Stable Carbon Pool Research Articles

Understanding how corn-yellow melilot intercropping combined with straw return increased stable organic carbon pool in Mollisols by reducing the application of chemical fertilizer

Long-term cultivation with persistent use of chemical fertilizers causes a rapid decline in organic matter content of Mollisols. But practicing intercropping along with straw return instead of the use of chemical fertilizer have the potential to reverse the decline and improve soil organic carbon (SOC) content. A field experiment with five treatments at two sites of Songnen Plain of Northeast China included the following treatments (1) corn monoculture with no chemical fertilizers and no straw return (c), (2) corn monoculture with chemical fertilizers and no straw return (c + F), (3) corn-yellow melilot intercropping and straw return combined with full application of chemical fertilizers (c&m + S + F), (4) corn-yellow melilot intercropping and straw return combined with half dose of chemical fertilizers (c&m + S + F/2), and (5) corn-yellow melilot intercropping and straw return without chemical fertilizers (c&m + S). The results showed that c&m + S + F, c + F, and c&m + S + F/2 decreased specific gravity and c&m + S + F increased water content. The c&m + S + F, c&m + S + F/2, and c&m + S improved SOC with 18.7%, 7.95%, and 8.17% (P < 0.05), respectively. The c&m + S + F and c&m + S + F/2 increased mineral-associated organic carbon, humus organic carbon, and heavy fraction organic carbon content. These findings provide a method for utilizing intercropping and straw return to replace chemical fertilizers to improve SOC and stable organic carbon content to ameliorate soil quality.

PhytOC sequestration characteristics and phytolith carbon sink capacity of the karst grasslands in southwest China

Grassland is an important component of terrestrial ecosystems and plays a crucial role in the global carbon cycle. PhytOC (phytolith-occluded organic carbon) is an extremely important long-term and stable carbon pool in terrestrial ecosystems. Southwest China karst soil exhibits obvious characteristics of alkalinity, high silicon content, and rich calcium, which can significantly influence the characteristics and mechanisms of PhytOC sequestration in vegetation. To elucidate the sequestration characteristics and mechanisms of PhytOC in the karst grasslands, three typical karst grasslands of tropical shrub tussock (TST), warm-temperate shrub tussock (WST), and mountain meadow (MM) from Guizhou province of southwest China were studied. The following results and conclusions were obtained that: 1) the range of PhytOC content of aboveground plant parts, underground roots, and soil in the karst grasslands was 4.03–16.54 g·kg−1, 10.67–33.92 g·kg−1, and 0.63–1.89 g·kg−1, respectively. The underground roots are an important site for phytolith carbon sequestration in grassland ecosystems, and the PhytOC content of underground roots may be higher than that of the aboveground parts. 2) The PhytOC sequestration rate of vegetation was 7.34–15.93 kg·ha−1·yr−1, and the annual sequestration amount of PhytOC of the whole grasslands in southwest China could reach 0.48 × 103–1.48 × 103 t CO2. Compared to grasslands in non-karst regions of China, karst grasslands in southwest China have a higher sequestration rate of PhytOC in vegetation and a greater capacity for phytolith carbon sequestration. 3) Soil available silicon, pH, and stoichiometric characteristics of C, N and P nutrients significantly affected the phytolith carbon sequestration of vegetation and the soil accumulation of PhytOC in the karst grasslands. The research results are of great significance for estimating the phytolith carbon sequestration capacity of grassland ecosystems and for grassland construction and management based on enhancing carbon sequestration.

Divergent assembly of soil microbial necromass from microbial and organic fertilizers in Chimonobambusa hejiangensis forest.

