Soil priming effect and its responses to nutrient addition along a tropical forest elevation gradient

Soil organic matter priming: The pH effects

Priming of soil organic matter (SOM) decomposition by microorganisms is a key phenomenon of global carbon (C) cycling. Soil pH is a main factor defining priming effects (PEs) because it (i) controls microbial community composition and activities, including enzyme activities, (ii) defines SOM stabilization and destabilization mechanisms, and (iii) regulates intensities of many biogeochemical processes. In this critical review, we focus on prerequisites and mechanisms of PE depending on pH and assess the global change consequences for PE. The highest PEs were common in soils with pH between 5.5 and 7.5, whereas low molecular weight organic compounds triggered PE mainly in slightly acidic soils. Positive PEs up to 20 times of SOM decomposition before C input were common at pH around 6.5. Negative PEs were common at soil pH below 4.5 or above 7 reflecting a suboptimal environment for microorganisms and specific SOM stabilization mechanisms at low and high pH. Short-term soil acidification (in rhizosphere, after fertilizer application) affects PE by: mineral-SOM complexation, SOM oxidation by iron reduction, enzymatic depolymerization, and pH-dependent changes in nutrient availability. Biological processes of microbial metabolism shift over the short-term, whereas long-term microbial community adaptations to slow acidification are common. The nitrogen fertilization induced soil acidification and land use intensification strongly decrease pH and thus boost the PE. Concluding, soil pH is one of the strongest but up to now disregarded factors of PE, defining SOM decomposition through short-term metabolic adaptation of microbial groups and long-term shift of microbial communities.

Greenhouse gas emissions of sewage sludge land application in urban green space: A field experiment in a Bermuda grassland

Sewage sludge land application is recognized as a strategy for recycling resource and replenishing soil nutrients. However, the subsequent greenhouse gas emissions following this practice are not yet fully understood, and the lack of quantitative research and field experiments monitoring these emissions hampers the establishment of reliable emission factors. This study investigated the greenhouse gas emission characteristics of sewage sludge land application through a field experiment that monitoring soil greenhouse gas fluxes. Seven nitrogen input treatments were implemented in a typical Bermuda grassland in China, with D and C representing the amendment of digested and composted sludge, respectively, at the nitrogen input rate of 0, 100, 200, and 300 kg N ha-1. Soil CH4, CO2, and N2O fluxes were measured throughout the entire experimental period, and soil samples from different treatments at various growth stages were analyzed. The results revealed that sewage sludge land application significantly increased soil N2O and CO2 emissions while slightly reducing soil CH4 uptake. The increased CO2 emissions were biogenic and carbon-neutral, mainly due to enhanced plant root respiration. The N2O emissions were the primary greenhouse gas emissions of sewage sludge land application, which were mainly concentrated in two 50-day periods following base and topdressing fertilization, respectively. N2O emissions following base fertilization by rotary tillage were substantially lower than those following topdressing fertilization. A logarithmic response relationship between N input rates and increased soil N2O emissions was observed, suggesting lower N2O emissions from sewage sludge land application compared to conventional N fertilizers at the same N input level. Future field experiments and meta-analysis are necessary to develop reliable greenhouse gas emission factors for sewage sludge land application.

Experimental warming accelerates positive soil priming in a temperate grassland ecosystem

Unravelling biosphere feedback mechanisms is crucial for predicting the impacts of global warming. Soil priming, an effect of fresh plant-derived carbon (C) on native soil organic carbon (SOC) decomposition, is a key feedback mechanism that could release large amounts of soil C into the atmosphere. However, the impacts of climate warming on soil priming remain elusive. Here, we show that experimental warming accelerates soil priming by 12.7% in a temperate grassland. Warming alters bacterial communities, with 38% of unique active phylotypes detected under warming. The functional genes essential for soil C decomposition are also stimulated, which could be linked to priming effects. We incorporate lab-derived information into an ecosystem model showing that model parameter uncertainty can be reduced by 32-37%. Model simulations from 2010 to 2016 indicate an increase in soil C decomposition under warming, with a 9.1% rise in priming-induced CO2 emissions. If our findings can be generalized to other ecosystems over an extended period of time, soil priming could play an important role in terrestrial C cycle feedbacks and climate change.

