Organic Carbon Mineralization and Bacterial Community of Active Layer Soils Response to Short-Term Warming in the Great Hing’an Mountains of Northeast China

As a buffer layer for the energy and water exchange between atmosphere and permafrost, the active layer is sensitive to climate warming. Changes in the thermal state in active layer can alter soil organic carbon (SOC) dynamics. It is critical to identify the response of soil microbial communities to warming to better predict the regional carbon cycle under the background of global warming. Here, the active layer soils collected from a wetland-forest ecotone in the continuous permafrost region of Northeastern China were incubated at 5 and 15°C for 45 days. High-throughput sequencing of the 16S rRNA gene was used to examine the response of bacterial community structure to experimental warming. A total of 4148 OTUs were identified, which followed the order 15°C > 5°C > pre-incubated. Incubation temperature, soil layer and their interaction have significant effects on bacterial alpha diversity (Chao index). Bacterial communities under different temperature were clearly distinguished. Chloroflexi, Actinobacteria, Proteobacteria, and Acidobacteria accounted for more than 80% of the community abundance at the phylum level. Warming decreased the relative abundance of Chloroflexi and Acidobacteria, while Actinobacteria and Proteobacteria exhibited increasing trend. At family level, the abundance of norank_o__norank_c__AD3 and Ktedonobacteraceae decreased significantly with the increase of temperature, while Micrococcaccac increased. In addition, the amount of SOC mineralization were positively correlated with the relative abundances of most bacterial phyla and SOC content. SOC content was positively correlated with the relative abundance of most bacterial phyla. Results indicate that the SOC content was the primary explanatory variable and driver of microbial regulation for SOC mineralization. Our results provide a new perspective for understanding the microbial mechanisms that accelerates SOC decomposition under warming conditions in the forest-wetland ecotone of permafrost region.

The Response of Critical Microbial Taxa to Litter Micro-Nutrients and Macro-Chemistry Determined the Agricultural Soil Priming Intensity After Afforestation

Afforestation with trees and shrubs around cropland can effectively decrease soil degradation and avoid sand storms, but subsequent modification of litter quality accelerates the degradation of native organic matter via the soil priming effect (PE). Although carbon accumulation in agricultural soils after afforestation was widely studied, little is known about the extent to which soil organic carbon (SOC) mineralization is induced by complex residue input in agro-forest-grass composite ecosystems. Here, we mixed corn field soil and litter of afforestation tree and shrub species together in a micro-environment to quantify the effects of litter-mixture input on farmland soil priming associated with afforestation. Additionally, we studied the responses of bacterial and fungal species to litter chemistry, with the aim to identify the litter and microbial driver of soil priming. The results showed that soil priming was accelerated by different litter addition which varied from 24 to 74% of SOC mineralization, suggesting that priming intensity was relatively flexible and highly affected by litter quality. We also find that the macro-chemistry (including litter carbon, nitrogen, lignin, and cellulose) directly affects priming intensity, while micro-chemistry (including litter soluble sugar, water-soluble phenol, methanol-soluble phenol, and condensed tannin) indirectly influences priming via alteration to dominant bacterial taxa. The stepwise regression analysis suggested that litter nitrogen and cellulose were the critical litter drivers to soil priming (r2 = 0.279), and the combination of bacterial phylum Proteobacteria, Firmicutes, Bacteroidetes, Acidobacteria, and fungal taxa Eurotiomycetes was a great model to explain the priming intensity (r2 = 0.407).

Soil Organic Carbon Mineralization and its Temperature Sensitivity Along a Vegetation Restoration Gradient in Subtropical China

Background and aims Soil organic carbon (SOC) mineralization produces important CO2 flux from terrestrial ecosystems which can provide feedbacks to climates. Vegetation restoration can affect SOC mineralization and its temperature sensitivity (Q10), but how this effect is related to soil moisture remains uncertain. Methods We performed a laboratory incubation using soils of different vegetation restoration stages (i.e., degraded vegetation [DS], plantation [PS], and secondary natural forest [SFS]) maintained under different moisture and temperature conditions to explore the combined effects of vegetation restoration and soil moisture on SOC mineralization and Q10. Results We found that cumulative SOC mineralization in PS and SFS were about 11.7 times higher than that in the DS, associated with higher SOC content and microbial biomass. Increased soil moisture and temperature led to higher SOC mineralization in the SFS and PS. However, in the DS, soil moisture did not affect SOC mineralization, but temperature enhancement solely increased (158.7%) SOC mineralization at the 60%MWHC treatment. Furthermore, significant interactive effect of vegetation restoration and soil moisture on Q10 was detected. At the 60%MWHC treatment, Q10 declined with vegetation restoration age. Nevertheless, at the 30%MWHC treatment, Q10 was lower in the DS than that in the PS. Higher soil moisture did not affect Q10 in the PS and SFS, but enhanced Q10 in the DS. Conclusions Our results highlight that the responses of SOC mineralization and Q10 to vegetation restoration were highly dependent on soil moisture and substrate availability, and vegetation restoration reduced the influence of soil moisture on Q10.

