Μg CO2-C Research Articles

Evaluating the tradeoffs between water conservation, aesthetic value, evaporative cooling and CO2 emissions in St. augustinegrass and buffalograss

Urban lawns comprise a significant portion of urban greenery and provide several ecosystem services. Nevertheless, maintaining lawns comes with significant water costs in semi-arid inland southern California, as they require consistent irrigation to stay healthy and productive. The main objective of this study was to evaluate the effect of a wide range of irrigation rates and frequencies applied autonomously by a smart ET-based controller on the aesthetic value, cooling potential, and CO2 efflux of two warm-season turfgrass species. For three years, we studied the responses of Buffalograss and St. Augustinegrass to six irrigation rates and two irrigation frequencies in Riverside, California. Under historical average climate conditions, the minimum irrigation rate for Buffalograss was 93 % ETo, and for St. Augustinegrass, it was 74 % ETo. Under projected future climate conditions, the estimated minimum irrigation rate for Buffalograss did not change, but for St. Augustinegrass, it increased by 4 % and 7 % in 2100 under low emission (RCP 4.5) and high emission (RCP 8.5) scenarios, respectively. On average, canopy minus air temperature in Buffalograss was 6.2 ℃, and in St. Augustinegrass, it was 1.1 ℃. The average CO2 efflux in Buffalograss was 122.3 µg CO2-C m−2 s−1, and in St. Augustinegrass, it was 182.8 µg CO2-C m−2 s−1. Our results showed that turfgrass aesthetic values, cooling potential, and CO2 efflux diminished as the irrigation rate decreased, but at different rates for each turfgrass species.

Biochar and Organic Fertilizer Co-Application Enhances Soil Carbon Priming, Increasing CO2 Fluxes in Two Contrasting Arable Soils.

Biochar soil amendments, along with non-tillage agriculture, are often proposed as a strategy for carbon sequestration. It is still questionable how the quality of biochar might influence the priming effect on soil organic matter and whether the addition of unprocessed organic amendments will affect biochar stability. In the study, six different biochars and three exogenous organic matter sources were added to two distinct arable soils. CO2 emission was monitored for 100 days of incubation and CO2 flux was estimated. Results showed that biochar increased soil CO2 fluxes. The highest peaks, up to 162 µg C-CO2 h-1 100 g-1, were recorded in treatments with food waste biochars, suggesting that they serve as a source of easily available carbon to soil microbes. Co-application of raw organic materials (manure and fresh clover biomass) enhanced CO2 emission and carbon losses, especially in sandy soil, where 0.85-1.1% of total carbon was lost in the short-term experiment. Biochar properties and content of labile C can stimulate CO2 emission; however, in a long-term period, this contribution is negligible. The findings of our study showed that more attention should be paid to priming effects caused by the addition of exogenous organic matter when applied to biochar-amended soils.

Disentangling carcass size and climate effects on soil biogeochemistry during decomposition

When an animal dies the decomposition of its carcass releases multiple compounds that become available to soil microbiota altering soil biogeochemistry. This zone of affected substrate is characterized by significant changes to numerous soil biogeochemical parameters including pH, conductivity, microbial respiration rates, dissolved organic carbon (DOC), C/N ratios, carbon and nitrogen stable isotopes (δ13C and δ15N), and major/trace elemental concentrations. Although the effects of animal decomposition on these parameters have been recorded for various organisms and in a variety of ecosystems several knowledge gaps remain, including: the effect of carcass size; and decomposition effects in prairie ecosystems (i.e., semi-arid climates). the effect of carcass size; and decomposition effects in prairie ecosystems (i.e., semi-arid climates). Here, we investigate how soil biogeochemical parameters are affected by the decomposition of a horse carcass (~660 kg) in a prairie ecosystem in comparison to beavers (~18 kg) in a temperate forest. Three parameters were identified as significantly influenced by carcass size: conductivity, DOC, and microbial respiration rate. Average soil conductivity underneath the horse carcass was three to fifteen times higher than control soils and reached a maximum at 1114 ADD, while maximum soil conductivity values were observed after 160 ADD in beaver-associated soils. Maximum DOC concentrations were observed after 160 ADD in beaver-associated soil (67 ± 40 mg C gdw-1) and after 1114 ADD in horse-associated soil (326 ± 115 mg C gdw-1). Microbial respirations rates were both greater in horse-associated soil and longer-lasting compared to beaver-associated soil. Respiration rates were greatest in two of the three horse-associated soils after 1114 ADD (~733 µg CO2-C gdw-1 day-1), which significantly differs compared to beaver-associated soils (300 ± 90 µg CO2-C gdw-1 day-1 at 160 ADD). Taken together, these results demonstrate that larger carcasses result in a greater release of decomposition products, including C, and greater stimulation of soil heterotrophic communities. Other measured biogeochemical indicators of decay suggest a mixed influence due to carcass size and climate, notably δ15N values of the soil, and major element concentrations. Climate was revealed to be more important in controlling changes to pH and gravimetric moisture than carcass size, with baseline soil conditions and type playing critical roles. Additionally, in semi-arid regions such as western South Dakota, the role of wet-dry cycles on carcasses undergoing decomposition may help to explain the pulses observed with C concentrations and C cycling. This study provides the first direct comparison of soil biogeochemistry associated with the decomposition of two different sized taxa decayed in two different climates. Attempting to normalize climate data using ADD was found to be an imperfect system that does not account for all climate variables affecting decomposition. Unravelling patterns in decomposition can better help to constrain nutrient cycling in modern and ancient ecosystems.

