Soil Incubation Research Articles

Biochar reduced the mineralization of native and added soil organic carbon: evidence of negative priming and enhanced microbial carbon use efficiency

Biochar has been widely recognized for its potential to increase carbon (C) sequestration and mitigate climate change. This potential is affected by how biochar interacts with native soil organic carbon (SOC) and fresh organic substrates added to soil. However, only a few studies have been conducted to understand this interaction. To fill this knowledge gap, we conducted a 13C-glucose labelling soil incubation for 6 months using fine-textured agricultural soil (Stagnosol) with two different biochar amounts. Biochar addition reduced the mineralization of SOC and 13C-glucose and increased soil microbial biomass carbon (MBC) and microbial carbon use efficiency (CUE). The effects were found to be additive i.e., higher biochar application rate resulted in lower mineralization of SOC and 13C-glucose. Additionally, soil density fractionation after 6 months revealed that most of the added biochar particles were recovered in free particulate organic matter (POM) fraction. Biochar also increased the retention of 13C in free POM fraction, indicating that added 13C-glucose was preserved within the biochar particles. The measurement of 13C from the total amino sugar fraction extracted from the biochar particles suggested that biochar increased the microbial uptake of added 13C-glucose and after they died, the dead microbial residues (necromass) accumulated inside biochar pores. Biochar also increased the proportion of occluded POM, demonstrating that increased soil occlusion following biochar addition reduced SOC mineralization. Overall, the study demonstrates the additional C sequestering potential of biochar by inducing negative priming of native SOC as well as increasing CUE, resulting in the formation and stabilization of microbial necromass.Graphical

Ecological relevance of flagellar motility in soil bacterial communities.

Flagellar motility is a key bacterial trait as it allows bacteria to navigate their immediate surroundings. Not all bacteria are capable of flagellar motility, and the distribution of this trait, its ecological associations, and the life history strategies of flagellated taxa remain poorly characterized. We developed and validated a genome-based approach to infer the potential for flagellar motility across 12 bacterial phyla (26 192 unique genomes). The capacity for flagellar motility was associated with a higher prevalence of genes for carbohydrate metabolism and higher maximum potential growth rates, suggesting that flagellar motility is more prevalent in environments with higher carbon availability. To test this hypothesis, we applied a method to infer the prevalence of flagellar motility in whole bacterial communities from metagenomic data and quantified the prevalence of flagellar motility across four independent field studies that each captured putative gradients in soil carbon availability (148 metagenomes). We observed a positive relationship between the prevalence of bacterial flagellar motility and soil carbon availability in all datasets. Since soil carbon availability is often correlated with other factors that could influence the prevalence of flagellar motility, we validated these observations using metagenomic data from a soil incubation experiment where carbon availability was directly manipulated with glucose amendments. This confirmed that the prevalence of bacterial flagellar motility is consistently associated with soil carbon availability over other potential confounding factors. This work highlights the value of combining predictive genomic and metagenomic approaches to expand our understanding of microbial phenotypic traits and reveal their general environmental associations.

Methane flux from transplanted soil monoliths depends on moisture, but not origin

