Soil Solution Concentrations Research Articles

Mechanisms by Which Soil Solution Fe2+ Affects Seedling Growth of Rice Under Rice Straw Return

Rice straw return plays an important role in sustainable agricultural development, but the impact of rice straw return on rice growth remains inconclusive. In this experiment, we employed a combination of soil and water cultivation experiments to investigate the impact of rice straw return on seedling growth of rice in Northeast China. This study demonstrated that rice straw return inhibited rice plant growth within 37 days but was no longer significant after 37 days. Rice straw return resulted in an increase in iron(II) (Fe2+) concentration of soil solution. The hydroponic test demonstrated that a high concentration of Fe2+ significantly increased the uptake of Fe, reduced the H2O2 content in root, facilitated the formation of root iron plaque, and inhibited the dry weight at the rice seedling stage, in comparison to a nutrient solution with a normal Fe2+ concentration. The increased Fe2+ concentration of soil solution under rice straw return may be a significant factor in the inhibition of rice seedling growth. Moreover, in field management, this study also linked the amount of rice straw returned and soil solution Fe2+ to provide a clear quantitative guide without affecting plant growth.

Identification of dissipation pathways for pharmaceuticals in soils - a modelling approach.

Concentrations of pollutants like pharmaceuticals in soils typically decrease over time, though it often remains unclear whether this dissipation is caused by the transformation of the pollutant or a decreasing extractability. We developed a mathematical model that (1) explores the plausibility of different dissipation pathways, and (2) allows the quantification of concentration differences between aqueous soil extracts and soil solution. The model considers soil particles as uniform spheres, kinetic sorption towards an equilibrium (Freundlich model), and two dissipation pathways, i.e. irreversible transformation and mineralization (following 1st order kinetics) as well as the formation of non-extractable residues via intraparticle diffusion. Applying the model to 19 published datasets (eight pharmaceuticals, three soils) showed that intraparticle diffusion accounts for approximately two thirds of all simulated mass fluxes. Reversible formation of non-extractable residues could play a major role for dissipation, while irreversible transformation and mineralization cannot be excluded either based on the available datasets. The difference between concentrations in aqueous extracts and soil solution quantified by the model is below typical model and measurement uncertainties for most of the investigated pharmaceuticals. Larger differences might be linked to a positive ionic charge of the pollutant. The model helps in disentangling different dissipation pathways in soils and optimizing experiments elucidating the long-term fate of pollutants in soils.

Amelioration of different acidic soils in China using seafood shell biochars.

Soil acidification limits crop and pasture production and leads to the degradation of agroecosystems. A substantial volume of seafood shells are discarded each year, which creates enormous environmental and social pressures. In this study, the anaerobic pyrolysis characteristics of four types of seafood shells (clam, scallop, oyster, and mussel) were evaluated. Then, outdoor pot experiments using canola (Brassica napus L.) were conducted and the soil solution was collected in situ to determine the pH, organic matter, nutrient content, and aluminum (Al) species in soil liquid and solid phase. Finally, the relationships between the growth of canola and the concentration of phytotoxic Al were investigated to evaluate the amelioration efficacy and mechanism of seafood shell biochars applied to three typical acidic soils in China. The results showed that seafood shell biochars pyrolyzed under 800°C anaerobic conditions had characteristics of inorganic and organic amendments. The addition of 2gkg-1 four seafood shell biochars pyrolyzed at 800°C could increase pH of three kinds of severely acidified soils and soil solutions to more than 5.0, and adding 4gkg-1 could increase they to more than 6.0. After adding 4gkg-1 four seafood shell biochars in Oxisol-GD, Ultisol-AH and Ultisol-JX, the Al concentration of soil solution decreased by more than 89%, 88% and 80%, respectively; the effects were similar to lime. The application of four seafood shell biochars promoted the transformation of water-soluble Al and exchangeable Al in acidic soils into organically bound Al, adsorbed hydroxyl Al, and more stable Al, thus greatly reducing the concentration of Al in soil solution; this improved the growth of canola and increased the yield.

