The multi-pathway mechanism of soil organic carbon accumulation in Pinus massoniana plantations under nitrogen and phosphorus addition: from soil biota to carbon stability

Mixing native broadleaved tree species is a widely used method for renovating Pinus massoniana plantations. Soil microbial necromass carbon and organic carbon fractions are important parameters for evaluating the impacts of tree species mixing and soil organic carbon (SOC) stability. However, their responses to the mixing and renovation of P. massoniana plantation has not been understood yet. Here, we selected a pure P. massoniana plantation (PP) and a mixed P. massoniana and Castanopsis hystrix plantation, with ages of 16 (MP16) and 38 years (MP38), respectively, as the research objects. We quantified soil physical and chemical properties, microbial necromass carbon content, and organic carbon components at different soil layers to reveal whether and how the introduction of C. hystrix into P. massoniana plantation affected soil microbial necromass carbon and organic carbon components. The results showed that the mixed P. massoniana and C. hystrix plantation significantly reduced fungal necromass carbon content and the ratio of fungal/bacterial necromass carbon in the 0-20 cm and 20-40 cm soil layers. There were no significant differences in microbial necromass carbon contents, bacterial necromass carbon contents, and their contributions to SOC among the different plantations. The contribution of fungal necromass carbon to SOC was higher than that of bacterial necromass carbon in all plantation types. The contribution of soil mineral-associated organic carbon (MAOC) to SOC was higher than that of occluded particulate organic carbon (oPOC) and light-free particulate organic carbon (fPOC) for all plantation types. Mixing the precious broadleaved tree species (i.e., C. hystrix) with coniferous species (P. massoniana) significantly increased MAOC content and the contribution of MAOC, oPOC, and fPOC to SOC in the 0-20 cm and 20-40 cm soil layers. The MAOC of MP38 was significantly higher than that of PP in all soil layers and the MAOC of MP38 stands were significantly higher than MP16 stands in the 20-40 cm, 40-60 cm, and 60-100 cm soil layers, indicating that hybridization enhanced SOC stability and that the SOC of MP38 stands were more stable than MP16 stands. SOC and total nitrogen contents were the main environmental factors driving the changes in soil microbial necromass carbon, while soil total nitrogen and organically complexed Fe-Al oxides were the primary factors affecting organic carbon fraction. Therefore, SOC stability can be enhanced by introducing native broadleaved species, such as C. hystrix, during the management of the P. massoniana plantation.

Tidal marshes serve as critical carbon (C) sinks, yet face increasing threats from global environmental changes. While previous research has documented how nitrogen (N) loading and sea‐level rise affect total C pools individually, their impacts on soil organic carbon (SOC) stabilization remain critically underexplored, particularly when these factors co‐occur in tidal marsh ecosystems. Through a 3‐yr field experiment, we analyzed how these factors, alone and combined, impact SOC stabilization by examining SOC fraction dynamics. Results showed that N loading increased particulate organic carbon (POC) by 18% and decreased mineral‐associated organic carbon (MAOC) by 13%, reducing SOC stabilization. Conversely, increased inundation raised MAOC by 31% and decreased POC by 19%, promoting SOC stabilization. The decreased MAOC under N loading stemmed from reduced fungal necromass C, while the increased POC related to lower phenol oxidase activity. In contrast, with increased inundation, MAOC rose due to iron‐bound organic C (Fe‐OC) accumulation, while POC declined from increased phenol oxidase activity. When both factors were applied together, SOC stabilization remained at control levels. This occurred because the combined effect maintained oxidative enzyme activities and thus retained POC levels. The simultaneous reduction in fungal necromass C and enhancement of Fe‐OC associations established complementary mechanisms that maintained MAOC at levels equivalent to control. Our findings reveal that N loading and increased inundation drive contrasting patterns of SOC stabilization, while their combination produces uniquely stabilized C dynamics. This insight challenges single‐factor predictions and underscores the importance of multi‐factor experiments in understanding ecosystem responses under concurrent global change scenarios.

