Increasing Soil Microbial Necromass Carbon Under Climate Change in Chinese Terrestrial Ecosystems: A Meta-Analysis

Abstract

Soil necromass carbon (C) has significant potential for C sequestration in combination with minerals. Global warming and nitrogen (N) deposition affect necromass C, but these effects vary greatly across different climate conditions, land uses, and soil properties, and the role of regional specificity remains poorly understood. We synthesized 197 paired observations from 50 climate change studies to investigate these effects in China. Our results indicate that warming and N addition significantly increase necromass C accumulation by 17% and 9%, respectively. Warming strongly enhanced necromass C incroplands (+44%), cool (+16%) and semi-arid regions (+15%), and soils rich in soil organic carbon (SOC) (+17%) or loam (+22%), whereas N addition further promoted sequestration in croplands (+12%), forests (+10%) SOC-rich soils (+16%), and clay (+7%) or clay–loam (+12%) soils. In the context of climate change, soil C management requires attention to land use, climate, and soil properties. However, heterogeneous responses of microbial necromass C to global warming and N addition are still poorly understood. In the future, global warming is projected to enhance necromass C in croplands, cool or semiarid regions, SOC-rich and loam soils, whereas N addition is expected to further promote its sequestration in croplands, forests, SOC-rich, and clay-based soils. These findings demonstrate the targeted management of necromass C, particularly through optimized nitrogen application in clay-rich croplands and conservation tillage in cool and semiarid regions, offering a nature-based solution to complement global climate mitigation.

1. Introduction

The stability and function of terrestrial ecosystems are greatly affected by climate change. Global warming and N deposition are two relatively key influencing factors [1,2,3]. According to the relevant content of the CMIP6 (Coupled Model Intercomparison Project Phase 6), this modeling framework provides a systematic and accurate depiction of the spatial patterns and temporal dynamics of global climate change [4]. Assessments indicate that middle- and low-latitude regions are more vulnerable to global warming and increased N deposition, with China’s ecosystems being particularly sensitive, and therefore subject to elevated potential risks [4,5,6]. Although China’s land area accounts for only 6.5% of the world’s total land area, its SOC reserves represent a relatively high proportion on a global scale [7]. Due to climate change, China may lose SOC faster than other regions [2]. N and temperature will have certain impacts on soil microorganisms and the growth of plants. Plants and microorganisms are very crucial for the storage of SOC [3]. More N deposition and global warming will alter the dynamics of SOC. It may cause changes in C storage and have an impact on ecosystem functions [3,8].

The formation of soil organic matter is more influenced by the anabolic metabolism of microorganisms than by plant-derived substances in natural or semi-natural ecosystems [9,10]. Shao et al. [11] have reported that microbial necrotic C constitutes approximately 10–50% of SOC. When these microbial necrotic C combine with soil minerals, it is even less likely to be decomposed. Glucosamine is derived from the cell walls of bacteria, and lactic bactericic acid is derived from the cell walls of fungi. Both are regarded as markers of C sources in microbial necrotic blocks [12,13].

Concern over the potential effects of climate change on microbial necrotic mass C has grown in recent years [8,9,12]. However, it is still unclear what causes C production in a necrotic mass and how climate change affects it globally. Both N deposition and global warming increase the microbial turnover of SOC and plant-derived C input, though the dynamics of necromass C remain controversial [1,3,14,15,16,17,18,19]. Warming alters microbial turnover and necromass C accumulation, which speeds up necromass C dynamics [20]. N deposition increases nutrition and decreases pH, which accelerates necromass C breakdown [15]. It has been demonstrated that warming and N addition have beneficial effects on necromass C [15,20] that are neutral [21,22] and negative [23,24], likely linked to climatic conditions, land uses, and soil properties [25,26,27]. Warming in cool regions causes more necromass C to build up than in warm regions [20,23]. Adding N to clay soils helps C accumulate better than in sandy soils [25,26]. This happens because clay holds water and nutrients better than sandy soil [27]. Grasslands react more to warming and N than forests do. Forests have more complex and stable ecosystems [27]. Croplands face strong human interference. This makes it harder to predict how climate change affects necromass C [27]. Current studies may not fully account for three factors: local climates, land uses, and soil textures. These differences explain why necromass C responds differently to climate change. China’s temperature patterns will change unevenly between 2030 and 2052. Average temperatures will rise 1.5 °C overall [28]. Northern areas will warm noticeably. Northeast areas might cool down significantly [28]. N deposition decreases from southeast to northwest. But North, Central, and Northeast China keep getting more N over time [29,30]. Together, these changes threaten necromass C storage.