Soils both produce and consume methane (CH4), a potent greenhouse gas that contributes to climate change. In coastal forests, upland soils are shifting from being CH4 sinks to sources as sea levels rise, increasingly flooding soils with little prior inundation history. Ecosystem CH4 budgets are highly uncertain due in part to the difficulty in separating fluxes measured at the soil surface into individual production and consumption processes which are likely to have different responses to future environmental conditions. We measured growing season CH4 fluxes from soil monoliths transplanted four years prior along an inundation and salinity gradient to determine how changes in abiotic conditions control CH4 flux rates. To parse net fluxes measured at the soil surface into their component gross rates, we paired field measurements with a stable isotope pool dilution incubation of surface soils. Throughout the growing season, net soil surface CH4 flux was positively correlated with soil moisture (p < 0.01), with lowland-located soils tending towards CH4 sources (mean 0.349 ± 1.11 mg CH4–C m−2 h−1, error is standard deviation) and upland-located soils tending towards CH4 sinks (−0.003 ± 0.003 mg CH4–C m−2 h−1). Transplanted soils’ fluxes were statistically identical to their native neighbors once microtopography-driven differences in soil moisture were controlled for. The pool dilution experiment revealed that production and consumption rates were similar in upland and lowland surface soils (2.82 ± 3.29 μmol CH4 g−1 dry soil d−1 production, 3.47 ± 2.03 μmol CH4 g−1 dry soil d−1 consumption), indicating the majority of production likely occurs at depth in lowland soils. Both gross and net fluxes from transplanted soils showed no effect of soil origin after four years, suggesting low resistance of CH4 cycling to global change drivers. Our results indicate the strength of the coastal forest CH4 sink is likely to decrease in proportion to sea-level rise.

Projected soil carbon loss with warming in constrained Earth system models

The soil carbon-climate feedback is currently the least constrained component of global warming projections, and the major source of uncertainties stems from a poor understanding of soil carbon turnover processes. Here, we assemble data from long-term temperature-controlled soil incubation studies to show that the arctic and boreal region has the shortest intrinsic soil carbon turnover time while tropical forests have the longest one, and current Earth system models overestimate intrinsic turnover time by 30 percent across active, slow and passive carbon pools. Our constraint suggests that the global soils will switch from carbon sink to source, with a loss of 0.22–0.53 petagrams of carbon per year until the end of this century from strong mitigation to worst emission scenarios, suggesting that global soils will provide a strong positive carbon feedback on warming. Such a reversal of global soil carbon balance would lead to a reduction of 66% and 15% in the current estimated remaining carbon budget for limiting global warming well below 1.5 °C and 2 °C, respectively, rendering climate mitigation much more difficult.

Application of soil amendments: investigation of incubation time on some soil solution properties of Kien Giang, Vietnam

Soil amendments can affect soil properties such as pH, EC, available phosphorus (P), and ammonium nitrogen (NH4+). The aim of this study was to determine the potential influence of submergence incubation time on some soil properties using soil amendments under controlled laboratory conditions suitable to the characteristics of rice-growing areas in Kien Giang, Vietnam. The experiment included treating soil supplemented with lime (at ratios of 0.0, 0.4, 0.6, and 1.0%) and soil supplemented with lime (0.4%) combined with alkalized biochar (at ratios 0.0, 0.2, 0.4, and 0.8%). The treatments were incubated in the laboratory under submerged conditions, and pH, EC, available phosphorus (P), and ammonium nitrogen (NH4+) parameters were measured at 7, 14, 21, 35, and 60 days. The results showed that soil supplemented with lime improved pH and EC, and maintained high phosphorus release for 21 days, but did not improve NH4+ release. In contrast, soil supplemented with lime (0.4%) combined with alkalized biochar improved pH and EC, maintained high levels of phosphorus and NH4+ content up to 35 days. The results showed that using lime combined with alkalized biochar to improve soil in rice-growing areas has many advantages compared to using lime alone. Therefore, understanding the contribution of soil incubation time with amendments to soil properties is an important prerequisite for developing a strategy to ensure crop productivity.

Emerging investigator series: Coprecipitation with glucuronic acid limits reductive dissolution and transformation of ferrihydrite in an anoxic soil.

