Soil Incubation Research Articles

Succession of soil bacterial communities and network patterns in response to conventional and biodegradable microplastics: A microcosmic study in Mollisol

Significant soil contamination of microplastics (MPs) by the application of agricultural mulching films has aroused global concern, however, the effects of conventional and biodegradable MPs on the dynamics of soil microbial communities and network patterns have not been sufficiently reported. In this study, we conducted a soil microcosmic experiment by adding low-density polyethylene and biodegradable MPs (PE and BD) into a black soil at the dosages of 0 % (CK), 0.1 % (low-dose, w/w), 1 % (medium-dose, w/w) and 5 % (high-dose, w/w), and soils were sampled on the 15th, 30th, 60th and 90th day of soil incubation for high-throughput sequencing. The results showed that the incubation time was the most influential factor driving the variations in bacterial community structures, and significant effects of MP dosages and types were also detected. With the increase in MP dosage, bacterial diversity markedly increased and decreased at the beginning (D15) and end of sampling day (D90), respectively. Compared to CK, BD induced a larger community dissimilarity than PE and tended to enrich environmentally friendly taxa, while PE likely promoted the growth of hazardous taxa. Moreover, BD simplified interspecies interactions compared to the networks of PE and CK, and Nitrospira was identified as a keystone species in both PE and BD networks. These findings provide new insights into the influences of conventional and biodegradable MPs on the succession patterns of soil bacterial communities, and further studies are needed to explore the soil metabolic potentials affected by the presence of MPs.

Immobilization of Cd and Pb in soil facilitated by magnetic biochar: metal speciation and microbial community evolution.

The preparation of magnetic biochar from sewage sludge and rice straw for heavy metal contaminated soil remediation has greater application prospects, but its remediation mechanism was rarely considered by combining soil physicochemical properties with microbial community. In this study, the effects of magnetic sewage sludge biochar (SSB) and rice straw biochar (RSB) on Cd and Pb immobilization in paddy soil were compared and analyzed by 60-day soil incubation experiments. The results illustrated that DTPA-Cd and DTPA-Pb were reduced by 51.53% (43.07%) and 53.57% (50.47%), while the percentage of residual fraction of the BCR procedure was enhanced by 31.27% (30.78%) of Cd and 27.25% (23.22%) of Pb in the SSB (RSB) treatment, respectively. Fe was detected on both SSB and RSB surfaces, but SSB had rougher and a larger specific surface area compared to RSB. The addition of SSB and RSB in paddy soil increased soil pH and TOC content, and affected the diversity and species of soil microbial community. Compared with the CK group, the relative abundance of Proteobacteria, Bacteroidota, and Lysobacter decreased, and the relative abundance of Actinobacteriota, Pontibacter, and Alkaliphilus increased with SSB and RSB treatments, all of which reflected the bioavailability of Cd and Pb reduction.

The Response of Cd Chemical Fractions to Moisture Conditions and Incubation Time in Arable Land Soil

Heavy metal pollution in soils is an issue of global concern, and many scholars have focused on Cadmium (Cd) because of its strong biological migration and toxicity. This study explored arable land soil, changes in external Cd contamination processes and its response to soil moisture conditions, and indoor simulation. After adding an external source of 5 mg/kg d.w., the distribution of soil Cd fractions content, EXC-Cd, CAB-Cd, FMO-Cd, OM-Cd, and RES-Cd, were continuously monitored under different water management regimes, and correlation analysis and regression equations were calculated. The results show that after external Cd entered arable land soils, the binging strength of pollutants and soil gradually increased with incubation time, and the distribution of Cd chemical forms was more stable under different water management regimes. The oversaturated water content promotes the transformation of EXC-Cd to other forms. The transformation of CAB-Cd fractions can be accelerated to other fractions by field capacity, and the active conversion period was 30–60 d. Not all Cd fractions correlated between each other, under the four water management regimes, but it seems that the reducibility of the soil environment was more conducive to external Cd fixation and stability. The response surface design method (RSM) was used to establish quantitative regimes between Cd fractions with incubation time and soil moisture, and the soil moisture content and incubation time had an obvious effect on FMO-Cd content, with R2 = 0.9542.

