Soil Organic C Mineralization Research Articles

Understanding how conservation tillage promotes soil carbon accumulation: Insights into extracellular enzyme activities and carbon flows between aggregate fractions

Conservation tillage has been shown to mitigate climate change by promoting the sequestration of soil carbon (C) in agroecosystems. However, knowledge on how conservation tillage accumulates soil organic C (SOC), especially at the aggregate scale, remains limited. This study aimed to clarify the effects of conservation tillage on SOC accumulation by measuring hydrolytic and oxidative enzyme activities and C mineralization in aggregates and developing an extended scheme of C flows between aggregate fractions using the 13C natural abundance (δ13C) method. Topsoils (0–10 cm) were sampled from a 21-year tillage experiment located in the Loess Plateau of China. Compared with conventional (CT) and reduced tillage with straw removal (RT), no-till (NT) and subsoiling with straw mulching (SS) enhanced the proportions of macro-aggregates (> 0.25 mm) (by 12–26%) and SOC contents in bulk soils and all aggregate fractions (by 12–53%). In bulk soils and all aggregate fractions, SOC mineralization and the activities of hydrolases (β-1,4-glucosidase, β-acetylglucosaminidase, β-xylosidase, and cellobiohydrolase) and oxidases (peroxidase and phenol oxidase) were 9–35% and 8–56% lower, respectively, under NT and SS than under CT and RT. Partial least squares path model revealed that reductions in the activities of hydrolases and oxidases and increases in macro-aggregation decreased SOC mineralization in bulk soils and macro-aggregates. Furthermore, Δ13C values (aggregate-associated δ13C − bulk-soil δ13C) increased with decreasing size of soil aggregates, suggesting that C is younger in larger aggregates than in smaller aggregates. The probability of C flows from large to small soil aggregates was lower under NT and SS than under CT and RT, indicating that young SOC with low rates of decomposition in macro-aggregates was better protected under NT and SS. Overall, NT and SS enhanced SOC accumulation in macro-aggregates by decreasing the activities of hydrolases and oxidases and C flows from macro- to micro-aggregates, which promoted C sequestration in soils. The present study provides improved insights into the mechanism and prediction of soil C accumulation under conservation tillage.

Biochar Amendment Increases C and N Retention in the Soil–Plant Systems: Its Implications in Enhancing Plant Growth and Water-Use Efficiency Under Reduced Irrigation Regimes of Maize (Zea mays L.)

Biochar influences soil biophysicochemical processes and nutrient availability, yet the effects of different biochar and soil water dynamics on carbon (C) and nitrogen (N) retention in the soil–plant systems remain unknown. Maize plants were grown in split-root pots filled with clay loam soil amended with wheat straw pellet biochar (WSP) and softwood pellet biochar (SWP) at 2% (w/w) and were either irrigated daily to 90% of water-holding capacity (FI) or irrigated with 70% volume of water used for FI to the whole root-zone (DI) or alternately to half root-zone (PRD) from the fourth leaf to grain-filling stage. Compared to the unamended controls, biochar amendment enhanced plant biomass and water-use efficiency, particularly when combined with PRD. Although the WSP amendment tended to decrease soil net N mineralization rate, it significantly increased C and N retention in the soil–plant systems. Compared to DI, PRD significantly increased soil respiration rate while lowering soil total organic C content. Moreover, PRD increased soil inorganic N content, which might be related to increased mineralization of soil organic C (SOC) and soil organic N (SON). Such effects might implicate that PRD outperformed DI in enhancing the mineralization of soil organic matter. Although PRD alone might not be a sustainable irrigation method because of greater C and N losses, biochar addition could alleviate these undesirable effects via depressing SOC and SON mineralization. Biochar amendment, especially WSP combined with PRD, could be a promising practice to increase maize growth and water-use efficiency while sustaining C and N retention in the soil–plant systems.