China became the world’s third-largest emitter of greenhouse gases between 2009 and 2022, with its emissions increasing from 7.71 billion tons to 11.48 billion tons [31]. The continuous process of urbanization and industrialization emphasizes the potential risks of C loss due to possible climate changes [32]. It is essential to consider the regulatory roles of climate conditions, land uses, and characteristics of the soil in the research area in order to gain a deeper comprehension of the effect on necromass C of warming and N deposition. In this regard, we gathered 208 paired observations from 50 published studies on warming and N addition in China. We proposed and investigated the following assumptions: (1) warming is predicted to boost microbiological activity and decomposition rates, which will hasten the buildup of necromass C [33,34,35]; (2) N addition is anticipated to enhance nutrient cycling and encourage plant and microorganism growth [13,17]. The role that necromass C plays in SOC will, therefore, eventually rise [25,36,37]; and (3) climate variability, land uses, and soil characteristics may affect necromass C dynamics under climate change [36,38,39,40,41,42].

2. Materials and Methods

2.1. Data Collection

The sources of the data were Google Scholar (scholar.google.com), Web of Science (https://www.webofscience.com), and the National Knowledge Infrastructure of China (https://www.cnki.net/). The search was limited to studies that looked at how microbial necromass accumulation was impacted by climate change, and it included papers that were published prior to October 2023 that addressed how climate change affected either amino sugars or microbial necromass. There were many different keyword combinations used, including (“global warming” OR “climate change” OR “warm” OR “warming” OR “elevated temperature” OR “nitrogen deposition” OR “nitrogen addition”) AND (“microbial necromass” OR “fungal necromass” OR “bacterial necromass” OR “microbial residues” OR “amino sugars” OR “glucosamine” OR “muramic acid”).

In order to mitigate publication bias, the following criteria were used to screen articles [43]: (1) research in China is required (3°51′ N–53°33′ N, 73°33′ E–135°05′ E); (2) articles must include the contents of fungal necromass carbon (FNC) and bacterial necromass carbon (BNC) or amino glucosamine and muramic acid; (3) each experiment must have control and treatment groups with a minimum of three replications; (4) control and treatment plots must be established under comparable environmental conditions, encompassing both native vegetation and agricultural land uses; and (5) sample sizes, means, and the data must contain the standard deviations for the treatment and control groups. The following is how the standard deviation was determined if only the standard error was provided SD = $\mathrm{SE}\sqrt{\mathrm{n}}$, and (6) all studies based on laboratory incubation experiments were excluded. Implementing these criteria led to the selection of 50 articles on climate change, totaling 197 paired observations to analyze microbial necromass responses to climate change.

Additional information collected included latitude, longitude, elevation, mean annual temperature (MAT), mean annual precipitation (MAP), soil pH, initial SOC content, soil total N, C/N ratio, microbial biomass C and N, soil texture, and treatment variation (e.g., the magnitude of warming, N addition level, and precipitation reduction). The data in the figures were obtained using the Web Plot Digitizer. The Global Aridity and PET Database (https://cgiarcsi.community/data/global-aridity-and-pet-database, accessed on 17 August 2025) served as the source of the aridity index. In the absence of soil texture data, the Harmonized World Soil Database v2.0 (https://www.fao.org/soils-portal/data-hub, accessed on 17 August 2025) was used to supplement the information. To further explore the regulatory mechanisms of microbial necromass under climate change through background conditions, land uses were categorized into cropland, forest, and grassland. The aridity index was classified as semiarid (AI ≤ 0.65) or humid (AI > 0.65) [44]. Following the USDA soil texture classification system, data were further classified into clay, clay loam, and loam [13]. The MAT of the study sites was used to classify the data into two climatic conditions: warm (subtropical and tropical climates) and cool (temperate and Mediterranean climates) [43]. Initial SOC content was categorized as low (<12 g C/kg) or high (>12 g C/kg) [45] (Figure 1).

