Trade-offs between ligninase and cellulase and their effects on soil organic carbon in abandoned Moso bamboo forests in southeast China

A vast expanse of Moso bamboo (Phyllostachys pubescens J.Houz.) forests in subtropical areas was once intensively managed but has been abandoned in recent years. However, the response of soil organic carbon (SOC) to abandonment management remains unclear, partly because how carbon-degrading enzymes vary with abandonment management and the role of this change in the soil carbon cycle are still poorly understood, which restricts the scientific evaluation of carbon sink benefits of these abandoned Moso bamboo forests. The results of the survey, based on 40 Moso bamboo forests, showed that compared with intensive management, abandonment management for 7-10 and 11-14 years exhibited a significant decrease in ligninase activities (a reduction of 12.14 % and 44.41 %, respectively) and a significant increase in SOC content (an increase of 49.39 % and 52.64 %, respectively). However, abandonment management did not affect cellulase activities or easily oxidizable organic carbon content (p > 0.05), but significantl increased non-easily oxidizable organic carbon (p < 0.05). Furthermore, the total nitrogen (TN) content and pH value increased with prolonged abandonment, and these trade-offs between ligninase and cellulase were primarily driven by pH and TN. The ligninase-to-cellulase activities ratio is the most key factor affecting NEOC and SOC changes in abandoned Moso bamboo forests. Together, these findings demonstrate the response of carbon-degrading enzyme trade-offs to abandonment management and highlight the role of these trade-offs in controlling SOC accumulation. In addition, the different responses of different SOC fractions to abandonment management deserve attention in future studies.

Soil organic carbon fractionation of spoiled and unspoiled soils under different land use and land cover systems

Alteration in land use and land cover is the key factor affecting the soil carbon fractions and its distribution. A study was carried out to estimate the carbon fractions in soils of agricultural, forest and pasture lands in two different areas separated on the basis of industrial activities (spoiled and unspoiled) to get an insight on the long-term soil carbon storage potential. The results showed that the mean values of the total organic carbon (TOC) and various fractions are significantly different between the land use types (p < 0.05). Irrespective of the land uses, the forest land showed significantly higher TOC (7.97) than agricultural land (6.98) and pasture lands (6.68). Further, evaluation of carbon management index (CMI) indicated that forest lands had highest CMI value compared to the other land uses. The spoiled area had significantly higher TOC and carbon fractions than their respective counterparts in the unspoiled area (p < 0.05) due to the negative industrial impact on soil biological processes. The PCA separates the sources of different carbon fractions and revealed an association of N (nitrogen) and K (potassium) with VL (very labile) and L (labile) fractions and the association of P (phosphorous) with stable R (recalcitrant) form. Therefore, it can be inferred from the present study that alterations in land use not only result in soil quality degradation but also trigger a reduction in potential for long term soil C sequestration.

Sources and chemical stability of soil organic carbon in natural and created coastal marshes of Louisiana

Coastal marshes are globally important for sequestering carbon, yet sea-level rise and anthropogenic stressors can reduce their capacity as carbon sinks. Marsh restoration can offset a portion of carbon loss through the degradation of natural marshes, but potential differences in the sources and stability of soil organic carbon (SOC) between created and natural marshes may affect their function as a long-term carbon sink. Here, we examine the sources and chemical stability of SOC in natural and created marshes across the Gulf coast of Louisiana, USA. Marshes were examined along an estuarine salinity gradient in a former interdistributary basin of the Mississippi River Delta and in six created marshes across a 32-year chronosequence and a natural reference marsh (n = 6) in the Chenier Plain. Carbon source was assessed using δ¹³C analysis and chemical stability was determined through an acid hydrolysis digestion that removed labile carbon (LC). Soil δ¹³C values suggested that the local vegetation dominated SOC in all natural marshes although brackish marshes had a mix of sources and degradation of SOC. Recalcitrant carbon (RC) was 72.2 ± 0.5 % of SOC across fresh, brackish and saline marshes. The depth-averaged RC accumulation rate was almost three times greater than LC accumulation rate, yet both contributed significantly to accretion and long-term SOC accumulation (124-132 g m^-2 y^-1 in natural marshes). RC and SOC accumulation rate increased with mineral sediment accumulation rate. For the created marshes, SOC became increasingly recalcitrant due to an increase in in-situ plant inputs, but accumulation rates were lower than the natural marshes. Overall, this study illustrates that natural marshes have a large stock of RC from the vegetation while dredge sediment created marshes have no plant-derived carbon initially, which accumulates slowly thereafter. Restoration practices may be improved by preserving and augmenting these deteriorating but carbon-rich natural marshes.

