Organic Matter Decomposition Rates Research Articles

Using aquatic animals as partners to increase yield and maintain soil nitrogen in the paddy ecosystems.

Whether species coculture can overcome the shortcomings of crop monoculture requires additional study. Here, we show how aquatic animals (i.e. carp, crabs, and softshell turtles) benefit paddy ecosystems when cocultured with rice. Three separate field experiments and three separate mesocosm experiments were conducted. Each experiment included a rice monoculture (RM) treatment and a rice-aquatic animal (RA) coculture treatment; RA included feed addition for aquatic animals. In the field experiments, rice yield was higher with RA than with RM, and RA also produced aquatic animal yields that averaged 0.52-2.57 t ha-1. Compared to their corresponding RMs, the three RAs had significantly higher apparent nitrogen (N)-use efficiency and lower weed infestation, while soil N contents were stable over time. Dietary reconstruction analysis based on 13C and 15N showed that 16.0-50.2% of aquatic animal foods were from naturally occurring organisms in the rice fields. Stable-isotope-labeling (13C) in the field experiments indicated that the organic matter decomposition rate was greater with RA than with RM. Isotope 15N labeling in the mesocosm experiments indicated that rice used 13.0-35.1% of the aquatic animal feed-N. All these results suggest that rice-aquatic animal coculture increases food production, increases N-use efficiency, and maintains soil N content by reducing weeds and promoting decomposition and complementary N use. Our study supports the view that adding species to monocultures may enhance agroecosystem functions.

Vertically Divergent Responses of SOC Decomposition to Soil Moisture in a Changing Climate

AbstractThe role of soil moisture for organic matter decomposition rates remains poorly understood and underrepresented in Earth System Models (ESMs). We apply the Dual Arrhenius Michaelis‐Menten (DAMM) model to a selection of ESM soil temperature and moisture outputs to investigate their effects on decomposition rates, at different soil depths, for a historical period and a future climate period. Our key finding is that the inclusion of soil moisture controls has diverging effects on both the speed and direction of projected decomposition rates (up to ±20%), compared to a temperature‐only approach. In the top soil, the majority of these changes is driven by substrate availability. In deeper soil layers, oxygen availability plays a relatively stronger role. Owing to these different moisture controls along the soil depth, our study highlights the need for depth‐resolved inclusion of soil moisture effects on decomposition rates within ESMs. This is particularly important for C‐rich soils in regions which may be subject to strong future warming and vertically opposing moisture changes, such as the peat soils at northern high latitudes.

Dweller and tunneler dung beetles synergistically accelerate decomposition of cattle and horse dung in a semi-arid steppe

Aboveground and belowground invertebrates play a key role in decomposition of organic matter in terrestrial ecosystems. Dung beetles are well known for their functions in breaking down and transporting ungulate dung into soils thus promoting dung decomposition. However, the effects of functionally different dung beetles on the decomposition processes of the dung from different ungulates remain less explored. We did an in situ experiment using the 'dung and soil' mesocosm to investigate the separate and combined effects of two functionally contrasting dung beetle species, a dweller Aphodius sordecens and a tunneler Onthophagus gibbulus, on the decomposition processes of the dung from horse and cattle. We determined the changes in dung mass, dung C and N contents, microbial respiration from the 'dung and soil' mesocosm or from the soil beneath the dung pats, and examined the amount of dung balls transferred by beetles into different soil layers, under five dung beetle treatments (C – soil, no dung nor beetles; D – dung on soil with no beetles; DB - dung on soil with dweller beetles; TB - dung on soil with tunneler beetles; DTB - dung on soil with dweller and tunneler beetles) were applied to the dung of two major grazers with different digestive systems in native steppes in northern China, over a two-month period following dung deposition. We found that (1) tunneler beetles (in TB and DTB) significantly enhanced the dry mass loss of cattle dung but not horse dung. (2) Dung C and N contents declined more rapidly with the presence of any dung beetle species (DB-DTB) than that without (D); and dung N content showed different decreasing rates between horse and cattle dung in different decomposition periods. (3) The presence of the two functionally contrasting dung beetle species together had a synergistic, positive effect on soil microbial respiration and thus on the decomposition rate of soil organic matter; however, no significant effects of beetles on net microbial respiration was detected in the 'dung + soil' mesocosm. (4) The direct effect of dung beetles on dung mass loss was stronger for cattle dung than for horse dung. In contrast, the decline in moisture content of horse dung with decomposition time affected the microbial respiration in dung, and thus indirectly affected the dry mass loss and the C and N contents. Our results suggest it critically important to incorporate the functions of soil invertebrates into nutrient cycling models to improve their accuracy, and to promote invertebrate diversity in grazed grassland to maintain ecosystem health and services.

