Application of mixed straw and biochar meets plant demand of carbon dioxide and increases soil carbon storage in sunken solar greenhouse vegetable production

Solar vegetable greenhouse soils show low soil organic carbon content and thus also low rates of soil respiration. Processing vegetable residues to biochar and mixing biochar with maize straw might improve soil respiration and increase soil organic carbon stocks, while preventing the spread of soil-borne diseases carried by vegetable residues. In an incubation experiment, we tested how additions of maize straw (S) and biochar (B) added in varying ratios (100S, 75S25B, 50S50B, 25S75B, 100B and 0S0B (control)) affect soil respiration and fraction of added C remaining in soil. Daily CO₂ emissions were measured over 60 days incubation, the natural abundance of ¹³C in soil and in the added biochar and maize straw were analysed. Our result shows that (a) soil CO₂ emissions were significantly increased compared to soil without the straw additions, while addition of biochar only decreased soil respiration; (b) cumulative CO₂ emissions decreased with increasing ratio of added biochar to maize straw; (c) the abundance of soil ¹³C was significant positively correlated with cumulative CO₂ emissions, and thus with the ratio of straw addition. Our results indicate that incorporation of maize straw in greenhouse soils is a meaningful measure to increase soil respiration and to facilitate greenhouse atmosphere CO₂ limitation while producing vegetables. On the other hand, additions of biochar from vegetable residues will increase soil organic carbon concentration. Therefore, the simultaneous application of maize straw and biochar obtained from vegetable residues is an effective option to maintain essential soil functions for vegetable production in sunken solar greenhouses.     

Global climate change and its impacts on the terrestrial Arctic carbon cycle with special regards to ecosystem components and the greenhouse‐gas balance

The climatic changes on earth may have serious implications for the carbon (C) cycle in the terrestrial Arctic throughout the 21st century. Arctic vegetation takes up carbon dioxide (CO₂) from the atmosphere producing biomass. In a cold and often moist soil environment, dead organic matter is preferentially preserved as soil organic matter (SOM) due to the inhibition of decomposition processes. However, viable soil microbes exhale huge amounts of CO₂ and methane (CH₄) annually. Hence, Arctic ecosystems exhibit annual fluxes of both carbon‐based (CO₂ and CH₄) greenhouse gases (GHGs) that are in an order of magnitude of millions of tons. Rising Arctic temperatures lead to the degradation of much of today's permafrost in the long run. As a result, large quantities of frozen SOM may become available for decomposers, and GHGs that are entrapped in permafrost may be released. At the same time, warming tends to stimulate the growth, development, and reproduction of many Arctic plants, at least transiently. The present northward migration of boreal shrubs and trees into southern tundra areas may be amplified by that, increasing the ecosystems' gross primary production and, thus, their C sequestration. On the other hand, rising temperatures boost SOM decomposition and microbial respiration rates. In general, soil temperature and soil moisture are key environmental variables to control the intensity of aerobic and anaerobic respiration by microbes, and autotrophic respiration by plants. On the basis of published data on Arctic CO₂ and CH₄ fluxes, the calculations on the terrestrial C‐based Arctic GHG balance made in this review reveal a current annual GHG exchange that ranges between a weak storage of ≤ 225 Tg CO₂ equivalent (eq.) y^–1 and a huge release of ≤ 1990 Tg CO₂ eq. y^–1. Hence, the Arctic GHG balance does apparently already contribute positively to the climatic changes at present. Regarding the future, the relative development of the uptake and release of CO₂ and CH₄ by northern ecosystems is fundamental to the overall GHG status of the Arctic under scenarios of continued climate change.     