Semi-continuous C supply reveals that priming due to N-mining is driven by microbial growth demands in temperate forest plantations

Nitrogen (N) availability is a powerful controller of soil carbon (C) cycling in temperate forests, affecting plant productivity and microbial activities. Enhanced rhizosphere input from increased plant productivity can stimulate the decomposition of native soil organic matter (SOM), termed the “priming effect”, with an effect size that is affected by N availability. Using soils from N-fertilization field-experiments conducted in larch and mixed forest plantations, we investigated how N availability influenced the priming of soil organic C (SOC) mineralization and soil organic N (SON) mineralization by adding 13C-labelled glucose semi-continuously (every third day) to simulate the semi-continuous delivery of rhizosphere inputs. We found that semi-continuous additions of glucose induced a repeating pattern for the priming of SOC mineralization with an initial decrease followed by an increase. This repeating pattern of SOC mineralization reversely coincided with repeating dynamics in the level of bacterial growth which first increased followed by a decrease. The labile C additions induced a gradually increasing priming of SON mineralization, which was greater than the priming of SOC mineralization (i.e. “selective microbial N-mining”). The priming of SOC mineralization and SON mineralization were both reduced by N fertilization. Increased priming of SON mineralization was related to stimulated bacterial growth and changes in the microbial C use efficiency. These responses suggested that increased microbial demand for N drove the observed N-mining responses. This microbial N-mining could not be explained by increased oxidative enzyme activities, and was instead linked to microbial growth. In the larch forest with high C-quality, the high priming of SOM mineralization were linked to bacterial growth, while in the mixed forest with low C-quality, the high levels of priming of SOM mineralization was linked to fungal growth. We also observed that labile C input initially decoupled the C and N mineralization from SOM. Within 16 days, however, a new equilibrium developed, where both C and N mineralization from SOM were similarly enhanced by labile C input. Overall, our results suggest that semi-continuous rhizosphere inputs can induce a sustained priming of SOM mineralization driven by the microbial demand for N – increasing the release of CO2 – but that N fertilization could reduce the soil C loss, contributing to enhanced soil C sequestration.

Land-Use Change Enhanced SOC Mineralization but Did Not Significantly Affect Its Storage in the Surface Layer.

To achieve carbon (C) neutrality and mitigate climate change, it is crucial to understand how converting natural forests to agricultural plantations influences soil organic C (SOC) mineralization. In this study, we investigated the impact of converting evergreen broadleaf forests (EBF) to extensively managed Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau) plantations (MBP) in subtropical China on SOC mineralization rate; the concentrations of labile SOC fractions such as dissolved organic C (DOC), microbial biomass C (MBC), and readily oxidizable C (ROC); the activities of C-degrading enzymes (cellobiohydrolase and phenol oxidase); and the abundance of C-degrading enzyme-encoding genes (cbhI and lcc). Three paired soil samples were taken from the surface layer (0–20 cm) of adjacent EBF-MBP sites in Anji County, Zhejiang province. Results showed that converting EBF to MBP significantly increased the SOC mineralization rate as well as soil pH, MBC, cellobiohydrolase, and phenol oxidase activities, and cbhI gene abundance, but did not change other soil properties described above. In addition, structural equation modelling (SEM) showed that the conversion increased SOC mineralization rate through increasing soil pH, cbhI gene abundance, MBC, and cellobiohydrolase and phenol oxidase activities. Our novel finding that converting EBF to extensively managed MBP enhanced SOC mineralization via increasing the activities of C-degrading enzymes suggests that C-degrading enzymes were a key factor regulating SOC mineralization in the extensively managed subtropical bamboo plantations.

Increased soil organic matter after 28 years of nitrogen fertilization only with plastic film mulching is controlled by maize root biomass

Nitrogen (N) fertilization and plastic film mulching (PFM) are two widely applied management practices for crop production. Both of them impact soil organic matter individually, but their interactive effects as well as the underlying mechanisms are unknown. Soils from a 28-year field experiment with maize monoculture under three levels of N fertilization (0, 135, and 270 kg N ha−1 yr−1) and with or without PFM were analyzed for soil organic C (SOC) content, total soil nitrogen (N), root biomass, enzyme activities, and SOC mineralization rates. After 28 years, N fertilization increased root biomass and consequently, SOC by 26% (averaged across the two fertilizer application rates) and total soil N by 25%. These increases, however, were only in soil with PFM, as PFM reduced N leaching and loss, as a result of a diurnal internal water cycle under the mulch. The SOC mineralization was slower with N fertilization, regardless of the PFM treatment. This trend was attributed to the 43% decrease of β-glucosidase activity (C cycle enzyme) and 51% drop of leucine aminopeptidase (N cycle) with N fertilization, as a result of a strong decrease in soil pH. In conclusion, root biomass acting as the main source of soil C, resulted in an increase of soil organic matter after 28 year of N fertilization only with PFM.