Rice straw carbon mineralization is affected by the timing of exogenous glucose addition in flooded paddy soil

Crop straw amendment is a farm management approach for enhancing soil carbon sequestration. Moreover, crop straw residue decomposition rates are affected by irregular exogenous organic carbon secretion or metabolism such as that from root exudates or microbial metabolites. To quantify the mineralization rate of straw residue affected by exogenous organic carbon addition and elucidate the underlying mechanisms, in this study, a total of 0.5 g 13C- labeled dry rice straw was added into pre-incubated soil (equivalent to 200 g dry soil) and incubated for 20 days. Then,1mL of a solution with 50 mg glucose was added only once into the soil on day 21 (G(1a)) or day 35 (G(1b)), twice with 25 mg glucose every 14 days (G(2)) or four times with 12.5 mg glucose every 7 days (G(4)). Soil without glucose addition served as a control. The frequent addition of glucose induced higher soil respiration than that induced by only one addition, and stimulated the mineralization of rice straw carbon compared to that in the control in the following order: G(4) (14.6%) > G(1a) (12.7%) > G(2) (12.3%) > G(1b) (11.2%) = control (11.2%). In general, frequent addition of glucose decreased the soil redox potential (Eh) and fungi to bacteria ratio. Structural equation modeling analysis showed that the rice straw carbon mineralization rate was positively correlated with the 13C-dissolved organic carbon (13C-DOC) content and negatively regulated by Eh. Microbial bioactivity exerted indirect positive effects on rice straw carbon mineralization by influencing the 13C-DOC content. A decrease in the fungi to bacteria ratio induced by frequent addition of glucose improved the microbial bioactivity and enhanced rice straw carbon mineralization. Our results suggested that single exogenous organic carbon additions may underestimate residue or soil organic matter decomposition rates in the field.

Biowaste composting process - comparison of a rotary drum composter and open container

Composting is recognized as a sustainable waste management approach in which microorganisms treat and stabilize biodegradable waste under aerobic conditions to obtain compost as a final product. In this paper, composting of biowaste in a rotary drum composter (closed system) and an open container (open system) was compared. Temperature, pH, electrical conductivity, a carbon-to-nitrogen mass ratio (C/N ratio) and contents of moisture, carbon and dry and volatile matter, were measured during composting. Results showed decreasing profiles for moisture, volatile matter, and carbon contents, as well as for the C/N ratio, while increasing profiles for the dry matter content and electrical conductivity during composting in both systems. Leachates were formed only during the first three days of composting and were characterized with high organic loads, high ammonia concentrations, low pH, and high conductivity and turbidity. The organic matter content data during the composting process were analysed according to the first order kinetic model. Results suggested that there was a difference in the rate of organic matter decomposition, which was higher when composting in the open vessel than in the rotary drum composter.