Plant-biochar interactions drive the negative priming of soil organic carbon in an annual ryegrass field system

There is a knowledge gap on biochar carbon (C) longevity and its priming effects on soil organic carbon (SOC) and recent root-derived C under field conditions. This knowledge would allow the potential of biochar in long-term soil C sequestration to be established. However, most studies on biochar C longevity and its priming effect have been undertaken in plant-free laboratory incubations.A 388 d field study was carried out in the presence of an annual ryegrass (C₃) growing on a rhodic ferralsol with established C₃/C₄ plant-derived SOC (δ¹³C: −20.2‰) in a subtropical climate. A ¹³C-depleted hardwood biochar (δ¹³C: −35.7‰, produced at 450 °C) was applied at 0 and 30 dry t ha⁻¹ and mixed into the top 100-mm soil profile (equivalent to 3% w/w). We report on the differentiation and quantification of root respiration and mineralisation of soil-C and biochar-C in the field. Periodic ¹³CO₂ pulse labelling was applied to enrich δ¹³C of root respiration during two separate winter campaigns (δ¹³C: 151.5–184.6‰) and one summer campaign (δ¹³C: 19.8–31.5‰). Combined soil plus root respiration was separated from leaf respiration using a novel in-field respiration collar. A two-pool isotope mixing model was applied to partition three C sources (i.e. root, biochar and soil). Three scenarios were used to assess the sensitivity associated with the C source partitioning in the planted systems: 1) extreme positive priming of recent SOC derived from the current ryegrass (C₃) pasture; 2) equivalent magnitude of priming of SOC and labile root C; and 3) extreme positive priming of the native C4-dominant SOC.We showed that biochar induced a significant negative priming of SOC in the presence of growing plants but no net priming was observed in the unplanted soil. We also demonstrated the importance of experimental timeframe in capturing the transient nature of biochar-induced priming, from positive (day 0–62) to negative (day 62–388). The presence/absence of plants had no impact on biochar-C mineralisation in this ferralsol during the measurement period. Based on a two-pool exponential model, the mean residence time (MRT) of biochar varied from 351 to 449 years in the intensive pasture system to 415–484 years in the unplanted soils.     

Three‐source partitioning of CO2 efflux from maize field soil by 13C natural abundance

A natural‐¹³C‐labeling approach—formerly observed under controlled conditions—was tested in the field to partition total soil CO₂ efflux into root respiration, rhizomicrobial respiration, and soil organic matter (SOM) decomposition. Different results were expected in the field due to different climate, site, and microbial properties in contrast to the laboratory. Within this isotopic method, maize was planted on soil with C₃‐vegetation history and the total CO₂ efflux from soil was subdivided by isotopic mass balance. The C₄‐derived C in soil microbial biomass was also determined. Additionally, in a root‐exclusion approach, root‐ and SOM‐derived CO₂ were determined by the total CO₂ effluxes from maize (Zea mays L.) and bare‐fallow plots. In both approaches, maize‐derived CO₂ contributed 22% to 35% to the total CO₂ efflux during the growth period, which was comparable to other field studies. In our laboratory study, this CO₂ fraction was tripled due to different climate, soil, and sampling conditions. In the natural‐¹³C‐labeling approach, rhizomicrobial respiration was low compared to other studies, which was related to a low amount of C₄‐derived microbial biomass. At the end of the growth period, however, 64% root respiration and 36% rhizomicrobial respiration in relation to total root‐derived CO₂ were calculated when considering high isotopic fractionations between SOM, microbial biomass, and CO₂. This relationship was closer to the 50% : 50% partitioning described in the literature than without fractionation (23% root respiration, 77% rhizomicrobial respiration). Fractionation processes of ¹³C must be taken into account when calculating CO₂ partitioning in soil. Both methods—natural ¹³C labeling and root exclusion—showed the same partitioning results when ¹³C isotopic fractionation during microbial respiration was considered and may therefore be used to separate plant‐ and SOM‐derived CO₂ sources.     

In situ respiration determination as tool for classifying soils according to soil organic matter content