Enantiomer-Selective Characterization of the Adsorption, Dissipation, and Phytotoxicity of the Plant Monoterpene Pulegone in Soils.

Plant monoterpenes have received attention for their ecological functions and as potential surrogates for synthetic herbicides, but very little is known about the processes that govern their behavior in the soil environment, and even less about the possible enantioselectivity in the functions and environmental behavior of chiral monoterpenes. We characterized the adsorption and dissipation of the two enantiomers of the chiral monoterpene pulegone in different soils, and their phytotoxicity to different plant species through Petri dish and soil bioassays. R- and S-pulegone displayed a low-to-moderate non-enantioselective adsorption on the soils that involved weak interaction mechanisms. Soil incubation experiments indicated that, once in the soil, R- and S-pulegone are expected to suffer rapid volatilization and scarcely enantioselective, biodegradation losses. In Petri dishes, the phytotoxicity of pulegone and its enantioselectivity to Lactuca sativa, Hordeum vulgare, and Eruca sativa was species-dependent. Lactuca sativa was the most sensitive species and showed higher susceptibility to S- than to R-pulegone. Biodegradation and volatilization losses greatly reduced the phytotoxic activity of S-pulegone applied to soil, but the addition of a highly-adsorptive organoclay stabilized the monoterpene and increased its phytotoxic effect. Stabilization by adsorption may represent an important mechanism by which the bioactivity of plant monoterpenes in soils can be increased.

Differential Responses of the Catalytic Efficiency of Ammonia and Nitrite Oxidation to Changes in Temperature.

Microbially mediated nitrification plays an important role in the nitrogen (N) cycle, and rates of activity have been shown to change significantly with temperature. Despite this, the substrate affinities of nitrifying bacteria and archaea have not been comprehensively measured and are often assumed to be static in mathematical models of environmental systems. In this study, we measured the oxidation kinetics of ammonia- (NH3) oxidizing archaea (AOA), NH3-oxidizing bacteria (AOB), and two distinct groups of nitrite (NO2–)-oxidizing bacteria (NOB), of the genera Nitrobacter and Nitrospira, by measuring the maximum rates of apparent activity (Vmax(app)), the apparent half-saturation constant (Km(app)), and the overall catalytic efficiency (Vmax(app)/Km(app)) over a range of temperatures. Changes in Vmax(app) and Km(app) with temperature were different between groups, with Vmax(app) and catalytic efficiency increasing with temperature in AOA, while Vmax(app), Km(app), and catalytic efficiency increased in AOB. In Nitrobacter NOB, Vmax(app) and Km(app) increased, but catalytic efficiency decreased significantly with temperature. Nitrospira NOB were variable, but Vmax(app) increased while catalytic efficiency and Km(app) remained relatively unchanged. Michaelis–Menten (MM) and Haldane (H) kinetic models of NH3 oxidation and NO2– oxidation based on the collected data correctly predict nitrification potential in some soil incubation experiments, but not others. Despite previous observations of coupled nitrification in many natural systems, our results demonstrate significant differences in response to temperature strategies between the different groups of nitrifiers; and indicate the need to further investigate the response of nitrifiers to environmental changes.

Vermiculite Mining Waste Enriched with Elemental Sulfur as a Chemical Conditioner for Alkaline Saline Soils