Ferrihydrite, a poorly crystalline Fe(III)-oxyhydroxide, is abundant in soils and is often found associated with organic matter. Model studies consistently show that in the presence of aqueous Fe(II), organic carbon (OC)-associated ferrihydrite undergoes less transformation than OC-free ferrihydrite. Yet, these findings contrast microbial reductive dissolution studies in which the OC promotes the reductive dissolution of Fe(III) in ferrihydrite and leads to the release of associated OC. To shed light on these complex processes, we quantified the extent of reductive dissolution and transformation of native Fe minerals and added ferrihydrite in anoxic soil incubations where pure 57Fe-ferrihydrite (57Fh), pure 57Fe-ferrihydrite plus dissolved glucuronic acid (57Fh + GluCaq), a 57Fe-ferrihydrite-13C-glucuronic acid coprecipitate (57Fh13GluC), or only dissolved glucuronic acid (13GluCaq) were added. By tracking the transformation of the 57Fe-ferrihydrite in the solid phase with Mössbauer spectroscopy together with analysis of the iron isotope composition of the aqueous phase and chemical extractions with inductively coupled plasma-mass spectrometry, we show that the pure 57Fe-ferrihydrite underwent more reductive dissolution and transformation than the coprecipitated 57Fe-ferrihydrite when identical amounts of glucuronic acid were provided (57Fh + GluCaqversus57Fh13GluC treatments). In the absence of glucuronic acid, the pure 57Fe-ferrihydrite underwent the least reductive dissolution and transformation (57Fh). Comparing all treatments, the overall extent of Fe(III) reduction, including the added and native Fe minerals, determined with X-ray absorption spectroscopy, was highest in the 57Fh + GluCaq treatment. Collectively, our results suggest that the limited bioavailability of the coprecipitated OC restricts not only the reductive dissolution of the coprecipitated mineral, but it also limits the enhanced reduction of native soil Fe(III) minerals.

Effect of Sterilization on the Change of Available Phosphorus During the Wetting and Drying Process

Wetting and drying are typical natural events in arid and semiarid areas. In order to explore the contribution of microorganisms to the change in soil phosphorus components during wetting and drying processes, soil incubation experiments were conducted on calcareous soil from Inner Mongolia grasslands. By comparing the change in various phosphorus fractions between sterilized and unsterilized soils, the microbial effect on the change in available phosphorus with soil moisture changes was studied. The results indicated that there was no significant difference in APi (NaHCO3-extractable inorganic phosphate) content between the two treatments. Sterilization caused the content of APo (NaHCO3-extractable inorganic phosphate) to significantly increase. These results suggested that soil microorganisms have a negligible influence on changes of soil available inorganic phosphorus (APi) in the present soil sample during the wetting and drying process. The change in APo may be related to the high-temperature and high-pressure sterilization processes. These results enrich our knowledge about the microbial influence on dynamics of soil available phosphorus during wetting and drying events.

Legacy effects of earthworms on soil microbial abundance, diversity, and community dynamics

The earthworm species Lumbricus terrestris L. feeds on plant litter mixed with surrounding soil. Here, we analyzed with a mesocosm approach and soil incubations how that activity and subsequent ageing of casts (feces) affects the abundance and diversity of the soil microbiome. Earthworms were fed either with straw of sainfoin (SA, Onobrychis viciifolia; C/N ratio 22) or winter wheat (WW, Triticum aestivum, C/N ratio 101). The gut transit increased the abundances of bacteria and fungi, but reduced archaea. As indicated at the DNA and RNA level, main beneficiaries of the facilitated access to nutrients were members of Bacteroidota, especially Flavobacteriales with an estimated generation time of only 2 h. While Alphaproteobacteria were reduced, Gammaproteobacteria also increased in abundance and activity. SA was more nutritious for L. terrestris, and supported a higher bacterial abundance, probably because more N was available for growth and denitrification. During cast ageing, prokaryotic community compositions became increasingly similar to bulk soil communities. However, they remained distinguishable even after 168 d, suggesting that effects can last beyond a vegetation period. Dry-wet conditions preserved these differences better than continuous moisture. During ageing, more complex prokaryotic networks were detected with WW and dry-wet conditions. Thus, N and water limitations appeared to enhance cooperation rather than competition between the prokaryotes. Overall, this study demonstrates that earthworm soil interactions strongly affect the diversity and temporal dynamics of the soil microbiome. Legacy effects of earthworm activities should thus be kept in mind when investigating the environmental variation of soil microbiomes.