Variability in microbial residues within soil aggregates are becoming progressively essential to the nutritive and sustainability of soils, and are therefore broadly regarded as an indispensable part of soil organic matter. It is unexplored how the widespread implementation of microbial fertilisers in agricultural production impacts soil organic nutrients, in particular the microbial residue fraction. We performed a three-year field experiment to verify the distinct impacts of microbial and organic fertilizers on carbon accumulation in soil microbial leftovers among aggregate fractions. Microbial residual carbon was shown to decrease insignificantly during the application of microbial fertilizer and to rise marginally afterwards with the utilization of organic fertilizer. However, the combined effects of the two fertilizers had substantial impacts on the accumulation of microbial residual carbon. Changes in the structure of the fungi and bacteria shown in this study have implications for the short-term potential of microbial fertilizer shortages to permanent soil carbon sequestration. Additionally, our findings revealed variations in microbial residue accumulation across the microbial fertilizers, with Azotobacter chroococcum fertilizer being preferable to Bacillus mucilaginosus fertilizer due to its higher efficiency. In this scenario of nutrient addition, fungal residues may serve as the primary binding component or focal point for the production of new microaggregates, since the quantity of SOC provided by fungal residues increased while that supplied by bacterial residues decreased. Our findings collectively suggested that the mechanisms behind the observed bacterial and fungal MRC (microbial residue carbon) responses to microbial fertilizer or organic fertilizer in bamboo forest soils are likely to be distinct. The application of microbial fertilizers for a limited duration led to a decline soil stable carbon pool, potentially influencing the regulation of soil nutrients in such hilly bamboo forests.

Soil's Hidden Power: The Stable Soil Organic Carbon Pool Controls the Burden of Persistent Organic Pollutants in Background Soils.

Persistent organic pollutants (POPs) tend to accumulate in cold regions by cold condensation and global distillation. Soil organic matter is the main storage compartment for POPs in terrestrial ecosystems due to deposition and repeated air-surface exchange processes. Here, physicochemical properties and environmental factors were investigated for their role in influencing POPs accumulation in soils of the Tibetan Plateau and Antarctic and Arctic regions. The results showed that the soil burden of most POPs was closely coupled to stable mineral-associated organic carbon (MAOC). Combining the proportion of MAOC and physicochemical properties can explain much of the soil distribution characteristics of the POPs. The background levels of POPs were estimated in conjunction with the global soil database. It led to the proposition that the stable soil carbon pools are key controlling factors affecting the ultimate global distribution of POPs, so that the dynamic cycling of soil carbon acts to counteract the cold-trapping effects. In the future, soil carbon pool composition should be fully considered in a multimedia environmental model of POPs, and the risk of secondary release of POPs in soils under conditions such as climate change can be further assessed with soil organic carbon models.

Permafrost degradation and its consequences for carbon storage in soils of Interior Alaska

Permafrost soils in the northern hemisphere are known to harbor large amounts of soil organic matter (SOM). Global climate warming endangers this stable soil organic carbon (SOC) pool by triggering permafrost thaw and deepening the active layer, while at the same time progressing soil formation. But depending, e.g., on ice content or drainage, conditions in the degraded permafrost can range from water-saturated/anoxic to dry/oxic, with concomitant shifts in SOM stabilizing mechanisms. In this field study in Interior Alaska, we investigated two sites featuring degraded permafrost, one water-saturated and the other well-drained, alongside a third site with intact permafrost. Soil aggregate- and density fractions highlighted that permafrost thaw promoted macroaggregate formation, amplified by the incorporation of particulate organic matter, in topsoils of both degradation sites, thus potentially counteracting a decrease in topsoil SOC induced by the permafrost thawing. However, the subsoils were found to store notably less SOC than the intact permafrost in all fractions of both degradation sites. Our investigations revealed up to net 75% smaller SOC storage in the upper 100 cm of degraded permafrost soils as compared to the intact one, predominantly related to the subsoils, while differences between soils of wet and dry degraded landscapes were minor. This study provides evidence that the consideration of different permafrost degradation landscapes and the employment of soil fractionation techniques is a useful combination to investigate soil development and SOM stabilization processes in this sensitive ecosystem.