No thermal adaptation in soil extracellular enzymes across a temperate grassland region

Soil organic matter decomposition rates are thermally adapted, in relation to the ambient mean annual temperature of the soil environment. This could be because of temperature sensitivity of soil extracellular enzymes involved in decomposition. The objective of this study was to determine if soil extracellular enzyme activity and its temperature sensitivity (measured as Q10) were related to the mean annual temperature across a grassland region. We collected surface soil (0-10 cm) from 57 temperate grassland sites along a latitudinal gradient in northern China. The Q10 of soil extracellular enzymes (β-1,4-glucosidase, β-1,4-N acetylglucosaminidase, leucine aminopeptidase and acid phosphatase) was measured at four assay temperatures (5 °C, 15 °C, 25 °C, and 35 °C). There was no relationship between the mean annual temperature and standardized extracellular enzyme activity when other influential factors (soil pH, substrates, texture) were controlled, implying no thermal adaption of extracellular enzyme activity. Soil edaphic factors like soil pH, substrates and texture explained more of the spatial variation in the standardized extracellular enzyme activity than the mean annual temperature. Although the Q10 of all four enzymes increased significantly with mean annual temperature, the data converged at the maximum activity, and the average Q10 values were 1.17 for β-1,4-glucosidase, 1.21 for β-1,4-N acetylglucosaminidase, 1.11 for leucine aminopeptidase and 1.08 for acid phosphatase. The positive correlation between the Q10 of extracellular enzyme activity and mean annual temperature is not consistent with the thermodynamic theory, which predicts higher Q10 at sites with lower mean annual temperature. This may be due to the fact that extracellular enzymes operate outside of biological cells and thus are not directly related to physiological process rates of the microbial biomass. This study contributes to our understanding of the thermal adaptation of soil extracellular enzymes, and show that grassland sites with lower mean annual temperature do not necessarily induce higher temperature sensitivity of extracellular enzymes.

A Feasibility Study on Effect of Food Waste Leachate Additions in the Full-Scale Waste Leachate Treatment Facility after the African Swine Fever Outbreak in South Korea

Due to the ban on the use of food waste as wet swine feed after the outbreak of African swine fever (ASF), 2900 tons/day of food waste and 1083 tons/day of food waste leachate were additionally generated. As an alternative treatment method for food waste leachate, the input of food waste leachate on weekends (5% of food waste leachate inflow, 100 tons/day) and its increased input (merge ratio 21.7%, 227.01 tons/day → 27.8%, 453.71 tons/day) into the digesters operated in a public sewage treatment plant were investigated. Additionally, the feasibility of the method was evaluated by analyzing the operation parameters, evaluating the operation efficiency, and identifying other environmental effects on the sewage treatment plant. In the case of input on weekends, the organic matter decomposition rate and gas production rate decreased by 8.0% and 9.5%, respectively, although the input on weekends was judged to be feasible, considering that the inflow into the WWTP increased by up to 206% due to the long-term (7 weeks) monsoon. In the case of the increased input of food waste leachate, the organic matter decomposition and gas production rates increased by 2.5% and 4.8%, respectively, compared with before the increased input. The results of this study confirmed that the additional input of food waste leachate into existing biogasification facilities is feasible. When performed in a stable manner, the use of food waste for anaerobic digestion is considered an appropriate alternative treatment method to wet feed. This will lead to technological and industrial development.

Effects of Copper Fungicides Spray on Nutrient Contents in Soils of Cocoa Growing Areas of Southwestern Nigeria

The present study aimed at evaluating the effects of copper fungicides spray on physical and chemical properties of soils of cocoa growing areas of southwestern Nigeria. Samples were collected from selected cocoa farms and adjacent plots across three states (9 samples from Osun State, 9 samples from Ondo State and 7 samples from Ekiti State), within rainforest zone of Nigeria. The physical and chemical properties of the soil samples were determined. Copper contents of the cocoa plantation soils were significantly higher (16.23 mg Cu/kg) than the adjacent plots (6.31 mg Cu/kg) indicating accumulation of Cu in the soil because of long-term Cu fungicides spray. Other chemical properties determined (pH, Zn, % organic matter, % total N, Ca, Mg and Na) also had higher values in cocoa plantations compared to adjacent plots. Significant decrease was observed in available P in soil samples from cocoa plantations (24.05 mg P/kg) when compared with adjacent plots (30.36 mg P/kg). Management practices like applications of K and P fertilizers are highly recommended for viable cocoa growth and optimum yield in the study areas. Reduction of canopy formed by cocoa trees will allow easy penetration of sunlight into cocoa plantation thereby increase the rate of organic matter decomposition.
 Keywords: Copper, fungicide spray, cocoa farms, black pod disease.