Determination of Kaolin Optimum Dose to Inhibit Iron Toxic Effect on Rice Plant in Lowland Rice Cultivation

In tropical areas, lowland rice cultivation with or without water control is often faced with iron toxicity constraint. This problem causes a drop of the rice grain yield when the iron concentration in soil solution is more than 300 ppm. To resolve iron toxicity problem, drainage and fertilizers as industrial silicon applying have been recommended. However, the used of industrial silicon is not adopted by the farmers because of his high cost and his accessibility. Kaolin contains 54.7% of SiO2 the aim of this research is to explore the potential of kaolin as a natural substitute of industrial silicon. A pot test was carried in 900 ppm iron toxicity condition. of Fe2+ the condition out for two successive rice cultivation cycles by applying 900ppm of Fe2+. Five kaolin treatments were analyzed (T0 = 0, T1 = 366, T2 = 735, T3 = 1097 and T4 = 1465 kg kaolin ha-1) in a complete randomized block design with 5 repetitions. The dose of 366 kg ha-1 reduced the symptoms of iron toxicity on rice plant. kaolin applying improved the grain yield of rice (0 tha-1 to 1.8 tha-1). The dose of 1465 kg kaolin ha-1 gave the best grain yield (1.08 t ha-1). The response of rice to kaolin doses was an equation RDG = 0.98 ×Dose more linear than quadratic and the dose of 1465 kg kaolin ha-1 was recommended as the optimal dose in the conditions of 900 ppm Fe2+.

Improving Phosphate Acquisition from Soil via Higher Plants While Approaching Peak Phosphorus Worldwide: A Critical Review of Current Concepts and Misconceptions.

Phosphate (P) is the plant macronutrient with, by far, the lowest solubility in soil. In soils with low P availability, the soil solution concentrations are low, often below 2 [µmol P/L]. Under these conditions, the diffusive P flux, the dominant P transport mechanism to plant roots, is severely restricted. Phosphate is sorbed into various soil solids, Fe/Al oxides, clay minerals and, sometimes overlooked, humic Fe/Al surfaces. The immobilization of P in soil is often the result of the diffusion of P into the internal surfaces of oxides or humic substances. This slow reaction between soil and P further reduces the availability of P in soil, leading to P fixation. The solubilization of soil P by root-released carboxylates is a promising way to increase the acquisition and uptake of P from P-fixing soils. Citrate and, sometimes, oxalate are effective with respect to additional P solubilization or P mobilization, which may help increase the diffusive P flux into the roots by increasing the P solution concentrations in the rhizosphere. The mobilization of humic-associated P by carboxylates may be an effective way to improve soil P solubility. Not only orthophosphate anions are mobilized by root-released carboxylates, but also higher phosphorylated inositol phosphates, as the main part of P esters in soil are mobilized by carboxylates. Because of the rather strong bonding of higher phosphorylated inositol phosphates to the soil solid phase, the mobilization step by carboxylates appears to be essential for plants to acquire inositol-P. The ecological relevance of P mobilization by carboxylates and its effect on the uptake of P by crops and grassland species are, at best, partially understood. Plant species which form cluster roots such as white lupin (Lupinus albus L.) or yellow lupin (Lupinus luteus L.) release high rates of carboxylates, mainly citrate from these root clusters. These plant species acquire fixed or low available P which is accessible to plants at rates which do not satisfy their P demand without P mobilization. And white lupin and yellow lupin make soil P available to other plants in mixed cropping systems or for subsequent plant species in crop rotations. The mobilization of P by carboxylates is probably also important for legume/grass mixtures for forage production. Species such as alfalfa, red clover or white clover release carboxylates. The extent of P mobilization and P uptake from mobilized P by legume/grass mixtures deserves further research. In particular, which plant species mostly benefit from P mobilization by legume-released carboxylates is unknown. Organic farming systems require such legume/grass mixtures for the introduction of nitrogen (N) by forage legumes into their farming system. For this agricultural system, the mobilization of soil P by carboxylates and its impact on P uptake of the mixtures are an important research task.