Multi-ecosystem services differently affected by over-canopy and understory nitrogen additions in a typical subtropical forest

Obtaining a holistic understanding of the impacts of atmospheric nitrogen deposition on multiple ecosystem services of forest is essential for developing comprehensive and sustainable strategies, particularly in heavy N deposition regions such as subtropical China. However, such impacts remain incompletely understood, with most previous studies focus on individual ecosystem function or service via understory N addition experiments. To address this knowledge gap, we quantified the effects of over-canopy and understory N additions on multiple ecosystem services based on a 7-year large-scale field experiment in a typical subtropical forest. Our results showed continued over-canopy N addition with 50 kg ha-1  year-1 over a period of 4-7 years significantly increased plant nutrient retention, but did not affect the services of soil nutrient accumulation, water yield, C sequestration (in plants and soil), or oxygen release. There were trade-offs between the soil and plant on providing the services of nutrient accumulation/retention and C sequestration under over-canopy N addition. However, without uptake and retention of tree canopy, the trade-off between soil and plant were more weaken under the understory N addition with 50 kg ha-1  year-1 , and their relationships were even synergetic under the understory N addition with 25 kg ha-1  year-1 . The results suggest that understory N addition cannot accurately simulate the effects of atmospheric N deposition on multiple services, along with mutual relationships. Interestingly, the services of plant N, P retention, and C sequestration exhibited a synergetic increase under the over-canopy N addition but a decrease under the understory N addition. Our results also found tree layer plays a primary role in providing plant nutrient retention service and is sensitive to atmospheric N deposition. Further studies are needed to investigate the generalized effects of forest canopy processes on alleviating the threaten of global change factors in different forest ecosystems.

Glucose input profit soil organic carbon mineralization and nitrogen dynamics in relation to nitrogen amended soils

Exogenous carbon (C) inputs stimulate soil organic carbon (SOC) decomposition, strongly influencing atmospheric concentrations and climate dynamics. The direction and magnitude of C decomposition depend on the C and nitrogen (N) addition, types and pattern. Despite the importance of decomposition, it remains unclear whether organic C input affects the SOC decomposition under different N-types (Ammonium Nitrate; AN, Urea; U and Ammonium Sulfate; AS). Therefore, we conducted an incubation experiment to assess glucose impact on N-treated soils at various levels (High N; HN: 50 mg/m2, Low N; LN: 05 mg/m2). The glucose input increased SOC mineralization by 38% and 35% under HN and LN, respectively. Moreover, it suppressed the concentration of NO3--N by 35% and NH4+-N by 15% in response to HN and LN soils, respectively. Results indicated higher respiration in Urea-treated soils and elevated net total nitrogen content (TN) in AS-treated soils. AN-amended soil exhibited no notable rise in C mineralization and TN content compared to other N-type soils. Microbial biomass carbon (MBC) was higher in glucose treated soils under LN conditions than control. This could result that high N suppressed microbial N mining and enhancing SOM stability by directing microbes towards accessible C sources. Our results suggest that glucose accelerated SOC mineralization in urea-added soils and TN contents in AS-amended soils, while HN levels suppressed C release and increased TN contents in all soil types except glucose-treated soils. Thus, different N-types and levels play a key role in modulating the stability of SOC over C input.

Unveiling the crucial role of soil microorganisms in carbon cycling: A review

Soil microorganisms, by actively participating in the decomposition and transformation of organic matter through diverse metabolic pathways, play a pivotal role in carbon cycling within soil systems and contribute to the stabilization of organic carbon, thereby influencing soil carbon storage and turnover. Investigating the processes, mechanisms, and driving factors of soil microbial carbon cycling is crucial for understanding the functionality of terrestrial carbon sinks and effectively addressing climate change. This review comprehensively discusses the role of soil microorganisms in soil carbon cycling from three perspectives: metabolic pathways, microbial communities, and environmental influences. It elucidates the roles of different microbial species in carbon cycling and highlights the impact of microbial interactions and environmental factors on carbon cycling. Through the synthesis of 2171 relevant papers in the Web of Science Core database, we elucidated the ecological community structure, activity, and assembly mechanisms of soil microorganisms crucial to the soil carbon cycle that have been widely analyzed. The integration of soil microbial carbon cycle and its driving factors are vital for accurately predicting and modeling biogeochemical cycles and effectively addressing the challenges posed by global climate change. Such integration is vital for accurately predicting and modeling biogeochemical cycles and effectively addressing the challenges posed by global climate change.