Interspecific variations in leaf litter decomposition and nutrient release from tropical mangroves

Efficient nutrient cycling through decomposition of leaf litter often regulates the high productivity and subsequent carbon sequestration of mangrove ecosystems along the land-ocean boundary. To understand the characteristics and the potentials of mangrove leaf litter in supplying organic carbon and nutrients to the coastal waters, four major mangrove species (A. officinalis, R. mucronata, H. littoralis and S. apetala) of Bhitarkanika mangrove forest, Odisha, India, were examined in controlled environmental conditions. Half-life time (t₀.₅), estimated for decomposition of those mangrove leaf litter materials ranged from 18 to 52 days. During the incubation experiment, organic carbon from mangrove leaf litter was released primarily through physical processes and was available for heterotrophic respiration. Among the four species, leaf litter of S. apetala with the lowest initial C/N ratios, released organic carbon with low molecular weight (labile substances) that has a relatively higher potential to support the aquatic food web. On the contrary, leaf litter of R. mucronata released organic material with relatively higher molecular weight (humic substances, higher aromaticity), which revealed its superior non-labile characteristics in this unique environment. The mean total heterotrophic bacterial (THB) population in the incubation was around nine-fold higher than the control. THB population growth and Chromophoric Dissolved Organic Matter (CDOM) spectral data further suggested the rapid release of highly labile and recalcitrant carbon from S. apetala and R. mucronata (between 7th and 21st day of incubation), respectively. The mean litter fall from the Bhitarkanika mangrove forest was estimated to be 11.32 ± 1.57 Mg ha^-1 y^-1 and its corresponding carbon content was 5.43 ± 0.75 Mg C ha^-1. The study revealed the role of leaf litter leachates as an important food source to microbial communities in the adjacent coastal waters, in addition to a potential carbon sequesterer through long-term burial in mangrove soil and export to the deep sea.

Response of mixed community anammox biomass against sulfide, nitrite and recalcitrant carbon in terms of inhibition coefficients and functional gene expressions

Anaerobic ammonium oxidation (anammox) has evolved as a carbon and energy-efficient nitrogen management bioprocess. However, factors such as inhibitory chemicals still challenge the easy operation of this powerful bioprocess. This research systematically evaluated the inhibition kinetics of sulfide, nitrite, and recalcitrant carbon under a genomic framework. The inhibition at the substrate and genetic levels of sulfide, nitrite and recalcitrant carbon on anammox activity was studied using batch tests. Nitrite inhibition of anammox followed substrate inhibition and was best described by the Aiba model with an inhibition coefficient [Formula: see text] of 324.04 mg N/L. Hydrazine synthase (hzsB) gene (anammox biomarker) expression was increased over time when incubated with nitrite up to 400 mg N/L. However, despite having the highest specific nitrite removal (SNR), the expression of hzsB at 100 and 200 mg N/L of nitrite was more muted than in most other samples with lower SNRs. Sulfide severely inhibited anammox activities. The inhibition was fitted with a Monod-based model with a [Formula: see text] of 4.39 mg S/L. At a sulfide concentration of 5 mg/L, the hzsB expression decreased throughout the experiment from its original value at he beginning. Recalcitrant carbon of filtrate from thermal hydrolysis process pretreated anaerobic digester had a minimal effect on maximum specific anammox activity (MSAA), and thus the value of the inhibition coefficient could not be calculated. At the same time, its hzsB expression profile was similar to that in the control. Resiliency and recovery tests indicated that the inhibition of nitrite (up to 400 mg N/L) and recalcitrant carbon (in 100% filtrate) were reversible. About 32% of MSAA was recovered after repeated exposures to sulfide at 2.5 mg/L, while at 5 mg/L, the inhibition was irreversible. Findings from this study will be helpful for the successful design and implementation of anammox in full-scale applications.

Carbon stock stability in drained peatland after simulated plant carbon addition: Strong dependence on deeper soil

Peatlands are vital soil carbon sinks, yet this function is jeopardized by plant carbon which could change the decomposition rate of soil organic carbon, knowing as "priming effect". How the priming effect depends on depth is a critical question in drained peatland given the heterogeneity of soil layers defined by the water table, which include the surface acrotelm, inter-mesotelm and deep catotelm. Here, through incubation, we quantified the response of these three soil layers to addition of ¹³C-labeled oxalate, glucose, cellulose, or cinnamic acid under anoxic or oxic conditions on the Zoige Plateau in Tibet. Soil carbon in the inter-mesotelm showed the greatest decomposition, with the highest humification index and lowest microbial biomass carbon, while the soil carbon at the surface acrotelm was least decomposed. Under anoxic conditions, exogenous carbon addition reduced CO₂ emission by 12.2% at the surface acrotelm but increased by 59.8% in the inter-mesotelm and 23.5% in the deep catotelm. In the inter-mesotelm, oxalate addition significantly increased CO₂ emission by 63.9%, while cinnamic acid significantly increased it by 92.9%. In the deep catotelm, cinnamic acid significantly increased CO₂ emission by 55.3%. These results suggested that deeper soil organic carbon was more sensitive to plant carbon, particularly complex or recalcitrant carbon, than surface acrotelm soil. Under oxic conditions, carbon addition increased surface soil CO₂ emission by 18.9%, and triggered even greater increase at inter-mesotelm and deep catotelm soil, with proportions of 48.3% and 32.0%, respectively. Under both conditions, peat profile CO₂ release increased by 17.2-31.4% after exogenous carbon addition, and more than 77.8% of the increase came from the deeper two layers. These findings highlighted the need to take full account of priming effect of deeper soil in order to assess and predict the stability of carbon stocks in drained peatland.