Bacteria-loaded biochar for the immobilization of cadmium in an alkaline-polluted soil.

The combination of biochar and bacteria is a promising strategy for the remediation of Cd-polluted soils. However, the synergistic mechanisms of biochar and bacteria for Cd immobilization remain unclear. In this study, the experiments were conducted to evaluate the effects of the combination of biochar and Pseudomonas sp. AN-B15, on Cd immobilization, soil enzyme activity, and soil microbiome. The results showed that biochar could directly reduce the motility of Cd through adsorption and formation of CdCO3 precipitates, thereby protecting bacteria from Cd toxicity in the solution. In addition, bacterial growth further induces the formation of CdCO3 and CdS and enhances Cd adsorption by bacterial cells, resulting in a higher Cd removal rate. Thus, bacterial inoculation significantly enhances Cd removal in the presence of biochar in the solution. Moreover, soil incubation experiments showed that bacteria-loaded biochar significantly reduced soil exchangeable Cd in comparison with other treatments by impacting soil microbiome. In particular, bacteria-loaded biochar increased the relative abundance of Bacillus, Lysobacter, and Pontibacter, causing an increase in pH, urease, and arylsulfatase, thereby passivating soil exchangeable Cd and improving soil environmental quality in the natural alkaline Cd-contaminated soil. Overall, this study provides a systematic understanding of the synergistic mechanisms of biochar and bacteria for Cd immobilization in soil and new insights into the selection of functional strain for the efficient remediation of the contaminated environments by bacterial biochar composite.

Straw incorporation into microplastic-contaminated soil can reduce greenhouse gas emissions by enhancing soil enzyme activities and microbial community structure

Microplastic (MP) contamination poses a substantial threat to agroecosystems, disrupting soil properties, nutrient cycles, and microbial communities and ultimately affecting plant growth and ecosystem resilience. The effects of straw addition on the storage of soil organic carbon (SOC) and greenhouse gas emissions have been extensively explored, but these effects have not been examined in the context of MP contamination. To assess the impacts of legume straw and polyethylene microplastics on SOC fractions and carbon dioxide (CO2) and nitrous oxide (N2O) emissions, 7-month soil incubation experiments were performed. The results revealed that the inclusion of legume straw in soil considerably increased microbial SOC compared to the control. However, straw addition to MP-contaminated soil reduced microbial SOC compared to that in soil containing only straw. In contrast, the addition of straw to MP-contaminated soil elevated (+44%) the SOC mineral relative to the sole application of straw. Intriguingly, straw incorporation into MP-contaminated soil reduced microbial biomass carbon and nitrogen relative to soil containing only straw. Straw addition to MP-contaminated soil enhanced the nitrification activity and reduced the relative expression of AOBamoABC gene compared to sole straw-incorporated soil and the control. Greenhouse gas emissions were also modulated; for instance, straw incorporation into MP-contaminated soil reduced CO2 and N2O emissions by −11% and −46%, compared to straw incorporation alone. The urease and phosphatase activities were decreased (−58% and −12%) in the MP-polluted soil with straw incorporation compared with those in the soil in which only straw was applied. However, invertase and catalase activities were upregulated in the straw-incorporated soil contaminated with MPs. Straw addition in the MP-polluted soil considerably enhanced (+2%) the microbial community structure (indicated by PLFA) compared to the sole straw application. These results provide a comprehensive perspective on the role of legume straw incorporation in addressing MP pollution, showcasing its potential for sustainable agricultural practices in the face of evolving environmental challenges.

