Climate warming alters the relative importance of plant root and microbial community in regulating the accumulation of soil microbial necromass carbon in a Tibetan alpine meadow

Effect of 9-year water and nitrogen additions on microbial necromass carbon content at different soil depths and its main influencing factors

Microbial necromass carbon (MNC) is a major source of soil organic carbon (SOC) pool, significantly influencing soil carbon sequestration. The effects of long-term water and nitrogen addition on MNC in soils at different depths, as well as their interactions, remain poorly understood. In this study, we examined the influence of water addition (+51.67 mm), nitrogen addition (25, 50, 100 kg N ha-1 yr-1), and their interactions on MNC accumulation at different soil depths in temperate grasslands. The addition of water and nitrogen over nine years has been observed to exhibit a decreasing trend in the MNC at different soil depths. Notably, MNC in the topsoil layer (0-10 cm) decreased significantly by 18.56 % under low nitrogen addition treatment, while MNC in the subsoil layer (40-60 cm) declined significantly by 27.19 % under high nitrogen addition treatment. Fungal microbial necromass carbon (FNC) contributed 3.25 times more to SOC than bacterial microbial necromass carbon (BNC). In the 0-10 cm soil layer, MNC is primarily governed by both microbial attributes and the physicochemical properties of the soil, in the 20-40 cm soil layer, the physicochemical properties of the soil predominantly control MNC, in the 40-60 cm layer, microbial characteristics exert a more significant influence on MNC. Collectively, our observations suggest that soil MNC decreased with the addition of water and nitrogen in the 0-60 cm soil slope, which could enable the accurate prediction of global change impacts on the accumulation of soil carbon, thus facilitating the implementation of strategies to augment soil carbon sequestration.

Necromass responses to warming: A faster microbial turnover in favor of soil carbon stabilisation

Microbial byproducts and residues (hereafter 'necromass') potentially play the most critical role in soil organic carbon (SOC) sequestration. However, little is known about the influence of climate warming on necromass accumulation in the agroecosystem and the underlying mechanisms associated with microbial life strategies. In order to address these knowledge gaps, we used amino sugars as biomarkers of microbial necromass, and investigated their variation through an 8-year trial in an agroecosystem with two warming levels (+1.6 and + 3.2 °C) compared to ambient temperature. The results showed that the lower warming level had no impact on total microbial necromass carbon. Conversely, warming the soil 3.2 °C above ambient increased total microbial necromass by 17 % and its contribution to SOC by 21.3 %, mainly by increasing fungal necromass (+19.8 %), whereas +3.2 °C warming had no impact on bacterial necromass. At the phylum level, compared with the ambient control, +3.2 °C warming induced an increase in the abundance of Proteobacteria and a decrease in both Acidobacteria and Actinobacteria, whereas in the fungal community, Ascomycota increased and Mortierellomycota decreased. This indicates that r-strategists outcompete K-strategists in warmer climates, which led to increased microbial necromass production and accumulation, as supported by the positive correlation between r-strategists and microbial necromass. Stronger microbial competition for resources also resulted in a higher biomass turnover rate, greater cell death, and greater production of microbial necromass. This was supported by the lower bacterial and fungal network complexity and trophic links under warming conditions. In addition, the necromass generated from accelerated microbial turnover further offsets warming-induced deceases in microbial biomass. Consequently, bulk SOC did not change, despite microbial necromass having a much greater response to warming than the soil C pool. Therefore, future climate warming may influence the composition and persistence of SOC during microbial degradation.

Plateau Pika Disturbance Changes Soil Bacterial Functions and Exoenzyme Abundance to Modulate the C Cycle Pathway in Alpine Grasslands