A series of nitrogen (N) and phosphorus (P) addition experiments using treatments of N0(0 kg N·hm-2·a-1), N1(50 kg N·hm-2·a-1), N2(100 kg N·hm-2·a-1), P (50 kg P·hm-2·a-1), N1P and N2P were conducted at Cunninghamia lanceolata plantations in subtropical China. The responses of soil organic carbon (SOC), particulate organic carbon (POC) and water-soluble organic carbon (WSOC) to the nutrient addition treatments after 3 years were determined. The results showed that N and P additions had no significant effects on SOC concentration in 0-20 cm soil layer, while P addition significantly decreased soil POC content in 0-5 cm soil layer by 26.1%. The responses of WSOC to N and P addition were mainly found in 0-5 cm soil layer, and low level N and P addition significantly increased the WSOC content in 0-5 cm soil layer. Nitrogen addition had no significant effect on POC/SOC, while the POC/SOC significantly decreased by 15.9% in response to P addition in 0-5 cm soil layer. In 5-10 cm and 10-20 cm soil layers, POC/SOC was not significantly altered in N and P addition treatments. Therefore, the forest soil C stability was mainly controlled by P content in subtropical areas. P addition was liable to cause the decomposition of surface soil active organic C and increased the soil C stability in the short term treatment.

Increased atmospheric nitrogen (N) deposition alters the formation and stability of soil organic carbon (SOC) in fragile ecosystems. While biochar (BC) amendment represents a promising strategy for augmenting soil carbon sequestration, its impact on the stability of the SOC pool under high N deposition remains unclear. In this study, we conducted a two-year field trial with three replicates to investigate the effects of combined N (0 and 9 g N·m−2·yr−1) and BC (0, 20, and 40 t·ha−1) addition on the stability of the SOC pool in restored grasslands on the Loess Plateau. We assessed SOC pool stability by examining the influence of soil microbial carbon utilization efficiency (CUE), metabolic constraints, and community composition on the content of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). The results indicate that in comparison to the control treatment (N0BC0), the addition of both high N (N9BC0) and BC (N0BC20 and N0BC40) significantly promoted the accumulation of POC by 15.78%, 9.87%, and 11.05%, respectively. Conversely, the content of MAOC was suppressed under the N9BC0 (−10.64%) and N0BC40 (−8.29%) treatments. However, the combination of high N and BC treatments resulted in increased levels of SOC, POC, and MAOC, while simultaneously reducing the MAOC/POC ratio, with all parameters reaching their peak under the N9BC40 treatment. Meanwhile, high N and BC additions led to differences in bacterial community structure, increased CUE, and enzyme vector angle. Notably, high N shifted the dominant factor of BC on MAOC/POC from physicochemical properties to biological factors. Microbes drive CUE to influence changes in MAOC by adapting to metabolic limitations and stoichiometric imbalances. In contrast, POC is primarily influenced by physicochemical properties. Overall, high additions of N and BC have been shown to reduce the stability of SOC by promoting the accumulation of POC. However, an addition rate of 40 t·ha−1 of BC was found to be more effective in mitigating the negative impacts of high N addition on MAOC. This strategy can serve as an effective management approach for enhancing SOC sequestration in vulnerable regions of the Loess Plateau.

Numerous studies have explored the impacts of nitrogen (N) deposition on soil organic carbon (SOC) dynamics. However, limited research has investigated the modulatory role of N deposition in urban to rural forests and the underlying microbial mechanisms. We carried out a 5-year field study to explore the links between microbial properties (microbial biomass carbon (MBC), microbial diversity, community composition and functions) and the different SOC fractions (particulate organic carbon, POC and mineral-associated organic carbon, MAOC) submitted to three levels of N addition rates (0, 50, and 100 kg N ha-1 yr-1) in urban–rural gradient forests in eastern China.We discovered that N addition raised the soil ammonium nitrogen concentration in urban and suburban forests. However, it had no effect on soil acidification or POC or SOC accumulation，and in urban forest the stability was due to the 105 % to 110 % increase in the mineral-associated organic carbon (MAOC) through enhancing peroxidase activity and microbial biomass carbon. On the contrary, high nitrogen input significantly reduced SOC stability in the suburban and rural forest stands. High nitrogen input contributed to the loss of MAOC (-33.6 %) in the suburban forest stand due to the enhancement of microbial biomass nitrogen. High nitrogen addition also decreased the ratio of MAOC to SOC in the rural forest stand by 29.8 % through indirect pathways mediated by the soil Ca2+ concentration and polyphenol oxidase activity. We concluded that SOC in the urban forest was stable when subjected to increased nitrogen deposition, primarily due to the enhancement of MAOC driven by microbial function. This finding has contributed to a better understanding  in predicting forest carbon cycling under conditions of global climate change and urban expansion.