ABSTRACT About 60 to 80% of the material extracted in vermiculite mining consists of unused waste, composed of minerals containing plant nutrients that can chemically improve the soil. The objective was to evaluate the potential of vermiculite mining residue associated with elemental sulfur in the chemical attributes of two alkaline saline soils. Two experiments were carried out in a completely randomized design in a 6 × 2 factorial scheme with four replications. In the first experiment, six doses of vermiculite waste enriched with 5% (50 g kg−1) elemental sulfur (0, 10, 20, 30, 40 and 50 t ha−1), and two alkaline soils (sodic soil and other non-sodic) were tested. In the second experiment, six doses of elemental sulfur (0; 0.25; 0.50; 0.75; 1.0; 1.25 g dm−3) and two soils (sodic soil and other non-sodic) were tested. At 30 and 105 days after soil incubation, the hydrogenionic potential (pH) values, electrical conductivity, concentrations of available sulfate (SO4 −2), phosphorus (P), and exchangeable calcium (Ca2+), potassium (K+) and sodium (Na+) were determined. The vermiculite waste associated with elemental sulfur can be used as an alternative source of K+ and contribute to reduce the pH of the alkaline soil and adding Ca2+ and SO4 −2 to the soil. However, in soils with poor drainage, this compound should be used with caution due to the risk of soil sodification. Elemental sulfur only, reduced the pH and increased the levels of Ca2+ and SO4 −2 in both studied soils and reduced levels of Na+ in sodic soil after 105 days of incubation.

Optimization for the determination of phenol oxidase activity in subtropical forest soils developed on sandstone

Phenol oxidase plays an important role in the degradation of soil organic matter. There was no standard method to determine soil phenol oxidase activity. To fill such knowledge gap, we investigated the effects of substrate type, pH, soil storage conditions, storage time, substrate concentration, water-soil ratio, incubation time and incubation temperature on soil phenol oxidase activity in three different subtropical forest soils developed on sandstone. The pH of extraction buffers significantly affected the phenol oxidase activity. Using 2,2'-azinobis-(-3-ethylbenzo-thiazoline-6-sulfononic acid)-diammonium salt (ABTS) as substrate acquired higher oxidase activity and was applicable to wider pH range than using 3-(3,4-Dihydroxyphenyl)-L-alanine (L-DOPA) as substrate, indicating that ABTS was more suitable as a substrate for measuring phenol oxidase activity in acidic soils of subtropical forests. The storage condition significantly affected phenol oxidase activity. The phenol oxidase activity declined with time in all the three types of soil. The decreasing rate was air-dried > 4 ℃ refrigerated > -20 ℃ frozen > -80 ℃ frozen, suggesting that the frozen storage method was better than others in maintaining soil phenol oxidase activity if the determination of phenol oxidase activity in fresh soil samples cannot be immediately done. Substrate concentration, water-soil ratio, and incubation time and temperature all affected the activity of soil phenol oxidase. The condition of soil: buffer ratio of 1:100, 2 mmol·L-1 concentration of ABTS with an incubation time of 4 h at 25-30 ℃ was optimal for measuring phenol oxidase activity in acidic soils of subtropical forests, with high repeatability and sensitivity.

Development and characterization of slow release N and Zn fertilizer by coating urea with Zn fortified nano-bentonite and ZnO NPs using various binders

The fertilizer use-efficiencies particularly of nitrogen (N), phosphorus (P), and zinc (Zn) in alkaline soils are often seriously low. The surplus N released from the conventional fertilizers pollutes the environment and causes climate change. Slow or controlled release N fertilizer can improve its use efficiency and decrease its environmental impacts. Previously, we have shown that the ZnO nanoparticles (ZnO NPs) has higher use efficiency than the conventional Zn fertilizers. In this study, we focused on the development of Zn coated slow-release urea fertilizer. For this purpose, ZnO NPs and Zn fortified nano-bentonite were coated on urea by using various binders. The coated fertilizers ZU1& ZU2: nano-bentonite (4%) & ZnO NPs (2%) + vegetable oil respectively, ZU3 & ZU4: nano-bentonite (4%) & ZnO NPs (2%)+stearic acid+ paraffin oil+ paraffin wax respectively was prepared. The coated granules were characterized by using SEM, XRPD techniques. Zn and N release was evaluated in a soil incubation experiment. The results showed that ZU4 and ZU2 released higher amounts of Zn (3.9 and 3.42 mg kg −1) respectively while the prolonged release of Zn (>30 days) was observed from ZU3 and ZU4. Similarly, a significant reduction in N release from ZU3 and ZU4 (15 days) was observed while the rapid release of N from ZU1, ZU2, and control (first 5 days) was recorded. It was concluded that coated urea with ZnO NPs and Zn fortified nano-bentonite using stearic acid, paraffin oil, and paraffin wax as binder can act as an efficient slow-release macro micronutrient fertilizer.