Integrated leachate management approach incorporating nutrient recovery and removal

This manuscript presents an integrated management scheme for leachate which employed struvite precipitation to recover ammonia nitrogen and phosphorus, aerobic granular sludge process for carbon oxidation (in the form of BOD and sCOD) and single stage anaerobic ammonia oxidation (ANAMMOX) for nitrogen management. The influent fed to the integrated treatment scheme was a mixture of anaerobic digester centrate and real leachate in 4:1 ratio. Almost 77% recovery of phosphorus and 25% removal of NH₄⁺-N were accomplished through struvite precipitation at an optimum pH of 9. High pH contributed to free ammonia loss during struvite precipitation experiments. In the aerobic granular sludge reactor overall, BOD₅, COD and NH₄⁺-N removal percentages were 74%, 45% and 35% and in the PN/A reactor, overall 35% removal of total inorganic nitrogen (TIN) was observed. More than 80% BOD removal was recorded in the granular reactor with soluble COD (sCOD) removal fluctuating between 28 and 57% depending on the operational phase. High-throughput amplicon sequencing of 16S rRNA gene targeting V4 region revealed a dominance of phylum Planctomycetes, in the PN/A reactor system. Presence of Rhodobacteraceae, Xanthomonadaceae, Flavobacteriaceae in the granular biomass confirmed the defined redox zones inside mature granules indicating simultaneous removal of nitrogen (N) and organics in aerobic granular sludge technology. Exposing the synthetically cultured aerobic granules directly to the mixture of leachate and centrate unveiled an alteration in physical characteristics of granules; however, reactor operational data and microbial community analysis ascertain the effectiveness of the treatment scheme treating two urban waste-streams.

Stoichiometric constraints on the microbial processing of carbon with soil depth along a riparian hillslope

Soil organic matter (SOM) content is a key indicator of riparian soil functioning and in the provision of ecosystem services such as water retention, flood alleviation, pollutant attenuation and carbon (C) sequestration for climate change mitigation. Here, we studied the importance of microbial biomass and nutrient availability in regulating SOM turnover rates. C stabilisation in soil is expected to vary both vertically, down the soil profile and laterally across the riparian zone. In this study, we evaluated the influence of five factors on C mineralisation (C_min): (i) substrate quantity, (ii) substrate quality, (iii) nutrient (C, N and P) stoichiometry, (iv) soil microbial activity with proximity to the river (2 to 75 m) and (v) as a function of soil depth (0-3 m). Substrate quality, quantity and nutrient stoichiometry were evaluated using high and low molecular weight ¹⁴C-labelled dissolved organic (DOC) along with different nutrient additions. Differences in soil microbial activity with proximity to the river and soil depth were assessed by comparing initial (immediate) C_min rates and cumulative C mineralised at the end of the incubation period. Overall, microbial biomass C (MBC), organic matter (OM) and soil moisture content (MC) proved to be the major factors controlling rates of C_min at depth. Differences in the immediate and medium-term response (42 days) of C_min suggested that microbial growth increased and carbon use efficiency (CUE) decreased down the soil profile. Inorganic N and/or P availability had little or no effect on C_min suggesting that microbial community growth and activity is predominantly C limited. Similarly, proximity to the watercourse also had relatively little effect on C_min. This work challenges current theories suggesting that areas adjacent to watercourse process C differently from upslope areas. In contrast, our results suggest that substrate quality and microbial biomass are more important in regulating C processing rates rather than proximity to a river.

Land-use impacts on profile distribution of labile and recalcitrant carbon in the Ili River Valley, northwest China

There is a growing evidence that the decomposition of recalcitrant carbon (C) can be stimulated by environmental changes, such as fresh C supply and increased temperature. However, the effect of land-use on profile distribution of recalcitrant C content is still poorly understood. In this study, soil samples were collected to a depth of 100cm from pastures and four major croplands including maize field, wheat field, paddy and apple orchard in the Ili River Valley, northwest China, to investigate the effects of land-use on profile distribution of labile organic C (LOC), semi-labile organic C (SLOC), recalcitrant organic C (ROC) and their relative proportions in total organic C (TOC), and evaluate whether such effects can be different between topsoil (0-20cm) and subsoil (20-100cm). The results showed that soil ROC accounting for 49.4-66.3% of TOC for different land-uses, implying that ROC is the major form of soil organic C (SOC). Soil TOC contents of croplands were 20.4-85.2% lower than those of pastures along the soil profile, indicating that SOC pool may be decreased by agricultural land-uses. The lower contents of LOC, SLOC and ROC in croplands than in pastures suggested that the decreases in TOC content in croplands are not only due to the decreases in labile C pool but also the reductions in recalcitrant C pool. The differences in SOC fractions among land-uses were similar in topsoil and subsoil, while the proportions of each SOC fraction in TOC did not differ significantly between the two soil layers in most cases, indicating that each SOC fraction in subsoil can be also influenced by land-use types. Therefore, it is suggested that the ROC in subsoil, which plays a crucial role in C sequestration, should be taken into account when estimating the effect of land-use on SOC kinetic.