Divergent accumulation of amino sugars and lignins mediated by soil functional carbon pools under tropical forest conversion

Natural forests vs. plantations: A meta-analysis of consequences for soil organic carbon functional fractions

Plantations are becoming more common globally as one of the important initiatives to mitigate global climate change, but the results on whether the soil organic carbon (SOC) can reach the level of natural forests are still inconsistent. Here, we conducted a meta-analysis of 418 paired observations, comparing plantations to adjacent natural forests (primary and secondary forests), from 47 published studies to explore the global patterns and associated drivers of SOC functional fractions (particulate OC, POC; mineral-associated OC, MAOC) and their ratios (the ratio of POC to MAOC, POC:MAOC; the ratio of POC to SOC, POC:SOC; the ratio of MAOC to SOC, MAOC:SOC). We found significant reductions of POC (42.4%, 35.9%), MAOC (19.4%, 15.2%), POC:MAOC (29.0%, 25.5%), and POC:SOC (18.2%, 18.9%) in plantations compared to primary and secondary forests. In contrast, MAOC:SOC in plantations had no change. The effects of plantations on POC, MAOC and their ratios were significantly affected by tree species, plantation age, soil type, and soil depth. Moreover, soil physical properties (soil bulk density, mean weight diameter), element contents (total phosphorus) and microbial communities (microbial biomass C) appeared to be drivers of lower POC, MAOC and their ratios in plantations. Our findings suggest that the reduction of SOC in plantations is mainly distributed in relative labile POC. The results reveal that the SOC functional fractions in plantations developed over time and were comparable to adjacent secondary forests after about 60 years, and plantations with native species will be more conducive to the formation of POC and MAOC. We emphasize that primary forests are not replaceable, and plantations with native species might be a reliable way for restoring a stable distribution of SOC.

Microbial necromass carbon contributed to soil organic carbon accumulation and stabilization in the newly formed inland wetlands

Inland wetlands might be an important "carbon sink", and the chronosequence development of newly formed inland wetlands offers a natural and suitable opportunity for studying the dynamic effect of plant and microbial necromass carbon (PlantC and MNC) on the soil organic carbon (SOC) stabilization. The space-for-time chronosequence approach was used and plots were established in the three ages of newly formed inland wetlands (2, 5, and 16 years). Soil samples were collected in the surface (0-10 cm) and subsurface soil (20-30 cm). Results showed that accumulation of SOC, PlantC, and MNC were significantly larger in the surface than those in the subsurface soil. Moreover, MNC stocks were more abundant than PlantC in the wetland ecosystem both in the surface and subsurface soil. During the chronosequence development, dynamics of SOC and its components accumulation were similar to MNC, both exhibiting an increasing and then decreasing trend in the surface and subsurface soil, except for free particulate organic carbon in the subsurface soil. Structural equation models revealed that changes of MNC affected by environmental variables were the main cause of MAOC dynamics both in the surface and subsurface soil, suggesting that contribution of MNC to MAOC would be the key way of carbon stabilization in the newly formed inland wetlands. Furthermore, MNC accumulation in the surface soil was closely linked to pH, CEC, and soil texture, while in the subsurface soil affected by soil nutrients (TN and NH4+-N). Particularly, despite the decreasing SOC stocks in the 16-year wetland, the stability has significantly enhanced due to the increasing persistent individual amino sugars. This study provides new information on the dynamics of SOC accumulation and highlights the significance of MNC on the SOC sequestration in the newly formed inland wetlands, which is important for the understanding of wetland SOC stock dynamics and stabilization mechanisms.

Planting Enhances Soil Resistance to Microplastics: Evidence from Carbon Emissions and Dissolved Organic Matter Stability

Microplastics (MPs) have become a global hotspot due to their widespread distribution in recent years. MPs frequently interact with dissolved organic matter (DOM) and microbes, thereby influencing the carbon fate of soils. However, the role of plant presence in regulating MPs-mediated changes in the DOM and microbial structure remains unclear. Here, we compared the mechanisms of soil response to 3 common nonbiodegradable MPs in the absence or presence of radish (Raphanus sativus L. var. radculus Pers) plants. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) analysis revealed that MPs reduced the chemodiversity and biodiversity of dissolved organic matter (DOM). MPs enhanced the degradation of lignin-like compounds and reduced the DOM stability. Comparative analysis showed that MPs caused less disturbance to the microbial composition and metabolism in planted soil than in unplanted soil. In unplanted soil, MPs stimulated fermentation while upregulating photoautotrophic activity in planted soil, thereby enhancing system stability. The rhizosphere effect mitigated MPs-induced CO2 emissions. Overall, our study highlights the crucial role of rhizosphere effects in maintaining ecosystem stability under soil microbe-DOM-pollutant interactions, which provides a theoretical basis for predicting the resistance, resilience, and transitions of the ecosystem upon exposure to the anthropogenic carbon source.