Cu Concentration in Vegetable-Cultivated Soil: Comparison Between Open Fields and Protected Environments in Brazil

High copper (Cu) concentration in soils used for vegetable production is an agricultural, social and environmental issue. The excess of Cu in soil can cause toxicity in plants, reducing growth and yields. Moreover, Cu can also be absorbed and accumulated in edible organs, increasing risks for human health. This study aimed to compare Cu fractions in soils: (1) non-cultivated, natural soil (NC), (2) soils cultivated with open field crops (FCs) and (3) soils cultivated in controlled environments (CEs). The survey was carried out on 25 sites in Rio Grande do Sul (Southern Brazil), with each site containing the three types of soils described above. From these sites, the four with the highest soil Cu concentrations were selected to compare soluble (Cu-CaCl2), available (Cu-EDTA) and total (Cu-EPA) Cu. Both total and available soil Cu concentration in soil solution, in CE and FC areas were higher than in NC. At sites 23R1 (Oxisol) and 11R1 (Molisol) the soluble and available Cu content was higher in the FC environment than in CEs, and the same was observed for the total Cu content at sites 23R1 (Oxisol) and 9R1 (Molisol). Some FC soils showed total Cu concentrations higher than the limits established by Brazilian environmental legislation, being, as a consequence, potentially contaminated. Soil monitoring and cropping practices must be adopted to reduce the Cu content in soils of vegetable crops in this region.

Interactions between arsenic and nitrogen regulate nitrogen availability and arsenic mobility in flooded paddy soils

In paddy soils, arsenic (As) stress influences nitrogen (N) transformation while application of N fertilizers during rice cropping affects As transformation. However, specific interactive effects between As and N in flooded paddy soils on As mobility and N availability were unclear. Here, we examined N and As dynamics in flooded paddy soils treated with four As levels (0, 30, 80 and 150 mg kg−1) and three urea additions (0, 4 and 8 mmol N kg−1). Arsenic contamination inhibited diazotrophs (nifH) and fungi but promoted AOA and denitrification genes (narG, nirK, nirS), decreasing dissolved organic N, NH4+-N and NO3–-N. Besides, urea application stimulated As- and Fe-reducing bacteria (arrA and Geo) coupled with anammox. On Day 28, the addition of 8 mmol N kg–1 increased total As concentrations in solutions of soils treated with 30 and 80 mg As kg−1 by 2.4 and 1.8 times compared with the nil-N control. In contrast, at 150 mg As kg−1, it decreased the total As concentration in soil solution by 63 % through facilitating As(III) oxidation coupled with NO3–-N reduction. These results indicate that As contamination decreases N availability, but urea application affects As mobility, depending on As contamination level.

Bioaccumulation of Atmospherically Deposited Cadmium in Soybean: Three-Year Field Experiment Combined with Cadmium Isotopes.

Atmospheric deposition plays a significant role in introducing cadmium (Cd) into agroecological systems; however, accurately determining its accumulation in crops through foliar and root uptake presents challenges. This study investigated the bioaccumulation of atmospherically deposited Cd in soybean using a three-year fully factorial atmospheric exposure experiment incorporating Cd isotope analysis. Results shown that atmospheric deposition accounted for 1-13% of soil Cd pools, yet contributed 11-72% of Cd to soybean tissues during the growing seasons. Over the course of soil exposure to atmospheric deposition ranging from 1 to 3 years, no notable variations were observed in Cd concentrations in soil solutions and soybean tissues, nor in isotope ratios. Newly deposited Cd was a major source in soybean plants, and the bioavailability of deposited Cd rapidly aged in soils. Atmospheric Cd enriched in lighter isotopes induced negative isotope shifts in soybean plants. By employing an optimized isotope mixing model in conjunction with a mass balance approach, foliar Cd uptake contributed 13-51%, 16-45%, and 21-56% to stem, leaf, and seed, respectively. This study highlights substantial contribution of foliar uptake of atmospheric deposition to Cd levels in soybean and controlling foliar uptake as a potential strategy in agroecological systems experiencing high atmospheric Cd deposition.