Nitrogen Enriched Organic fertilizer (NEO) elevates nitrification rates shortly after application but has no lasting effect on nitrification in agricultural soils

Amidst population growth, escalating food costs, limited arable land, and farmland degradation, the adoption of innovative technologies—like organic waste recycling and nutrient recovery—is crucial for enhancing the resilience of global agri-food systems. Nitrogen-Enriched Organic fertilizer (NEO) is produced using a new method, where dinitrogen (N2) is captured from the air through a plasma process and mixed with slurries or digestates as nitrate (NO3-) and nitrite (NO2-). This process leads to solid slurry acidification and a high NO2- content, potentially yielding toxic inorganic or organic N compounds. This study investigated the impact of NEO derived from cattle slurry and biogas digestate on soil nitrification—conversion of NH4+ to NO2- and NO3- by aerobic autotrophic bacteria and archaea. We investigated and compared the potential nitrification rates in soil samples from two agricultural trials (cereal and grass) treated with NEO and other fertilizers after two consecutive fertilization years. Additionally, we examined the immediate nitrification response to NEO through 73-hour soil incubations. Our results revealed that NEO significantly stimulated nitrification rates in agitated soil slurries, regardless of the feedstock used, surpassing rates observed in ammonium controls. Similarly, this pattern was also observed in loosely placed soil samples, with high nitrification rates occurring with NEO and ammonium chloride. Interestingly, the differences in nitrification rates between field-fertilized soil samples were minimal and inconsequential, suggesting that while NEO exhibits a rapid boost in nitrification rates shortly after application, this effect is not sustained ≈ six months after fertilization under field conditions. Consequently, NEO indicates its potential as an environmentally benign fertilizer without adversely affecting soil nitrification.

Response of soil nitrogen mineralization to warming temperatures depends on soil management history

Rising global temperatures have the potential to increase soil nitrogen (N) mineralization from soil organic matter (SOM). By increasing SOM over time, management practices that increase SOM through the addition of soil amendments, such as compost, have been recognized as effective strategies for mitigating the effects of climate change and building resilience in agricultural ecosystems. However, the effects of these strategies on temperature-induced changes to soil N cycling are unclear, particularly when soils are managed to increase SOM. To determine how agricultural management history and compost amendments affect net N mineralization, net nitrification, and nitrous oxide (N2O) production, we performed a laboratory incubation of soils with two distinct agricultural management histories under three incubation temperatures. Three compost treatments (green-waste compost, food-waste compost, and no compost) were applied, each with and without the addition of synthetic urea fertilizer. We found that organically managed soil exhibited higher rates of net N mineralization and nitrification than conventionally managed soil, leading to greater nitrate production. The rate of N mineralization in organically managed soil was also more sensitive to temperature increases. Although compost addition stimulated microbial activity, it did not affect the N-cycling processes measured in this study at any temperature. Therefore, the implementation of climate change resilience and mitigation strategies aimed at augmenting stocks of soil carbon may render agricultural soils more susceptible to increased N mineralization and subsequent losses under warming, particularly if plant uptake of the mineralized N does not occur concurrently. Moreover, the effects of compost application to stimulate the immobilization of excess N is likely limited in soils with low background C.

Physicochemical Improvements in Sandy Soils through the Valorization of Biomass into Biochar

Forestry management operations that are designed to prevent wildfires while also protecting the environmental compartments that are directly affected by them, such as soil, are of major relevance. The valorization of residual forestry biomass into biochar that is then used to be turned back into forest soils is an approach that meets Circular Economy principles. However, the effects on soil of the application of biochar that is produced from low-grade forestry biomass are unknown. In this work, a soil incubation assay was performed with a sandy soil that was amended with biochar produced from residual forestry biomass (Acacia) to assess its effectiveness in terms of boosting soil quality. The factorial study comprised the effects of biochar at two pyrolysis temperatures (450 °C and 550 °C), four application rates (0%, 3%, 6%, and 10% (w/w)), and three particle size classes (S &lt; 0.5 mm, M = [0.5; 3.15], and L &gt; 3.15 mm). The soil pH increased for all treatments to suitable agronomic values (5–7), and the water-holding capacity increased by 69% to 325% when compared to the control soil. The bioavailability of the plant nutrient elements also increased with the application rate, especially for treatments with small particles of biochar. Biochar that is made of low-grade biomass from forestry maintenance operations can be efficiently recycled back into forest soils to improve the physicochemical properties of agronomic relevance, thus allowing for a reduced water demand and better soil quality. However, studies on biochar applications in different soils are needed in order to assess the effectiveness of this approach.