Grassland degraded patchiness reduces microbial necromass content but increases contribution to soil organic carbon accumulation

Plant and microbially derived carbon (C) are the two major sources of soil organic carbon (SOC), and their ratio impacts SOC composition, accumulation, stability, and turnover. The contributions of and the key factors defining the plant and microbial C in SOC with grassland patches are not well known. Here, we aim to address this issue by analyzing lignin phenols, amino sugars, glomalin-related soil proteins (GRSP), enzyme activities, particulate organic carbon (POC), and mineral-associated organic carbon (MAOC). Shrubby patches showed increased SOC and POC due to higher plant inputs, thereby stimulating plant-derived C (e.g., lignin phenol) accumulation. While degraded and exposed patches exhibited higher microbially derived C due to reduced plant input. After grassland degradation, POC content decreased faster than MAOC, and plant biomarkers (lignin phenols) declined faster than microbial biomarkers (amino sugars). As grassland degradation intensified, microbial necromass C and GRSP (gelling agents) increased their contribution to SOC formation. Grassland degradation stimulated the stabilization of microbially derived C in the form of MAOC. Further analyses revealed that microorganisms have a C and P co-limitation, stimulating the recycling of necromass, resulting in the proportion of microbial necromass C in the SOC remaining essentially stable with grassland degradation. Overall, with the grassland degradation, the relative proportion of the plant component decreases while than of the microbial component increases and existed in the form of MAOC. This is attributed to the physical protection of SOC by GRSP cementation. Therefore, different sources of SOC should be considered in evaluating SOC responses to grassland degradation, which has important implications for predicting dynamics in SOC under climate change and anthropogenic factors.

Responses of soil organic carbon compounds to phosphorus addition between tropical monoculture and multispecies forests

Tropical forests are sensitive to nitrogen (N) and phosphorus (P) availability, and under nutrient application the variation of soil organic carbon (SOC) preserving mechanism remains to be explored. To reveal the forest-specific SOC preservation via biochemical selection in response to nutrient application, we investigated a monoculture (Acacia plantation) and a multispecies forest both with chronic fertilization in subtropical regions, and measured specific fingerprints of plant- and microbial-derived C compounds. In addition, to quantify the effect of P application on SOC content among tropical forests, we conducted a meta-analysis by compiling 125 paired measurements in field experiments from 62 studies. In our field experiment, microbial community composition and activity mediated forest-specific responses of SOC compounds to P addition. The shift of community composition from fungi towards Gram-positive bacteria in the Acacia plantation by P addition led to the consumption of microbial residual C (MRC) as C source; in comparison, P addition increased plant species with less complex lignin substrates and induced microbial acquisition for N sources, thus stimulated the decomposition of both plant- and microbial-derived C. Same with our field experiment, bulk SOC content had neutral response to P addition among tropical forests in the meta-analysis, although divergences could happen among experimental durations and secondary tree species. Close associations among SOC compounds with biotic origins and mineral associated organic C (MAOC) in the multispecies forest suggested contributions of both plant- and microbial-derive C to SOC stability. Regarding that fungal MRC closely associated with MAOC and consisted of soil N pool which tightly coupled to SOC pool, the reduce of fungal MRC by chronic P addition was detrimental to SOC accumulation and stability in tropical forests.

Continuous planting of Chinese fir monocultures significantly influences dissolved organic matter content and microbial assembly processes

Monoculture plantations in China, characterized by the continuous cultivation of a single species, pose challenges to timber accumulation and understory biodiversity, raising concerns about sustainability. This study investigated the impact of continuous monoculture plantings of Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) on soil properties, dissolved organic matter (DOM), and microorganisms over multiple generations. Soil samples from first to fourth-generation plantations were analyzed for basic chemical properties, DOM composition using Fourier transform ion cyclotron resonance mass spectrometry, and microorganisms via high-throughput sequencing. Results revealed a significant decline in nitrate nitrogen content with successive rotations, accompanied by an increase in easily degradable compounds like carbohydrates, aliphatic/proteins, tannins, Carbon, Hydrogen, Oxygen and Nitrogen- (CHON) and Carbon, Hydrogen, Oxygen and Sulfur- (CHOS) containing compounds. However, the recalcitrant compounds, such as lignin and carboxyl-rich alicyclic molecules (CRAMs), condensed aromatics and Carbon, Hydrogen and Oxygen- (CHO) containing compounds decreased. Microorganism diversity, abundance, and structure decreased with successive plantations, affecting the ecological niche breadth of fungal communities. Bacterial communities were strongly influenced by DOM composition, particularly lignin/CRAMs and tannins. Continuous monoculture led to reduced soil nitrate, lignin/CRAMs, and compromised soil quality, altering chemical properties and DOM composition, influencing microbial community assembly. This shift increased easily degraded DOM, accelerating soil carbon and nitrogen cycling, ultimately reducing soil carbon sequestration. From environmental point of view, the study emphasizes the importance of sustainable soil management practices in continuous monoculture systems. Particularly the findings offer valuable insights for addressing challenges associated with monoculture plantations and promoting long-term ecological sustainability.