Impact of Irrigation Management Decisions on the Water Footprint of Processing Tomatoes in Southern Spain

The water footprint is an increasingly demanded environmental sustainability indicator for certifications and labels in agricultural production. Processing tomatoes are highly water-intensive, and existing studies on water footprint have uncertainties and do not consider the impact of different irrigation configurations (e.g., surface drip irrigation (SDI) and subsurface drip irrigation (SSDI)) and irrigation strategies. This study presents a two-year experimental investigation to determine the water footprint of processing tomatoes grown in southern Spain (Andalusia) and the impact of SSDI and deficit irrigation. Five irrigation treatments were established: SDI1 (surface drip irrigation without water limitation), SDI2 (surface drip irrigation without water limitation adjusted by soil moisture readings), SSDI1 (subsurface drip irrigation without water limitation and a dripline depth of 15 cm), SSDI2 (similar to SSDI1 but with mild/moderate water deficit during the fruit ripening stage), and SSDI3 (subsurface drip irrigation without water limitation and a dripline depth of 35 cm (first year) and 25 cm (second year)). Measurements included crop vegetative growth, leaf water potential, leaf gas exchange, nitrate concentration in soil solution, and crop yield and quality. The soil water balance components (actual evaporation, actual transpiration, deep drainage), necessary for determining the total crop water footprint, were simulated on a daily scale using Hydrus 2D software. Results indicated that SSDI makes more efficient use of irrigation water than SDI. The water footprint of SSDI1 was 20–35% lower than that of SDI1. SSDI2 showed similar water footprint values to SDI1 under highly demanding environmental conditions and significantly lower values (≈40%) in a year with lower evaporative demand. The dripline depth in SSDI was critical to the water footprint. With a 35 cm installation depth, SSDI3 had a significantly higher water footprint than the other treatments, while the values were similar to SSDI1 when the depth was reduced to 25 cm.

Amended soils with weathered coal exhibited greater resistance to aggregate breakdown than those with biochar: From the viewpoint of soil internal forces

Soil erosion is the first threat to soil functions. Reducing the soil aggregate breakdown strength is a key step to improve the soil’s ability to resist rainfall splash erosion. Soil internal forces have been found to be the initial and important forces driving aggregate turnover. The application of exogenous organic materials can effectively improve soil aggregate stability and the resistance to rainfall erosion of agricultural soils. However, from the perspective of soil internal forces, information about the reduction effects of the exogenous organic materials application on soil aggregate breakdown is scarce, especially in comparing the effects of different materials. In this study, weathered coal and biochar were individually applied to loamy clay soil at rates of 0 %, 1 %, 2 %, and 3 % (w/w). Soil internal forces, aggregate breakdown strength, and splash erosion rate of different amended soils were then examined after four years. The results showed that compared with unamended soils (0 %), both weathered coal and biochar applications clearly increased the van der Waals attractive pressure and thus decreased the positive net pressure between soil particles. Additionally, these materials reduced soil aggregate breakdown strength and splash erosion rate. The application effects of the two materials were increased with their application rates. Under a lower electrolyte concentration in soil solution (0.0001 mol L−1), the aggregate breakdown strength in the soils amended with weathered coal was lower than that with biochar by 9.6 %, 23.2 %, and 17.7 % (when the diameter of broken aggregate was < 10 μm) and by 10.3 %, 20.8 %, and 17.5 % (when the diameter of broken aggregate was < 20 μm) at the 1 %, 2 %, and 3 % application rates, respectively (P < 0.05). Additionally, soils amended with weathered coal exhibited lower splash erosion rates compared to those amended with biochar, particularly at the higher application rate of 3 %. From the viewpoint of soil internal forces, weathered coal appears to be a suitable exogenous organic material for improving soil aggregate stability and anti-erosion ability during rainfall events. Our findings provide valuable insights into utilizing exogenous materials to improve soil resistance to rainfall splash erosion, assisting agricultural soil management in areas frequently affected by rainfall erosion.