Abstract

Soil respiration is indicative of biological status of the soil and high respiration is correlated to high contents of available carbon (C) in soil and/or organic matter content. Because of soil respiration's relationship to soil organic matter status and content, soil respiration is considered one measurement that could aid in determining the quality of soil. In the global scale, the cycling of C in soil is important because the rise in CO₂ in the atmosphere is linked to global climate change. In situ measurement of CO₂ using instruments that are portable and perform analyses quickly are important to obtain sufficient number of measurements in the field to overcome spatial variability. Soil respiration tests were conducted in plots amended with fertilizer or organic amendments of agricultural or municipal residues since 1994. Besides CO₂, moisture and temperature were measured over a period where the moisture varied from near saturation to below wilting point. It was found that flux was curvilinearly related to moisture from 5 to 40% (v/v). Maximum flux occurred for all plots between 30 and 40% saturation. The ratio of flux normalized by temperature to the volumetric soil moisture divided soils into two categories, those with soil organic matter (SOM) content above or below 4.5%. The determinations of CO₂ flux, moisture and temperature uses equipment that is portable so that several sites in a field can be analyzed to reduce spatial variation. The only limitation is that the determinations must be performed on soils with less than 40% saturation or 25% moisture (v/v) because the normalized function is no longer linear above this moisture content. More than two SOM categories might be found if studies are expanded to soils with a wider range of SOM content.     

Dynamics of soil organic matter turnover and soil respired CO2 in a temperate grassland labelled with 13C

The fate of carbon (C) in grassland soils is of particular interest since the vast majority in grassland ecosystems is stored below ground and respiratory C‐release from soils is a major component of the global C balance. The use of ¹³C‐depleted CO₂ in a 10‐year free‐air carbon dioxide enrichment (FACE) experiment, gave a unique opportunity to study the turnover of the C sequestered during this experiment. Soil organic matter (SOM), soil air and plant material were analysed for δ¹³C and C contents in the last year of the FACE experiment (2002) and in the two following growing seasons. After 10 years of exposure to CO₂ enrichment at 600 ppmv, no significant differences in SOM C content could be detected between fumigated and non‐fumigated plots. A ¹³C depletion of 3.4‰ was found in SOM (0–12 cm) of the fumigated soils in comparison with the control soils and a rapid decrease of this difference was observed after the end of fumigation. Within 2 years, 49% of the C in this SOM (0–12 cm) was exchanged with fresh C, with the limitation that this exchange cannot be further dissected into respiratory decay of old C and freshly sequestered new C. By analysing the mechanistic effects of a drought on the plant‐soil system it was shown that rhizosphere respiration is the dominant factor in soil respiration. Consideration of ecophysiological factors that drive plant activity is therefore important when soil respiration is to be investigated or modelled.     

Plant biomass influences rhizosphere priming effects on soil organic matter decomposition in two differently managed soils

We used a continuous labeling method of naturally ¹³C-depleted CO₂ in a growth chamber to test for rhizosphere effects on soil organic matter (SOM) decomposition. Two C3 plant species, soybean (Glycine max) and sunflower (Helianthus annus), were grown in two previously differently managed soils, an organically farmed soil and a soil from an annual grassland. We maintained a constant atmospheric CO₂ concentration at 400±5 ppm and δ¹³C signature at −24.4‰ by regulating the flow of naturally ¹³C-depleted CO₂ and CO₂-free air into the growth chamber, which allowed us to separate new plant-derived CO₂-C from original soil-derived CO₂-C in soil respiration. Rhizosphere priming effects on SOM decomposition, i.e., differences in soil-derived CO₂-C between planted and non-planted treatments, were significantly different between the two soils, but not between the two plant species. Soil-derived CO₂-C efflux in the organically farmed soil increased up to 61% compared to the no-plant control, while the annual grassland soil showed a negligible increase (up to 5% increase), despite an overall larger efflux of soil-derived CO₂-C and total soil C content. Differences in rhizosphere priming effects on SOM decomposition between the two soils could be largely explained by differences in plant biomass, and in particular leaf biomass, explaining 49% and 74% of the variation in primed soil C among soils and plant species, respectively. Nitrogen uptake rates by soybean and sunflower was relatively high compared to soil C respiration and associated N mineralization, while inorganic N pools were significantly depleted in the organic farm soil by the end of the experiment. Despite relatively large increases in SOM decomposition caused by rhizosphere effects in the organic farm soil, the fast-growing soybean and sunflower plants gained little extra N from the increase in SOM decomposition caused by rhizosphere effects. We conclude that rhizosphere priming effects of annual plants on SOM decomposition are largely driven by plant biomass, especially in soils of high fertility that can sustain high plant productivity.     