Microbial attributes as structural quality index for physical health of an Oxisol under compaction levels

Microbial activity is a sensitive indicator of soil structure improvement provided by plant species in the physical recovery of compacted soils. This study aimed to assess the impacts of crop species on soil aggregate classes for soil recovery after compaction and identify which microbial attributes best describe the structural quality index for soil physical health in an Oxisol under no-tillage. The experiment comprised three crop species—maize, ruzigrass, and black oats—cultivated under four degrees of compactness produced by chiseling or machine traffic under long-term no-tillage. Soil aggregates were separated by dry sieving into five diameter classes to assess the basal respiration, microbial biomass carbon, metabolic quotient for CO2 (qCO2), labile carbon, cellulase activity, and total organic carbon. The values of microbial indicators for the bulk soil were estimated as a weighted mean of each attribute using aggregate size distribution. The lowest values for total organic carbon, labile carbon, and microbial biomass carbon were observed in aggregates > 4 mm. The basal respiration values increased from 8.2 µg C-CO2 g d−1 in the aggregates > 4 mm, to 22.1 µg C-CO2 g soil d−1 in the aggregates < 0.5 mm (+170%). Similarly, the cellulase activity increased approximately four times comparing aggregates > 4 mm (38 µg glucose g−1 d−1) to those < 1 mm (181 µg glucose g−1 d−1). The total organic carbon and microbial biomass carbon were found to be little affected by crops. However, the plots cultivated with black oats or ruzigrass exhibited higher cellulase activity, basal respiration, and qCO2 in comparison with maize, primarily due to the greater content of labile carbon. Increasing the mean weight diameter from 3 mm to 8 mm led to a decrease in the labile carbon (−27.4%), microbial biomass carbon (−25.5%), cellulase activity (−70%), and basal respiration (−21.6%), thereby revealing negative effects of soil compaction on microbial attributes. The degree of compactness did not influence the values of total organic carbon and qCO2. The response of the microbial indicators depends on the aggregate diameter classes. The results of this study indicate that cultivation of cover crops decreases the negative impacts of soil structure degradation on soil microbial quality. Cellulase activity and labile carbon were considered as new potential indicators of soil compaction for soil physical health studies—highly sensitive and quickly responsive to managements that induce soil structure changes in an Oxisol.

THE EFFECT OF MINERAL FERTILIZERS ON SOIL RESPIRATION IN URBAN LAWNS

Due to the need to develop technologies for reducing carbon dioxide emissions and increasing the C-absorption capacity of natural and anthropogenic ecosystems, the possibility of using mineral fertilizers to regulate microbial respiration and CO2 emissions from urban lawn soils was evaluated. The studies were carried out on the territory of the Botanical Garden of Moscow State University on the Lenin Hills in a small-plot experiment with the fractional application of four types of complex fertilizers (NPKS 27:6:6:2, NPKS 21:10:10:2, NPK 15:15:15 and NPK 18:18:18 + 3 MgO + trace elements (TE)) at doses of 60 and 120 kg N ha-1 during the growing season. The carbon content of microbial biomass (Cmic) using substrate-induced respiration was studied, the basal respiration (BR) of soils was estimated, and the emission of CO2 from soils using closed static chambers was determined. Тhe Cmic content in the soil of the control plot in the summer period varied within 1300–1450 μg·g-1. The application of NPKS 21:10:10:2 and NPK 18:18:18 + 3 MgO + TE at a low dose increased the Cmic content by 12–35% for two weeks, then it de-creased. The application of all fertilizers for a short period increased basal respiration of soils and CO2 emission, maximum on the 6th day. Aft er two weeks, the increase in the intensity of BR and CO2 emission decreased or it reached the control values (1,5 μg C-CO2·g-1·h-1 and 500 mg·CO2 ·m-2·h-1 respectively) or less. In the short term, the lowest intensity of CO2 emission and a surge in basal respiration, along with an increase in microbial biomass, were observed with the addition of fertilizer NPKS 21:10:10:2. The greatest change in the functioning of the soil microbial community, revealed by the maximum qCO2, occurred with the introduction of NPKS 27:6:6:2. Th e dy-namics of CO2 emission from the soils of the small-plot experiment from April to October correlated with the soil temperature (rS = 0.66, p&lt;0.05, n=135). In all periods of the study, the minimum CO2 emissions were noted for the plot with NPKS 21:10:10:2 fertilizer.