Afforestation, Natural Secondary Forest or Dehesas? Looking for the Best Post-Abandonment Forest Management for Soil Organic Carbon Accumulation in Mediterranean Mountains

Forest expansion in Mediterranean mountain areas is a widespread phenomenon resulting from the abandonment of agricultural and pastoral activities during the last century. Therefore, knowledge of the long-term storage capacity of soil organic carbon (SOC) in Mediterranean forests is of great interest in the context of global change. However, the effects of these land uses and covers (natural secondary forest, afforestation with conifers and silvo-pastoral ecosystems (dehesas)) on SOC dynamics are still uncertain. The main objectives of this study were to evaluate physico-chemical soil properties, SOC and nitrogen stocks, and SOC fractions in Mediterranean forests and to assess the effects of tree species, the soil environment (acidic and alkaline), and land management. We selected five land uses and land covers: managed and non-managed afforestation and dehesa (except for alkaline dehesa) and a stage of succession when tree species begin to become established after abandonment. This study concludes that although total SOC stocks are higher in afforested systems with conifers, SOC is stored in less stable carbon pools than in broadleaf forests. In addition, this study confirms that there are marked differences in the results between acidic and alkaline environments. Finally, the management system is also a significant factor, particularly for afforested sites.

Response of soil microbial necromass carbon to litter and root carbon inputs in a mid-subtropical Castanopsis carlesii plantation

Microbial necromass carbon (MNC) is a crucial source for stable soil carbon pool, and understanding its response to carbon inputs from both aboveground (litter) and belowground (roots) in subtropical forest soils is essential for assessing soil carbon stocks in global ecosystems. In a Castanopsis carlesii plantation at the Sanming Forest Ecosystem National Observation and Research Station in Fujian Province, we conducted an experiment with five treatments, including root removal (NR), aboveground litter removal (NL), no litter input (removals of both aboveground litter and roots, NI), double aboveground litter addition (DL), and control (CK). After seven years, we collected soil samples in the 0-10 cm soil layer to examine changes in MNC content and its contribution to soil organic carbon (SOC). Results showed that NR treatment reduced MNC, bacterial necromass carbon (BNC), and fungal necromass carbon (FNC) by 15.9%, 20.2%, and 14.5%, respectively, while other treatments did not induce significant changes. The NR, NL, NI, and DL treatments did not affect the contributions of BNC, FNC, and MNC to SOC. Correlation and path analyses revealed that litter and root carbon input treatments could alter the MNC content directly or indirectly through changing soil available substrates and microbial community structure. Our results suggested that roots exert a stronger influence on the maintenance of MNC than aboveground carbon source in the mid-subtropical plantations.

Forest Structure and Carbon Reserve in Natural and Replanted Mangrove Forests in Different Years in the Limpopo Estuary, Gaza Province, Mozambique

The Limpopo estuary mangrove forest covers about 928 ha; however, 382 ha remain intact, and 546 ha were degraded after the 2000 floods. Mangrove replanting campaigns were carried out at the site. This study assesses the ability of restored forests to provide carbon storage functions. The results showed that A. marina was the dominant species in all study areas. The carbon reserve of living biomass above and below ground in the natural forest was 67.9 ± 100.9 MgCha−1 and 65.0 ± 77.1 MgC ha−1, respectively; in the planted forests (2016, 2014, 2010), it was 1.1 ± 0.5 MgCha−1 and 2.1 ± 1.0 MgCha−1, 1.8 ± 1.0 MgCha−1 and 3.6 ± 2.0 MgCha−1, 3.7 ± 2.0 MgCha−1 and 5.3 ± 2.5 MgCha−1. Soil carbon reserve was 229.4 ± 119.4 MgCha−1 in natural forest and 230.3 ± 134.8 MgCha−1, 234.8 ± 132.7 MgC ha−1, 229.4 ± 119.4 MgCha−1 in planted forests (2016, 2014, 2010). The total carbon reserve in the natural forest was 362.3 MgCha−1; and 233.5 MgCha−1, 240.2 MgCha−1 and 246.4 MgCha−1 in the planted forests (2016, 2014, 2010), respectively. Natural and restored forests had similar amounts of soil carbon, which reinforces the idea that soil is a stable carbon pool. Moreover, restored forests failed to store the same amount of live biomass (carbon), which supports the idea that it is better to prevent habitat degradation than to restore it.