Seaweed supplementation to organic dairy cows may reduce climate impact of manure in pasture soils during a laboratory incubation

AbstractIntroductionEnteric methane (CH4) emissions are one of the largest components of the anthropogenic CH4 budget, and with accelerating climatic changes, there are calls to reduce anthropogenic greenhouse gas emissions. Certain seaweeds fed as supplements can reduce enteric CH4 emissions from ruminant animals by as much as 80%; however, these studies have yet to analyze downstream impacts that may arise from the deposit of affected manures on pastures or agricultural fields.Materials and MethodsHere we conducted a 28‐day soil and manure incubation utilizing manures collected from dairy cows in a seaweed feeding trial to analyze the impacts of manure on greenhouse gas fluxes and nutrient cycling. Cows were fed different diets with a control group (no seaweed supplementation), and a 3% and 6% by dry‐weight seaweed (Chondrus crispus) supplementation. Three soil moisture treatments were also tested, with a field moist capacity [67% gravimetric water content (GWC)], a drier group (50% GWC), and a saturated treatment (84% GWC).ResultsWe found that manure from seaweed‐fed cows reduced emissions of carbon dioxide and had similar emissions of methane and nitrous oxide to the control. Higher moisture levels increased CH4 emissions and nitrous oxide emissions with CH4 emissions trending higher in seaweed supplements trending higher in. Inorganic nitrogen turnover varied more based on moisture levels than seaweed supplements, as did the magnitude of methane and nitrous oxide emissions. Total carbon and nitrogen mostly decreased except in the 3% seaweed supplementation, which increased carbon and nitrogen with a drop in C:N in the 50% GWC group.ConclusionSeaweed supplements may partially reduce climate impacts of manures from cows fed seaweed supplements or at least not cause harm or impact manure quality. Given feeds may reduce enteric CH4 emissions, they may be a part of a successful strategy to reduce climate impacts of cattle.

Co-exposure of dimethomorph and imidacloprid: effects on soil bacterial communities in vineyard soil.

In Taiwan, the pesticides dimethomorph and imidacloprid are recommended for pest control in vineyards. Therefore, tank-mixing of these two pesticides is usually a routine practice before application. This study analyzed the influence of vineyard soil microbial flora under the recommended and high dosages (100 times the recommended dosage) of dimethomorph and imidacloprid. Individual and combined applications of pesticides were also tested through batches of soil incubation experiments. Four treatments-control (C), dimethomorph (DT), imidacloprid (IM), and mixed application of dimethomorph and imidacloprid (ID)-were used in the experimental design. From the soil metabolism, no significant reaction was observed after 2 months in the recommended dosage group, regardless of whether the pesticides were being applied individually or combined. For the high dosage, imidacloprid showed a higher effect than the co-exposure treatments, showing a possible prolonged effect after its repetitive application. From PCoA analysis, pesticide treatments altered the soil ecology after 2 months, and the effect of imidacloprid can be explicitly observed at high dosages. At the phylum level, Acidobacteria can indicate pesticide application around the recommended dosage. It was inhibited by ID on day 7 and was augmented by all pesticides on day 63. The effect of the recommended dosage of pesticide mixtures after 2 months of incubation was revealed in the minor families Gemmataceae and Pirellulaceae, while the high dosage treatments affected both the core and the minor families. Our findings verified the changes in the composition of microbial communities upon pesticide application, which would affect carbon, nitrogen, sulfur, phosphorous cycles, and contaminant removal ability within the vineyard.