2.2. Microbial Necromass Calculation

The molar ratio of muramic acid to glucosamine in bacterial cells is 2:1, according to earlier studies [46,47,48]. Formulas (1)–(3) were employed to calculate, using this ratio, the C content of total necromass C (TNC), fungal necromass C (FNC), and bacterial necromass C (BNC):

$$\mathrm{BNC} = \mathrm{MurN} \times 45$$

(1)

$$\mathrm{FNC}=\left({\displaystyle \frac{\mathrm{GluN}}{179.17}}-2\times {\displaystyle \frac{\mathrm{MurN}}{251.23}}\right)\times 179.17\times 9$$

(2)

$$\mathrm{TNC}=\mathrm{BNC}+\mathrm{FNC}$$

(3)

The ratios specified in Formulas (4)–(6), respectively, were used to assess the roles that BNC, FNC, and TNC play in SOC [27]:

$$\mathrm{Contribution} \mathrm{of} \mathrm{bacterial} \mathrm{necromass} \mathrm{C} \mathrm{to} \mathrm{SOC} = {\displaystyle \frac{\mathrm{BNC}}{\mathrm{SOC}}}$$

(4)

$$\mathrm{Contribution} \mathrm{of} \mathrm{fungal} \mathrm{necromass} \mathrm{C} \mathrm{to} \mathrm{SOC}={\displaystyle \frac{\mathrm{FNC}}{\mathrm{SOC}}}$$

(5)

$$\mathrm{Contribution} \mathrm{of} \mathrm{total} \mathrm{necromass} \mathrm{C} \mathrm{to} \mathrm{SOC}={\displaystyle \frac{\mathrm{TNC}}{\mathrm{SOC}}}$$

(6)

2.3. Meta and Statistical Analysis

We conducted a formal meta-analysis using effect sizes (lnRR) to quantify the impacts of warming and N addition on microbial necromass C. According to Hedges et al. [47], the ratio of natural logarithms was utilized, or lnRR_i, to determine how microbial necromass C is affected by climate change. The calculation formula is presented in Equation (7):

$${\mathrm{l}\mathrm{n}\mathrm{R}\mathrm{R}}_{\mathrm{i}}=\mathrm{l}\mathrm{n}\left({\displaystyle \frac{X_t}{X_c}}\right)=\mathrm{l}\mathrm{n}\left(X_t\right)-\mathrm{l}\mathrm{n}\left(X_c\right)$$

(7)

In this equation, lnRR_i represents the effect size, X_t is the mean of the treatment group, and X_c is the mean of the control group. The variance of lnRRi is calculated using Equation (8):

$$v_i = {\displaystyle \frac{S{D}_t^2}{n_t{X}_t^2}}+{\displaystyle \frac{S{D}_c^2}{n_c{X}_c^2}}$$

(8)

where SD_t and SD_c represent the standard deviations of the treatment and control, respectively. n_t and n_c are the sample sizes for the treatment group and the control group, respectively. To enhance the accuracy of the results, cases with greater variances were assigned lower weights, whereas those with lower variances received higher weights. The weight w_i for each case was calculated, as shown in Equation (9):

$$w_i = {\displaystyle \frac{1}{v_i}}$$

(9)

The weighted effect size lnRR was calculated as per Equation (10):

$$\mathrm{lnRR} = {\displaystyle \frac{\sum \left(w_i{\times \mathrm{lnRR}}_i\right)}{\sum w_i}}$$

(10)

The standard error of lnRR (SlnRR) and 95% confidence intervals (95%CI) were computed using Equations (11) and (12), respectively:

$${\mathrm{S}}_{\mathrm{lnRR}} = \sqrt{{\displaystyle \frac{1}{w_i}}}$$

(11)

$$95\%\mathrm{CI}=\mathrm{lnRR}\pm 1.96\times {\mathrm{S}}_{\mathrm{lnRR}}$$

(12)