Arbuscular mycorrhizal fungi and goethite promote carbon sequestration via hyphal-aggregate mineral interactions

Goethite is known to contribute to the co-precipitation of rhizodeposits and thus benefit carbon (C) sequestration, while arbuscular mycorrhizal fungi (AMF) play significant role in soil organic C (SOC), however, the combined effect is less known. To address this paucity in knowledge, we compared the physicochemical stabilization and microbial mineralization of rhizodeposits from maize (Zea mays L.) and the rhizosphere priming effect (RPE) in soils with a combination of goethite addition and AMF inoculation. Here, we showed that compared to the control: i) Co-amendment of AMF and goethite resulted in a 0.6-fold decrease of rhizodeposit derived CO2, and a 2.8-fold larger allocation of rhizodeposits into macro-aggregates, most likely due to precipitation by goethite and macro-aggregate formation stimulated by AMF hyphae. Analyses using μ-FTIR confirmed the spatial distribution of polysaccharides overlapped with Fe–O minerals within macro-aggregates, supporting the concomitant processes of rhizodeposit stabilization and aggregate formation via hyphal-aggregate mineral interactions; ii) Inoculation with AMF accelerated SOC turnover by increasing the RPE (by 6.1 mg C kg−1 day−1, 74% increase) and rhizodeposit stabilization (by 6.2 mg C kg−1 soil day−1, 47% increase). The larger soil priming effect stimulated by AMF was associated with several genera including Solirubrobacter, Pseudomonas and Talaromyces, suggesting these hyper-symbionts were involved in nutrient acquisition (mining hypothesis). Our results enabled the comparison between rhizodeposit stabilization versus rhizodeposit and SOC mineralization, and highlighted the contributions of both goethite (abiotic contribution) and AMF (biotic contribution) to C accrual in a soil-plant system.

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

BackgroundDespite 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).ResultsCumulative 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.ConclusionsSoil 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.

Dinotefuran alters Collembola-fungi-bacteria interactions that control mineralization of maize and soil organic carbon

Rare studies investigated influence of neonicotinoid insecticides on the whole soil biota including non-target invertebrates and microorganisms. And less is known about the consequent intervention on soil C processes. This study aimed to decipher Collembola-fungi-bacteria interactive effects on pathways of maize C translocation, combining isotopic tracer analysis of relevant compartments with high-throughput sequencing for bacterial and fungal genetic profiles. Dinotefuran was applied at 0 or 100 μg kg−1 (a simulating residual dosage) to microcosms containing soils, Collembola and 13C labelled maize. Dinotefuran drastically reduced the density and maize-derived biomass C of Collembola, while intensifying antagonistic associations between soil organisms, with flourishing growth of Ascomycota and Actinobacteria, e.g., Streptomyces. This led to higher soil organic C (SOC) mineralization (elevated by 9.8–10.5%) across soils, attributing to the shift in microbial taxonomic and functional guild, e.g., with the increased abundance of genes aligned to cytochrome P450. Maize decomposition was controlled by Collembola that primarily fed on maize, via grazing behavior that facilitated labile maize C preferred decomposers, e.g., Xanthomonadaceae. These findings elucidate the influence of minute dinotefuran on intra-linkages between biomes (Collembola, fungi and bacteria), and highlight such legacy effects on maize and SOC mineralization.