Accumulation of soil microbial extracellular and cellular residues during forest rewilding: Implications for soil carbon stabilization in older plantations

Soil microbial residues are a critical component of stable soil organic carbon pools in terrestrial ecosystems. Plantations are anticipated to increase soil stable carbon stocks and contribute toward addressing global climate change. How rewilding, as a strategy to restore the multiple ecological functions of plantations, affects soil microbial residues remains to be elucidated. Here, we divided microbial residues into extracellular residues (using extracellular polymeric substances, i.e., EPS, as a proxy) and cellular residues (estimated by amino sugars) and investigated the changes in their accumulation over four decades (6–45 years) of Metasequoia glyptostroboides plantation development. It was found that forest rewilding resulted in a linear accumulation of soil microbial residues and enhanced their contributions to the soil organic carbon. Specifically, EPS-polysaccharide increased by 126.5%, while the total cellular residue carbon, fungal cellular residue carbon, and bacterial cellular residue carbon increased by 73.4%, 77.2%, and 54.3%, respectively. Correspondingly, the contributions of EPS-polysaccharide and total cellular residue carbon to the soil organic carbon increased by 66.1% and 32.1%, respectively. The main drivers behind extracellular and cellular residues differed, with fine root biomass being the main driver of extracellular residues, while soil nitrogen and organic carbon content were the main drivers of cellular residues. Our study demonstrated that through rewilding, microbial extracellular and cellular residues could continuously accumulate in soils and contribute significantly to the sequestration of atmospheric carbon, highlighting the need to incorporate both microbial residues into soil carbon models to address global climate change. The results suggested that plantations should be allowed to grow older rather than being prematurely harvested, as they promote soil carbon sequestration and stabilization, which has important implications for the establishment of stable soil carbon pools in plantations and can be an invaluable asset in the fight against climate change.

Formation of soil organic carbon pool is regulated by the structure of dissolved organic matter and microbial carbon pump efficacy: A decadal study comparing different carbon management strategies.

To achieve long-term increases in soil organic carbon (SOC) storage, it is essential to understand the effects of carbon management strategies on SOC formation pathways, particularly through changes in microbial necromass carbon (MNC) and dissolved organic carbon (DOC). Using a 14-year field study, we demonstrate that both biochar and maize straw lifted the SOC ceiling, but through different pathways. Biochar, while raising SOC and DOC content, decreased substrate degradability by increasing carbon aromaticity. This resulted in suppressed microbial abundance and enzyme activity, which lowered soil respiration, weakened in vivo turnover and ex vivo modification for MNC production (i.e., low microbial carbon pump "efficacy"), and led to lower efficiency in decomposing MNC, ultimately resulting in the net accumulation of SOC and MNC. In contrast, straw incorporation increased the content and decreased the aromaticity of SOC and DOC. The enhanced SOC degradability and soil nutrient content, such as total nitrogen and total phosphorous, stimulated the microbial population and activity, thereby boosting soil respiration and enhancing microbial carbon pump "efficacy" for MNC production. The total C added to biochar and straw plots were estimated as 27.3-54.5 and 41.4 Mg C ha-1 , respectively. Our results demonstrated that biochar was more efficient in lifting the SOC stock via exogenous stable carbon input and MNC stabilization, although the latter showed low "efficacy". Meanwhile, straw incorporation significantly promoted net MNC accumulation but also stimulated SOC mineralization, resulting in a smaller increase in SOC content (by 50%) compared to biochar (by 53%-102%). The results address the decadal-scale effects of biochar and straw application on the formation of the stable organic carbon pool in soil, and understanding the causal mechanisms can allow field practices to maximize SOC content.

High intensity fire accelerates accumulation of a stable carbon pool in permafrost peatlands under climate warming