Soil recalcitrant but not labile organic nitrogen mineralization contributes to microbial nitrogen immobilization and plant nitrogen uptake

Soil organic nitrogen (N) mineralization not only supports ecosystem productivity but also weakens carbon and N accumulation in soils. Recalcitrant (mainly mineral-associated organic matter) and labile (mainly particulate organic matter) organic materials differ dramatically in nature. Yet, the patterns and drivers of recalcitrant (MNrec) and labile (MNlab) organic N mineralization rates and their consequences on ecosystem N retention are still unclear. By collecting MNrec (299 observations) and MNlab (299 observations) from 57 15N tracing studies, we found that soil pH and total N were the master factors controlling MNrec and MNlab, respectively. This was consistent with the significantly higher rates of MNrec in alkaline soils and of MNlab in natural ecosystems. Interestingly, our analysis revealed that MNrec directly stimulated microbial N immobilization and plant N uptake, while MNlab stimulated the soil gross autotrophic nitrification which discouraged ammonium immobilization and accelerated nitrate production. We also noted that MNrec was more efficient at lower precipitation and higher temperatures due to increased soil pH. In contrast, MNlab was more efficient at higher precipitation and lower temperatures due to increased soil total N. Overall, we suggest that increasing MNrec may lead to a conservative N cycle, improving the ecosystem services and functions, while increasing MNlab may stimulate the potential risk of soil N loss.

Land Use Change from Natural Tropical Forests to Managed Ecosystems Reduces Gross Nitrogen Production Rates and Increases the Soil Microbial Nitrogen Limitation

Understanding the underlying mechanisms of soil microbial nitrogen (N) utilization under land use change is critical to evaluating soil N availability or limitation and its environmental consequences. A combination of soil gross N production and ecoenzymatic stoichiometry provides a promising avenue for nutrient limitation assessment in soil microbial metabolism. Gross N production via 15N tracing and ecoenzymatic stoichiometry through the vector and threshold element ratio (Vector-TER) model were quantified to evaluate the soil microbial N limitation in response to land use changes. We used tropical soil samples from a natural forest ecosystem and three managed ecosystems (paddy, rubber, and eucalyptus sites). Soil extracellular enzyme activities were significantly lower in managed ecosystems than in a natural forest. The Vector-TER model results indicated microbial carbon (C) and N limitations in the natural forest soil, and land use change from the natural forest to managed ecosystems increased the soil microbial N limitation. The soil microbial N limitation was positively related to gross N mineralization (GNM) and nitrification (GN) rates. The decrease in microbial biomass C and N as well as hydrolyzable ammonium N in managed ecosystems led to the decrease in N-acquiring enzymes, inhibiting GNM and GN rates and ultimately increasing the microbial N limitation. Soil GNM was also positively correlated with leucine aminopeptidase and β-N-acetylglucosaminidase. The results highlight that converting tropical natural forests to managed ecosystems can increase the soil microbial N limitation through reducing the soil microbial biomass and gross N production.

Vegetation restoration altered the soil organic carbon composition and favoured its stability in a Robinia pseudoacacia plantation

Soil organic carbon (SOC) stabilization is vital for the mitigation of global climate change and retention of soil carbon stocks. However, there are knowledge gaps on how SOC sources and stabilization respond to vegetation restoration. Therefore, we investigated lignin phenol and amino sugar biomarkers, SOC physical fractions and chemical structure in one farmland and four stands of a Robinia pseudoacacia plantation. We observed that the content of SOC increased with afforestation, but the different biomarkers had different contributions to SOC. Compared to farmland, the contribution of lignin phenols to SOC decreased in the plantations, whereas there was no difference among the four stand ages, likely resulting from the balance between increasing lignin derivation input and increasing lignin degradation. Conversely, vegetation restoration increased the content of microbial necromass carbon (MNC) and the contribution of MNC to SOC, mainly because microbial residue decomposition was inhibited by decreasing the activity of leucine aminopeptidase, while microbial necromass preservation was promoted by adjusting soil variables (soil water content, clay, pH and total nitrogen). In addition, vegetation restoration increased the particulate organic carbon (POC), mineral-associated organic carbon (MAOC) pools and the O-alkyl C intensify. Overall, vegetation restoration affected SOC composition by regulating lignin phenols and microbial necromass and also altered SOC stabilization by increasing the physically stable MAOC pool during late afforestation. The results of this study suggest that more attention should be given to SOC sequestration and stability during late vegetation restoration.