cm), RR increased the MNC contribution to SOC concentration by 21.2%-33.4% and mitigated SOC mineralization by 12.6% in micro-aggregates (P < 0.05). Besides, RR increased soil β-glucosidase and peroxidase activities but decreased soil phenol oxidase activity in micro-aggregates (P < 0.05). These indicated that RR might accelerate cellulose degradation and conversion to stable microbial necromass C, and thus RR improved SOC stability because SOC occluded in micro-aggregates were more stable. Interestingly, SOC concentration was mainly regulated by MNC, while SOC mineralization was by dissolved organic carbon under RR, both of which were affected by soil carbon, nitrogen, and phosphorus associated nutrients and enzyme activities. The findings of this study emphasize that the paths of RR-induced SOC accumulation and mineralization were different, and depended on stable and labile C, respectively. Overall, long-term RR increased topsoil carbon quantity and subsoil carbon quality.

Global synthesis on the response of soil microbial necromass carbon to climate-smart agriculture

Climate-smart agriculture (CSA) supports the sustainability of crop production and food security, and benefiting soil carbon storage. Despite the critical importance of microorganisms in the carbon cycle, systematic investigations on the influence of CSA on soil microbial necromass carbon and its driving factors are still limited. We evaluated 472 observations from 73 peer-reviewed articles to show that, compared to conventional practice, CSA generally increased soil microbial necromass carbon concentrations by 18.24%. These benefits to soil microbial necromass carbon, as assessed by amino sugar biomarkers, are complex and influenced by a variety of soil, climatic, spatial, and biological factors. Changes in living microbial biomass are the most significant predictor of total, fungal, and bacterial necromass carbon affected by CSA; in 61.9%-67.3% of paired observations, the CSA measures simultaneously increased living microbial biomass and microbial necromass carbon. Land restoration and nutrient management therein largely promoted microbial necromass carbon storage, while cover crop has a minor effect. Additionally, the effects were directly influenced by elevation and mean annual temperature, and indirectly by soil texture and initial organic carbon content. In the optimal scenario, the potential global carbon accrual rate of CSA through microbial necromass is approximately 980 Mt C year-1, assuming organic amendment is included following conservation tillage and appropriate land restoration. In conclusion, our study suggests that increasing soil microbial necromass carbon through CSA provides a vital way of mitigating carbon loss. This emphasizes the invisible yet significant influence of soil microbial anabolic activity on global carbon dynamics.

Microbial life-history strategies mediate microbial carbon pump efficacy in response to N management depending on stoichiometry of microbial demand

The soil microbial carbon pump (MCP) is increasingly acknowledged as being directly linked to soil organic carbon (SOC) accumulation and stability. Given the close coupling of carbon (C) and nitrogen (N) cycles and the constraints imposed by their stoichiometry on microbial growth, N addition might affect microbial growth strategies with potential consequences for necromass formation and carbon stability. However, this topic remains largely unexplored. Based on two multi-level N fertilizer experiments over 10 years in two soils with contrasting soil fertility located in the North (Cambisol, carbon-poor) and Southwest (Luvisol, carbon-rich), we hypothesized that different resource demands of microorganism elicit a trade-off in microbial growth potential (Y-strategy) and resource-acquisition (A-strategy) in response to N addition, and consequently on necromass formation and soil carbon stability. We combined measurements of necromass metrics (MCP efficacy) and soil carbon stability (chemical composition and mineral associated organic carbon) with potential changes in microbial life history strategies (assessed via soil metagenomes and enzymatic activity analyses). The contribution of microbial necromass to SOC decreased with N addition in the Cambisol, but increased in the Luvisol. Soil microbial life strategies displayed two distinct responses in two soils after N amendment: shift toward A-strategy (Cambisol) or Y-strategy (Luvisol). These divergent responses are owing to the stoichiometric imbalance between microbial demands and resource availability for C and N, which presented very distinct patterns in the two soils. The partial correlation analysis further confirmed that high N addition aggravated stoichiometric carbon demand, shifting the microbial community strategy toward resource-acquisition which reduced carbon stability in Cambisol. In contrast, the microbial Y-strategy had the positive direct effect on MCP efficacy in Luvisol, which greatly enhanced carbon stability. Such findings provide mechanistic insights into the stoichiometric regulation of MCP efficacy, and how this is mediated by site-specific trade-offs in microbial life strategies, which contribute to improving our comprehension of soil microbial C sequestration and potential optimization of agricultural N management.

Long-term biogas slurry application increases microbial necromass but not plant lignin contribution to soil organic carbon in paddy soils as regulated by fungal community

Biogas slurry (BS) is widely considered as a source of organic matter and nutrients for improving soil organic carbon (SOC) sequestration and crop production in agroecosystems. Microbial necromass C (MNC) is considered one of the major precursors of SOC sequestration, which is regulated by soil microbial anabolism and catabolism. However, the microbial mechanisms through which BS application increases SOC accumulation in paddy soils have not yet been elucidated. A 12-year field experiment with four treatments (CK, no fertilizers; CF, chemical fertilizer application; BS1 and BS2, biogas slurry application at two nitrogen rates from BS) was conducted in rice paddy fields. The results showed that long-term BS application had no effect on lignin phenols proportion in SOC relative to CF. In contrast, BS application elevated the MNC contribution to SOC by 15.5-20.5 % compared with the CF treatment. The proportion of fungal necromass C (FNC) to SOC increased by 16.0 % under BS1 and by 25.8 % under BS2 compared with the CF treatment, while no significant difference in bacterial necromass C (BNC) contribution to SOC was observed between the BS and CF treatments. The MNC was more closely correlated with fungal community structures than with bacterial community structures. We further found that fungal genera, Mortierella and Ciliophora, mainly regulated the MNC, FNC and BNC accumulation. Collectively, our results highlighted that fungi play a vital role in SOC storage in paddy soils by regulating MNC formation and accumulation under long-term BS application.