Plateau pika (Ochotona curzoniae) is crucial to soil organic carbon (SOC) storage in the Qinghai-Tibetan plateau (QTP), but its role in bacterial SOC metabolisms across different degraded alpine grasslands remains unclear. In this study, we investigated the soil physicochemical properties and the composition and function of the bacterial communities in control and pika-disturbed grasslands experiencing different degradation levels (undegraded, UDM; lightly, LDM; moderately, MDM and severely, SDM). The results demonstrate that (i) the primary bacterial phyla include Proteobacteria, Acidobacteriota, Actinobacteriota, Bacteroidota and Verrucomicrobiota. Soil physicochemical properties significantly impact the composition of the bacterial communities and determine the influence of pika disturbance. Pika disturbance increases bacterial OTUs by 7.5% in LDP (p > 0.05) and by 50.5% in MDP (p < 0.05), while decreases OTUs by 21.4% in SDP (p < 0.05). (ii) Pika disturbance downregulates the exoenzyme abundance associated with simple and complex organic matter decomposition by 9.5% and 13.9% in LDP, and 29.4% and 26.3% in MDP (p < 0.05), while upregulates these exoenzymes by 23.6% and 37.9% in SDP (p < 0.05). These changes correspond to the increase in TC and SOC in LDP and MDP but declines in SDP. (iii) Plateau pika disturbance can enhance SOC accumulation through upregulating the C cycle pathway of ethanol production in LDP and MDP. However, it upregulates the pathway of pyruvate to CO2 conversion in SDP, leading to negative influence on SOC storage.

Warming-induced shifts in alpine soil microbiome: An ecosystem-scale study with environmental context-dependent insights

Climate warming is a pressing global issue with substantial impacts on soil health and function. However, the influence of environmental context on the responses of soil microorganisms to warming remains largely elusive, particularly in alpine ecosystems. This study examined the responses of the soil microbiome to in situ experimental warming across three elevations (3850 m, 4100 m, and 4250 m) in the meadow of Gongga Mountain, eastern Tibetan Plateau. Our findings demonstrate that soil microbial diversity is highly resilient to warming, with significant impacts observed only at specific elevations. Furthermore, the influence of warming on the composition of the soil microbial community is also elevation-dependent, underscoring the importance of local environmental context in shaping microbial evolution in alpine soils under climate warming. Notably, we identified soil moisture at 3850 m and carbon-to-nitrogen ratio at 4250 m as indirect predictors regulating the responses of microbial diversity to warming at specific elevations. These findings underscore the paramount importance of considering pre-existing environmental conditions in predicting the response of alpine soil microbiomes to climate warming. Our study provides novel insights into the intricate interactions between climate warming, soil microbiome, and environmental context in alpine ecosystems, illuminating the complex mechanisms governing soil microbial ecology in these fragile and sensitive environments.

Long-term soil warming decreases soil microbial necromass carbon by adversely affecting its production and decomposition

Microbial necromass carbon (MNC) accounts for a large fraction of soil organic carbon (SOC) in terrestrial ecosystems. Yet our understanding of the fate of this large carbon pool under long-term warming is uncertain. Here, we show that 14 years of soil warming (+4°C) in a temperate forest resulted in a reduction in MNC by 11% (0-10 cm) and 33% (10-20 cm). Warming caused a decrease in the content of MNC due to a decline in microbial biomass carbon and reduced microbial carbon use efficiency. This reduction was primarily caused by warming-induced limitations in available soil phosphorus, which, in turn, constrained the production of microbial biomass. Conversely, warming increased the activity of soil extracellular enzymes, specifically N-acetylglucosaminidase and leucine aminopeptidase, which accelerated the decomposition of MNC. These findings collectively demonstrate that decoupling of MNC formation and decomposition underlie the observed MNC loss under climate warming, which could affect SOC content in temperate forest ecosystems more widespread.

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.

Long-Term Benefits of Cenchrus fungigraminus Residual Roots Improved the Quality and Microbial Diversity of Rhizosphere Sandy Soil through Cellulose Degradation in the Ulan Buh Desert, Northwest China