The effect of extreme temperatures on soil organic matter decomposition from Atlantic oak forest ecosystems

SummaryThis work designs a heatwave with a calorimeter to analyze the response of soils from oak forest ecosystems to increasing temperature from 20°C to 60°C and to cooling from 60°C to 20°C. Calorimetry measures the heat rate of the soil organic matter decomposition and the response to increasing and decreasing temperatures directly. It was applied to soil samples representing different soil horizons with organic matter at different degree of decomposition given by their heat of combustion, calculated by differential scanning calorimetry. Results showed temperature-dependent decomposition rates from 20°C to 40°C or 50°C typical for enzymatic activity. From 40°C to 60°C, changes in the rates are less predictable. Data analysis during cooling showed that all samples suffered losses of their enzymatic capacity and that only those with the heat of combustion values close to that of carbohydrates resisted the heat wave.

Limitations of the Q10 Coefficient for Quantifying Temperature Sensitivity of Anaerobic Organic Matter Decomposition: A Modeling Based Assessment

AbstractThe Q10 coefficient is the ratio of reaction rates at two temperatures 10°C apart, and has been widely applied to quantify the temperature sensitivity of organic matter decomposition. However, biogeochemists and ecologists have long recognized that a constant Q10 coefficient does not describe the temperature sensitivity of organic matter decomposition accurately. To examine the consequences of the constant Q10 assumption, we built a biogeochemical reaction model to simulate anaerobic organic matter decomposition in peatlands in the Upper Peninsula of Michigan, USA, and compared the simulation results to the predictions with Q10 coefficients. By accounting for the reactions of extracellular enzymes, mesophilic fermenting and methanogenic microbes, and their temperature responses, the biogeochemical reaction model reproduces the observations of previous laboratory incubation experiments, including the temporal variations in the concentrations of dissolved organic carbon, acetate, dihydrogen, carbon dioxide, and methane, and confirms that fermentation limits the progress of anaerobic organic matter decomposition. The modeling results illustrate the oversimplification inherent in the constant Q10 assumption and how the assumption undermines the kinetic prediction of anaerobic organic matter decomposition. In particular, the model predicts that between 5°C and 30°C, the decomposition rate increases almost linearly with increasing temperature, which stands in sharp contrast to the exponential relationship given by the Q10 coefficient. As a result, the constant Q10 approach tends to underestimate the rates of organic matter decomposition within the temperature ranges where Q10 values are determined, and overestimate the rates outside the temperature ranges. The results also show how biogeochemical reaction modeling, combined with laboratory experiments, can help uncover the temperature sensitivity of organic matter decomposition arising from underlying catalytic mechanisms.

Are structural and functional endpoints of soil communities similarly affected by metal mixtures? – A terrestrial model ecosystem approach