Carbon sequestration in aggregates from native and cultivated soils as affected by soil stoichiometry

Quantitative influence and underlying mechanisms of nutrient stoichiometry on mineralization of native Soil organic C (SOC) and straw C in different aggregate classes from cultivated and non-cultivated soils are still unclear. Soil samples (Mollisols) from a native woodlot and a farmland converted from woodlot were sieved into three aggregate classes (mega-aggregates (6.3-2 mm), macro-aggregates (2-0.25 mm), and micro-aggregates (< 0.25 mm)) and incubated (180 days) under different nutrient rates (nil, low, and high supplies of N and P) with or without 13C-enriched straw amendment. Significantly higher percentage of native SOC was mineralized from mega- and macro-aggregates (65.8-82.2%) compared with micro-aggregates (48.3-52.0%) in woodlot soil. Nutrient addition significantly increased aggregate-associated C in both soils with straw, and the increase was greater in farmland than in woodlot soil and in large-sized aggregates than in micro-aggregates. These results suggested that large aggregates serve as a C reservoir of labile C, while micro-aggregates that contained C was not easily mineralized even with abundant nutrients. Differences of C mineralization among aggregate size classes were significant in woodlot soil but not in farmland soil. Depletion of SOC was greater with increasing nutrient addition rates in farmland aggregates without straw, while the depletion in woodlot aggregates showed no difference among nutrient treatments, suggesting that microbial activity was nutrient-limited in farmland aggregates. The results improved our knowledge on SOC mineralization in response to residue-nutrient management in different aggregate classes from cultivated and non-cultivated soils, which have important implications for strategies to improve soil fertility or mitigate climate change via increased SOC.

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

Stoichiometric control of input substrate (glucose) and native soil organic C (SOC) mineralization was assessed by performing a manipulation experiment based on N or P fertilization in paddy soil. Glucose mineralization increased with nutrient addition up to 11.6% with combined N and P application compared with that without nutrient addition. During 100 days of incubation, approximately 4.5% of SOC was mineralized and was stimulated by glucose addition. Glucose and SOC mineralization increased exponentially with dissolved organic C (DOC):NH4+-N, DOC:Olsen P, and microbial biomass (MB)C:MBN ratios. The relative abundances of Clostridia and β-Proteobacteria (r-strategists) were increased with combined C and NP application at the beginning of the experiment, while the relative abundances of Acidobacteria (K-strategists) were enhanced with the exhaustion of available resource at the end of incubation. The bacteria abundance and diversity were negatively related to the DOC:NH4+-N and DOC:Olsen P, which had direct positive effects (+ 0.63) on SOC mineralization. Combined glucose and NP application decreased the network density of the bacterial community. Moreover, P addition significantly decreased the negative associations among bacterial taxa, which suggested that microbial competition for nutrients was alleviated. The relative abundances of keystone species showed significant positive correlations with SOC mineralization in the soils without P application, revealing that microbes increased their activity for mining of limited nutrients from soil organic matter. Hence, bacteria shifted their community composition and their interactions to acquire necessary elements by increasing SOC mineralization to maintain the microbial biomass C:N:P stoichiometric balance in response to changes in resource stoichiometry.

Interactions between aged biochar, fresh low molecular weight carbon and soil organic carbon after 3.5 years soil-biochar incubations

Biochar has been considered a stable additive for long-term soil carbon (C) storage and climate change mitigation. However, our understanding of the long-term interactions between aged biochar, soil organic C (SOC), and fresh low molecular weight C compounds (LMW-C) is still limited. In addition to the changes in the chemical properties of biochar and the SOC after aging, biochar stability and C sequestration potentials might be affected by the LMW-C availability. After 3.5 years of incubation, aged soils and aged soil-biochar mixtures (4 atom‰ 13C enriched grass-derived biochar, 10% addition rate) of two different soil types were used in this study, with or without the addition of LMW-C. The 13C natural abundance of the LMW-C was manipulated by mixing cane sucrose (C4) and beet sugar (C3) to make it different from or equal to the native SOC isotopic signature of the two soil types, which allowed the partitioning of the LMW-C and biochar C from the SOC in non-biochar and biochar addition treatments, respectively. We traced the 13C in the cumulative CO2, microbial biomass C (MBC), and dissolved organic C (DOC) at the beginning and the end of the 28 days of incubation. Results indicated that this aged biochar was not as effective for SOC protection as it was when fresh. Aged biochar caused significant increases in the native SOC-derived CO2 compared to non-biochar treatment during the 3.5 years of aging. Within the 28 days of incubation, aged biochar not only promoted the total CO2 emission but also did not limit the priming effect of LMW-C on native SOC mineralization. SOC-derived DOC in the aged soil-biochar mixture was significantly higher than that in the aged soil, which further increased the SOC loss. A decline in the SOC stability with aged biochar might be associated with the attenuated sorption of SOC on aged biochar, the decline in bulk density with the high biochar addition rate, and a modified microbial community. The priming of aged SOC mineralization with the addition of LMW-C might be associated with the enhanced microbial N demand in aged soils and soil-biochar mixtures. The effect of aged biochar on the soil DOC and the effect of LMW-C on the aged biochar and SOC stabilities should be further investigated in field conditions, under continuous fresh C inputs, to better evaluate the effects of biochar on long-term SOC storage.