Nitrogen addition reduced carbon mineralization of aggregates in forest soils but enhanced in paddy soils in South China

Background Despite the crucial role of nitrogen (N) availability in carbon (C) cycling in terrestrial ecosystems, soil organic C (SOC) mineralization in different sizes of soil aggregates under various land use types and their responses to N addition is not well understood. To investigate the responses of soil C mineralization in different sized aggregates and land use types to N addition, an incubation experiment was conducted with three aggregate-size classes (2000, 250, and 53 μm) and two land use types (a Chinese fir plantation and a paddy land). Results Cumulative C mineralization of the < 53-μm fractions was the highest and that of microaggregates was the lowest in both forest and paddy soils, indicating that soil aggregates enhanced soil C stability and reduced the loss of soil C. Cumulative C mineralization in all sizes of aggregates treated with N addition decreased in forest soils, but that in microaggregates and the < 53-μm fraction increased in paddy soils treated with 100 μg N g−1. Moreover, the effect sizes of N addition on C mineralization of forest soils were below zero, but those of paddy soils were above zero. These data indicated that N addition decreased SOC mineralization of forest soils but increased that of paddy soils. Conclusions Soil aggregates play an important role in soil C sequestration, and decrease soil C loss through the increase of soil C stability, regardless of land use types. N addition has different effects on soil C mineralization in different land use types. These results highlight the importance of soil aggregates and land use types in the effects of N deposition on the global terrestrial ecosystem C cycle.

The effect of dolomite amendment on soil organic carbon mineralization is determined by the dolomite size

Background The size of lime material is vital for the efficiency of ameliorating soil acidity, thereby influencing soil biochemical processes. However, the effects of different sized lime material application on soil organic carbon (SOC) mineralization are yet to be elucidated. Therefore, a 35-day incubation experiment was conducted to determine the effects of three particle size fractions (0.5 to 0.25, 0.25 to 0.15, and < 0.15 mm) of dolomite on SOC mineralization of two acidic paddy soils. Results CO2 emission was increased by 3–7%, 11–21%, and 32–49% for coarse-, medium-, and fine-sized dolomite treatments, respectively, compared to the control in both soils. They also well conformed to a first-order model in all treatments, and the estimated decomposition rate constant was significantly higher in the fine-sized treatment than that of other treatments (P < 0.05), indicating that SOC turnover rate was dependent on the dolomite size. The finer particle sizes were characterized with higher efficiencies of modifying soil pH, consequently resulting in higher dissolved organic carbon contents and microbial biomass carbon, eventually leading to higher CO2 emissions. Conclusions The results demonstrate that the size of dolomite is a key factor in regulating SOC mineralization in acidic paddy soils when dolomite is applied to manipulate soil pH.

Phenolic acid-degrading Paraburkholderia prime decomposition in forest soil

Plant-derived phenolic acids are metabolized by soil microorganisms whose activity may enhance the decomposition of soil organic carbon (SOC). We characterized whether phenolic acid-degrading bacteria would enhance SOC mineralization in forest soils when primed with 13C-labeled p-hydroxybenzoic acid (PHB). We further investigated whether PHB-induced priming could explain differences in SOC content among mono-specific tree plantations in a 70-year-old common garden experiment. The activity of Paraburkholderia and Caballeronia dominated PHB degradation in all soils regardless of tree species or soil type. We isolated the principal PHB-degrading phylotype (Paraburkholderia madseniana RP11T), which encoded numerous oxidative enzymes, including secretion signal-bearing laccase, aryl-alcohol oxidase and DyP-type peroxidase, and confirmed its ability to degrade phenolics. The addition of PHB to soil led to significant enrichment (23-fold) of the RP11T phylotype (RP11ASV), as well as enrichment of other phylotypes of Paraburkholderia and Caballeronia. Metabolism of PHB primed significant loss of SOC (3 to 13 µmols C g-1 dry wt soil over 7 days). In contrast, glucose addition reduced SOC mineralization (−3 to -8 µmols C g-1 dry wt soil over 7 days). RP11ASV abundance and the expression of PHB monooxygenase (pobA) correlated with PHB respiration and were inversely proportional to SOC content in the field. We propose that plant-derived phenolics stimulate the activity of phenolic acid-degrading bacteria thereby causing soil priming and SOC loss. We show that Burkholderiaceae dominate soil priming in diverse forest soils and this observation counters the prevailing view that priming phenomena are a generalized non-specific response of community metabolism.