Temperature sensitivity of soil organic matter decomposition varies with biochar application and soil type

Biochar application has the potential to improve soil fertility and increase soil carbon stock, especially in tropical regions. Information on the temperature sensitivity of carbon dioxide(CO_2) evolution from biochar-amended soils at very high temperatures, as observed for tropical surface soils, is limited but urgently needed for the development of region-specific biochar management targeted to optimize biochar effects on soil functions. Here, we investigated the temperature sensitivity of soil respiration to the addition of different rates of Miscanthus biochar(0, 6.25, 12.5, and 25 Mg ha~(-1)) in two types of soils with contrasting textures. Biochar-amended soil treatments and their controls were incubated at constant temperatures of 20, 30, and 40℃. Overall, our results show that: i) considering data from all treatments and temperatures, the addition of biochar decreased soil CO_2 emissions when compared to untreated soils;ii) CO_2 emissions from biochar-amended soils had a higher temperature sensitivity than those from biochar-free soils; iii) the temperature sensitivity of soil respiration in sandy soils was higher than that in clay soils; and iv) for clay soils, relative increases in soil CO_2 emissions from biochar-amended soils were higher when the temperature increased from 30 to 40℃, while for sandy soils, the highest temperature responses of soil respiration were observed when increasing the temperature from 20 to 30℃. Together, these findings suggest a significantly reduced potential to increase soil organic carbon stocks when Miscanthus biochar is applied to tropical soils at high surface temperatures, which could be counteracted by the soil-and weather-specific timing of biochar application.     

Interactions between biochar stability and soil organisms: review and research needs

The stability of biochar in soils is the cornerstone of the burgeoning worldwide interest in the potential of the pyrolysis/biochar platform for carbon (C) sequestration. While biochar is more recalcitrant in soil than the original organic feedstock, an increasing number of studies report greater C‐mineralization in soils amended with biochar than in unamended soils. Soil organisms are believed to play a central role in this process. In this review, the variety of interactions that occur between soil micro‐, meso‐ and macroorganisms and biochar stability are assessed. In addition, different factors reported to influence biochar stability, such as biochar physico‐chemical characteristics, soil type, soil organic carbon (SOC) content and agricultural management practices are evaluated. A meta‐analysis of data in the literature revealed that biochar‐C mineralization rates decreased with increasing pyrolysis temperature, biochar‐C content and time. Enhanced release of CO₂ after biochar addition to soil may result from (i) priming of native SOC pools, (ii) biodegradation of biochar components from direct or indirect stimulation of soil organisms by biochar or (iii) abiotic release of biochar‐C (from carbonates or chemi‐sorbed CO₂). Observed biphasic mineralization rates suggest rapid mineralization of labile biochar compounds by microorganisms, with stable aromatic components decomposed at a slower rate. Comparatively little information is available on the impact of soil fauna on biochar stability in soil, although they may decrease biochar particle size and enhance its dispersion in the soil. Elucidating the impacts of soil fauna directly and indirectly on biochar stability is a top research priority.     

Microbial mineralization of biochar and wheat straw mixture in soil: A short-term study

A short-term incubation study was carried out to investigate the effect of biochar addition to soil on CO₂ emissions, microbial biomass, soil soluble carbon (C) nitrogen (N) and nitrate–nitrogen (NO₃–N). Four soil treatments were investigated: soil only (control); soil + 5% biochar; soil + 0.5% wheat straw; soil + 5% biochar + 0.5% wheat straw. The biochar used was obtained from hardwood by pyrolysis at 500 °C. Periodic measurements of soil respiration, microbial biomass, soluble organic C, N and NO₃–N were performed throughout the experiment (84 days). Only 2.8% of the added biochar C was respired, whereas 56% of the added wheat straw C was decomposed. Total net CO₂ emitted by soil respiration suggested that wheat straw had no priming effect on biochar C decomposition. Moreover, wheat straw significantly increased microbial C and N and at the same time decreased soluble organic N. On the other hand, biochar did not influence microbial biomass nor soluble organic N. Thus it is possible to conclude that biochar was a very stable C source and could be an efficient, long-term strategy to sequester C in soils. Moreover, the addition of crop residues together with biochar could actively reduce the soil N leaching potential by means of N immobilization.     