Catalytic power of enzymes decreases with temperature: New insights for understanding soil C cycling and microbial ecology under warming.

Most current models of soil C dynamics predict that climate warming will accelerate soil C mineralization, resulting in a long-term CO2 release and positive feedback to global warming. However, ecosystem warming experiments show that CO2 loss from warmed soils declines to control levels within a few years. Here, we explore the temperature dependence of enzymatic conversion of polymerized soil organic C (SOC) into assimilable compounds, which is presumed the rate-limiting step of SOC mineralization. Combining literature review, modelling and enzyme assays, we studied the effect of temperature on activity of enzymes considering their thermal inactivation and catalytic activity. We defined the catalytic power of enzymes (Epower ) as the cumulative amount of degraded substrate by one unit of enzyme until its complete inactivation. We show a universal pattern of enzyme's thermodynamic properties: activation energy of catalytic activity (EAcat )<activation energy of thermal inactivation (EAinact ). By investing in stable enzymes (high EAinact ) having high catalytic activity (low EAcat ), microorganisms may maximize the Epower of their enzymes. The counterpart of such EAs' hierarchical pattern is the higher relative temperature sensitivity of enzyme inactivation than catalysis, resulting in a reduction in Epower under warming. Our findings could explain the decrease with temperature in soil enzyme pools, microbial biomass (MB) and carbon use efficiency (CUE) reported in some warming experiments and studies monitoring the seasonal variation in soil enzymes. They also suggest that a decrease in soil enzyme pools due to their faster inactivation under warming contributes to the observed attenuation of warming effect on soil C mineralization. This testable theory predicts that the ultimate response of SOC degradation to warming can be positive or negative depending on the relative temperature response of Epower and microbial production of enzymes.

Converting natural evergreen broadleaf forests to intensively managed moso bamboo plantations affects the pool size and stability of soil organic carbon and enzyme activities

Land-use change significantly affects the soil organic C (SOC) dynamics and microbial activities. However, the roles of chemical composition of SOC and enzyme activity in the change in the SOC mineralization rate caused by land-use change are poorly understood. This study aimed to investigate the impact of land-use conversion from natural evergreen broadleaf forests to intensively managed moso bamboo (Phyllostachys edulis) plantations on the pool size and mineralization rate of SOC, as well as the activities of C-cycling enzymes (invertase, β-glucosidase, and cellobiohydrolase) and dehydrogenase. Four paired soil samples in two layers (0–20 and 20–40 cm) were taken from adjacent evergreen broadleaf forest-moso bamboo plantation sites in Lin’an County, Zhejiang Province, China. Soil water-soluble organic C (WSOC), hot-water-soluble organic C (HWSOC), microbial biomass C (MBC), readily oxidizable C (ROC), the activities of C-cycling enzymes and dehydrogenase, and mineralization rates of SOC were measured. The chemical composition of SOC was also determined with 13C-nuclear magnetic resonance spectroscopy. The conversion of broadleaf forests to bamboo plantations reduced SOC stock as well as WSOC, HWOC, MBC, and ROC concentrations (P < 0.05), decreased O-alkyl, aromatic, and carbonyl C contents, but increased alkyl C content and the alkyl C to O-alkyl (A/O-A) ratio, suggesting that the land-use conversion significantly altered the chemical structure of SOC. Further, such land-use change lowered (P < 0.05) the SOC mineralization rate and activities of the four enzymes in the 0–20-cm soil. The decreased SOC mineralization rate associated with the land-use conversion was closely linked to the decreased labile organic C concentration and soil enzyme activities. The results demonstrate that converting broadleaf forests to moso bamboo plantations markedly decreased the total and labile SOC stocks and reveal that this conversion decreased the mineralization rate of SOC via changing the chemical composition of SOC and decreasing activities of C-cycling enzymes. Management practices that enhance C input into the soil are recommended to mitigate the depletion of SOC associated with land-use conversion to moso bamboo plantations.