Low carbon availability in paleosols nonlinearly attenuates temperature sensitivity of soil organic matter decomposition.

Temperature sensitivity (Q10 ) of soil organic matter (SOM) decomposition is an important parameter in models of the global carbon (C) cycle. Previous studies have suggested that substrate quality controls the intrinsic Q10 , whereas environmental factors can impose large constraints. For example, physical protection of SOM and its association with minerals attenuate the apparent Q10 through reducing substrate availability and accessibility ([S]). The magnitude of this dampening effect, however, has never been quantified. We simulated theoretical Q10 changes across a wide range of [S] and found that the relationship between Q10 and the log10 -transformed [S] followed a logistic rather than a linear function. Based on the unique Holocene paleosol chronosequence (7 soils from ca. 500 to 6900years old), we demonstrated that the Q10 decreased nonlinearly with soil age up to 1150years, beyond which Q10 remained stable. Hierarchical partitioning analysis indicated that an integrated C availability index, derived from principal component analysis of DOC content and parameters reflecting physical protection and mineral association, was the main explanatory variable for the nonlinear decrease of Q10 with soil age. Microbial inoculation and 13 C-labelled glucose addition showed that low C availability induced by physical protection and minerals association attenuated Q10 along the chronosequence. A separate soil incubation experiment indicated that Q10 increased exponentially with activation energy (Ea ) in the modern soil, suggesting that SOM chemical complexity regulates Q10 only when C availability is high. In conclusion, organic matter availability strongly decreased with soil age, whereas Michelis-Menten kinetics defines the Q10 response depending on C availability, but Arrhenius equation describes the effects of increasing substrate complexity.

Exploring Key Soil Parameters Relevant to Arsenic and Cadmium Accumulation in Rice Grain in Southern China

Paddy soils in some areas of southern China are contaminated by arsenic (As) and cadmium (Cd), threatening human health via the consumption of As- and/or Cd-tainted rice. To date, a quantitative understanding of how soil characteristics control As and Cd accumulation in rice grains under field conditions is still deficient. Based on 31 paired soil-grain samples collected in southern China, we statistically explored which soil parameter or parameter combination from various soil analyses best estimates As and Cd in rice. We found that CaCl2 extraction of field-moist soil collected at rice harvest provided the best estimation (R2adj = 0.47–0.60) for grain Cd followed by dry soil CaCl2 extraction (R2adj = 0.38–0.49), where CaCl2 extractable Cd from moist or dry soil was the dominant soil parameter. Compared to soil totals, parameters from neither dry soil ascorbate-citrate extraction nor anoxic soil incubation improved model performance for grain As (R2adj ≤ 0.44), despite their closer relevance to soil redox conditions during plant As uptake. A key role of soil-available sulfur in controlling grain As was suggested by our models. Our approach and results may help develop potential soil amendment strategies for decreasing As and/or Cd accumulation from soils.

Methanogenic community linked to organic acids fermentation from root exudates are affected by rice intensification in rotational soil systems