Relationships between greenhouse gas production and landscape position during short-term permafrost thaw under anaerobic conditions in the Lena Delta

Abstract. Soils in the permafrost region have acted as carbon sinks for thousands of years. As a result of global warming, permafrost soils are thawing and will potentially release greenhouse gases (GHGs) such as methane (CH4) and carbon dioxide (CO2). However, small-scale spatial heterogeneities of GHG production have been neglected in previous incubation studies. Here, we used an anaerobic incubation experiment to simulate permafrost thaw along a transect from upland Yedoma to the floodplain on Kurungnakh Island. Potential CO2 and CH4 production was measured during incubation of the active layer and permafrost soils at 4 and 20 ∘C, first for 60 d (approximate length of the growing season) and then continuing for 1 year. An assessment of methanogen abundance was performed in parallel for the first 60 d. Yedoma samples from upland and slope cores remained in a lag phase during the growing season simulation, while those located in the floodplain showed high production of CH4 (6.5×103 µg CH4-C g−1 C) and CO2 (6.9×103 µg CO2-C g−1 C) at 20 ∘C. The Yedoma samples from the permafrost layer started producing CH4 after 6 months of incubation. We conclude that landscape position is a key factor triggering CH4 production during the growing season time on Kurungnakh Island.

Rapid Permafrost Thaw Removes Nitrogen Limitation and Rises the Potential for N2O Emissions

Ice–rich Pleistocene permafrost deposits (Yedoma) store large amounts of nitrogen (N) and are susceptible to rapid thaw. In this study, we assess whether eroding Yedoma deposits are potential sources of N and gaseous carbon (C) losses. Therefore, we determined aerobic net ammonification and nitrification, as well as anaerobic production of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) in laboratory incubations. Samples were collected from non-vegetated and revegetated slump floor (SF) and thaw mound (TM) soils of a retrogressive thaw slump in the Lena River Delta of Eastern Siberia. We found high nitrate concentrations (up to 110 µg N (g DW)−1) within the growing season, a faster transformation of organic N to nitrate, and high N2O production (up to 217 ng N2O-N (g DW)−1 day−1) in revegetated thaw mounds. The slump floor was low in nitrate and did not produce N2O under anaerobic conditions, but produced the most CO2 (up to 7 µg CO2-C (g DW)−1 day−1) and CH4 (up to 65 ng CH4-C (g DW)−1 day−1). Nitrate additions showed that denitrification was substrate limited in the slump floor. Nitrate limitation was rather caused by field conditions (moisture, pH) than by microbial functional limitation since nitrification rates were positive under laboratory conditions. Our results emphasize the relevance of considering landscape processes, geomorphology, and soil origin in order to identify hotspots of high N availability, as well as C and N losses. High N availability is likely to have an impact on carbon cycling, but to what extent needs further investigation.

Distinct Changes in Abundance of Culturable Microbial Community and Respiration Activities in Response to Mineral–Organic Mixture Application in Contaminated Soil

The availability and affordability of fertilizers are the main risks currently faced by the fertilizer market. Therefore, there is a need to look for other sources of nutrient supply for plants, while taking care of soil properties. The application of fertilizers with the addition of functionalized materials could help in the efficient use of nutrients. The aim of the study was to assess the impact of the application of mixtures with the addition of zeolite–vermiculite composites (NaX–Ver) on the culturable microorganisms and selected soil properties. A two-year pot experiment was conducted on soil with elevated contents of cadmium, zinc, and lead. The test treatments included soil mixed with NPK and additives in two doses of NaX–Ver combined with leonardite (Leo) or lignite (L). The test plant used in the experiment was maize. The soil material was analyzed for the number of bacteria, mold fungi, actinomycetes, and ammonifiers. Furthermore, soil pH, EC, N total, and SOC contents, as well as soil respiration activity, were tested. The applied fertilizer mixtures had a great effect on changes in the N total and SOC contents. The N total increase was 45.5% in NaX–Ver3%L3% and 51% in NaX–Ver9%Leo6%, and the largest SOC increase (24.3%) was recorded in the NaX–Ver3%Leo3% treatment. The highest respiration activity was determined in NaX–Ver3%Leo3% and NaX–Ver9%Leo6%: 2.12 µg C-CO2 g−1 DM h−1 and 2.14 µg C-CO2 g−1 DM h−1, respectively. A significant correlation between pH values and the number of culturable microorganisms was found. The number of soil microorganisms depended on the type of fertilization used. The best stimulation of the number of culturable soil microorganisms was found in treatments with the addition of 3% of L or Leo in combination with NaX–Ver. The percentage increases in the number of the analyzed culturable microorganisms after the application of leonardite-based fertilization in combination with the zeolite–vermiculite composite were, on average: bacteria, 1096%; mold fungi, 1529%; actinomycetes, 1477%; ammonifiers, 910%.