Response of soil organic carbon fractions and CO2 emissions to exogenous composted manure and calcium carbonate

The addition of exogenous substances has an important influence on soil organic matter mineralization. The aim of the present study was to investigate the effect of calcium carbonate (CaCO3) and composted manure (CM) on soil organic C (SOC) fractions and soil CO2 emission in calcareous soil. We conducted a 105-day incubation experiment involving six treatments of two soils from Yangling (Y) and Tongchuan (T) on the Loess plateau, China, which had different CaCO3 and total organic C (TOC) content. Treatments consisted of 0, 30, and 50 g kg−1 CaCO3 (Y0, Y30, Y50, T0, T30, and T50) and 0, 30, and 50 g kg−1 CaCO3 plus 20 g kg−1 CM (Y0-M, Y30-M, Y50-M, T0-M, T30-M and T50-M). Soil pH, CaCO3, CO2 emissions, TOC, microbial biomass C (MBC), dissolved organic C (DOC), readily oxidizable organic C (ROC), and bound humus (B-Hs) C were determined for each of the treatments. The application of CM increased MBC, DOC, and ROC, promoting the mineralization of SOC. However, the effects of CaCO3 addition on SOC fractions differed based on soil type and CM amendment. The addition of CaCO3 stimulated soil microbial population in the soils of Y and T. When soil was amended with CaCO3 alone, CaCO3 decreased DOC and ROC but increased B-Hs C, resulting in a reduction in soil CO2 emissions. When soil was amended with both CM and CaCO3, positive and negative net increments in soil CO2 emissions are possible, resulting in different soil cumulative CO2 emissions. These differences in interaction resulted from the priming effect of CaCO3 and CM on soil microbial community activity and the protective effect of CaCO3 on SOC. Soil CO2 emissions are determined by the combined effect of the protective effect of CaCO3 on SOM and the priming effect of soil microbes on SOC mineralization. The effect of the application of CM plus CaCO3 on SOC cycle in calcareous soils is affected by the degree of protection of SOC from SIC on the Loess plateau, China.

Effects of temperature, soil substrate, and microbial community on carbon mineralization across three climatically contrasting forest sites.

How biotic and abiotic factors influence soil carbon (C) mineralization rate (R S) has recently emerged as one of the focal interests in ecological studies. To determine the relative effects of temperature, soil substrate and microbial community on R s, we conducted a laboratory experiment involving reciprocal microbial inoculations of three zonal forest soils, and measured R S over a 61‐day period at three temperatures (5, 15, and 25°C). Results show that both R s and the cumulative emission of C (R cum), normalized to per unit soil organic C (SOC), were significantly affected by incubation temperature, soil substrate, microbial inoculum treatment, and their interactions (p < .05). Overall, the incubation temperature had the strongest effect on the R S; at given temperatures, soil substrate, microbial inoculum treatment, and their interaction all significantly affected both R s (p < .001) and R cum (p ≤ .01), but the effect of soil substrate was much stronger than others. There was no consistent pattern of thermal adaptation in microbial decomposition of SOC in the reciprocal inoculations. Moreover, when different sources of microbial inocula were introduced to the same soil substrate, the microbial community structure converged with incubation without altering the overall soil enzyme activities; when different types of soil substrate were inoculated with the same sources of microbial inocula, both the microbial community structure and soil enzyme activities diverged. Overall, temperature plays a predominant role in affecting R s and R cum, while soil substrate determines the mineralizable SOC under given conditions. The role of microbial community in driving SOC mineralization is weaker than that of climate and soil substrate, because soil microbial community is both affected, and adapts to, climatic factors and soil matrix.