Nitrogen and phosphorus addition mediate soil priming effects via affecting microbial stoichiometric balance in an alpine meadow

Priming effect (PE) plays a crucial role in regulating the decomposition of soil organic matter (SOM). Multiple empirical results have shown that nitrogen (N) and phosphorus (P) addition can significantly alter the direction and intensity of PE, which may significantly affect carbon turnover in grasslands, especially in alpine meadows that are sensitive to N and P enrichment. To evaluate the PE responses to N and/or P addition, we conducted an incubation experiment by adding 13C-labeled glucose and nutrient additions (+N, +P, and +NP) in soils collected from an alpine meadow. The soils were incubated for 30 days and soil/microbial properties and enzyme activities were measured. Partial correlation and linear regression analyses were then performed to investigate their correlations with PE. The results showed that mean PE intensity among all treatments was 0.61 mg C g-1 soil or 1.35 (ratio). Nitrogen addition increased PE intensity, which was attributed to the better match between soil resources and microbial demands and enhanced enzyme activities. However, the PE intensity in P-addition soils was lower than that in control soils. This discrepancy may be related to the P-induced decrease of N availability and stronger microbial C/N imbalance. No significant response of PE intensity to NP addition was detected, and this could be explained by the offset of positive N effects and negative P effects on microbial decomposition. In this experiment, N or P addition altered the PE intensity by mediating the match between soil C:N:P ratio and microbial demands, which supported the stoichiometric decomposition hypothesis. Overall, our study highlights the importance of considering the C, N and P coupling in regulating PE, and underscores the need for further investigation into the effects of soil P on microbial activity and SOM decomposition.

Microbial traits drive soil priming effect in response to nitrogen addition along an alpine forest elevation gradient

Priming effect is a critical process affecting soil organic carbon (SOC) cycle, however, its drivers and patterns responding to nutrient addition are still unclear in alpine forests. Here, we conducted a 28-day incubation experiment based on the collected soils along an elevational gradient (3500-4300 m) on the southeastern Tibetan Plateau with adding carbon and nitrogen sources. The priming effect and microbial traits were analyzed based on 13C-stable glucose and bioinformatics methods. Results revealed that the carbon priming effect (PEC) ranged from 0.45 to 1.63 mg C g-1 SOC along the altitude, which was significantly associated with both soil organic carbon and total nitrogen. The addition of nitrogen inhibited the PEC and showed a positive correlation with the activities of β-1,4-glucosidase, β-1,4-N-acetyl-glucosaminnidase, β-cellobiosidase and β-xylosidase, while microbial community network became more complex and stable in respond to nitrogen addition. Structural equation modeling indicated that microbial communities, especially fungal communities in alpine regions drove PEC in response to nitrogen addition. Soil enzymes were the important intermediaries which drove the mineralization of soil carbon by microorganisms after adding nitrogen. Microorganisms were more sensitive to nitrogen rather than carbon due to the specific climate of alpine regions. Collectively, our works revealed the response pattern of soil carbon decomposition to nutrient addition in alpine ecosystem, clarifying the contribution of soil microorganisms in regulating carbon decomposition and nutrient cycle along high-elevation gradients in the context of global environmental change.

Warming inhibits the priming effect of soil organic carbon mineralization: A meta-analysis