Peatland carbon pools store one-third of global soil carbon, but are increasingly threatened by wildfires, particularly high intensity wildfires, as a consequence of climate warming. However, with only a limited understanding of fire history reconstruction available, the long-term impacts of fire intensity on the stability of the peatland carbon pool remains poorly understood. Here, based on Fourier transform infrared spectroscopy and chemical analysis of PyC and organic matter in Hongtu (HT) peat core in the northern Great Khingan Mountains (China), historical fire intensity and fuel sources during the last 700 years were reconstructed and their effects on carbon stability evaluated. Our results showed that the major stable carbon pool (i.e. aromatics) and the retained labile carbon pool (i.e. iron-bound organic carbon, Fe-OC) in HT peatland are 278.1 ± 6.2 mg·g−1 and 6.78 ± 3.85 mg·g−1, respectively. High-intensity herb fires in peatlands occurred more easily under wet conditions and caused more PyC accumulation than shrub fires. Both climate warming and high-intensity fire promoted more aromatics and Fe-OC accumulation, increasing the overall stability of peatland carbon pool. High-intensity fire under warm climate conditions resulted in Fe-OC accumulation rates threefold higher (ca. 0.02 mg·cm−2 yr−1 to ca. 0.06 mg·cm−2 yr−1) but had no marked effects on the aromatic and PyC accumulation rates. Overall, our results suggest that high-intensity fires can accelerate stable carbon pool accumulation in peatlands during climate warming.

Microbial necromass carbon in estuarine tidal wetlands of China: Influencing factors and environmental implication

Microbial necromass is an important component of the stable soil organic carbon (SOC) pool. However, little is known about the spatial and seasonal patterns of soil microbial necromass and their influencing environmental factors in estuarine tidal wetlands. In the present study, amino sugars (ASs) as biomarkers of microbial necromass were investigated along the estuarine tidal wetlands of China. Microbial necromass carbon (C) contents were in the range of 1.2–6.7 mg g−1 (3.6 ± 2.2 mg g−1, n = 41) and 0.5–4.4 mg g−1 (2.3 ± 1.5 mg g−1, n = 41), which accounted for 17.3–66.5 % (44.8 % ± 16.8 %) and 8.9–45.0 % (31.0 % ± 13.7 %) of the SOC pool in the dry (March to April) and wet (August to September) seasons, respectively. At all sampling sites, fungal necromass C predominated over bacterial necromass C as a component of microbial necromass C. Compared to bacterial necromass C, fungal necromass C showed a stronger connection with ferrous oxides (Fe2+) and total Fe concentrations. Both fungal and bacterial necromass C contents revealed large spatial heterogeneity and declined in the estuarine tidal wetlands with the increase in latitude. Statistical analyses showed that the increases in salinity and pH in the estuarine tidal wetlands suppressed the accumulation of soil microbial necromass C.

Changes of Soil Dissolved Organic Matter and Its Relationship with Microbial Community along the Hailuogou Glacier Forefield Chronosequence.

Glacier-retreated areas are ideal areas to study soil biogeochemical processes during vegetation succession, because of the limited effect of other environmental and climatic factors. In this study, the changes of soil dissolved organic matter (DOM) and its relationship with microbial communities along the Hailuogou Glacier forefield chronosequence were investigated. Both microbial diversity and DOM molecular chemodiversity recovered rapidly at the initial stage, indicating the pioneering role of microorganisms in soil formation and development. The chemical stability of soil organic matter enhanced with vegetation succession due to the retaining of compounds with high oxidation state and aromaticity. The molecular composition of DOM affected microbial communities, while microorganisms tended to utilize labile components to form refractory components. This complex relationship network between microorganisms and DOM components played an important role in the development of soil organic matter as well as the formation of stable soil carbon pool in glacier-retreated areas.

Responses of microbial necromass carbon and microbial community structure to straw- and straw-derived biochar in brown earth soil of Northeast China.