Verification of the Solid–Liquid Separation of Waterlogged Reduced Soil via a Centrifugal Filtration Method

The efficient separation of solid and liquid phases of soil under reductive conditions is of the utmost importance to study soil chemistry and to predict the mobility and bioavailability of nutrients and toxic contaminants in waterlogged reduced soils (WRSs). However, there is no established method for efficiently separating the solid and liquid phases of WRS within a short time while maintaining its reductive conditions. This study aimed to verify the applicability of a simple centrifugal filtration method (CFM) for the efficient separation of solid and liquid phases of a WRS and examine the CFM-extracted soil solution to confirm that the reductive condition was maintained during the solid–liquid separation process. Incubation experiments were performed under reductive conditions with or without ethanol/molasses used as additional organic material (OM), while the soil solution was collected by both a suction method and CFM at different centrifugation speeds (700, 2760, and 11,000 rpm) and times (1–7 min). The results showed that the soil pH increased with time while the Eh decreased, indicating that its reducing state was enhanced during the incubation experiments. The addition of OM promoted the reductive conditions in the first days of the experiments. Centrifugation speed, rather than time, was found to be the key to extract the maximum amount of soil solution, while a higher centrifugation speed (11,000 rpm), which represents the permanent wilting point, was found to be most effective for extracting the maximum amount of soil solution. The results exhibited no significant difference in solute (As, Fe(II), and Mn) concentrations when varying amounts of CFM-extracted soil solution were measured. The statistical analysis also indicated no significant (p &gt; 0.05) difference between the solute concentrations in the CFM-extracted soil solution and the solute concentrations in the soil solution extracted by the suction method, confirming that the reductive condition was maintained during solid–liquid separation by CFM. This study suggests that CFM operating at a higher centrifugation speed could potentially be employed as a simple and highly effective technique to efficiently separate the solid and liquid phases of WRS (sandy clay loam) within a short time while maintaining its reductive conditions.

Electrochemical study characterizations of pyrite weathering in simulated acidic soil: Iron transformation, sulfur conversion and environmental implications

Pyrite in natural environments is susceptible to weathering, which results in acid mine drainage (AMD) and heavy metal pollution-related environmental problems. The increase in mining activities has led to a large amount of pyrite entering surroundings, which may aggravate soil acidification and heavy metal pollution. In situ electrochemical techniques were used to investigate the weathering behavior of pyrite in simulated acidic soil solutions, which is conducive to understanding the weathering mechanism and environmental impact. In this study, the polarization curves and electrochemical impedance spectroscopy reveal that a higher soil solution concentration, temperature and/or acidity accelerate pyrite weathering rate, with an activation energy of 21.21 kJ·mol-1. Further investigations reveal that the pyrite is initially oxidized to initially oxidized to S0 and released Fe(II), the Fe(II) is easily transformed into Fe(III), and part of the Fe species ultimately transforms into goethite γ-FeOOH and hematite α-Fe2O3. The S0 ultimately transform into sulfate, accompanied by the release of hydrogen ions (H+), resulting in serious soil acidification. These results specifically reveal the mechanism of pyrite weathering, especially the process of iron transformation and sulfur conversion, and they also quantitatively identify the pyrite weathering rate and the quantities of heavy metal ions released, providing an experimental basis for the risk assessment of pyrite in acid soils.