Fresh organic carbon (C) input will accelerate or inhibit the mineralization of native soil organic carbon (SOC), which is called positive or negative priming effect (PE), respectively. However, little is known about how warming affects the PE. Here, we adopted a widely-used ratio of SOC mineralization between substrate-added and unadded-control treatments to represent PE intensity and used the PE difference between ambient-control temperature and elevated temperature to indicate the effect of warming on PE (ΔPE). By conducting a meta-analysis of 146 observations from 57 independent soils worldwide, we found that experimental warming significantly decreased the PE by 0.26 (unitless). Among ecosystems, warming significantly suppressed the PE of cropland and grassland soils by 0.43 and 0.21 respectively, but did not change the PE of forest soils. Moreover, we found significant positive correlations of ΔPE with the initial soil C/N ratio and the effect size of warming on microbial biomass. Between substrate types (i.e., containing N or not), warming significantly decreased the PE induced by N-containing substrates. These results suggested that the response of PE to warming is likely regulated by soil N availability and warming-induced changes in microbial biomass. As such, we proposed a conceptual framework-the microbial N mining hypothesis dominates in soils with low C/N ratio where warming inhibits PE by promoting N mineralization, while the stoichiometric decomposition hypothesis dominates in soils with high C/N ratio where warming stimulates PE by promoting N mineralization. Collectively, these findings provide important insights into how warming affects SOC dynamics via inhibiting PE, which may weaken the positive feedback between soil C emission and climate warming.

Microbial ecological clusters structured by environments drive maize residue decomposition at the continental scale

Environmental factors (e.g., climate and edaphic factors) indirectly regulate residue decomposition via microbial communities. Microbial ecological clusters (eco-clusters) structured by specific environmental factors have consequences for ecosystem functions. However, less is known about how microbial eco-clusters affect residue decomposition, especially over broad geographic scales. We collected agricultural soils from adjacent pairs of upland and paddy fields along a latitudinal gradient from the cold-temperature zone to the tropical zone, and conducted a microcosm experiment with 13C-labelled maize residue to explore the continental pattern of maize residue-derived 13CO2 (RDC), and whether and how microbial eco-clusters drive and predict RDC. Results showed that RDC decreased with latitude in both upland and paddy fields. Further, we identified 21 well-defined eco-clusters according to microbial environmental preferences, which explained 51.15 % of the spatial variations in RDC. The eco-clusters of high-total annual precipitation (TAP), high-mean annual temperature (MAT), low-pH, and some low-nutrient-associated exerted a positive effect on RDC. These eco-clusters contained many taxa belonging to the Actinobacteriota, Firmicutes, and Sordariomycetes, and their relative abundance decreased with latitude. Upland soils displayed 2.40-fold of RDC over paddy soils. Low-pH and high-organic matter (OM) eco-clusters were found to be the most prominent predictors of RDC in upland and paddy fields, respectively. Finally, we constructed a continental atlas of RDC in both upland and paddy fields based on eco-clusters and high-resolution climate and soil data. Overall, our study provides important evidence that historical environment-shaped microbial eco-clusters can drive and predict residue decomposition, providing new insights into how environmental factors indirectly regulate residue decomposition.

Long-term nitrogen deposition inhibits soil priming effects by enhancing phosphorus limitation in a subtropical forest

It is widely accepted that phosphorus (P) limits microbial metabolic processes and thus soil organic carbon (SOC) decomposition in tropical forests. Global change factors like elevated atmospheric nitrogen (N) deposition can enhance P limitation, raising concerns about the fate of SOC. However, how elevated N deposition affects the soil priming effect (PE) (i.e., fresh C inputs induced changes in SOC decomposition) in tropical forests remains unclear. We incubated soils exposed to 9 years of experimental N deposition in a subtropical evergreen broadleaved forest with two types of 13 C-labeled substrates of contrasting bioavailability (glucose and cellulose) with and without P amendments. We found that N deposition decreased soil total P and microbial biomass P, suggesting enhanced P limitation. In P unamended soils, N deposition significantly inhibited the PE. In contrast, adding P significantly increased the PE under N deposition and by a larger extent for the PE of cellulose (PEcellu ) than the PE of glucose (PEglu ). Relative to adding glucose or cellulose solely, adding P with glucose alleviated the suppression of soil microbial biomass and C-acquiring enzymes induced by N deposition, whereas adding P with cellulose attenuated the stimulation of acid phosphatase (AP) induced by N deposition. Across treatments, the PEglu increased as C-acquiring enzyme activity increased, whereas the PEcellu increased as AP activity decreased. This suggests that P limitation, enhanced by N deposition, inhibits the soil PE through varying mechanisms depending on substrate bioavailability; that is, P limitation regulates the PEglu by affecting soil microbial growth and investment in C acquisition, whereas regulates the PEcellu by affecting microbial investment in P acquisition. These findings provide new insights for tropical forests impacted by N loading, suggesting that expected changes in C quality and P limitation can affect the long-term regulation of the soil PE.