Soil microbial organisms are conducive to SOC sequestration. However, little attention has been given to the contributions of living MBC and microbial necromass carbon to the SOC pool under biochar and straw amendments. The aims of the study were to explore (1) the effects of maize straw and biochar on MBC, POC, MAOC, DOC and microbial necromass carbon; (2) the contribution of MBC and microbial necromass carbon to the SOC pool; and (3) the relationships among the soil microbial community structure, microbial necromass carbon and other SOC fractions under maize straw and biochar application for nine consecutive years. Three treatments were studied: CK (applied chemical fertilizer only), BC (biochar applied annually at a rate of 2.625 t ha−1 combined with chemical fertilizer), and SR (straw applied annually at a rate of 7.5 t ha−1). Both biochar and straw increased the SOC contents after nine successive maize plant seasons; the DOC and MAOC contents were also increased by biochar and straw amendments. Biochar had advantages in increasing POC contents compared to straw. Biochar and straw increased MBC contents by 48.54% and 60.83% compared to CK, respectively. Straw significantly increased the Galn, GluN, MurA, ManN and total amino contents (P < 0.05); however, biochar significantly increased the Galn and GluN contents (P < 0.05) but had no impact on the MurA contents and decreased the ManN contents. Biochar mainly increased the fungal-derived necromass carbon contents but had no effect on the bacterial-derived necromass carbon, and straw increased both the bacterial- and fungal-derived necromass carbon contents. Straw had no influence on the ratios of microbial necromass carbon accounting for SOC and MAOC, but biochar decreased the ratios in the current study. Similarly, biochar mainly increased the fungal PLFA and total PLFA contents compared to CK, but straw increased bacterial PLFAs, fungal PLFAs and Actinomycetes PLFAs. Maize yield were increased by 7.44 and 9.16% by biochar and straw application, respectively. These results indicate that biochar stimulates fungal activities and turnover to contribute to the stable soil carbon pool and that biochar also improves POC contents to improve the soil organic carbon sink.

Forests have a higher soil C sequestration benefit due to lower C mineralization efficiency: Evidence from the central loess plateau case

Soil carbon (C) mineralization varies with vegetation restoration and plays an essential role in the soil C cycle. However, the response of soil C mineralization to different vegetation restoration types remains unclear. In this study, three vegetation restoration types for the restoration of cropland, namely forestland (Robinia pseudoacacia), shrubland (Caragana korshinskii), and grassland (Artemisia sacrorum and Stipa bungeana), were selected to determine how vegetation restoration types affect soil C mineralization. The vegetation in the assessed areas had been restored 20 years before this study. The results showed that vegetation restoration significantly increased soil CO2 efflux, whereas soil C mineralization efficiency (CME) decreased after cropland conversion. The lowest soil CO2 efflux and CME were observed in forestland, with the highest stable soil organic carbon (SOC) pool. Vegetation restoration significantly increased SOC and its C fraction stocks, and the variation in soil C fractions was regulated by vegetation restoration type. Soil microbial biomass carbon (MBC) and dissolved organic carbon (DOC) effectively reflected the variation in soil C mineralization and stability of the SOC pool, respectively. Soil CO2 efflux was controlled by litter biomass, macroaggregates, particulate organic C, MBC, and DOC, whereas CME was mainly influenced by aboveground biomass, pH, silt and clay content, macroaggregates and DOC. The study suggested that forestland should be selected as the preferred vegetation restoration type in the central Loess Plateau owing to its lower CME with similar C stocks.

Nitrogen availability and mineral particles contributed fungal necromass to the newly formed stable carbon pool in the alpine areas of Southwest China

Nitrogen (N) addition can essentially enhance soil carbon (C) storage in terrestrial ecosystems. However, there is little information concerning how N availability regulates new C sequestration and potential microbial mechanisms in different forest soils. Here, we investigated the effects of N addition on the distribution of newly formed C in particulate organic C (POC) and mineral-associated organic C (MAOC) fractions, as well as the newly formed amino sugars C (i.e. biomarkers for fungal and bacterial-derived microbial necromass) in MAOC under coniferous and broad-leaved forest soils in alpine areas of Southwest China, based on 60-day incubation experiments by adding 13C labeled glucose and NH4Cl. The newly formed C derived from glucose was heavily distributed in MAOC fractions, with 90.8% in coniferous forest soil and 83.2% in broad-leaved forest soil, reflecting that new C was predominantly stabilized by soil mineral particles under glucose addition. Despite the divergent soil organic C (SOC) content and quality (C:N ratio) in both forest soils, we detected that N addition decreased the newly formed C by lowering the new POC and MAOC. Conversely, N addition significantly enhanced the new microbial necromass, with more fungal necromass associated with mineral particles, resulting in a high ratio of new fungal/bacterial necromass in the MAOC fraction in both forest soils. However, the lower ratio of new fungal/bacterial necromass was observed in the broad-leaved forest soil with higher mineral particles, which was primarily attributed to the preferential accumulation of bacterial necromass on mineral particles. These results further underpinned the importance of soil mineral particles in regulating the contribution of fungal necromass to a new stable C pool. Collectively, this study revealed that N availability and soil mineral particles determined the contribution of fungal necromass to the new stable C pool, thereby providing novel insights for accurately assessing the soil C stability of various forest types under climate change.