Carbon Mineralization under Different Saline—Alkali Stress Conditions in Paddy Fields of Northeast China

Soil organic carbon (SOC) mineralization (conversion of carbonaceous material to carbon dioxide) plays a central role in global carbon cycle. However, the effects of SOC mineralization under different saline–alkali stress conditions are poorly understood. In order to understand the carbon mineralization processes, four paddy fields with different saline and alkali degrees were chosen as the experimental samples and the soil CO2 emission fluxes at nine different time steps of the whole simulation experiment were observed. The physical and chemical properties of soils of four field conditions were compared for the dynamic changes of CO2 flux in the progress of paddy field cultivation simulations. The results showed that the first three fields (P1, P2, and P3) were weakly alkaline soils and the last one (P4) was strongly alkaline soil. The SOC content of each plot was significantly different and there was a near-surface enrichment, which was significantly negatively correlated with the degree of alkalization. The accumulation process of the SOC mineralization during the incubation time was consistent with the first-order kinetic model. In the initial stage of mineralization, the amount of CO2 released massively, and then the release intensity decreased rapidly. The mineralization rate decreased slowly with time and finally reached a minimum at the end of the incubation period. This study indicates that the SOC mineralization process is affected by a variety of factors. The main factors influencing SOC mineralization in the saline–alkaline soils are the exchangeable sodium percentage (ESP), followed by enzyme activities. Salinization of the soils inhibits the rate of soil carbon cycle, which has a greater impact on the carbon sequestration than on the carbon source process. The intensity and completeness of the SOC mineralization reactions increase with increasing SOC contents and decrease with increasing ESP levels.

Predicting bi-decadal soil organic carbon mineralization with Rock-Eval® thermal analysis

&lt;p&gt;The organic carbon reservoir of soils is a key component of climate change, calling for an accurate knowledge of the residence time of soil organic carbon (SOC). Existing proxies of the labile SOC pool such as particulate organic carbon or basal respiration tests are time consuming and unable to consistently predict SOC mineralization over years to decades. Similarly, models of SOC dynamics often yield unrealistic values of the size of SOC kinetic pools. Rock-Eval&amp;#174; 6 (RE6) thermal analysis of bulk soil samples has recently been shown to provide useful and cost-effective information regarding the long-term in-situ decomposition of SOC. The objective of this study was to design a method based on RE6 indicators to assess for a given soil, the proportion of SOC that will be mineralized in the coming 20 years.&lt;/p&gt;&lt;p&gt;To do so, we needed samples ready to be analyzed using RE6 with a known proportion of SOC mineralized in 20 years. We used archived soil samples from 4 long-term bare fallows and 8 C&lt;sub&gt;3&lt;/sub&gt;/C&lt;sub&gt;4&lt;/sub&gt; chronosequences. For each sample, the value of bi-decadal SOC mineralization was obtained from the observed SOC dynamics of its long-term bare fallow plot or the calculated C&lt;sub&gt;3&lt;/sub&gt;-derived SOC decline following the conversion to C&lt;sub&gt;4&lt;/sub&gt; plants. Those values ranged from 0.3 to 14.3 gC&amp;#183;kg&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; (concentration data), representing 8.6 to 52.6% of total SOC (proportion data). All samples were analyzed using RE6 and simple linear regression models were used to predict bi-decadal SOC loss (concentration and proportion data) from 4 RE6 parameters: 1) HI (the amount of hydrogen-rich effluents formed during the pyrolysis phase of RE6; mgCH.g&lt;sup&gt;-1&lt;/sup&gt; SOC), 2) OI&lt;sub&gt;RE6&lt;/sub&gt; (the O recovered as CO and CO&lt;sub&gt;2&lt;/sub&gt; during the pyrolysis phase of RE6; mgO&lt;sub&gt;2&lt;/sub&gt;.g&lt;sup&gt;-1&lt;/sup&gt; SOC), 3) PC/SOC (the amount of organic C evolved during the pyrolysis phase of RE6; % of total SOC) and 4) T50 CO&lt;sub&gt;2&lt;/sub&gt; oxidation (the temperature at which 50% of the residual organic C was oxidized to CO&lt;sub&gt;2&lt;/sub&gt; during the RE6 oxidation phase; &amp;#176;C).&lt;/p&gt;&lt;p&gt;The RE6 HI parameter yielded the best predictions of bi-decadal SOC mineralization, for both concentration and proportion data. PC/SOC and T50 CO&lt;sub&gt;2&lt;/sub&gt; oxidation parameters also yielded significant regression models. The OI&lt;sub&gt;RE6&lt;/sub&gt; parameter was not a good predictor of bi-decadal SOC loss, with non-significant regression models. The results showed that SOC chemical composition (HI is a proxy for SOC H/C ratio), and to a lesser degree SOC thermal stability, are related to bi-decadal SOC dynamics. The RE6 thermal analysis method can therefore provide a quantitative and accurate estimate of SOC biogeochemical stability.&lt;/p&gt;