Rhizodeposition-induced decomposition increases N availability to wild and cultivated wheat genotypes under elevated CO2

Elevated CO₂ may increase nutrient availability in the rhizosphere by stimulating N release from recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed to elucidate how CO₂-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO₂. To quantify root-derived soil carbon (C) input and release of N from stable SOM pools, plants were grown for 1 month in microcosms, exposed to ¹³C labeling at ambient (392 μmol mol⁻¹) and elevated (792 μmol mol⁻¹) CO₂ concentrations, in soil containing ¹⁵N predominantly incorporated into recalcitrant SOM pools. Decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial-¹³C was enhanced by 50% under elevated compared to ambient CO₂. Concurrently, plant ¹⁵N uptake increased (+7%) under elevated CO₂ while ¹⁵N contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO₂. This led to increased stable C decomposition, but not to increased N acquisition for the wild genotypes. Data suggest that increased rhizodeposition under elevated CO₂ can stimulate mineralization of N from recalcitrant SOM pools and that contrasting C allocation patterns cannot fully explain plant mediated differential responses in soil N cycling to elevated CO₂.     

Contrasting effects of bamboo leaf and its biochar on soil CO2 efflux and labile organic carbon in an intensively managed Chinese chestnut plantation

The effects and associated mechanisms of the application of organic residues or their derived biochar on the dynamics of soil organic C and soil CO₂ efflux in planted soils are poorly understood. This paper investigated the impact of bamboo leaf and the derived biochar applications on soil CO₂ efflux and labile organic C in an intensively managed Chinese chestnut plantation in a 12-month field study. The treatments studied included Control, application of bamboo leaf (Leaf), and application of biochar (Biochar). The Leaf treatment increased (P?2 efflux and concentrations of water-soluble organic C (WSOC) and microbial biomass C (MBC). The Biochar treatment increased soil CO₂ efflux and WSOC and MBC only in the first month after application, but such effects diminished thereafter. The annual cumulative soil CO₂ emission was increased by 16 % by the Leaf treatment as compared to the Control, but there was no difference between the Biochar and Control treatments. The soil organic C (SOC) storage was increased by biochar addition but not by bamboo leaf addition. An exponential relationship between soil temperature and soil CO₂ efflux was observed regardless of the treatment. Soil CO₂ efflux was correlated to soil WSOC (P?Q ₁₀) of soil CO₂ efflux was ranked as Leaf?>?Biochar?>?Control. In comparison with the application of fresh bamboo leaf, pyrolyzed bamboo leaf (biochar) application decreased CO₂ effluxes and increased C sequestration in the soil.     

Relating the biological stability of soil organic matter to energy availability in deep tropical soil profiles

Tropical subsoils contain large reservoirs of carbon (C), most of which is stored in soil organic matter (SOM). Subsoil OM is thought to be particularly stable against microbial decomposition due to various mechanisms and its position in the soil profile, potentially representing a long-term C sink. However, few experiments have explicitly investigated SOM stability and microbial activity across several orders of magnitude of soil C concentrations as a function of soil depth. The objective of this study was to evaluate the biological stability of SOM in the upper 1.4 m of tropical forest soil profiles. We did so by measuring CO₂ evolution during a 90-day laboratory incubation experiment on a sample set that was previously characterized for C and nutrient concentrations and microbial biomass. We concurrently measured the energy content of SOM using differential scanning calorimetry (DSC) as an index of the energy available for microbial metabolism, with the hypothesis that the biological stability of SOM would be inversely related to the energy contained within it. Cumulative CO₂ evolution, mean respiration rates, and the energy density of SOM (energy released during combustion normalized to soil C) all declined with soil depth (P < 0.01). Biological indices of C stability were well correlated with measures of SOM energy. There was no change in the mean respiration rate as a function of depth when normalized to soil C, and a trend toward increased respiration per-unit microbial biomass (P = 0.07). While reduced microbial respiration in subsoils suggests an increase in the biological stability of SOM, we suggest this is driven principally by concurrent declines in energy availability as measured by DSC and the size of the microbial biomass pool. On a per-unit biomass basis, subsoil OM may be as prone to decomposition and destabilization as surface SOM.     