Circular utilization of urban tree waste contributes to the mitigation of climate change and eutrophication

Circular utilization of urban tree waste contributes to the mitigation of climate change and eutrophication

Microbial Residue Distribution in Microaggregates Decreases with Stand Age in Subtropical Plantations

Soil microbial residues contribute to the majority of stable soil organic carbon (SOC) pools, and their distribution among aggregate fractions determines long-term soil carbon (C) stability and, consequently, soil productivity. However, how microbial residue accumulation and distribution respond to stand age remains unexplored. To fill this knowledge gap, we investigated microbial residues in bulk soil and soil aggregate fractions under a chronosequence of Chinese fir (Cunninghamia lanceolata [Lamb.] Hook) plantations with stands aged 3, 17, 27, and 36 years. The results showed that microbial residues in topsoil did not change across the different stand ages, but the residues in the subsoil increased from 3 to 17 years of age and then remained constant. Moreover, microbial residue distribution in microaggregates decreased with stand age, and the residue distribution in small macroaggregates was lower at age 17 years than at other stand ages. The effect of stand age on microbial residue distribution was due to the fact of their effect on aggregate distribution but not microbial residue concentrations in aggregate fractions. Collectively, our results indicate that microbial residue stability decreased with stand age, which has significant implications for the management of SOC in subtropical plantations.

Tree biomass allocation differs by mycorrhizal association

Tree biomass allocation to leaves, roots, and wood affects the residence time of carbon in forests, with potentially dramatic implications for ecosystem carbon storage. However, drivers of tree biomass allocation remain poorly quantified. Using a combination of global data sets, we tested the relative importance of climate, leaf habit, and tree mycorrhizal associations on biomass allocation. We show that trees that associate with arbuscular mycorrhizal (AM) fungi allocate roughly 4% more of their biomass to root tissue than trees that associate with ectomycorrhizal (ECM) fungi. Further, the effect of mycorrhizal association on root biomass allocation was greater than that of climate and similar in magnitude to that of leaf habit (evergreen vs. deciduous). These patterns in whole-plant biomass allocation are likely due to differences in carbon investment toward root versus fungal tissues, where trees with AM fungi favor root production while trees with ECM fungi favor fungal tissue production. These results suggest that considering tree mycorrhizal associations could improve our understanding of ecosystem carbon storage in terrestrial biosphere models: specifically, that greater within-tree allocation to root biomass in AM-associated tree species may contribute to stable soil carbon pools in forests dominated by AM fungi.

The accumulation of microbial residues and plant lignin phenols are more influenced by fertilization in young than mature subtropical forests

Soil microbial residues and plant-derived compounds are important components of the stable carbon (C) pool in forest soil. It is not well understood how microbial residues and plant lignin phenols respond to nutrient inputs in subtropical forests of different ages, especially in mature forest stands. Here, a 5-year nitrogen (N) and phosphorus (P) fertilization (addition) management experiment was conducted in two stages of subtropical forests (<40- and > 80-year). We found that the concentrations of amino sugars (i.e., microbial residues) and plant lignin phenols and their contributions to soil organic carbon (SOC) were significantly higher in the > 80-year forests than in the < 40-year forests. In the < 40-year forests, N addition increased amino sugars and lignin phenols and their contributions to SOC, whereas P addition decreased muramic acid (bacterial-derived) and its contribution to SOC. In the > 80-year forests, however, N and P addition had no significant effect on amino sugars and lignin phenols and their contributions to SOC. Overall, soil total amino sugars and lignin phenols could be directly and indirectly affected by forest age and N addition, and could be only indirectly impacted by P addition. Our results suggest that microbial residues and plant lignin phenols are sensitive to the nutrient additions in the young forests, which have a greater potential for the soil stable C sequestration in comparison with the mature forests.