Interactions of soil particulate organic matter chemistry and microbial community composition mediating carbon mineralization in karst soils

Particulate organic matter (POM) chemistry and microbial degraders are important determinants of soil carbon (C) mineralization, but the effects of their interactions on C mineralization are largely unknown. Two contrasting soils with different POM chemical composition from karst ecosystems were sampled, sterilized, and cross-inoculated with microorganisms for 124 days to explore the relationships between POM chemical composition, microbial community composition, and soil organic C (SOC) mineralization. Pyrolysis-gas chromatography/mass spectrometry analysis was used to determine POM chemistry, and phospholipid fatty acid (PLFA) analysis was used to characterize the soil microbial community. Microbial cross-inoculation showed that the microbial community composition did not change in response to changes in soil C and that the relatively labile POM components, e.g., fatty acids and n-alkenes, explained 61.2% of the variation in microbial community composition. Microbial community composition, such as PLFAs cy19:0, i17:0, 10Me16:0, and 18:2ω6,9c, strongly influenced POM chemical composition, explained 94.5% of the variation in POM chemical composition. These results indicated that soil microbial communities could adapt to changes in POM and served as main drivers of POM chemistry alterations. In addition, mineralized soil C (% of SOC) was significantly influenced by microbial community composition, soil source, and their interactions. Redundancy analysis and Mantel tests further revealed that SOC mineralization was strongly affected by POM chemical composition (e.g., the content of ketones and p-hydroxyphenyl) and microbial community composition (e.g., the content of PLFAs 16:1ω7c and 10Me16:0), and that the influence of microbial community composition on SOC mineralization was highly dependent on POM chemical composition, suggesting that the interactions of POM chemistry and microbial community composition mediate SOC mineralization. These analyses indicate a tight relationship between POM chemistry and microbial community composition, and highlight the importance of their interactions in mediating the persistence of organic matter in karst soils.

Composition and mineralization of soil organic carbon pools in four single-tree species forest soils

Forest soil carbon (C) is an important component of the global C cycle. However, the mechanism by which tree species influence soil organic C (SOC) pool composition and mineralization is poorly understood. To understand the effect of tree species on soil C cycling, we assessed total, labile, and recalcitrant SOC pools, SOC chemical composition by 13C nuclear magnetic resonance spectroscopy, and SOC mineralization in four monoculture plantations. Labile and recalcitrant SOC pools in surface (0–10 cm) and deep (40–60 cm) soils in the four forests contained similar content. In contrast, these SOC pools exhibited differences in the subsurface soil (from 10 to 20 cm and from 20 to 40 cm). The alkyl C and O-alkyl C intensities of SOC were higher in Schima superba and Michelia macclurei forests than in Cunninghamia lanceolata and Pinus massoniana forests. In surface soil, S. superba and M. macclurei forests exhibited higher SOC mineralization rates than did P. massoniana and C. lanceolata forests. The slope of the straight line between C 60 and labile SOC was steeper than that between C 60 and total SOC. Our results suggest that roots affected the composition of SOC pools. Labile SOC pools also affected SOC mineralization to a greater extent than total SOC pools.

Tillage and corn residue harvesting impact surface and subsurface carbon sequestration.