Effect of dry-wet cycles on carbon dioxide release from two different volcanic ash soils in a Japanese temperate forest

ABSTRACT

In the present study, two volcanic ash soils (soil A and B) from a temperate broad-leaved forest in eastern Japan were aerobically incubated under repeated dry-wet cycles and continuously constant moisture conditions. The primary aims were to quantify the potential for enhancement of carbon dioxide (CO₂) release owing to increased water fluctuation and to examine differences in the responses of volcanic ash soils with different physicochemical properties. Soil B, rather than soil A, was a typical Andosol. During incubation at 20°C for 120 days with five dry-wet cycles, the CO₂ release rate was measured periodically. Abundance of the stable carbon isotope in CO₂ (δ¹³C-CO₂) was measured to capture changes in the origin of decomposed soil organic matter (SOM) owing to the dry-wet cycles. The CO₂ release rate under the dry-wet cycles was up to 49% higher than the values predicted from a parabolic relationship between CO₂ release and water content during incubation under the continuously constant moisture condition. The magnitude of CO₂ release enhancement was 2.7-fold higher in soil B relative to that in soil A. The δ¹³C-CO₂ value in the dry-wet cycles was enriched by 0.3–2.3‰ compared to that during incubation under the continuously constant moisture conditions, suggesting that the decomposition of well-metabolized and/or old SOM was enhanced by the dry-wet cycles. Thus, the present study suggests that Andosols, which have been believed to have a strong SOM stabilization ability, are vulnerable to dry-wet cycles. Then, increased water fluctuation in a future warmer world would have significant potential to stimulate CO₂ release from soils.     

Abiotic processes dominate CO2 fluxes in Antarctic soils

Ecosystems within the McMurdo Dry Valleys of Antarctica are highly sensitive to environmental change. Increases in soil temperature and/or moisture content may dramatically change rates of soil respiration and soil carbon (C) turnover. Present estimates of soil respiration rates and C turnover times are based on surface carbon dioxide (CO₂) fluxes and soil organic C content. However, the assumption that surface CO₂ fluxes are purely biological in origin has not been rigorously tested. We use concentration and, for the first time, the stable C isotopic composition of surface soil CO₂ fluxes and subsurface CO₂ profiles to: 1) examine mechanisms of soil CO₂ uptake and release, 2) identify the location of potential CO₂ sources and sinks within the soil profile, and 3) discriminate between biotic and abiotic contributions to CO₂ fluxes in soils of Taylor Valley. Surface CO₂ fluxes and subsurface CO₂ profiles confirm that these soils take up and release CO₂ on a daily basis (during the austral summer), associated with small changes in soil temperature. Shifts in the C isotopic composition of soil CO₂ are inconsistent with biological mechanisms of CO₂ production and consumption. Instead, the isotopic shifts can be accounted for by Henry's Law dissolution and exsolution of CO₂ into a solution of high pH, driven by changes in soil temperature. Our results constrain the biological component of soil CO₂ fluxes in Taylor Valley to less than 25% (and likely to be significantly less). This finding implies that previous measurements of surface soil CO₂ fluxes are overestimates of soil respiration, thus C turnover times calculated from them are underestimates. Discriminating between biotic and abiotic contributions to CO₂ fluxes in Antarctic dry valley soils is essential if the effects of climate change on these sensitive ecosystems are to be accurately identified.     