Biochar affects organic carbon composition and stability in highly acidic tea plantation soil

滇南地区桃树种植模式对土壤有机碳组分及碳库管理指数的影响

The integration of manure and straw substantially affects soil organic carbon (SOC) dynamics, transformation, and long-term stabilization in agricultural systems. Dissolved organic carbon (DOC), particulate organic carbon (POC), and mineral-associated organic carbon (MOC) are the three main components of the SOC pool, each influencing soil carbon dynamics and nutrient cycling. Current research gaps remain regarding how combined fertilization practices affect the inputs of plant-originated and microbe-derived carbon into SOC pools and stability mechanisms. Our investigation measured SOC fractions (DOC, POC, MOC), SOC mineralization rate (SCMR), microbial necromass carbon, lignin phenols, enzyme activities, and microbial phospholipid fatty acids (PLFAs) over a long-term (17 years) field experiment with four treatments: mineral fertilization alone (CF), manure-mineral combination (CM), straw-mineral application (CS), and integrated manure-straw-mineral treatment (CMS). The CMS treatment exhibited notably elevated levels of POC (7.42 g kg−1), MOC (10.7 g kg−1), and DOC (0.108 g kg−1), as well as a lower SCMR value (1.85%), compared with other fertilization treatments. Additionally, the CMS treatment stimulated the buildup of both bacterial and fungal necromass while enhancing the concentrations of ligneous biomarkers (vanillin, syringyl, and cinnamic derivatives), which correlated strongly with the elevated contents of fungal and bacterial PLFAs and heightened activity of carbon-processing enzymes (α-glucosidase, polyphenol oxidase, cellobiohydrolase, peroxidase, N-acetyl-β-D-glucosidase). Furthermore, fungal and bacterial microbial necromass carbon, together with lignin phenols, significantly contributed to shaping the composition of SOC. Through random forest analysis, we identified that the contents of bacterial and fungal necromass carbon were the key factors influencing DOC and MOC. The concentrations of syringyl phenol and cinnamyl phenols, and the syringyl-to-cinnamyl phenols ratio were the primary determinants for POC, while the fungal-to-bacterial necromass carbon ratio, as well as the concentrations of vanillyl, syringyl, and cinnamyl phenols, played a critical role in SCMR. In conclusion, the manure combined with straw incorporation not only promoted microbial growth and enzyme activity but also enhanced plant- and microbial-derived carbon inputs. Consequently, this led to an increase in the contents and stability of SOC fractions (DOC, POC, and MOC). These results suggest that manure combined with straw is a viable strategy for soil fertility due to its improvement in SOC sequestration and stability.

Soil organic carbon pool distribution and stability with grazing and topography in a Mongolian grassland

Greater variation of soil organic carbon in limestone- than shale-based soil along soil depth in a subtropical coniferous forest within a karst faulted basin of China

The stabilization of soil organic carbon (SOC) is influenced by soil microbes and environmental factors, particularly temperature, which significantly affects SOC decomposition. This study investigates the effects of temperature (ambient: 25 °C; elevated: 27.5 °C) and soil microbial diversity (low, medium, and high) on the formation of stabilized SOC, focusing on mineral-associated organic carbon (MAOC) and water-stable aggregates, through a 75-day model soil incubation experiment. We measured water-stable aggregates, microbial respiration, and SOC in different fractions. Our results demonstrate that microbial diversity is crucial for SOC mineralization; low diversity resulted in 3.93–6.26% lower total carbon and 8.05–17.32% lower particulate organic carbon (POC) compared to medium and high diversity under the same temperature. While total MAOC was unaffected by temperature and microbial diversity, macroaggregate-occluded MAOC decreased by 8.78%, 38.36% and 9.40% under elevated temperature for low, medium and high diversity, respectively, likely driven by decreased macroaggregate formation. A negative correlation between macroaggregate-occluded POC and microbial respiration (r= -0.37, p < 0.05) suggested microbial decomposition of POC within macroaggregates contributed to respiration, with a portion of the decomposed POC potentially stabilized as microbial-derived MAOC. Notably, soils with medium microbial diversity exhibited the highest levels of both macroaggregate-occluded POC and MAOC at ambient temperature; however, elevated temperature disrupted this stabilization, reducing both POC retention and MAOC accumulation within macroaggregates. These findings underscore the temperature-sensitive interplay between microbial diversity and SOC stabilization, highlighting the need to disentangle microbial pathways governing C dynamics under climate change.