Seasonal variations in soil chemical and microbial indicators under conventional and organic vineyards

Studies regarding soil quality and health often need to be up-to-date, as they feed new models for quantifying agricultural impacts on the environment. This study was established to understand how types of vineyard cultivation (organic and conventional) affect soil chemical and microbial attribute dynamics throughout different seasons. Vineyard management had a strong effect on chemical soil attributes. Organic carbon and phosphorus were 2.8 and 2.0 times greater, respectively, in organic vineyards than in conventional vineyards. Metabolic quotient (qCO2) values were lowest in summer and autumn, with an average of 2.31-2.49 µg C-CO2 h-1 g-1 soil, under organic management, indicating greater microbial growing efficacy. Regardless of season and sampling position, organic soil had a higher C microbial biomass than conventional vineyards, with values ranging from 179.79 to 284.71 µg g-1 soil, which were similar to those of the adjacent forest soil. Overall, there were increases in both the microbial and the chemical attributes of soil under organic vineyards compared relative to conventional management, which might have been due to the continuous input of organic matter, crop rotation, and alternative plant protection and fertilizer compounds used in organic farming.

Progressive freeze-thaw redistributes water, solute and CO2 emissions across soil layers – The role of soil particle size

Progressive freeze-thaw redistributes water and solute among soil layers, and thus perturbs soil carbon mineralization. This potentially introduces variations when differently-sized soil aggregates experience progressive thawing and detachment. In this study, a Mollisol was dry-sieved into three particle sizes (<0.125, 0.125–0.5 and 0.5–1 mm). Each was refilled into a soil column, rewetted to saturation, frozen at −15 °C for 12 h, and then thawed at ambient temperature. During thawing, the soil columns were peeled by layers from the outermost to the inner core into six layers (T1 ∼ T6), to simulate soil detachment while progressive thawing. Our results show that: 1) The outermost layer T1, thawed earlier and thus peeled off first, had the highest water content (102.1–151.2 %) but the lowest electrical conductivity (EC, 63.2–68.3 μS·cm−1). On the contrary, the inner core T6, thawed the last, was driest with merely 24.1–28.4 % water content yet higher EC of 61.0–87.0 μS·cm−1. Such polarized distributions were most pronounced among the soil layers composed of fine aggregates. 2) The CO2 emission rates from the T1 were in general lower than that from the T6 (on average 56.1–65.6 vs. 70.0–84.8 μg CO2-C·g soil −1 7d −1). The soil columns composed of fine aggregates released significantly less CO2 than the coarser aggregates. Overall, the convection of free water during freezing and the diffusion of ice-insoluble materials from freezing front to unfrozen zones, collectively resulted in the polarized patterns of soil water, solute and CO2 emissions. This challenges the conventional investigations based on complete freeze-thaw, highlighting the necessity to distinguish inter-layer heterogeneity during progressive thawing. The varying responses of differently-sized aggregates call for systematic investigations on how progressively thawing and detachment affect slope-scale carbon biogeochemical processes.

Land use impact on carbon mineralization in well aerated soils is mainly explained by variations of particulate organic matter rather than of soil structure