Rice paddy soils are an important biogenic source of methane, a relevant greenhouse gas. Rice rotation with upland crops and pastures affects hydrolytic and fermentative bacteria and methanogenic archaea by sequential exposition to oxic and anoxic conditions. There are several ways to reduce methane emissions from rice paddy soils. Here we examine the effect of intensification in rice rotations with upland crops upon the microbial community structure and methanogenesis when dicarboxylic acids exudated by rice root were amended in microcosms assays. Soils under rotations rice-pasture, rice-soybean and continuous rice, with the last two more intensive systems recently installed in the field experiment, were sampled. Amplicon sequencing of 16S rRNA gene analysis showed that the rotation system was a main driver of the microbial community structure, explaining 33%, 38% and 55% of the differences in the structure of Bacteria, Archaea and methanogens, respectively. The density of mcrA gene copies was significantly higher in continuous rice soil than in other soils, and the relative abundance of methanogenic archaea was 33% for rice monoculture, whereas represented 15% and 14% for the rice-pasture and rice-soybean soils, respectively. The incubation of soils with tartrate and succinate confirmed functional differences among soils since rice monoculture showed similar or higher methane production and significantly less acetate and propionate accumulation than other soils. The T-RFLP analysis of mcrA gene in soils and incubated soils revealed that Rice Cluster I increased after incubation in all rotational soils, whereas Methanosarcinaceae and Methanobacteriaceae remained relevant after incubation. Furthermore, a more stable community of methanogenic archaea was established in soil under continuous rice, since T-RF profiles of soils and incubated soils with dicarboxylic acids grouped together with 62% similarity, while communities from other rotational soils experimented a great shift after incubation with these substrates. Altogether, these results showed that the conversion of rice-pasture rotational fields to a more intensive rice crop production strongly impacts the methanogenic community structure and their potential for methane emission.

Digestate-Derived Ammonium Fertilizers and Their Blends as Substitutes to Synthetic Nitrogen Fertilizers

Nutrient recovery from biomass streams generates novel recycling-derived fertilizers (RDFs). The effect of RDFs depends on their nutrient content and variability, which can aid or hinder their use by end-users. Detailed characterization of RDFs can help in evaluating product properties, whereas blending RDFs can optimize their nutrient ratios and reduce nutrient variability. This study assesses ammonium nitrate (AN) from stripping-scrubbing, ammonium water (AW) and concentrate (CaE) from evaporation, and two tailor-made blends (AN + CaE and AW + CaE), for their potential as nitrogen (N) fertilizers in the pot cultivation of lettuce. Parallelly, a soil incubation experiment was conducted to investigate the N release dynamics of the tested RDFs. The RDFs were compared against the commercial calcium ammonium nitrate (CAN) and an unfertilized control. AN and AW fertilization resulted in a similar crop yield and N uptake to the CAN treatment. CaE and blends exhibited poor yield and N uptake, possibly due to the sodium toxicity detected. AN and AW displayed N fertilizer replacement values above 100%, whereas CaE and blends exhibited poor results in the current experiments. The soil incubation experiment showed a positive soil priming effect in AN and AW treatment, as their N release was over 100%. Further research under uncontrolled field conditions utilizing AN and AW for diverse crop types can validate their N replacement potential.

Land use-land cover gradient demonstrates the importance of perennial grasslands with intact soils for building soil carbon in the fertile Mollisols of the North Central US

The impact of land use change and agricultural management on the cycling of soil organic carbon (SOC) is not well understood, limiting our ability to manage for, and accurately model, soil carbon changes at both local and regional scales. To address this issue, we combined long-term soil incubations with acid-hydrolysis and dry combustion to parse total SOC (Ct) into three operationally defined SOC pools (active, slow, and recalcitrant) from 9 long-term sites with varying land uses on current and former tallgrass prairie soil. Land uses represented a gradient of soil disturbance histories including remnant prairie, restored prairie, grazed pasture, annual crop rotations, and continuous maize. Dry combustion was used to estimate total carbon (Ct, physical), while acid hydrolysis of both the active (Ca) and slow (Cs) pools was used to estimate a recalcitrant carbon pool (Cr, chemical). Non-linear modeling of CO2 efflux data from the long-term incubations was then used to estimate Ca, and the decomposition rates of both Ca and Cs (ka and kr, biological). The size of the slow pools Cs was then defined mathematically as Ct-(Ca + Cr). Remnant prairie had the highest Ct, while cool-season pasture and a 35-y-old restored prairie had higher Ct than the other agricultural systems. All agricultural systems, including pasture, had the highest fraction of Ct as Cr (∼50%), whose mean residence time (MRT) in these soils is ≥500 years (Paul et al., 2001a) demonstrating that this fraction persists, while the more labile fractions were lost over the course of a few months (Ca) to a few decades (Cs) as a result of tillage-intensive agriculture. The two- to four-decade MRT time of Cs indicated a pool likely to be more responsive to the 20 to 40 years of land-use practices used at some of the sites. The Cs pool was largest in the remnant- and 35-y-old prairies indicating significant C accrual and stabilization compared to the agricultural ecosystems. Interestingly, the remnant prairie maintained the highest Ca pool as well, demonstrating the strong connection between the quantity of fresh C inputs and the potential for long-term C stabilization and accrual. The accumulation of C in active (≈labile) pools as a first step toward long-term stabilization highlights the tenuous nature of early carbon gains, which can be quickly lost in response to climate change or poor management.