Soils are habitat to a variety of flora and fauna in a linked ecosystem which provides essential ecosystem services. In soil, metals can accumulate at high concentrations, because of anthropogenic activities, leading to toxic effects, threatening the ecosystem and the services it provides. In most real-world contamination scenarios, metals occur as complex mixtures which can interact and produce different toxicity than predicted from individual metal data. Current regulatory guidelines are based on single species responses to individual metals and ignore indirect effects inherent to the inter-linked nature of ecosystems. Also, the evaluation of anthropogenic impacts to the soil communities is usually measured through structural endpoints (e.g. abundance) disregarding functional measurements (e.g. organic matter decomposition rates), which are often seen as tightly related, and thus, similarly affected. In this study we tested three mixture ratios of five metal oxides (lead, copper, nickel, zinc, cobalt) at three dose levels (Low, Med, High) in a terrestrial model ecosystem experiment and measured structural and functional endpoints. Exposure to metal mixtures for 16 weeks did not affect the microarthropod community, but produced severe effects on soil microbial activity (PNR and DHA) reducing activity below 50% compared to control levels, in all dosed treatments. Metal contamination also significantly affected feeding activity and organic matter decomposition, but effects were not as pronounced as on microbial activity. Data suggest that, in the risk assessment of metals and their mixtures, effects on ecosystem structure and functions must be considered to provide adequate environmental protection.

Plant legacies and soil microbial community dynamics control soil respiration

Through litter and root inputs, plants modulate soil properties and the soil microbial communities around them. In turn, these changes in soil properties and microbial community composition can impact plant performance (i.e., plant-soil feedbacks). Many studies have focused on how plant-soil feedbacks affect plant performance and successional patterns, but studies on the impact of plant-soil feedbacks on ecosystem processes are rare. Our research focused on the potential of species-specific plant-soil feedbacks to affect rates of soil organic matter (SOM) decomposition. We conducted a “home vs. away” plant-soil feedback greenhouse experiment using two C3 grass species (Bromus inermis and Pascopyrum smithii) grown in C4 tallgrass prairie soil. We used a closed-circuit CO2 trapping method and isotopic analysis to differentiate between root-derived and SOM-derived CO2 production. Contrary to our predictions, plant-soil feedbacks on plant biomass were independent of the effects of plant-soil interactions on SOM-derived CO2 production, but we did detect a significant legacy of conditioning by B. inermis on subsequent total belowground respiration (i.e., total belowground respiration was higher in soils originally conditioned by B. inermis regardless of which plant species was currently growing in those soils). We attribute these results to the differential effects of these plant species on soil chemistry and soil microbes during the original conditioning phase. This is supported by the observation that differences in soil chemistry and bacterial community composition persisted in soils conditioned by different plant species throughout the entire experiment. Together these results suggest that plant-soil history is important for soil respiration and that differences in soil microbial communities induced by conditioning with different plant species may have lasting effects on ecosystem processes.

Effect of Plant Growth Regulators on Protease Activity in Forest Floor of Norway Spruce Stand

Soil proteases are involved in organic matter transformation processes and, thus, influence ecosystem nutrient turnovers. Phytohormones, similarly to proteases, are synthesized and secreted into soil by fungi and microorganisms, and regulate plant rhizosphere activity. The aim of this study was to determine the effect of auxins, cytokinins, ethephon, and chlorocholine chloride on spruce forest floor protease activity. It was concluded that the presence of auxins stimulated native proteolytic activity, specifically synthetic auxin 2-naphthoxyacetic acid (16% increase at added quantity of 5 μg) and naturally occurring indole-3-acetic acid (18%, 5 μg). On the contrary, cytokinins, ethephon and chlorocholine chloride inhibited native soil protease activity, where ethephon (36% decrease at 50 μg) and chlorocholine chloride (34%, 100 μg) showed the highest inhibitory effects. It was concluded that negative phytohormonal effects on native proteolytic activity may slow down organic matter decomposition rates and hence complicate plant nutrition. The study enhances the understanding of rhizosphere exudate effects on soil microbial activity and soil nitrogen cycle.