Abstract. Land use is known to exert a dominant impact on a range of essential soil functions like water retention, carbon sequestration, organic matter cycling and plant growth. At the same time, land use management is known to have a strong influence on soil structure, e.g., through bioturbation, tillage and compaction. However, it is often unclear whether the differences in soil structure are the actual cause of the differences in soil functions or if they only co-occur. This impact of land use (conventional and organic farming, intensive and extensive meadow, extensive pasture) on the relationship between soil structure and short-term carbon mineralization was investigated at the Global Change Exploratory Facility, in Bad Lauchstädt, Germany. Intact topsoil cores (upper 10 cm, n=75) were sampled from all land use types at the early growing season. Soil structure and microbial activity were measured using X-ray-computed tomography and respirometry, respectively. Differences in microstructural properties between land uses were small in comparison to the variation within land uses. The most striking difference between land uses was larger macropore diameters in grassland soils due to the presence of large biopores that are periodically destroyed in croplands. Grasslands had larger amounts of particulate organic matter (POM), including root biomass, and also greater microbial activity than croplands, both in terms of basal respiration and rate of carbon mineralization during growth. Basal respiration among soil cores varied by more than 1 order of magnitude (0.08–1.42 µg CO2-C h−1 g−1 soil) and was best explained by POM mass (R2=0.53, p&lt;0.001). Predictive power was only slightly improved by considering all bulk, microstructure and microbial properties jointly. The predictive power of image-derived microstructural properties was low, because aeration did not limit carbon mineralization and was sustained by pores smaller than the image resolution limit (&lt;30 µm). The frequently postulated dependency of basal respiration on soil moisture was not evident even though some cores were apparently water limited, as it was likely disguised by the co-limitation of POM mass. This finding was interpreted in regards to the microbial hotspots which form on decomposing plant residues and which are decoupled from water limitation in bulk soil. The rate of glucose mineralization during growth was explained well by substrate-induced respiration (R2=0.84) prior to growth, which in turn correlated with total microbial biomass, basal respiration and POM mass, and was not affected by pore metrics. These findings stress that soil structure had little relevance in predicting carbon mineralization in well-aerated soil, as mineralization appeared to by predominantly driven by the decomposition of plant residues in intact soil. Land use therefore affects carbon mineralization in well-aerated soil mainly in the amount and quality of labile carbon.

Short-Term Mineralization of Belowground Biomass of Perennial Biomass Crops after Reversion to Arable Land

Little is known about the effect of perennial biomass crops (PBCs) removal on soil C dynamics. The belowground biomass (BGB) that is composed by plant belowground organs (PBO) such as rhizomes in the herbaceous PBCs and stumps in woody PBCs should be considered, together with fine roots (FR), as a huge input of exogenous organic matter (EOM) that is incorporated into the soil at the reversion. In this study, we mimic the incorporation of BGB of PBCs through a soil-residues incubation under controlled conditions to investigate the effects of adding FR and PBO (at real field rates) on soil C and N mineralization dynamics, and to understand decomposition controlling factors. A modified RothC model version, encompassing a better description of decomposable (DEOM) and resistant (REOM) pools, was fitted to C mineralization curves of respiration measured by CO2 evolution in incubated soil to quantify partitioning factors and decomposition rates of PBCs BGB components. After 1 month, PBO showed higher mineralization rates (498 µg CO2-C gsoil−1) than FR (196 µg CO2-C gsoil−1), with black locust having the highest amount of C respired (38% of added C). The emission peak occurred within 3 days from the beginning of the experiment for PBO and after 1 day for FR. Generally, according to the modified version of RothC model, PBO had higher proportion of REOM than FR, except for black locust. The decomposition constant rates from the optimized RothC model were higher for PBO (kDEOM: 20.9 y−1, kREOM: 12.1 y−1) than FR (kDEOM: 0.4 y−1, kREOM: 0.1 y−1), indicating that FR are less decomposable than PBO. The C/N ratio is not the main controlling factor of decomposition when residue N is not a limiting factor, while the availability of easily decomposable substrates (DEOM/REOM ratio) and cell-wall composition decomposition is a strong predictor of C and N mineralization of these EOM types. The explicit inclusion of crop-specific DEOM/REOM ratios within RothC or a similar soil C model will help to improve the predictions of long-term C sequestration trajectories (half-life &gt; 30 years) associated with PBCs cultivation, especially when dismission of such perennial cropping systems is addressed.

Biological Indicators of Soil Condition on the Kabanyolo Experimental Field, Uganda

Soil biological activity is an integral characteristic reflecting the state of soil fertility, biodiversity, and the activity of soil processes carried out by soil organisms. In Africa, studies of soil biological properties are few compared to the agrochemical research. In this paper, we present an assessment of multiple biochemical and microbiological properties of soil from an agricultural field located in the African tropical savanna. We determined basal respiration, substrate-induced respiration, C of microbial biomass, the potential activity of denitrification, nitrogen fixation activity, and estimated prokaryotic components in the soil microbial complex by quantitative PCR. Basal respiration of soils ranged from 0.77 ± 0.04 to 1.90 ± 0.23 μg C-CO2·g−1·h−1, and substrate-induced respiration ranged from 3.31 ± 0.17 to 7.84 ± 1.04 μg C-CO2·g−1·h−1. The C reserves of microbial biomass averaged 403.7 ± 121.6 μg C·g−1 of soil. The N2O emission from the upper layer on average amounted to 2.79 ng N-N2O·g−1·day−1, and the potential denitrification activity reached 745 ± 98 ng N-N2O·g−1·h−1. The number of copies of bacterial genes varied from (0.19 ± 0.02) × 108 to (3.52 ± 0.8) × 108 copies·g−1, and of archaea—from (0.10 ± 0.01) × 107 to (0.29 ± 0.01) × 107 copies·g−1 of soil. These results were in good agreement with the studies in other seasonally wet tropical regions: the biological activity was relatively low. The difference between biological indicators of the experimental field and the reference profile were insignificant except for nitrogen loss, which was higher in the ploughed field. Biological indicators strongly varied in space; we explained their heterogeneity by non-uniform management practices in the course of agrochemical field experiments in the past. The use of organic fertilisers may cause the release of climatically active gases due to intensive microbial respiration and denitrification, but the intensity of emission would strongly depend on the cultivation and management method.