Viral lysing can alleviate microbial nutrient limitations and accumulate recalcitrant dissolved organic matter components in soil

Viruses are critical for regulating microbial communities and biogeochemical processes affecting carbon/nutrient cycling. However, the role of soil phages in controlling microbial physiological traits and intrinsic dissolved organic matter (DOM) properties remains largely unknown. Herein, microcosm experiments with different soil phage concentrates (including no-added phages, inactive phages, and three dilutions of active phages) at two temperatures (15 °C and 25 °C) were conducted to disclose the nutrient and DOM dynamics associated with viral lysing. Results demonstrated three different phases of viral impacts on CO₂ emission at both temperatures, and phages played a role in maintaining Q₁₀ within bounds. At both temperatures, microbial nutrient limitations (especially P limitation) were alleviated by viral lysing as determined by extracellular enzyme activity (decreased V_angle with active phages). Additionally, the re-utilization of lysate-derived DOM by surviving microbes stimulated an increase of microbial metabolic efficiency and recalcitrant DOM components (e.g., SUV254, SUV260 and HIX). This research provides direct experimental evidence that the "viral shuttle" exists in soils, whereby soil phages increase recalcitrant DOM components. Our findings advance the understanding of viral controls on soil biogeochemical processes, and provide a new perspective for assessing whether soil phages provide a net "carbon sink" vs. "carbon source" in soils.

Niche-mediated bacterial community composition in continental glacier alluvial valleys under cold and arid environments

Introduction

Bacteria are an essential component of glacier-fed ecosystems and play a dominant role in driving elemental cycling in the hydrosphere and pedosphere. However, studies of bacterial community composition mechanisms and their potential ecological functions from the alluvial valley of mountain glaciers are extremely scarce under cold and arid environments.

Methods

Here, we analyzed the effects of major physicochemical parameters related to soil on the bacterial community compositions in an alluvial valley of the Laohugou Glacier No. 12 from the perspective of core, other, and unique taxa and explored their functional composition characteristics.

Results and discussion

The different characteristics of core, other, and unique taxa highlighted the conservation and difference in bacterial community composition. The bacterial community structure of the glacial alluvial valley was mainly affected by the above sea level, soil organic carbon, and water holding capacity. In addition, the most common and active carbon metabolic pathways and their spatial distribution patterns along the glacial alluvial valley were revealed by FAPTOTAX. Collectively, this study provides new insights into the comprehensive assessment of glacier-fed ecosystems in glacial meltwater ceasing or glacier disappearance.

5300-Year-old soil carbon is less primed than young soil organic matter

Soils harbor more than three times as much carbon (C) as the atmosphere, a large fraction of which (stable organic matter) serves as the most important global C reservoir due to its long residence time. Litter and root inputs bring fresh organic matter (FOM) into the soil and accelerate the turnover of stable C pools, and this phenomenon is termed the "priming effect" (PE). Compared with knowledge about labile soil C pools, very little is known about the vulnerability of stable C to priming. Using two soils that substantially differed in age (500 and 5300 years before present) and in the degree of chemical recalcitrance and physical protection of soil organic matter (SOM), we showed that leaf litter amendment primed 264% more organic C from the young SOM than from the old soil with very stable C. Hierarchical partitioning analysis confirmed that SOM stability, reflected mainly by available C and aggregate protection of SOM, is the most important predictor of leaf litter-induced PE. The addition of complex FOM (i.e., leaf litter) caused a higher bacterial oligotroph/copiotroph (K-/r-strategists) ratio, leading to a PE that was 583% and 126% greater than when simple FOM (i.e., glucose) was added to the young and old soils, respectively. This implies that the PE intensity depends on the chemical similarity between the primer (here FOM) and SOM. Nitrogen (N) mining existed when N and simple FOM were added (i.e., Glucose+N), and N addition raised the leaf litter-induced PE in the old soil that had low N availability, which was well explained by the microbial stoichiometry. In conclusion, the PE induced by FOM inputs strongly decreases with increasing SOM stability. However, the contribution of stable SOM to CO₂ efflux cannot be disregarded due to its huge pool size.