Corn stover harvesting is a common practice in the western U.S. Corn Belt. This 5-yr study used isotopic source tracking to quantify the influence of two tillage systems, two corn ( L.) surface residue removal rates, and two yield zones on soil organic C (SOC) gains and losses at three soil depths. Soil samples collected in 2008 and 2012 were used to determine C enrichment during SOC mineralization, the amount of initial SOC mineralized (SOC), and plant C retained in the soil (PCR) and sequestered C (PCR - SOC). The 30% residue soil cover after planting was achieved by the no-till and residue returned treatments and was not achieved by the chisel plow, residue removed treatment. In the 0- to 15-cm soil depth, the high yield zone had lower SOC (1.49 Mg ha) than the moderate yield zone (2.18 Mg ha), whereas in the 15- to 30-cm soil depth, SOC was higher in the 60% (1.38 Mg ha) than the 0% (0.82 Mg ha) residue removal treatment. When the 0- to 15- and 15- to 30-cm soil depths were combined, (i) 0.91 and 3.62 Mg SOC ha were sequestered in the 60 and 0% residue removal treatments; (ii) 2.51 and 0.36 Mg SOC ha were sequestered in the no-till and chisel plow treatments, and (iii) 1.16 and 1.65 Mg SOC ha were sequestered in the moderate and high yield zone treatments, respectively. The surface treatments influenced C cycling in the 0- to 15- and 15- to 30-cm depths but did not influence SOC turnover in the 30- to 60-cm depth.

Film-Mulched Ridge-Furrow Management Increases Maize Productivity and Sustains Soil Organic Carbon in a Dryland Cropping System

Alleviating the hydrothermal limitations to growth, clear film fully mulched ridge–furrow (FMRF) cropping significantly improves maize (Zea mays L.) grain yield on the Loess Plateau of China. Major concerns for FMRF cropping are the stability of maize productivity and whether the system is detrimental to the soil organic C (SOC) balance under changed hydrothermal conditions. We investigated the effects of maize production with FMRF and its effect on SOC concentration for five consecutive years from 2008 to 2012. Three treatments were imposed: no mulch (narrow ridges with the crop sown beside the ridges), half mulch (the same as no mulch, except the narrow ridges were mulched), and full mulch (i.e., FMRF; alternate narrow and wide ridges, all mulched, with maize in furrows). The 5-yr average of the grain yield was 3.8 Mg ha−1 under no mulch. Half and full mulch increased the grain yield by 68 and 102%, respectively, relative to no mulch. Root biomass was 69 and 104% greater under half and full mulch, respectively, than no mulch. The maize yield and biomass differed among years depending on the growing-season precipitation and its distribution, but the increased yield and biomass from mulching was consistent in all years. The mulch stimulated SOC mineralization and enzymatic activity but had no effect on light (density <1.8 g cm–3) and total SOC concentrations compared with no mulch. We conclude that increased SOC mineralization under FMRF was offset by increased SOC addition; FMRF cropping increased maize productivity without detriment to the SOC balance.

A long‐term nitrogen fertilizer gradient has little effect on soil organic matter in a high‐intensity maize production system

Global maize production alters an enormous soil organic C (SOC) stock, ultimately affecting greenhouse gas concentrations and the capacity of agroecosystems to buffer climate variability. Inorganic N fertilizer is perhaps the most important factor affecting SOC within maize-based systems due to its effects on crop residue production and SOC mineralization. Using a continuous maize cropping system with a 13 year N fertilizer gradient (0-269 kg N ha(-1) yr(-1)) that created a large range in crop residue inputs (3.60-9.94 Mg dry matter ha(-1) yr(-1)), we provide the first agronomic assessment of long-term N fertilizer effects on SOC with direct reference to N rates that are empirically determined to be insufficient, optimum, and excessive. Across the N fertilizer gradient, SOC in physico-chemically protected pools was not affected by N fertilizer rate or residue inputs. However, unprotected particulate organic matter (POM) fractions increased with residue inputs. Although N fertilizer was negatively linearly correlated with POM C/N ratios, the slope of this relationship decreased from the least decomposed POM pools (coarse POM) to the most decomposed POM pools (fine intra-aggregate POM). Moreover, C/N ratios of protected pools did not vary across N rates, suggesting little effect of N fertilizer on soil organic matter (SOM) after decomposition of POM. Comparing a N rate within 4% of agronomic optimum (208 kg N ha(-1) yr(-1)) and an excessive N rate (269 kg N ha(-1) yr(-1)), there were no differences between SOC amount, SOM C/N ratios, or microbial biomass and composition. These data suggest that excessive N fertilizer had little effect on SOM and they complement agronomic assessments of environmental N losses, that demonstrate N2 O and NO3 emissions exponentially increase when agronomic optimum N is surpassed.