Characterization of the Microbial Community in A Continuous Garbage Treatment Process

Previously, we developed a “static type” garbage treatment system whose performance exceeds that of conventional garbage treatment systems. However, we could not explain the system’s excellent performance especially from a microbial ecosystem’s stand point (Matsuda et al, 2017). Microbial activity causes the decomposition of organic materials in a garbage treatment process. However, we do not know which microbes are effective in garbage decomposition and what relationship exists between the succession of a microbial community and the decomposition of organic materials in a garbage composting process. 
 This study analyzed the relationship between a continuous operation garbage treatment system’s microbial community and the decomposition of organic matter and evaluated for effective microbes in the same garbage treatment system. Although many research articles have been published on the composting processes throughout the world, most of them employ batch processes and not continuous processes. Conversely, most real-world practical composting plants adopt a continuous operation system in which fresh feed is usually input once a day with continuous aeration and intermittent mixing. 
 To comprehensively analyze the microbial community, three different approaches were adopted in this study; 1) colony observation, 2) DNA analysis, and 3) the enzymatic activities of each colony.
 In our experiment, the working volume was 10 L of leaf mold and 40 g/(day L) of garbage (the organic load) was input to the reactor every day at a fixed time. Dog food with 70 % moisture content was used as a model garbage substrate. The reactor’s internal temperature, the total reactor’s weight and the sampled residue’s weight, moisture content and pH were measured before inputting garbage. Additionally, the reactor’s internal temperature was measured six hours later. The internal temperature was about 55 ℃ at the highest without heating. The organic matter decomposition rate was about 50 % and the weight reduction rate was over 90 %, implying successful garbage decomposition was achieved.
 The microbial composition and the number of the colonies seen on the medium plate changed every day and did not realize a “steady state.” Thus an extremely efficient microbe does not exist. Only eleven microbes were isolated; yet many more microbes must exist in the system but were not counted due to a high dilution rate.
 From our DNA analysis, the PCR-DGGE profile of the microbial community in the garbage residue showed that bands of isolated colonies were detected in the same positions as the bands of garbage residue, which contained all kinds of microbes. Nine microbes were identified using 16S rRNA genome from eleven isolated ones. The identified microbes were of different bacterial species and their characteristics were examined from the stand point of nutritional property and enzymatic activity. 
 The garbage decomposition process consists of two steps, solubilization and metabolization. Extracellular enzymes act during solubilization of solid garbage residue, and intracellular enzymes work when water-soluble substances are taken up into bacterial cells and metabolized. Protease and amylase activity were measured to assess extracellular enzymatic activity and dehydrogenase activity was measured to evaluate intracellular enzymatic activity. The enzyme activities of bacterial strains significantly differed by strain. Results from this study suggest complementary microbial activity and that Bordetella trematum, Bacillus cereus, Bacillus subtills subtills and Streptomyces thermocarboxydus effectively decompose garbage.

Ecological effects of imidacloprid on a tropical freshwater ecosystem and subsequent recovery dynamics

This study aimed to investigate the effect of imidacloprid on structural (invertebrates and primary producers) and functional (organic matter decomposition and physicochemical parameters) characteristics of tropical freshwaters using acute single species and mesocosm studies performed in Ethiopia. The recovery of affected endpoints was also studied by using a mesocosm study period of 21 weeks. Our acute toxicity test showed that Cloeon dipterum (96-h EC50 = 1.5 μg/L) and Caenis horaria (96-h EC50 = 1.9 μg/L) are relatively sensitive arthropods to imidacloprid. The mesocosm experiment evaluated the effects of four applications of imidacloprid with a weekly interval and the results showed that the macroinvertebrate and zooplankton community structure changed significantly due to imidacloprid contamination in mesocosms repeatedly dosed with ≥0.1 and ≥ 0.01 μg/L, respectively (time weighted average concentrations of 112 days (TWA112d) of ≥0.124 and ≥ ≈0.02 μg/L, respectively). The largest responses were found for C. dipterum, C. horaria, Brachionus sp. and Filinia sp. Chlorophyll-a concentrations of periphyton and phytoplankton significantly increased in the ≥0.1 μg/L treatments levels which are indirect effects as a result of the release of grazing pressure. A significant, but quantitatively small, decrease of organic matter decomposition rate was observed in mesocosms treated with repeated doses of 1 μg/L (TWA112d of 2.09 μg/L). No recovery was observed for the macroinvertebrates community during the study period of 21 weeks, but zooplankton recovered after 9 weeks. We observed spatio-temporal related toxicity differences between tropical and temperate aquatic taxa, with tropical taxa generally being more sensitive. This suggests that use of temperate toxicity data for the risk assessment of imidacloprid in tropical region is not recommended.