Priming of soil organic carbon mineralization and its temperature sensitivity in response to vegetation restoration in a karst area of Southwest China

Plant residue input alters native soil organic carbon (SOC) mineralization through the priming effect, which strongly controls C sequestration during vegetation restoration. However, the effects of different vegetation types on SOC priming and the underlying microbial mechanisms due to global warming are poorly understood. To elucidate these unknowns, the current study quantified soil priming effects using ¹³C-labeled maize residue amendments and analyzed the community structure and abundances in the soils of a vegetation succession gradient (maize field (MF), grassland (GL), and secondary forest (SF)) from a karst region under two incubation temperatures (15 °C and 25 °C). Results revealed that after 120 d of incubation, vegetation restoration increased the soil priming effects. Compared to MF, the priming effects of SF at 15 °C and 25 °C increased by 142.36 % and 161.09 %, respectively. This may be attributed to a high C/N ratio and low-N availability (NO₃^-), which supports the "microbial nitrogen mining" theory. Variations in soil priming were linked to changes in microbial properties. Moreover, with vegetation restoration, the relative abundance of Actinobacteria (copiotrophs) increased, while Ascomycota (oligotrophs) decreased, which accelerated native SOC decomposition. Co-occurrence network analysis indicated that the cooperative interactions of co-existing keystone taxa may facilitate SOC priming. Furthermore, structural equation modeling (SEM) indicated that changes in the priming effects were directly related to the fungal Shannon index and microbial biomass C (MBC), which were affected by soil C/N and NO₃^-. Warming significantly decreased soil priming, which may be attributed to the increase in microbial respiration (qCO₂) and decreased MBC. The temperature sensitivity (Q₁₀) of SOC mineralization was higher after residue amendment, but significant differences were not detected among the vegetation types. Collectively, our results indicated that the intensity of priming effects was dependent on vegetation type and temperature. Microbial community alterations and physicochemical interactions played important roles in SOC decomposition and sequestration.

Different mechanisms driving increasing abundance of microbial phosphorus cycling gene groups along an elevational gradient

Microbes play an integral role in forest soil phosphorus (P) cycling. However, the variation of microbial P-cycling functional genes and their controlling factors in forest soils is unclearly. We used metagenomics to investigate changes in the abundance of genes involved in P-starvation response regulation, P-uptake and transport, and P-solubilization and mineralization along the five elevational gradients. Our results showed the abundance of three P cycling gene groups increasing along the elevational gradient. Acidobacteria and Proteobacteria were the dominant microbial phyla determining the turnover of soil P-solubilization and immobilization. Along the elevational gradient, soil substrates are the major factor explaining variation in P-starvation response regulation genes. Soil environment is the main driver of P-uptake and transport and P-solubilization and mineralization genes. This study provided insights into the regulation of P-cycling from a microbial functional profile perspective, highlighting the importance of substrate and environmental factors for P-cycling genes in forest soils.

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P-cycling functional genes increased along the elevational gradient

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Acidobacteria and Proteobacteria are the key phyla for P cycle in forest soils

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Microbial functional gene groups for P-cycling were driven by different factors

Tropical forest strata shifts in plant structural diversity-aboveground carbon relationships along altitudinal gradients