C:N:P stoichiometry regulates soil organic carbon mineralization and concomitant shift of microbial community in paddy soil

&lt;p&gt;Soil carbon (C), nitrogen (N), and phosphorus (P) contents and their stoichiometric ratios play modifying the microbial metabolism of C. Microbial populations vary in their strategies for C and nutrient acquisition to maintain the microbial biomass C:N:P balance. However, the regulation of soil C mineralization and microbial activities by stoichiometric ratios in input substrates becomes unpredictable in flooded soils because of the frequent redox fluctuations and general oxygen limitation. Stoichiometric control on input substrate (glucose) and soil organic carbon (SOC) mineralization were assessed by a manipulation experiment based on N or P fertilization in paddy soil. Glucose mineralization increased by nutrient addition up to 11.6% with combined N and P applications compared with addition without nutrients. During 100-days incubation, about 4.5% of SOC was mineralized in all five treatments, being increased by glucose and reduced by P fertilization. Glucose and SOC mineralization increased exponentially with the dissolved organic carbon (DOC):NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;-N, DOC:Olsen P, and microbial biomass (MB)C:MBN ratios. The glucose mineralization was negatively associated with the MBC:MBP ratio, suggesting that P addition relieved P limitation for microorganisms and increased microbial activities of labile C mineralization. The shift of bacterial community structure was significantly affected by the soil available and microbial biomass C:N:P stoichiometric ratios. The decrease of negative associations between bacterial taxa in the P-added soil indicated that microbial competition for nutrients was alleviated. 16S rRNA amplicon sequencing showed that combined C and nutrients application stimulated the Clostridia and &amp;#946;-Proteobacteria (r strategists) and increased the enzyme activities of &amp;#946;-glucosidase and &amp;#946;-acetyl-glucosaminidase. In contrast, after 100-day incubation, when the available substrate was exhausted, Syntrophus (K strategist) was found as the keystone species. Hence, soil microbial communities shifted their keystone species to acquire necessary elements to maintain the microbial biomass C:N:P stoichiometric balance in response to the change of resource C:N:P stoichiometry.&lt;/p&gt;

Soil available resource C:N ratio regulates the priming effect by maintaining microbial C:N stoichiometric balance in long-term fertilized paddy soils

&lt;p&gt;Fertilization practices can influence the soil nutrients and fertility status, which subsequently induce changes in soil carbon (C):nitrogen (N) ratio and rebuilt C:N stoichiometric balances between microbial biomass and resources. In this study, we investigated how available resource C:N ratio can regulate the priming effect (PE) to maintain microbial C:N stoichiometric balance by adding &lt;sup&gt;13&lt;/sup&gt;C-labeled glucose to four long-term fertilized paddy soils [Control (no fertilization), NPK (fertilized with mineral NPK fertilizers) , NPKS (NPK combined with straw), NPKM (NPK combined with manure)]. Glucose addition significantly increased SOC mineralization and subsequently induced a positive priming effect at day 2 of incubation, whereas the PE became negative after 20 days. DOC contents were increased by more than 1000% with glucose addition at day 2, whereas they rapidly decreased to -10% to -50% compared with those in soils without glucose addition. With the changes in available and biomass C and N, the microbial C:N imbalance initially increased to 3.3&amp;#8211;6.8, and then reduced to the level as that in the soils without glucose addition. At the end of incubation, the microbial C:N imbalance in the glucose-treated soils was ranked as Control &lt; NPK &lt; NPKM &lt; NPKS. This suggested that, without organic fertilization, soils were highly susceptible to labile C and increased SOC mineralization, leading to C limitation. The PE was positively related to DOC and NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; ratio, but negatively associated with microbial C:N imbalance, suggesting that the labile C supplied stimulated microbial stoichiometric decomposition of SOM. Glucose addition modified enzyme activities after 20 days, to allow the microorganisms to break up complex C compounds for C source. Our findings suggested that soil microorganisms could regulate extracellular hydrolytic enzyme production and their relative stoichiometric ratios to obtain necessary elements, thereby adjusting the microbial biomass C:N to the resource stoichiometry.&lt;/p&gt;