Effects of Water Content and Mesh Size on Tea Bag Decomposition

The tea bag method provides a replicable and standardized method to study the effect of environmental variables on the decomposition of standard litter, which enables comparison of organic matter decomposition rates on a large scale. However, it remains uncertain whether tea bag decomposition in response to wetness is representative of that of local litters. We performed incubation experiments to examine whether the effect of soil water on tea bag decomposition becomes inhibitory at higher water contents, as is the case in local leaf litters. In addition, we performed field studies in a mixed forest and cedar plantation in Japan to compare two litter bag mesh sizes: 0.25-mm mesh, the size previously used by a major manufacturer of tea bags (Lipton), and nonwoven bags with mesh sizes finer than 0.25 mm, which are currently produced by Lipton. Both green tea and rooibos tea exhibited higher decomposition rates at higher water contents, but decomposition was inhibited at the highest water content; this was in contrast to our hypothesis based on a field observation but consistent with conceptual models of local litters. The nonwoven tea bags did not show lower decomposition rates, despite the finer mesh size. Rather, the nonwoven rooibos tea bags exhibited slightly higher decomposition rates than the 0.25-mm mesh bags in the cedar plantation, possibly due to a greater abundance of microorganisms that decompose litters in the nonwoven bags, due to the decrease in predation by mesofauna. Our findings provide essential information for future studies of tea bag decomposition.

Bentonite clay combined with organic amendments to enhance soil fertility in oasis agrosystem

Soils in arid and semi-arid lands are vulnerable to climate change, erosion, and mismanagement practices that contribute to organic matter depletion and nutrients imbalances. Oasis agrosystems are considered one of the major farming systems in some parts of these lands in the Middle East and North Africa region. However, oases soils are inherently low in organic carbon due to their sandy texture which retains little water and binds low organic matter contents, and the accelerated decomposition rates of organic matter under high temperature, which makes its accumulation difficult. Traditional farming practices in oasis agrosystems have always integrated organic matter derived from livestock to enhance organic carbon (OC) and total nitrogen (TN) stocks in the soil. This study is aimed to examine the potential effects of a new mixture of organic amendments and bentonite clay on organic carbon and total nitrogen stock accumulation in the soil. Five amendments were applied including sand and compost (SC), sand and manure (SM), sand, bentonite, and compost (SBC), sand, bentonite, and manure (SBM), and sand mixed with bentonite (SB). These treatments were compared to untreated soil (U). The results showed that OC and TN stocks increased significantly (p<0.05) in all three depths of the five treatments compared to the untreated soil. Higher organic carbon stocks (2862±3.4 g m−2) were detected in SBM treatment. The combined organic and mineral amendment retarded OC and TN decomposition. After two years of the amendments’ application, 30% of OC remained in the soil for SBC treatment. In conclusion, the mixed mineral and organic amendment is a better choice to preserve OC and, thus, maintain oasis soil fertility.

Responses of Soil Organic Carbon Mineralization and Microbial Communities to Leaf Litter Addition under Different Soil Layers