Soil aggregate size mediates the impact of different fertilization patterns on the diazotrophic community of mine soils

The development of the mining industry has led to significant environmental pollution around mining sites, affecting the nitrogen (N) cycle and associated microorganisms and disrupting the structure of soil aggregates. In this study, mine soil incubation experiments were conducted under the application of two types of N fertilizer (urea (U) and ammonium chloride (AC)) and nine different fertilization patterns, and the effects of the different N fertilization patterns on soil aggregate properties and diazotrophic community structure were compared. The results showed that carbon (C), phosphorus (P) and N were mainly enriched in megaaggregates (2–4 mm) and macroaggregates (0.25–2 mm). N fertilizer addition decreased the bioavailable heavy metal (HM) content of soil aggregates to different degrees. In addition, N fertilization resulted in significantly higher activities of cellulase, invertase, amylase, urease, protease and phosphatase in microaggregates (<0.25 mm) than in mega- and macroaggregates (p < 0.05). Among the nine fertilization patterns, AC application was performed at a low frequency and high concentration. In most cases, U application resulted in a higher nifH gene abundance than AC application. Specifically, nifH gene abundance in microaggregates in the four U treatments ranged from 9.75 × 106 to 16.63 × 106 gene copies g−1 dry soil, which was significantly higher than that in the control (p < 0.05). Proteobacteria was the major diazotrophic phylum, accounting for 52.9 % to 79.1 %, 46.4 % to 80.3 % and 49.1 % to 81.2 % in mega-, macro- and microaggregates, respectively, followed by Firmicutes and Actinobacteria. The application of N fertilizers had different impacts on soil physicochemical properties, which in turn indirectly affected the abundance and community composition of soil diazotrophs. Among the aggregate properties, microbial nitrogen (MBN) was an important variable that explained the variation in diazotrophic communities (p < 0.05). Our results suggest that the addition of N fertilizer affects the physicochemical properties and the structure and functions of diazotrophic community in soil aggregates.

Goethite-enriched biochar mitigates soil emissions of CO2 during arsenic passivation: Effect and mechanisms

Iron-enriched biochar has excellent adsorption capacity for contaminants, which is a sustainable multifunctional potential material to mitigate environmental pollution. Both biochar and Fe (hydr)oxides are important materials for accumulating and preserving soil organic carbon. However, it is unclear that the potential of iron-enriched biochar for mitigating CO2 emission from contaminated soil. In this study, soil incubation and extraction experiments were conducted to determine the effects and mechanisms of goethite-enriched wheat straw biochars (GWBs) prepared at two pyrolysis temperatures (GWB450 and GWB600) on mitigation of CO2 emissions in arsenic (As) contaminated soil. The results showed that compared to the control, GWBs significantly reduced the soil available arsenic (As), decreased soil Eh, and promoted the water stability of soil aggregates (<1mm). GWBs also regulated bacterial community structures, and efficiently improved microbial carbon use efficiency (CUE) in contaminated soil, thereby significantly reducing CO2 emission by 37.2 %−40.9 % in 37 days. Spectroscopic and microscopic analysis indicated that microbial colonization widely occurred on the surfaces of the goethite-enriched biochar, and promoted the formation of organo-mineral layers, which resulted in the significant decrease in the CO2 emission. These findings enhance the understanding of the mechanisms of soil carbon sequestration induced by metal-enriched biochar and indicate a new strategy of biochar-based multifunctional soil amendment that can mitigate heavy metal pollution and reduce CO2 emission simultaneously in contaminated soils.

Effect of Urea: Hydroxyapatite Composites for Controlled-Release Fertilization to Reduce P Complexation in Soils

ABSTRACT Phosphorous fixation in the soil is the most limiting factor for P-management, and previous studies have indicated that coupling nanoparticulated P sources (e.g. hydroxyapatite) in adequate matrices (nanocomposites) can increase nutrient uptake. However, how the nanocomposite affects the P fixation is still an open question. Herein we describe X-ray absorption near edge structure (XANES) experiments to investigate the role of phosphate fertilizer (P) composite in reducing P complexation in soil. Experiments in a greenhouse (maize cultivation) and soil incubation comparatively revealed that a composite formed by the dispersion of hydroxyapatite (phosphate source) into urea could increase the production of dry matter (compared to monoammonium phosphate, MAP), keeping the residual P available in the soil for future cultivations (i.e. not strongly bonded to minerals). This behavior was evidenced by P-XANES, which indicated that the composite induced the bonding of remnant P to more labile organic fractions instead of mineral phases, as seen in MAP cultivation.