Leaf litter decomposition in the air should not be ignored in subtropical plantations of China

Litter decomposition is an indispensable component of carbon and nutrient cycles in forests. Plant litter is often intercepted by tree branches or understory vegetation in the air, yet aerial litter decomposition dynamics are overlooked in these ecosystems. Here, we collected leaf litter of four common tree species (Liquidambar formosana, Schima superba, Pinus elliottii, and Pinus massoniana) from subtropical plantations of southern China, and used the litterbag method to measure litter mass remaining and microbial respiration on the ground and in the air by reciprocally transplanting leaf litter in the four plantations. The main objectives were to compare the differences in litter decomposition rates and the home-field advantage between on the ground and in the air over 360 days of decomposition. Irrespective of decomposition habitats and incubation periods, litter mass remaining was slightly higher in the air (37.0%) than on the ground (32.0%), and microbial respiration was much lower in the air (2.2 μg CO2-C h−1 g−1) than on the ground (18.2 μg CO2-C h−1 g−1). Litter mass remaining correlated negatively with microbial respiration on the ground, despite no significant relationship in the air. In addition, the home-field advantage during decomposition only occurred on the ground (4.86–12.33%), but disappeared in the air. These contrasting patterns indicated that litter decomposition was driven by microbial processes on the ground and abiotic processes in the air. Our findings highlight that decomposition position predominantly regulates leaf litter decomposition patterns, and suggest that litter decomposition in the air should not be neglected in subtropical forests.

TIME-INTEGRATED COLLECTION OF CO2FOR14C ANALYSIS FROM SOILS

ABSTRACTWe developed a passive sampler for time-integrated collection and radiocarbon (14C) analysis of soil respiration, a major flux in the global C cycle. It consists of a permanent access well that controls the CO2uptake rate and an exchangeable molecular sieve CO2trap. We tested how access well dimensions and environmental conditions affect collected CO2, and optimized cleaning procedures to minimize14CO2memory. We also deployed two generations of the sampler in Arctic tundra for up to two years, collecting CO2over periods of 3 days–2 months, while monitoring soil temperature, volumetric water content, and CO2concentration. The sampler collects CO2at a rate proportional to the length of a silicone tubing inlet (7–26 µg CO2-C day-1·m Si-1). With constant sampler dimensions in the field, CO2recovery is best explained by soil temperature. We retrieved 0.1–5.3 mg C from the 1st and 0.6–13 mg C from the 2nd generation samplers, equivalent to uptake rates of 2–215 (n=17) and 10–247 µg CO2-C day-1(n=20), respectively. The method blank is 8 ± 6 µg C (mean ± sd,n=8), with a radiocarbon content (fraction modern) ranging from 0.5875–0.6013 (n=2). The sampler enables more continuous investigations of soil C emission sources and is suitable for Arctic environments.

Greenhouse gas production and lipid biomarker distribution in Yedoma and Alas thermokarst lake sediments in Eastern Siberia.