Effects of warming on soil organic carbon pools mediated by mycorrhizae and hyphae on the Eastern Tibetan Plateau, China

Based on a long-term positioning experiment established in 2017 with biochar derived from Eucalyptus plantation waste branches pyrolyzed at high temperature （500℃） under anaerobic conditions， six treatments were established： CK （0%）， T1 （0.5%）， T2 （1.0%）， T3 （2%）， T4 （4%）， and T5 （6%）. The study explored the changes in soil organic carbon （SOC）， labile organic carbon （LOC）， carbon pool management index （CPMI）， and the chemical structure of organic carbon after a single application of different amounts of biochar over 5 years. The study produced several results： ① Compared to the control， biochar application significantly increased the contents of SOC， LOC， particulate organic carbon （POC）， non-labile organic carbon （NLOC）， and mineral-associated organic carbon （MOC） （P&lt;0.05）， with larger increments at high application rates （T4 and T5 treatments）. The soil CPMI showed a significant increasing trend with increase of biochar application. ② Biochar application increased the relative contents of alkyl carbon and aromatic carbon in the soil， while decreasing the relative contents of alkoxy carbon and carboxyl carbon （P&lt;0.05）. At high application rates， there was a significant increase in the alkyl carbon/alkoxy carbon ratio， hydrophobic carbon/hydrophilic carbon ratio， and aromatic carbon/alkoxy carbon ratio， and a decrease in the lipid carbon/aromatic carbon ratio （P&lt;0.05）， with an increased trend toward aromaticity. ③ Correlation and principal component analyses revealed that the soil CPMI was significantly positively correlated with SOC， LOC， POC， alkyl carbon， aromatic carbon， pH value， soil moisture content， total nitrogen， total phosphorus， total potassium， available nitrogen， available phosphorus， available potassium， microbial biomass carbon， microbial biomass nitrogen， and MOC （P&lt;0.01）. It was also positively correlated with MOC （P&lt;0.05） and negatively correlated with alkoxy carbon， carboxyl carbon， and soil bulk density （P&lt;0.01）. Redundancy analysis indicated that soil bulk density， LOC， SOC， total potassium， POC， microbial biomass carbon， and available potassium were key environmental factors affecting the soil CPMI and the chemical structure of organic carbon. In conclusion， the application of biochar to Eucalyptus plantation through 5 years improved soil quality， which is beneficial for enhancing soil carbon sequestration capacity and increasing the stability of soil organic carbon.

Background Soil organic carbon (SOC) represents the largest carbon sink in terrestrial ecosystems. Anthropogenic nitrogen inputs increase the levels of reactive nitrogen, significantly influencing SOC dynamics. Given the complexity of SOC, it remains unclear how its key components—particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)—respond to nitrogen addition over time, due to their fundamental differences in formation, persistence, and functions. We conducted a meta-analysis of 105 global studies and 499 observational results to investigate the responses of soil organic carbon components to long-term nitrogen addition. Results The results showed that nitrogen addition had a consistently positive impact on POC content, with the impact becoming increasingly pronounced with increasing nitrogen addition duration. Short-term nitrogen addition led to significant increases in MAOC content (9.47%), while long-term addition (over 20 years) resulted in a notable 12.19% reduction. The observed increase in aboveground biomass following nitrogen addition likely drives the rise in POC, whereas the inhibitory effects of long-term nitrogen addition on microbial activity may explain the negative effects on MAOC. Conclusions Overall, although long-term nitrogen addition will promote soil carbon sequestration to a certain extent, the stability of the soil carbon pool might be reduced. Therefore, future studies on the response mechanisms of soil carbon stability to nitrogen deposition need to take into account the impact of duration to improve our understanding of the spatial and temporal variations in soil carbon cycling amidst climate change challenges.

Responses of particulate and mineral-associated organic carbon to temperature changes and their mineral protection mechanisms: A soil translocation experiment