Chemical and microbial activation energies of soil organic matter decomposition

Present concepts emphasize that substrate quality exerts an important control over substrate decomposability and temperature sensitivity of heterotrophic soil respiration (Rh). In this context, soil organic matter (SOM) quality is defined by its molecular and structural complexity and determines the ease by which substrate is oxidized. However, temperature not only affects SOM oxidation rates but also equally the physiology of soil microorganisms, making it difficult to use respiration rates as indicative for the quality inherent to a substrate. One way to distinguish these two would be to measure organic matter oxidation by controlled combustion and to compare the temperature sensitivity of this chemical process to that of enzyme-catalyzed microbial respiration. We analyzed reaction rates, thermal stability indices, and activation energies (Ea) during (i) microbial respiration (Ea_Rh) and (ii) controlled combustion by differential scanning calorimetry (DSC) (Ea_DSC) of the same set of mineral and organic soils. A high thermal stability coincided with small heterotrophic respiration rates, indicating that thermal stability may be useful as a proxy for biological degradability. Under ambient conditions, enzymes greatly reduced Ea on average from 136 (Ea_DSC) to 87 (Ea_Rh) kJ mol^?1, thereby increasing CO₂ release by a factor of 1.5 * 10⁷ relative to the noncatalyzed chemical reaction. However, temperature dependency of chemical and microbial oxidation was not correlated, suggesting that they are determined by different sample properties. A high temperature sensitivity of microbial respiration is linked to parameters independent of chemical oxidizability, in our case, organic matter C/N ratio and soil pH. These factors are important controls for microbial, but not for chemical, oxidation.     

Carbon mineralization in subtropical alluvial arable soils amended with sugarcane bagasse and rice husk biochars

Subtropical recent alluvial soils are low in organic carbon (C). Thus, increasing organic C is a major challenge to sustain soil fertility. Biochar amendment could be an option as biochar is a C-rich pyrolyzed material, which is slowly decomposed in soil. We investigated C mineralization (CO₂-C evolution) in two types of soils (recent and old alluvial soils) amended with two feedstocks (sugarcane bagasse and rice husk) (1%, weight/weight), as well as their biochars and aged biochars under a controlled environment (25 ±2 ℃) over 85 d. For the recent alluvial soil (charland soil), the highest absolute cumulative CO₂-C evolution was observed in the sugarcane bagasse treatment (1 140 mg CO₂-C kg^-1 soil) followed by the rice husk treatment (1 090 mg CO₂-C kg^-1 soil); the lowest amount (150 mg CO₂-C kg^-1 soil) was observed in the aged rice husk biochar treatment. Similarly, for the old alluvial soil (farmland soil), the highest absolute cumulative CO₂-C evolution (1 290 mg CO₂-C kg^-1 soil) was observed in the sugarcane bagasse treatment and then in the rice husk treatment (1 270 mg CO₂-C kg^-1 soil); the lowest amount (200 mg CO₂-C kg^-1 soil) was in the aged rice husk biochar treatment. Aged sugarcane bagasse and rice husk biochar treatments reduced absolute cumulative CO₂-C evolution by 10% and 36%, respectively, compared with unamended recent alluvial soil, and by 10% and 18%, respectively, compared with unamended old alluvial soil. Both absolute and normalized C mineralization were similar between the sugarcane bagasse and rice husk treatments, between the biochar treatments, and between the aged biochar treatments. In both soils, the feedstock treatments resulted in the highest cumulative CO₂-C evolution, followed by the biochar treatments and then the aged biochar treatments. The absolute and normalized CO₂-C evolution and the mineralization rate constant of the stable C pool (K_s) were lower in the recent alluvial soil compared with those in the old alluvial soil. The biochars and aged biochars had a negative priming effect in both soils, but the effect was more prominent in the recent alluvial soil. These results would have good implications for improving organic matter content in organic C-poor alluvial soils.     

Responses of soil organic matter and microorganisms to freeze-thaw cycles

Soil organic matter (SOM) biomarker methods were utilized in this study to investigate the responses of fungi and bacteria to freeze-thaw cycles (FTCs) and to examine freeze-thaw-induced changes in SOM composition and substrate availability. Unamended, grass-amended, and lignin-amended soil samples were subject to 10 laboratory FTCs. Three SOM fractions (free lipids, bound lipids, and lignin-derived phenols) with distinct composition, stability and source were examined with chemolysis and biomarker Gas Chromatography/Mass Spectrometry methods and the soil microbial community composition was monitored by phospholipid fatty acid (PLFA) analysis. Soil microbial respiration was also measured before and during freezing and thawing, which was not closely related to microbial biomass in the soil but more strongly controlled by substrate availability and quality. Enhanced microbial mineralization (CO₂ flush), considered to be derived from the freeze-thaw-induced release of easily decomposable organic matter from microbial cell lyses, was detected but quickly diminished with successive FTCs. The biomarker distribution demonstrated that free lipids underwent a considerable size of decrease after repeated FTCs, while bound lipids and lignin compounds remained stable. This observation indicates that labile SOM may be most influenced by increased FTCs and that free lipids may contribute indirectly to the freeze-thaw-induced CO₂ flush from the soil. PLFA analysis revealed that fungal biomass was greatly reduced while bacteria were unaffected through the lab-simulated FTCs. Microbial community shifts may be caused by freezing stress and competition for freeze-thaw-induced substrate release. This novel finding may have an impact on carbon and nutrient turnover with predicted increases in FTCs in certain areas, because fungi and bacteria have different degradation patterns of SOM and the fungi-dominated soil community is considered to have a higher carbon storage capacity than a bacteria-dominated community.     