To directly reflect the effect size of climate change on microbial necromass C, the percent change was calculated using Equation (13):

$$\mathrm{Effect} \mathrm{size} = \left(e^{\mathrm{lnRR}}-1\right)\times 100\%$$

(13)

For all data analyses in this study, R version 4.3.1 was used [49]. To assess how warming and N addition affect TNC, BNC, FNC, and their contribution to SOC, the “rma. mv” function from the “metafor” package was utilized to compute lnRR and 95%CI [50]. Subgroup analyses were also performed to assess these impacts across different land uses, MAT, AI, initial organic C content, and soil texture. To evaluate publication bias, Fail–Safe analysis and Egger’s regression test were employed [51,52]. To examine the mechanisms underlying the impacts of warming and N addition on microbial necromass, structural equation modeling (SEM) was performed using the “lavaan” package [53].

3. Results

3.1. Effect of Climate Change on Microbial Necromass C

The ways that climate change affected microbial necromass C varied noticeably (Figure 2). Warming significantly increased the accumulation of TNC, BNC, and FNC by 17%, 10%, and 20%, respectively (p < 0.05), whereas N addition significantly increased them by 9%, 11%, and 9%, respectively (p < 0.01). Climate change caused changes in the FNC/SOC, BNC/SOC, and TNC/SOC ratios ranging from 2% to 19%; only warming substantially changed these contributions (p < 0.01; Figure 2).

3.2. Response of Microbial Necromass C to Climate Change Depending on Climatic Conditions, Land Uses, and Soil Properties

Climate conditions influenced how microbial necromass C responded to climate change (Figure 3a–c). In cool climates, warming and N addition significantly increased TNC by 16% and 8%, respectively (p < 0.05). Both treatments also significantly increased the BNC, TNC, and FNC contributions to SOC (p < 0.05). In warm climates, N addition significantly accelerated the accumulation of TNC (+11%), BNC (+11%), and FNC (+11%; p < 0.01). Only with the N addition did the amount of microbial necromass C in humid sites rise noticeably (p < 0.05; Figure 4a–c). In semiarid regions, N addition and warming both significantly raised TNC by 11% and 15%, respectively (p < 0.05), despite the fact that only the addition of N markedly raised BNC.

Necromass C responses to climate change varied across different land uses (Figure 5a–c). In croplands, warming significantly increased TNC and FNC (+44% and +44%, p < 0.05), respectively. In grasslands, FNC was increased (+16%, p < 0.05) by warming. N addition increased TNC, BNC, and FNC in croplands and forests (p < 0.05). Furthermore, warming increased TNC and BNC contributions to SOC (+18% and +18%, p < 0.05) in grasslands; however, in no land uses, the addition of N had an impact on these contributions (Figure 5d–f).

In soils where the starting SOC content is less than 12 g/kg, only BNC was significantly increased (+12%, p < 0.01) by N addition (Figure 6a–c). In soils with high initial SOC content (>12 g/kg), warming and N addition significantly enhanced both total necromass C and its fractions (p < 0.05) (Figure 6a–c). Warming significantly increased TNC/SOC, BNC/SOC, and FNC/SOC (+16%, +15%, and +18%, p < 0.05), whereas N addition significantly increased TNC/SOC, BNC/SOC, and FNC/SOC (+4%, +3%, and +4%, p < 0.05) (Figure 6a–c).

Additionally, there were variations in how different soil textures affected how microbial necromass C responded to climate change factors (Figure 7a–c). Microbial necromass C levels in loam soil increased with warming (TNC: 22%, BNC: 17%, and FNC: 35%), as did its contribution to SOC (TNC/SOC: 25%, BNC/SOC: 27%, and FNC/SOC: 37%) (p < 0.05). In clay soils, N addition led to an increase of 7% in TNC and 9% in BNC (p < 0.05). In clay loam soils, N addition significantly boosted TNC by 12%, BNC by 11%, and FNC by 12% (p < 0.01).