Permafrost thaw leads to thermokarst lake formation and talik growth tens of meters deep, enabling microbial decomposition of formerly frozen organic matter (OM). We analyzed two 17-m-long thermokarst lake sediment cores taken in Central Yakutia, Russia. One core was from an Alas lake in a Holocene thermokarst basin that underwent multiple lake generations, and the second core from a young Yedoma upland lake (formed ~70years ago) whose sediments have thawed for the first time since deposition. This comparison provides a glance into OM fate in thawing Yedoma deposits. We analyzed total organic carbon (TOC) and dissolved organic carbon (DOC) content, n-alkane concentrations, and bacterial and archaeal membrane markers. Furthermore, we conducted 1-year-long incubations (4°C, dark) and measured anaerobic carbon dioxide (CO2 ) and methane (CH4 ) production. The sediments from both cores contained little TOC (0.7±0.4 wt%), but DOC values were relatively high, with the highest values in the frozen Yedoma lake sediments (1620mgL-1 ). Cumulative greenhouse gas (GHG) production after 1year was highest in the Yedoma lake sediments (226±212µgCO2 -Cg-1 dw, 28±36µg CH4 -Cg-1 dw) and 3 and 1.5 times lower in the Alas lake sediments, respectively (75±76µgCO2 -Cg-1 dw, 19±29µg CH4 -Cg-1 dw). The highest CO2 production in the frozen Yedoma lake sediments likely results from decomposition of readily bioavailable OM, while highest CH4 production in the non-frozen top sediments of this core suggests that methanogenic communities established upon thaw. The lower GHG production in the non-frozen Alas lake sediments resulted from advanced OM decomposition during Holocene talik development. Furthermore, we found that drivers of CO2 and CH4 production differ following thaw. Our results suggest that GHG production from TOC-poor mineral deposits, which are widespread throughout the Arctic, can be substantial. Therefore, our novel data are relevant for vast ice-rich permafrost deposits vulnerable to thermokarst formation.

Reducing peat in growing media: impact on nitrogen content, microbial activity, and CO2 and N2O emissions

Renewable materials including coir, biochar, and composts are investigated worldwide in the horticultural industry to partially substitute peat in growing media. In this study, we assessed the effects of biochar and vermicompost as partial substitution of peat and compared these peat-based growing media with coir in terms of NH4+-N and NO3−-N content, CO2-C and N2O-N emissions and their microbial biomass carbon and nitrogen. Six growing media mixtures (peat; peat + biochar 9:1 v/v; peat + vermicompost 9:1 v/v; coir; coir + biochar 9:1 v/v; coir + vermicompost 9:1 v/v) replicated three times were incubated in growth chambers during a 60 d period. At day 0 of incubation (DAI), peat amended with biochar retained around 12.81% of NH4+-N compared with peat alone. The concentrations of NO3−-N peaked at 275 mg·kg−1 at 33 DAI for peat and 552 mg·kg−1 at 46 DAI for coir amended with vermicompost. The substitution of peat with biochar resulted in large CO2-C [2070 μg CO2-C·g−1 dry weight (DW)] and N2O-N (62.78 μg N2O-N·g−1 DW) emissions, but not coir. The substitution of coir with vermicompost increased N2O-N emissions at a much lower level (47.53 μg N2O-N·g−1 DW) than peat (111.82 μg N2O-N·g−1 DW). Our results showed that supplements of vermicompost in peat and coir improved N supply which could benefit plant growth, while substituting part of peat with biochar increased CO2-C and N2O-N emissions. In contrast, no effect of biochar was observed with coir, which is beneficial for the environmental footprint of short-cycle growing crops.

Effects of warming, wetting and nitrogen addition on substrate-induced respiration and temperature sensitivity of heterotrophic respiration in a temperate forest soil

Soil heterotrophic respiration and its temperature sensitivity are affected by various climatic and environmental factors. However, little is known about the combined effects of concurrent climatic and environmental changes, such as climatic warming, changing precipitation regimes, and increasing nitrogen (N) deposition. Therefore, in this study, we investigated the individual and combined effects of warming, wetting, and N addition on soil heterotrophic respiration and temperature sensitivity. We incubated soils collected from a temperate forest in South Korea for 60 d at two temperature levels (15 and 20 °C, representing the annual mean temperature of the study site and 5 °C warming, respectively), three moisture levels (10%, 28%, and 50% water-filled pore space (WFPS), representing dry, moist, and wet conditions, respectively), and two N levels (without N and with N addition equivalent to 50 kg N ha–1 year–1). On day 30, soils were distributed across five different temperatures (10, 15, 20, 25, and 30 °C) for 24 h to determine short-term changes in temperature sensitivity (Q10, change in respiration with 10 °C increase in temperature) of soil heterotrophic respiration. After completing the incubation on day 60, we measured substrate-induced respiration (SIR) by adding six labile substrates to the three types of treatments. Wetting treatment (increase from 28% to 50% WFPS) reduced SIR by 40.8% (3.77 to 2.23 µg CO2-C g–1 h–1), but warming (increase from 15 to 20 °C) and N addition increased SIR by 47.7% (3.77 to 5.57 µg CO2-C g–1 h–1) and 42.0% (3.77 to 5.35 µg CO2-C g–1 h–1), respectively. A combination of any two treatments did not affect SIR, but the combination of three treatments reduced SIR by 42.4% (3.70 to 2.20 µg CO2-C g–1 h–1). Wetting treatment increased Q10 by 25.0% (2.4 to 3.0). However, warming and N addition reduced Q10 by 37.5% (2.4 to 1.5) and 16.7% (2.4 to 2.0), respectively. Warming coupled with wetting did not significantly change Q10, while warming coupled with N addition reduced Q10 by 33.3% (2.4 to 1.6). The combination of three treatments increased Q10 by 12.5% (2.4 to 2.7). Our results demonstrated that among the three factors, soil moisture is the most important one controlling SIR and Q10. The results suggest that the effect of warming on SIR and Q10 can be modified significantly by rainfall variability and elevated N availability. Therefore, this study emphasizes that concurrent climatic and environmental changes, such as increasing rainfall variability and N deposition, should be considered when predicting changes induced by warming in soil respiration and its temperature sensitivity.