Long-term plant residue retention can effectively replenish soil quality and fertility. In this study, we collected rhizosphere soil from the residual roots of annual Cenchrus fungigraminus in the Ulan Buh Desert over the past 10 years. The area, depth, and length of these roots decreased over time. The cellulose content of the residual roots was significantly higher in the later 5 years (2018-2022) than the former 5 years (2013-2017), reaching its highest value in 2021. The lignin content of the residual roots did not differ across samples except in 2015 and reached its highest level in 2021. The total sugar of the residual roots in 2022 was 227.88 ± 30.69 mg·g-1, which was significantly higher than that in other years. Compared to the original sandy soil, the soil organic matter and soil microbial biomass carbon (SMBC) contents were 2.17-2.41 times and 31.52-35.58% higher in the later 3 years (2020-2022) and reached the highest values in 2020. The residual roots also significantly enhanced the soil carbon stocks from 2018-2022. Soil dehydrogenase, nitrogenase, and N-acetyl-β-D-glucosidase (S-NAG) were significantly affected from 2019-2022. The rhizosphere soil community richness and diversity of the bacterial and fungal communities significantly decreased with the duration of the residual roots in the sandy soil, and there was a significant difference for 10 years. Streptomyces, Bacillus, and Sphigomonas were the representative bacteria in the residual root rhizosphere soil, while Agaricales and Panaeolus were the enriched fungal genera. The distance-based redundancy analysis and partial least square path model results showed that the duration of residual roots in the sandy soil, S-NAG, and SMBC were the primary environmental characteristics that shaped the microbial community. These insights provide new ideas on how to foster the exploration of the use of annual herbaceous plants for sandy soil improvement in the future.

Response of the C-fixing bacteria community to precipitation changes and its impact on bacterial necromass accumulation in semiarid grassland

Climate change-induced warming has the potential to intensify drought conditions in certain regions, resulting in uneven precipitation patterns. However, the impact of precipitation-induced changes on soil C-fixing bacterial community composition to changes and their subsequent effect on the accumulation of microbial necromass in the soil remains unclear. To address this knowledge gap, we conducted an in-situ simulated precipitation control experiment in semi-arid grasslands, encompassing five primary precipitation gradients: ambient precipitation as a control (contr), decreased precipitation by 80% and 40% (DP80, DP40), and increased precipitation by 40% and 80% (IP80, IP40). Our findings indicate that while an increase in precipitation promotes greater total bacterial diversity, it reduces the diversity of cbbM-harboring bacteria. The dominance of drought-tolerant Proteobacteria within the cbbM-harboring bacterial community was responsible for the observed increase in their relative abundance, ranging from 8.9% to 15.6%, under conditions of decreased precipitation. In arid environments characterized by limited soil moisture and nutrient availability, certain dominant genera such as Thiobacillus, Sulfuritalea, and Halothiobacillus, which possess cbbM genes, exhibit strong synergistic effects with other bacteria, thereby leading to a high nutrient use efficiency. Linear regression analysis shows that bacterial necromass C was significantly negatively correlated with cbbM-harboring bacterial diversity but positively correlated with cbbM-harboring bacterial community composition. Consequently, in the extreme drought environment of DP80, the contribution of bacterial necromass C to SOC was dramatically reduced by 75% relative to the control. Although bacterial necromass C was preferentially consumed as nutrients and energy for microorganisms, C-fixing microorganisms supplemented the soil C pool by assimilating atmospheric CO2. Bacterial necromass was primarily controlled by accessible C and N rather than by the total bacterial community composition and relative abundance. Our results provide compelling evidence for the critical role of the composition of the bacterial community and its necromass in the accumulation of SOC in semiarid grassland ecosystems.

Assembly of typical steppe community and functional groups along the precipitation gradient from 1985 to 2022

Long-term observations have shown that structure and function of grasslands have changed due to climate change over the past decades. However, little is known about how grasslands respond to climate change along the precipitation gradient, and potential mechanisms remain elusive. Here, we utilize a long-term experiment in typical steppe to explore universal and differential mechanisms of community and functional groups assembly along the precipitation gradient. Our results indicated that the sensitivity of community and functional groups assembly to climate change was related to local precipitation. The strength of the positive effects of climate change on aboveground biomass, species richness, and their relationship of community decreased modestly with local precipitation. The mechanism behind this was the change in plant community composition of the precipitation-induced, annuals that was more responsive to climate change decreased as increased local precipitation. Furthermore, current and past climate both drove community and functional group assembly, and the role of past climate diminished with increasing local precipitation. Among them, climate fluctuation, average climate and current climate were the most critical climate indicators affecting community and functional groups assembly in low, medium and high precipitation sites, respectively. In conclusion, climatic change do not always exert identical effects on grasslands along the precipitation gradient. This could be critical importance for improving our ability to predict future changes in grassland ecosystems.