Aboveground carbon storage in forests can be influenced by both structural and compositional diversity of plant communities. However, the relative and interactive effects of structural and compositional diversity on multilevel aboveground carbon storage across forest strata and how these relationships vary with altitude and soil nutrients remain unclear. Using data obtained from 34 tropical forest plots (total area 8.5 ha) in Hainan Island, China, we analyzed the relationships between aboveground carbon at four levels (litter, understory, overstory, and whole-community) with structural diversity (diameter and height diversity) and compositional diversity (species diversity and evenness) in the understory and overstory. The direct and indirect effects of altitude, soil nutrients (total N and total P and N/P ratio), structural diversity, and compositional diversity on aboveground carbon were explored via Bayesian structural equation modeling. The results showed that structural diversity, rather than compositional diversity, in overstory stratum determined aboveground carbon. Specifically, overstory structural diversity was negatively associated with understory carbon, while positively associated with overstory and whole-community carbon. Furthermore, diversity‑carbon relationships were slightly affected by soil nutrients but strongly by altitude. Specifically, the relationship between overstory and whole-community carbon content with overstory tree height diversity weakened with altitude, while their relationship with overstory diameter diversity strengthened. Altitude directly and indirectly affected overstory tree height and diameter diversity through overstory species diversity, thereby reducing understory and increasing overstory and whole-community carbon. Altitude directly promoted litter carbon. We provide evidence that the effects of plant diversity on aboveground carbon storage are forest strata- and altitude-dependent. As overstory structural diversity plays a crucial role in storing aboveground carbon at all altitudes, we proposed that focusing on overstory structural diversity would be promising for predicting trends in how plant diversity affects aboveground carbon in response to climate change.

Rhizosphere Effects along an Altitudinal Gradient of the Changbai Mountain, China

Rhizosphere effects (REs) play important roles in regulating carbon (C) and nutrient cycling in terrestrial ecosystems. However, little is known about the REs of mature trees in the field, especially at the ecosystem scale. This study aimed to explore the variation and patterns of REs in natural ecosystems. Here, combining soil monoliths with an adhering soil (shaking fine roots) method was adopted to sample paired rhizosphere soil and bulk soil along an altitudinal gradient. Based on the relative REs and the percentage of rhizosphere soil mass, the REs on soil C and net nitrogen mineralization rates (Cmin and net Nmin) at the ecosystem scale were estimated. Our results showed that the REs on soil processes, soil microbial biomass C and extracellular enzyme activities (β-glucosidase and N-acetyl-glucosaminidase activities), and soil chemical properties (total C, total N, inorganic N, extractable P, K, Ca, Mg, Fe, and Mn) were significantly positive across altitudinal sites, while soil pH was significantly negative. Although the relative REs on investigated variables varied significantly among altitudes, the relative REs did not show a clear trend with the increased altitudes. Across altitudes, the mean magnitude of ecosystem-level REs on Cmin and net Nmin were 19% (ranging from 4% to 48%) and 16% (ranging from 3% to 34%), respectively. Furthermore, the magnitude of ecosystem-level rhizosphere effects increased linearly with the increased altitudes. The altitudinal patterns of ecosystem-level RE mainly depend on the percentage of rhizosphere soil mass. In conclusion, our results provided a set of new evidence for the REs, and highlighted the need to incorporate REs into land C and N models.

Responses of soil mineral-associated and particulate organic carbon to carbon input: A meta-analysis

A minor change of soil organic carbon (SOC) greatly influences atmospheric carbon dioxide concentration and climate change. Exogenous carbon (C) input into soils can induce SOC decomposition or sequestration. The response of SOC to C input can be better understood when SOC is separated into mineral-associated (MAOC) and particulate (POC) organic carbon. The objective of this study is to explore whether exogenous C input promote MAOC and POC increase or decrease and the controlling factors. We gained 1181 observations from 17 studies for this meta-analysis. The effect sizes of exogenous C input on MAOC and POC content, and MAOC decomposition were calculated. The key factors influencing the effect sizes were explored through subgroup analysis. Potential publication bias was explored by using funnel plots, trim and fill method, and Egger's test. Exogenous C input significantly increased MAOC and POC content, although promoted MAOC decomposition. The effect sizes were larger for MAOC content than for POC content irrespective of soil and substrate properties and experiment methods. The effects of C input on MAOC and POC content were more pronounced in forest soils, and depended on the C and nitrogen (N) content in soil and substrates as well as experiment methods. The effect size of C input on MAOC decomposition were larger with substrate input of below 200 g C kg^-1 in specific soils. The sensitivity analysis carried out by removing one observation indicated our results were robust. In conclusion, exogenous C input increases MAOC and POC content although stimulate MAOC decomposition, and the effect sizes were influenced mainly by ecosystem type, carbon and nitrogen content of substrates and soils, and fractionation methods. The findings indicate the importance of C and N content in substrates and soils in controlling the response of SOC rather than the ratio of C to N.