Tall wheatgrass response toward phosphorus deficiency: a constraint to its use as a forage resource in marginal soils

Tall wheatgrass is cultivated for pasture in marginal areas around the world due to its tolerance to abiotic stress including seasonal droughts and floods, as well as saline and alkaline soils. However, there is concern about its biomass productivity, which often does not exceed that of natural pastures. In this work we hypothesize that tall wheatgrass is a plant more sensitive to low P availability than commonly assumed. A glasshouse sand culture experiment was conducted to analyze morpho-physiological responses of tall wheatgrass to different P availability levels: 1 µmol L−1 phosphorus (severe deficiency, P1), 10 µmol L−1 phosphorus (representative value of soil solution in the region, P10) and 500 µmol L−1 phosphorus (full P supply, P500). Our results show that both limiting P conditions strongly decreased plant development and biomass accumulation, and altered above- and below-ground morphology, in comparison with P500 plants. The latter, in turn, had relative growth rate values comparable to those of other cultivated grasses under non-limiting conditions. In both P-deficient treatments, plant development was severely delayed starting from leaf number 3, even when P concentration in the pseudostem at early stages was similar or even higher than that of P500 plants. Our results suggest that developmental inhibition of tall wheatgrass at low P supply may be the consequence of a specific plant response to P concentration in soil solution, rather than to direct P starvation in plant tissues, and underlines the need of improvement of this species to enhance productivity in this environment.

Elemental mercury production from contaminated riparian soil suspensions under air and nitrogen bubbling conditions.

The dynamic change of redox conditions is a key factor in emission of elemental mercury (Hg0) from riparian soils. The objective of this study was to elucidate the influences of redox conditions on Hg0 emission from riparian soils. Soil suspension experiments were conducted to measure Hg0 emission from five Hg-contaminated soil samples in two redox conditions (i.e., treated with air or with N2). In four of the five samples, Hg0 emission was higher in air treatment than on N2 treatment. Remaining one soil, which has higher organic matter than other soils, showed no distinct difference in Hg0 production between air and N2 treatment. In soil suspensions subject to N2 treatment, the dissolved organic carbon (DOC) and Fe2+ concentrations were 3.38- to 1.34-fold and 1.44- to 2.28-fold higher than those in air treatment, respectively. Positive correlations were also found between the DOC and Fe2+ (r = 0.911, p < 0.01) and Hg2+ (r = 0.815, p < 0.01) concentrations in soil solutions, suggesting Fe2+ formation led to the release of DOC, which bound to Hg2+ in the soil and, in turn, limited the availability of Hg2+ for reduction to Hg0 in N2 treatment. On the other hand, for remaining one soil, more Hg2+ might be adsorbed onto the DOM in the air treatment, resulted in the inhibition of Hg0 production in air treatment. These results imply that the organic matter is important to prevent Hg0 production by changing redox condition. Further study is needed to prove the role of organic matter in the production of Hg0.

Survival and rapid resuscitation permit limited productivity in desert microbial communities

Microbial activity in drylands tends to be confined to rare and short periods of rain. Rapid growth should be key to the maintenance of ecosystem processes in such narrow activity windows, if desiccation and rehydration cause widespread cell death due to osmotic stress. Here, simulating rain with 2H2O followed by single-cell NanoSIMS, we show that biocrust microbial communities in the Negev Desert are characterized by limited productivity, with median replication times of 6 to 19 days and restricted number of days allowing growth. Genome-resolved metatranscriptomics reveals that nearly all microbial populations resuscitate within minutes after simulated rain, independent of taxonomy, and invest their activity into repair and energy generation. Together, our data reveal a community that makes optimal use of short activity phases by fast and universal resuscitation enabling the maintenance of key ecosystem functions. We conclude that desert biocrust communities are highly adapted to surviving rapid changes in soil moisture and solute concentrations, resulting in high persistence that balances limited productivity.