Cross-scale spatial variability and associations of carbon pools provide insight into regulating carbon sequestration in tropical montane rainforests

The spatial distribution of plant, soil, and microbial carbon pools, along with their intricate interactions, presents a great challenge for the current carbon cycle research. However, it is not clear what are the characteristics of the spatial variability of these carbon pools, particularly their cross-scale relationships. We investigated the cross-scale spatial variability of microbial necromass carbon (MNC), soil organic carbon (SOC) and plant biomass (PB), as well as their correlation in a tropical montane rainforest using multifractal analysis. The results showed multifractal spatial variations of MNC, SOC, and PB, demonstrating their adherence to power-law scaling. MNC, especially low MNC, exhibited stronger spatial heterogeneity and weaker evenness compared with SOC and PB. The cross-scale correlation between MNC and SOC was stronger than their correlations at the measurement scale. Furthermore, the cross-scale spatial variability of MNC and SOC exhibited stronger and more stable correlations than those with PB. Additionally, this research suggests that when SOC and PB are both low, it is advisable for reforestations to potentiate MNC formation, whereas when both SOC and PB are high some thinning can be advisable to favour MNC formation. Thus, these results support the utilization of management measures such as reforestation or thinning as nature-based solutions to regulate carbon sequestration capacity of tropical forests by affecting the correlations among various carbon pools.

Microbial-driven mechanisms for the effects of heavy metals on soil organic carbon storage: A global analysis

Heavy metal (HM) enrichment is closely related to soil organic carbon (SOC) pools in terrestrial ecosystems, which are deeply intertwined with soil microbial processes. However, the influence of HMs on SOC remains contentious in terms of magnitude and direction. A global analysis of 155 publications was conducted to integrate the synergistic responses of SOC and microorganisms to HM enrichment. A significant increase of 13.6 % in SOC content was observed in soils exposed to HMs. The response of SOC to HMs primarily depends on soil properties and habitat conditions, particularly the initial SOC content, mean annual precipitation (MAP), initial soil pH, and mean annual temperature (MAT). The presence of HMs resulted in significant decreases in the activities of key soil enzymes, including 31.9 % for soil dehydrogenase, 24.8 % for β-glucosidase, 35.8 % for invertase, and 24.3 % for cellulose. HMs also exerted inhibitory effects on microbial biomass carbon (MBC) (26.6 %), microbial respiration (MR) (19.7 %), and the bacterial Shannon index (3.13 %) but elevated the microbial metabolic quotient (qCO2) (20.6 %). The HM enrichment-induced changes in SOC exhibited positive correlations with the response of MBC (r = 0.70, p < 0.01) and qCO2 (r = 0.50, p < 0.01), while it was negatively associated with β-glucosidase activity (r = 0.72, p < 0.01) and MR (r = 0.39, p < 0.01). These findings suggest that the increase in SOC storage is mainly attributable to the inhibition of soil enzymes and microorganisms under HM enrichment. Overall, this meta-analysis highlights the habitat-dependent responses of SOC to HM enrichment and provides a comprehensive evaluation of soil carbon dynamics in an HM-rich environment.

Microbially mediated mechanisms underlie soil carbon accrual by conservation agriculture under decade-long warming

Increasing soil organic carbon (SOC) in croplands by switching from conventional to conservation management may be hampered by stimulated microbial decomposition under warming. Here, we test the interactive effects of agricultural management and warming on SOC persistence and underlying microbial mechanisms in a decade-long controlled experiment on a wheat-maize cropping system. Warming increased SOC content and accelerated fungal community temporal turnover under conservation agriculture (no tillage, chopped crop residue), but not under conventional agriculture (annual tillage, crop residue removed). Microbial carbon use efficiency (CUE) and growth increased linearly over time, with stronger positive warming effects after 5 years under conservation agriculture. According to structural equation models, these increases arose from greater carbon inputs from the crops, which indirectly controlled microbial CUE via changes in fungal communities. As a result, fungal necromass increased from 28 to 53%, emerging as the strongest predictor of SOC content. Collectively, our results demonstrate how management and climatic factors can interact to alter microbial community composition, physiology and functions and, in turn, SOC formation and accrual in croplands.

Afforestation increased the microbial necromass carbon accumulation in deep soil on the Loess Plateau

Microbial necromass carbon (MNC) is a stable part of SOC that makes up most of the C pool in land ecosystem. However, the contribution of MNC to SOC accumulation during afforestation is still unclear, particularly in the deep soil. Based on the collection and biomarker analysis of the forest succession sequence and soil profiles with significant depth on the Loess Plateau located China, we study the vertical distribution characteristics and control of MNC. The results found that MNC content increased with succession and decreased with soil depth. On average, the MNC content of a climax forest was 2.23 times higher than that of farmland. The FNC:BNC ratio increased with vegetation succession and decreased with soil depth. Although the MNC content decreased with soil depth, the necromass accumulation coefficient increased. The contribution of MNC to SOC in deep soil (60-100 cm) of pioneer forest was more than 10 % higher than that of farmland, suggesting that afforestation had a relatively positive effect on MNC stabilization and accumulation in deep soils. The microbial biomass and soil nutrient characteristics (i.e., TN, SOC, DOC, and DON) are important factors in mediating the accumulation of MNC in the succession of farmland to forest. These findings demonstrate the potential of MNC in deep soil and provide scientific guidance for sustainable reforestation management based on the carbon pump theory at regional scales.