The mechanism of how soil carbon pools change when coniferous forests are converted into conifer-broadleaf mixed forests is poorly understood. In this study, the impact of additional carbon inputs on soil organic carbon mineralization and microbial communities was evaluated. In a microcosm incubation experiment, three types of 13C-labeled litter (Pinustabulaeformis (PT), Robiniapseudoacacia (RP), and a mixture of PT and RP (1:1, PR)) were added in to top (0–20 cm) and deep (60–80 cm) soil collected from a Chinese pine plantation. The priming effect (PE) and specific microbial groups involved in PE were studied. PT and RP addition to topsoil induced a negative PE. In deep soil, the decomposition rates of soil organic matter (SOM) after adding PT and mixture increased by 16.6% and 34.6% compared to those without litter. The addition of RP with a lower C:N ratio had a stronger negative PE than adding PT or mixture. Moreover, the PE in deep soil was more intense after all litter additions. In topsoil, the litter-derived carbon was mainly incorporated into 16:0, 18:1ω9c, and 18:1ω7c fatty acids. In conclusion, the addition of broadleaf litter into coniferous plantations might be beneficial for enhancing deep soil carbon stocks.

Larger floods reduce soil CO2 efflux during the post-flooding phase in seasonally-flooded forests of Western Amazonia

Seasonally-flooded várzea forests of Western Amazonia are one of the most productive and biodiverse wetland forests in the world. However, data on their soil CO2 emissions, soil organic matter decomposition rates, and soil C stocks are scarce. This is a concern because hydrological changes are predicted to lead to increases in the height, extent, and duration of seasonal floods, which are likely to have a significant effect on soil C stocks and fluxes. However, with no empirical data, the impact of altered flood regimes on várzea soil C cycles remains uncertain. This study quantified the effects of maximum annual flood height and soil moisture on soil CO2 efflux rate (Rs) and soil organic matter decomposition rate (k) in the várzea forests of Pacaya Samiria National Reserve, Peru. The study was conducted between May and August 2017. The results showed that Rs (10.6–182.7 mg C m−2 h–1) and k (0.016–0.078) varied between and within sites, and were considerably lower than the values reported for other tropical forests. In addition, Rs was negatively affected by flood height (P < 0.01) and soil moisture (P < 0.001), and it decreased with decreasing river levels post flooding (P < 0.001). In contrast, k was not affected by any of the above-mentioned factors. Soil moisture was the dominant factor influencing Rs, and it was significantly affected by maximum flood height, even after the floods had subsided (P < 0.001). Consequently, we concluded that larger floods will likely lead to reduced Rs, whilst k could remain unchanged but with decomposition processes becoming more anaerobic.

Responses of Soil Respiration Dynamics and Bacterial Community Structure to Fertilizer and Irrigation Regimes Over 2 Years in Temperate Vineyards in North China

Fertilizer and irrigation regimes can have profound effects on soil carbon (C) emissions; however, information regarding the effects of fertilizer and irrigation management on the components of soil respiration (Rs) and the underlying bacterial community characteristics in vineyard ecosystems remains limited. Therefore, a 2-year field experiment was conducted in a wine-grape vineyard (WGV) and table-grape vineyard (TGV). For this experiment, each vineyard included two fertilizer and irrigation regimes: farmers’ practice (FP) and recommended practice (RP). The trenching method was employed to separate Rs into heterotrophic respiration (Rh) and autotrophic respiration (Ra) and soil bacterial community structure was examined after the 2-year experiment. Fertilizer and irrigation regimes have caused no effect on Ra, but RP treatment significantly (P < 0.05) decreased the average daily Rh by 15.13% and 17.11% in the WGV and TGV, respectively, leading to significant (P < 0.05) decreases in annual total Rs values at the scale of whole-vineyard by 8.93% and 11.78%, respectively. The lower annual total Rh was partially ascribed to the significant (P < 0.05) decline in Proteobacteria and Bacteroidetes relative abundance, which decreased the rate of organic matter decomposition. When compared with WGV, the higher fertilizer and water input in the TGV resulted in 11.53% and 15.74% higher annual total Rs and Rh values on the scale of whole-vineyard, respectively, while the lower grapevine density and more frequent summer pruning resulted in an 18.83% lower annual total Ra. Overall, RP treatment was found to be a suitable strategy for reducing soil C emissions and benefiting SOC storage in vineyards around North China.