Responses of Soil Microbial Community Structure and Activity to Incorporation of Straws and Straw Biochars and Their Effects on Soil Respiration and Soil Organic Carbon Turnover

Like straw, biochar incorporation can influence soil microorganisms and enzyme activities and soil carbon(C) responses; however,few studies have compared the various effects of straw and biochar and the underlying mechanisms. An experiment was performed to study the changes in soil respiration(SR) and soil organic C(SOC) fluxes in response to the incorporation of three kinds of straw(reed, smooth cordgrass, and rice) and their pyrolyzed products(biochars) at Chongming Island, China. In addition, the microbial activity and community structure of some amended soils were also analyzed to clarify the mechanisms of these responses. The results showed that all biochar incorporation(BC) induced lower SR than the corresponding unpyrolyzed straw incorporation(ST), and the average SR in the soils following BC and ST during the experimental periods was 21.69 and 65.32 μmol CO_2 m~(-2)s~(-1), respectively.Furthermore, the average SOC content was 16.97 g kg~(-1) following BC, which was higher than that(13.71 g kg~(-1)) following ST,indicating that compared to ST, BC was a low-C strategy, even after accounting for the C loss during biochar production. Among the BC treatments, reed-BC induced the lowest SR(17.04 μmol CO_2 m~(-2)s~(-1)), whereas smooth cordgrass-BC induced the highest SR(27.02 μmol CO_2 m~(-2)s~(-1)). Furthermore, in contrast with ST, BC significantly increased the abundance of some bacteria with poorer mineralization or better humification ability, which led to lower SR. The lower easily oxidizable C(EOC) and higher total C contents of biochars induced lower SR and higher SOC in the soil following BC compared to that following ST. Among the BC treatments,the higher total nitrogen content of rice biochar led to significantly higher soil microbial biomass, and the lower EOC content of reed biochar led to lower soil microbial activity and SR.     

Geogenic CO2 affects stabilization of soil organic matter

The soil on mofette sites is affected by ascending geogenic carbon dioxide (CO₂), which partially fills the soil atmosphere. We hypothesized that geogenic CO₂ affects the stabilization of soil organic matter (SOM) at lower partial pressures than had been discussed previously for mofette sites. We studied loamy Ah horizons (n = 22; pH 3.4–4) of the soil along a transect on a grassland mofette site in the northwest Czech Republic with CO₂ partial pressures (p(CO₂)) of up to 0.52. The samples were fractionated by particle size, density and solubility (water‐soluble organic matter (WSOM)), and analysed quantitatively for organic carbon (C) and total nitrogen (N) and qualitatively (¹³C‐NMR spectroscopy). Soil OM with a narrower C:N ratio accumulated in the clay fraction, but at p(CO₂) less than approximately 0.1 the proportion of SOM in the clay fraction relative to total SOM tended to decrease with increasing p(CO₂), whereas that of particulate organic matter (POM) fractions increased with increasing p(CO₂). We attribute the distribution of SOM among the mineral soil and POM to decreased interactions with minerals of the clay fraction. The formation of iron (Fe) hydroxides, which potentially sorb SOM, was not affected negatively by CO₂. The potential reactivity of Fe hydroxides was even positively affected by increased p(CO₂). Export of dissolved SOM into the subsoil might increase at mofette sites because of the large amounts of WSOM and decreasing interactions with minerals of the clay fraction. Therefore, our results show negative effects of CO₂ on SOM stabilization even at moderate p(CO₂).