Risk of Epidemic Development in Nurseries from Soil Inoculum ofPhytophthora ramorum.

Soilborne inoculum arising from buried, infested leaf debris may contribute to the persistence of Phytophthora ramorum at recurrently positive nurseries. To initiate new epidemics, inoculum must not only survive but also produce sporangia during times conducive to infection at the soil surface. To assess this risk, we performed two year-long experiments in a soil plot at the National Ornamentals Research Site at Dominican University of California. Inoculated rhododendron leaf disks were buried at a depth of 5 or 15 cm in the early summer of 2014 or 2015. Inoculum was baited at the soil surface with noninfested leaf disks (2014 only) and then retrieved to assess pathogen viability and sporulation capacity every 5 weeks. Two 14-week-long trials were conducted in 2016. We were able to consistently culture P. ramorum over all time periods. Soil incubation rapidly reduced the capacity of inoculum to sporulate, especially at 5 cm; however, sporulation capacity increased with the onset of seasonally cooler temperatures. P. ramorum was baited most frequently between November and January, especially from inoculum buried at 5 cm 1 day before the baiting period; in January we also baited P. ramorum from inoculum buried at 15 cm the previous June. We validate prior observations that P. ramorum poses a greater risk after exposure to cooler temperatures and provide evidence that infested leaf debris plays a role in the perpetuation of P. ramorum in nurseries. This work provides novel insights into the survival and epidemic behavior of P. ramorum in nursery soils.

Effects of microplastics and carbon nanotubes on soil geochemical properties and bacterial communities

A 100-day soil incubation experiment was conducted to explore the effects of conventional (high-density polyethylene, HDPE) and biodegradable (polylactic acid, PLA) microplastics (MPs) and multiwall carbon nanotubes (MWCNTs) on soil geochemical properties and bacterial communities. Generally, soil pH was increased by 10% HDPE and 10% PLA, but decreased by increasing MWCNTs. Soil dissolved organic carbon content was only increased by 10% PLA. NO3−-N content was significantly decreased by MPs, with a decrement of 99% by 10% PLA. Similarly, available P content was reduced by 10% MPs. The activities of urease and alkaline phosphatase were stimulated by 10% PLA, but generally inhibited by HDPE. Conversely, FDAse activity was stimulated by HDPE, but inhibited by 10% PLA, whereas invertase activity decreased with increasing MWCNTs. Overall, both MPs and MWCNTs changed soil bacterial diversity. Co-exposure to 10% MPs and MWCNTs of 1 and 10 mg/kg caused the lowest species richness and Shannon indexes. MPs especially at the 10% dose changed bacterial community composition and the associated metabolic pathways, causing the enrichment of specific taxa and functional genes. Our findings show that conventional and biodegradable MPs differently change soil geochemical properties and microbial community structure and functions, which can be further modified by co-existing MWCNTs.

Biochar application affects Nitrobacter rather than Nitrospira in plastic greenhouse vegetable soil