Valorizing lignite waste into engineered nitro-humic fertilizer: Advancing resource efficiency in the era of a circular economy

This study investigates using lignite waste as a carrier matrix for creating engineered nitro-humic fertilizer (NHF). The NHF is synthesized through an innovative ozone oxidation method and nitrogen enrichment. Agronomic efficiency was evaluated through maize growth responses, soil incubation experiments, and fertilizer performance, comparing NHF to commercial urea and NPK fertilizers. Nitrogen release patterns and kinetic models were studied to understand the nitrogen release mechanism of NHF. The engineered structure of NHF displayed globular-like, microporous, and heterogeneous properties, with a negative charge density of 48.85 mV and lower thermal stability of aliphatic carbons. The NHF exhibited a remarkable capability to increase water-holding capacity by up to 48% and extend the water-retention period by 57% during 30 d of soil application. NHF significantly reduced soil urease enzyme activity by more than two-fold. It exhibited a longer nitrogen release period (77 d) with a slow-release pattern, releasing 71.75% of total nitrogen compared to 88.5% in 14 d for urea. The simple Elovich model accurately predicted NHF's nitrogen release kinetics (R2 = 0.9943), identifying Fickian diffusion-based controlled release. Applying NHF improved multiple aspects of maize growth: height, stem diameter, biomass, root weight, leaf area, SPAD value, and leaf nitrogen concentration. Total chlorophyll, carotenoid, and nitrogen uptake also greatly increased compared to the control sample. NHF exhibited superior nitrogen use efficiency (41.5%) and agronomic fertilizer efficiency (36.22%) compared to urea and even surpassed commercial NPK fertilizers. Generally, the produced NHF is an eco-friendly alternative to slow-release fertilizers, offering the advantages of humic substances and nitrogen fertilizers.

Unraveling mechanisms underlying effects of wetting–drying cycles on soil respiration in a dryland

Rewetting of dry soils usually stimulates soil carbon (C) emission, a phenomenon known as the Birch effect. Soil C cycling in drylands, which store approximately one third of terrestrial soil organic C (SOC), is strongly affected by wetting–drying cycles. However, the physical, chemical, and biological mechanisms that link rewetting cycles with dryland soil C cycling have not been comprehensively studied, nor do we understand how these mechanisms interact with each other. Here, we conducted a dryland soil incubation experiment manipulating four factors related to global change (soil moisture content, soil moisture variability, C availability, and prior warming) in a factorial design. The experiment was divided into two periods: a rewetting period consisting of six 14-d wetting–drying cycles; and a recovery period lasting 28 days during which soil moisture content was held stable, allowing for examination of the legacy effects of the wet-dry cycles. Rewetting cycles decreased soil aggregate stability under some conditions, but their effects on soil microbial biomass and fungal communities, soil enzyme activities, soil priming, and soil dissolved C were not significant. We found lower average soil respiration under the wetting–drying treatment than the stable soil moisture treatment, and Birch effects were observed, but only under some conditions. This was probably because moisture variability exacerbated soil microbial metabolic stress, which showed itself as oxygen limitation during the initial precipitation pulse and as water limitation during soil drying. Notably, respiration rates remained low even after moisture fluctuations ceased, suggesting a legacy effect of rewetting cycles on dryland microbial communities. Overall, rewetting inhibited aggregate formation (physical mechanism), and suppressed soil respiration by inducing soil microbial metabolic stress (biological mechanism), ultimately leading to lower soil C loss under rewetting. Our findings indicate that Birch effects are mediated by the magnitude of moisture variability, the availability of C, and the degree of physiological stress microbes experience.