Microbial activity effect on aggregate stability after residue addition in a Mollisol and a Vertisol in the Pampas, Argentina

Crop residue addition to the soil promotes an increase in microbial activity and, as a consequence, may improve soil aggregate stability. However, this effect may be different in soils with contrasting aggregation agents. The aim of this study was to evaluate, in an incubation experiment, the role of microbial activity on soil aggregate stability after crop residue addition in two soils with contrasting clay mineralogy. Soybean and maize crop residues were added with three different frequencies on a Mollisol and a Vertisol, which were incubated during a 120-days period. It was studied the dynamics of soil aggregate stability after application of a fast wetting (MWDfw), slow wetting (MWDsw) and stirring after prewetting treatment (MWDst) which evaluates three different breakdown mechanism, i.e., slacking, microcracking and mechanical breakdown, respectively. Microbial activity dynamic was evaluated through measurements of soil respiration rate, microbial biomass-C (MBC) and hot-water extractable carbohydrate-C (HWEC). The Vertisol showed higher resistance to slaking (59% higher) but lower resistance to mechanical breakdown (92% lower) and microcracking (102% lower) than the Mollisol, with a scarce effect of residue quality and addition frequency. The discrete changes recorded in MWDfw throughout the experiment, were positively associated with changes in cumulative respiration (P < 0.001), MBC (P < 0.05) y HWEC (P < 0.05) in the Mollisol, and only with changes in MBC in the Vertisol (P < 0.05). Also, the small changes in MWDst were weakly and positively associated (P < 0.05) with changes in cumulative respiration in the Mollisol but not in the Vertisol. However, changes in MWDsw were closely associated with changes in cumulative soil respiration rate (P < 0.0001) and MBC (P < 0.001) in both soils, and with HWEC only in the Mollisol (P < 0.0001), with a higher effect on these variables than the other aggregate stability test. However, while in the Vertisol the increase in MWDsw as a function of cumulative soil respiration was lineal, in the Mollisol it was detected an evident increase in MWDsw up to a threshold of 500 μg C-CO2 g soil−1 of cumulative soil respiration. Similarly, changes in MWDsw associated with changes in MBC were higher in the Mollisol than in the Vertisol (i.e. 0.018 v. 0.004 mm of MWDsw per unit of MBC). Thus, this research added new evidence about the contrasting role of transitory aggregation agents that are provided by microbial activity on aggregate stability in two contrasting soils such as a Mollisol and a Vertisol.

Spatial distribution of microbial activities in Arsuz plain soils (Hatay, Turkey)

This study was carried out to determine spatial distribution of microbial activity in Arsuz plain soil located between the Mediterranean Sea and Amanos Mountains. This plain is selected since it is under the influence of several factors including soil types (colluvial and alluvial), deforestation, and agriculture (tillage and management). In addition, topographically, Arsuz plain narrows to the north and enlarges to the south. Its west and south part is plain while the north and east part is declined. For this purpose, a large survey study was carried out in the autumn of 2018 and soil sampling was done at 67 points in the Arsuz plain. Soil microbial activities such as soil respiration (CO2 production), dehydrogenase enzyme activity (DHA), and microbial biomass carbon (MBC) contents and some important chemical analyses such as pH, EC (1:5), and soil organic matter (SOM) were performed in soil samples. According to the results of the study, the overall average values of CO2 production, DHA, and MBC contents were determined as 181 μg CO2-C gds−1 day−1, 11.85 μg TPF gds−1, and 179 μg MBC-C gds−1, respectively. SOM content, pH, and EC (1:5) values were determined as 1.77, 8.65, and 567 μS cm−1, respectively. According to the results of this research, microbial activities in the Arsuz plain soils were found to be lower than those of fertile soils. Firstly, it is speculated that agriculture and deforestation appeared to be the main reasons for these low microbial activities. Secondly, as for the plain and large areas, heavy agricultural activities could be responsible for lower microbial activities. Finally, type of soils under colluvial effect may also assume to cause irregularities.