Nitrite oxidation, driven by nitrite-oxidizing bacteria, is an important step of nitrification and thus plays a vital role in biogeochemical nitrogen cycling in agricultural ecosystems. Although biochar has been widely recognized as a promising material for use in vegetable fields to slowly release nutrients, the current understanding of how nitrite oxidizers respond to the application of biochar in a plastic greenhouse vegetable field is very limited. A soil incubation experiment showed that soil nitrite oxidation was increased by 13.0%, 35.0% and 64.2% (P < 0.05) with the application of 0.5% (C0.5), 1.5% (C1.5), and 4.0% (C4.0) biochar (mass ratio), respectively. Methods including qPCR and Illumina MiSeq sequencing were also used to explore the impact of biochar on functional communities involved in nitrite oxidation. Nitrobacter and Nitrospira were the main genera of the nitrite-oxidizing bacteria and were assumed to be very important in the soil environment. Moreover, the increases in soil nitrite oxidation were positively correlated with the abundance of Nitrobacter-like NOB (P < 0.05) but not the Nitrospira-like NOB gene abundance. Furthermore, biochar application combined with nitrogen fertilizer had a significant effect on the Nitrobacter community composition, with lineages such as Nitrobacter alkalicus and Nitrobacter vulgaris being significantly enriched. Redundancy analysis indicated that the observed variations in the Nitrobacter community structure were associated with changes in soil microbial biomass nitrogen and nitrate nitrogen induced by biochar. These results implied that Nitrobacter rather than Nitrospira was significantly improved by biochar added to the soil for vegetable production.

Cellulose amendment promotes P solubilization by Penicillium aculeatum in non-sterilized soil

Successful application of microbial biofertilizers, such as phosphorus (P) solubilizing fungi to agroecosystems, is constrained from the lack of knowledge about their ecology; for example in terms of how they respond to an external input of carbon (C) to get established in the soil. In two soil incubation experiments we examined the performance of the P solubilizing fungus Penicillium aculeatum in non-sterile and semi-sterile (γ-irradiated) soil with different C and P sources. Results from the first experiment with C sources showed that starch and cellulose generally improved P solubilization by P. aculeatum measured as water extractable P (Pwep), though only significantly in non-sterile soil. This coincided with an increased population density of P. aculeatum measured with a hygromycin B resistant strain of this fungus. Soil respiration used to measure soil microbial activity was overall much higher in treatments with C compounds than without C in both non-sterile and semi-sterile soil. However, soil respiration was highest with cellulose in semi-sterile soil, especially in combination with P. aculeatum. Hence, for the second experiment with P sources (tricalcium phosphate (TCP) and sewage sludge ash) cellulose was used as a C source for P. aculeatum growth in all treatments. Main results showed that P. aculeatum in combination with cellulose soil amendment increased soil Pwep independent of soil sterilization and P source treatments. Soil resin P (Pres) and microbial P (Pmic), which represents stocks of potentially plant available P, were also affected from P. aculeatum inoculation. Increased soil Pres from TCP and sewage sludge ash was observed with P. aculeatum independent of soil type. On the other hand soil Pmic was higher after P. aculeatum inoculation only in semi-sterile soil. Population density of P. aculeatum measured with qPCR was maintained or increased in non-sterile and semi-sterile soil, respectively, compared to the original inoculum load of P. aculeatum. In conclusion, our results underline the importance of C source addition for P. aculeatum if used as a biofertilizer. For this, cellulose seems to be a promising option promoting P. aculeatum growth and P solubilization also in non-sterilized soil.

Nutrient Management Drives the Direction and Magnitude of Nitrous Oxide Flux in Crop Residue-Returned Soil Under Different Soil Moisture