3.3. Mechanisms of the Impact of Climate Change on Microbial Necromass C

The SEM provided the best fit for the data, revealing diverse mechanisms through which climate change influences microbial necromass C (Figure 8). Under warming, the correlation between SOC and muramic acid was significantly positive (p < 0.05), which significantly impacted the accumulation of TNC (p < 0.001). N addition positively influenced glucosamine (GluN) levels, which, in turn, affected TNC, thereby enhancing SOC (p < 0.05).

4. Discussion

4.1. Response of Microbial Necromass C to Climate Change

4.1.1. Response of Microbial Necromass C to Warming

In line with our original assumptions, warming enhanced the contribution of TNC to SOC, which encouraged its buildup (Figure 2). The 17% increase in TNC observed in our study differed from the unchanged TNC reported in a global meta-analysis that contained 80 studies on the warming effect (Figure 2) [54]. This difference might be attributed to the huge heterogeneity of soil properties and management [55]. Also, the different environmental conditions in our study sites could have played a role, as factors like soil texture and moisture can change how microbes respond to warming [33]. The rises in TNC and TNC/SOC under warming might be connected to increases in BNC (Figure 2 and Figure 8), showing that bacteria and fungi react differently [34,35]. Bacteria usually grow faster and are more active than fungi under warming, so they likely contribute more to TNC in SOC [33,34,35].

4.1.2. Response of Microbial Necromass C to N Addition

Our results indicate that N addition enhanced the accumulation of both necromass C and SOC (Figure 2 and Figure 8), which is partly consistent with three global meta-analyses [1,13,54]. SEM analysis showed that N addition directly and indirectly increased levels of GluN and muramic (MurN) acid, both sources of necromass C, conforming to meta-analysis results (Figure 8) [38]. The positive correlation between GluN and MurN demonstrated the strong interaction between the bacterial and fungal communities under N addition (Figure 8) [17]. This interaction indicates not only a cooperative relationship between microbial groups, but also potential shifts in community dynamics that could influence nutrient cycling [17,56]. Furthermore, this interaction enhances nutrient assimilation by microorganisms [56]. Nutrient enrichment also promotes plant and microbial growth, enhancing the efficiency of microbial carbon utilization, and enhancing both necromass C and SOC [13]. These discoveries emphasize the considerable role of N addition in promoting necromass C and SOC sequestration, with greater accumulation observed under higher levels of N addition.

4.2. Background Conditions Regulating Microbial Necromass C Under Climate Change

4.2.1. Background Conditions Regulating Microbial Necromass C Under Warming

Warming increased TNC in croplands (Figure 5a). This effect could be stronger because of farming methods, which change the availability of nutrients and microbial activity [1,57]. Croplands have more disturbance than forests and grasslands because tillage spreads nutrients more evenly [1,57]. Warming makes microbes use more soil nutrients, but it also raises how much TNC adds to SOC.

In croplands, warming may create more opportunities for necromass C to bind with minerals, thereby promoting C sequestration. Warming increased TNC, BNC, and FNC in cool and humid areas (Figure 3a–c). The climate in these areas plays a major role in necromass C accumulation [58]. Temperature and moisture together affect microbial activity [59]. In cool and humid conditions, plants and microbes grow slower, but warming speeds up their C cycling [60,61]. Cool and humid regions show stronger responses of necromass C to warming, so these areas need more study [62].

Warming affects necromass C differently depending on soil properties (Figure 6a–c and Figure 7a–c). Necromass C increased more in soils with high SOC or loam textures (Figure 6a and Figure 7a). This means soil texture and organic C may reduce warming’s impact on microbial necromass. Earlier studies showed that loam soils help oxygen, water, and nutrients move better, which helps microbes grow [25,63]. Warming speeds up microbial activity, causing these soils to accumulate more necromass C [13,64].

Warming increases necromass C (TNC, BNC, and FNC). This effect is the strongest in croplands, cool humid areas, and soils with high SOC or loam textures. Climate, land use, and soil properties all affect how necromass C responds to warming. Warming speeds up microbial activity, but can also help store more C, especially in soils good for microbial growth [13,64].