Crop residues as key organic carbon inputs have the potential for soil carbon sequestration. However, previous studies have shown an inconsistent effect of residue return on the direction and magnitude of soil nitrous oxide (N2O) emission. We used a laboratory-based soil incubation study to test the response of N2O emission to crop residue type, soil moisture, and how nutrient management modulates these responses. In this study, we incorporated crop residues with different qualities (wheat, rice, soybean, and maize) at two soil moisture contents {80% field capacity (FC) and 60% FC} and under seven nutrient levels: N0P0K0 (no nutrients), N0PK, N100PK, N150PK, N100PK + manure@ 5 Mg ha−1, N100PK + biochar@ 5 Mg ha−1, and N150PK + biochar@ 5 Mg ha−1. The results demonstrated significant (p &amp;lt; 0.01) differences in the magnitude of N2O emissions among treatments. However, only the interaction effect of residue × nutrient and nutrient × moisture was significant (p &amp;lt; 0.05). N100PK and N150PK at 80% FC mitigated N2O emission by approximately 20% in wheat residue-amended soil (cf. control soil without residue). In contrast, maize residue amendment (cf. control soil) increased N2O emission by 130% under N0P0K0 and 80% FC. Residue effects were negatively correlated with the C:N ratio, and a strong positive correlation (p &amp;lt; 0.01) was obtained between N2O emission and CO2 respiration, labile carbon, mineral N, and residue total nitrogen (TN). When no nutrients were added, N2O emission was higher in residue returned soil. However, cumulative fluxes of N2O decreased by 6–17% when maize and wheat residues (cf. control soil) were applied with nutrients. Negative fluxes of N2O indicating consumption were observed in every treatment after 57 days of incubation and were most pronounced in control soil without residue and nutrients. Decreasing the soil moisture from 80% FC to 60% FC, the N2O consumption rate increased by 6.6 times across residue types and nutrient management. The regression analysis and structural equation modeling (SEM) results showed that residue TN, soil CO2 emission, NO3-N, and labile SOC were the key predictor variables and could explain 82% variability in the soil N2O emission in the Vertisols of Central India. The results suggested that nutrient addition (NPK) could alter the magnitude and direction of soil N2O flux by residue type and soil moisture by influencing the underlying soil microbial processes of the C and N cycle in the Vertisol of subtropical India.

Limestone and Silicate Applications by Different Methods to Correct Soil Acidity

The study aimed to measure the variation in the values of pH, P, K, Ca, Mg, Al, H+Al, V, and Ca and Mg saturation after limestone and silicate applications as a function of different soil correction methods and incubation periods under a controlled environment. The research was carried out in a greenhouse at the FCA of the Federal University of Grande Dourados (UFGD). The experiment was completely randomized in a factorial scheme (5 &amp;times; 3 &amp;times; 5), with four replications. The main factors consisted of five incubation times: 0, 30, 60, 90, and 120 days; three soil classes: dystrophic Red Latosol (LVd), Dystroferric Red Latosol (LVdf), and dystrophic Gray Argisol (PACd); and five soil acidity correction methods: control, Ca/Mg balance for limestone and silicate, and 50% and 70% base saturation. Chemical analysses were performed after each incubation period. A regression analysis was carried out once a significant difference was observed between the means of the main factors of the analysis of variance, being adjusted to quadratic models for pH, P, Al, K, Ca, Mg, H+Al, and V. Statistical analyses were performed in the AgroStat software. The ideal soil incubation time to reach the maximum efficiency of correction of the chemical attributes of LVd, LVdf, and PACd soils by the studied methods ranges from 78 to 86 days. The application of limestone by balance of 60% Ca and 20% Mg and calcium and magnesium silicates achieved the best correction indexes of soil chemical attributes, enabling the proposed equation as a calcium and magnesium silicate calculation.

The Utilization of Phosphogypsum as a Sustainable Phosphate-Based Fertilizer by Aspergillus niger

Phosphogypsum (PG) is a by-product of the phosphorus (P) chemical industry, and usually contains ~2% residual phosphate. The P in PG would cause P pollution in the environment by the way of long-term stacking. Phosphate-solubilizing fungi (PSF) can effectively promote the release of P from insoluble phosphate. The aim of this study was to investigate the P release capacity between tricalcium phosphate (TCP) and PG by Aspergillus niger (ANG) via flask experiment. At the same time, the application of PG fertilizer (PGF) in soil was also performed. The dry biomass of ANG between TCP and PG treatment reached the maximum value of 0.71 and 0.38 g, respectively, after five days of incubation. The maximum P concentrations released from PG and TCP by ANG is five and seven days, i.e., 128 and 1103 mg/L, respectively. PG has a similar P release ratio compared with TCP by ANG, i.e., 52% vs. 55%. In the soil incubation experiment, PGF can significantly increase the soil available P content and continuously maintain the high concentration (17.2 mg/kg) after four weeks of incubation. The utilization of PG by ANG via the PGF is a feasible pathway to increase the soil P content, especially for the sustainable use of phosphate resources.