4.2.2. Background Conditions Regulating Microbial Necromass C Under N Addition

N addition increased necromass C in forests and croplands (Figure 5a–c). Croplands need more N than other land uses because crops grow fast [65]. The extra N may boost necromass C through crop roots [13]. Forests showed the strongest N shortage, with an average C/N ratio of 15.30. N addition helps microbes break down material more easily, speeding up C cycling [66].

Regardless of climate conditions, N addition has been found to promote the accumulation of necromass C, and this effect is more evident in warm or semiarid areas (as shown in Figure 3a–c and Figure 4a–c). High annual temperatures and a low aridity index indicate elevated microbial metabolic activity and soil permeability, which offer a conducive environment for microbial growth and turnover [9,13]. This demonstrates how microbial growth in warm or semiarid conditions can be stimulated by adding N, thereby increasing necromass C [36].

Necromass C was positively correlated with the sum of OC and TN contents when soil texture were clay, clay loam, or when SOC content >12 g/kg (Figure 6a–c and Figure 7a–c), because increasing N resulted in an increase in fungal hyphal mass and binding material, promoting aggregation formation and protecting necromass C against microbial decomposition [67,68].

Furthermore, the addition of N further acidifies clay and clay loam [69,70], resulting in an upward trend in the ion concentrations of Fe²⁺, Fe³⁺, and Al³⁺ [25,36]. This will promote the combination of microbial necromass C and soil minerals and enhance the soil’s resistance to degradation [37]. According to this relationship, N is an essential element for microbial necromass C, and it is also involved in soil texture to optimize C storage.

Necromass C increases with added N in forested lands [13], cropland systems [71], and clay-textured soils that are high in organic matter [72]. Moreover, human disturbances (such as N deposition and the application of N fertilizers) can have profound impacts on ecosystems [73]. These activities not only alter soil N availability, but also influence microbial community structure, soil C fluxes, and nutrient cycling efficiency [74]. Long-term N inputs may lead to soil acidification, nutrient imbalances, and shifts in microbial community functioning, thereby further influencing the accumulation and stability of microbial necromass C [75]. This indicates that human interventions must be considered when evaluating soil carbon management and C sequestration strategies. Furthermore, the effects of N addition on soil quality and ecological functions across different timescales warrant further investigation, especially in light of future climatic changes and evolving land management practices.

5. Conclusions

Our findings demonstrate how important climate, land uses, and soil characteristics are in determining how warming and N addition affect necromass C. Under specific environmental conditions, warming and N addition promote the accumulation of microbial necromass C more strongly. Warming strongly promoted microbial necromass C accumulation, particularly in croplands (+44%), cool regions (+16%), semi-arid regions (+15%), and soils rich in SOC, (+17%) or loam (+22%). Meanwhile, N addition further enhanced microbial necromass C sequestration, especially in croplands (+12%), forests (+10%), SOC-rich soils (+16%), and clay (+7%) or clay–loam (+12%) soils, highlighting the critical roles of climate condition land use, soil property in mediating the responses of necromass C to warming and nitrogen inputs.

While this meta-analysis provides insights into the effects of climate change on soil microbial necromass C in China, several limitations should be noted. First, the studies included are predominantly from China, and the regional and ecosystem-specific focus limits the generalizability of the findings at a global scale. Second, the analysis primarily considers the accumulation of microbial necromass C and does not fully account for potential increases in C losses under climate change, which may affect overall SOC stocks. Third, most of the studies are short-term experiments, potentially insufficient to capture long-term ecosystem responses. Moreover, the analysis cannot fully resolve complex ecosystem interactions, such as shifts in microbial community structure or soil–plant feedbacks. In addition, data gaps remain for specific climate types, land use practices, and soil properties, and incomplete information on the original climate and soil conditions of study sites constrains the development of more sophisticated models. Future research should further examine the impacts of climate change on microbial necromass C and integrate detailed climate and soil data from study sites to build robust models capable of accurately simulating and predicting the dynamics of global SOC.