Evaluating the effectiveness of sulfidated nano zerovalent iron and sludge co-application for reducing metal mobility in contaminated soil

Sewage sludge has long been applied to soils as a fertilizer yet may be enriched with leachable metal(loid)s and other pollutants. Sulfidated nanoscale zerovalent iron (S-nZVI) has proven effective at metal sorption; however, risks associated with the use of engineered nanoparticles cannot be neglected. This study investigated the effects of the co-application of composted sewage sludge with S-nZVI for the stabilization of Cd, Pb, Fe, Zn. Five treatments (control, Fe grit, composted sludge, S-nZVI, composted sludge and S-nZVI), two leaching fluids; synthetic precipitation leaching procedure (SPLP) and toxicity characteristic leaching procedure (TCLP) fluid were used, samples were incubated at different time intervals of 1 week, 1, 3, and 6 months. Fe grit proved most efficient in reducing the concentration of extractable metals in the batch experiment; the mixture of composted sludge and S-nZVI was the most effective in reducing the leachability of metals in the column systems, while S-nZVI was the most efficient for reducing about 80% of Zn concentration in soil solution. Thus, the combination of two amendments, S-nZVI incorporated with composted sewage sludge and Fe grit proved most effective at reducing metal leaching and possibly lowering the associated risks. Future work should investigate the longer-term efficiency of this combination.

Effects of 3-year biochar application on carbon sequestration, nitrogen retention and nitrate leaching of fluvo-aquic soil profiles in vegetable rotation fields

The impacts of biochar application on the carbon (C) and nitrogen (N) cycles in soil profiles in vegetable fields have rarely been reported. A three-year field experiment (CK, control; BC, biochar; N, nitrogen fertilizer; BCN, biochar and nitrogen fertilizer) was conducted in fluvo-aquic soil with a wild cabbage-Chinese cabbage rotation to investigate biochar effects on soil organic carbon (SOC) sequestration, N retention, and nitrate (NO3−) leaching in the soil profile (topsoil, 0–20 cm; subsoil, 20–50 cm; third-layer soil, 50–100 cm). The results showed that the biochar-induced increase in topsoil SOC under N fertilization was greater for Chinese cabbage season than for wild cabbage season. Excluding biochar-N, biochar application caused an extra increase in topsoil total nitrogen (TN) under N fertilization. Biochar decreased the NO3−-N concentration of third-layer soil solution, particularly under N fertilization, indicating that biochar reduced the potential source of NO3− leaching. Under N fertilization, biochar reduced the abundances of carbon-degrading bacteria (Streptomyces, Bacillus, Mycobacterium, and Sphingomonas) and genes (sga, xylA, lig, and pgu) in the topsoil, as well as the abundances of ammonia-oxidizing bacteria (Nitrosospira and Nitrosomonas) and gene (amoA) in the third-layer soil. However, biochar increased the abundance of ammonia-assimilating bacteria (Rhodococcus) and gene (glnA) in the topsoil. Biochar inhibited the microbial function associated with C degradation by affecting soil TN, thus enhancing topsoil SOC sequestration. Biochar promoted the soil microbial function related to ammonia assimilation by affecting dissolved organic carbon (DOC), subsequently enhancing ammonia assimilation. Moreover, biochar initially inhibited ammonia oxidation in the third-layer soil by increasing soil DOC, subsequently affecting nitrite oxidation and ultimately reducing soil NO3− leaching. Biochar significantly increased the N use efficiency of vegetables in the last two years. This study provides insights into biochar effects on the changes in soil microbial function, which promoted SOC sequestration, enhanced N retention and mitigated nitrate leaching in a vegetable rotation field.

Forms of Silicon in Rice Soils of Kerala, India

Aim: The plant-available silicon (PAS) content is highly influenced by different forms/fractions of silicon (Si) in which it exists in the soil. The solubility of these forms varies and affects the Si concentration in soil solution. Quantification of Si pools in soils is needed for a better understanding of biogeochemical processes that govern Si dynamics in soils, the magnitude of Si release, and Si-cycling between soil and plant systems. In this context, an attempt was made to elucidate the status and distribution of Si pools in major rice-growing soils of Kerala.&#x0D; Methodology: The soil samples were collected from five different locations representing the major rice growing tracts (Kuttand, Kole, Pokkali, sandy and lateritic soils) of Kerala, India. The processed soil samples were analyzed to estimate various physicochemical properties. Then, using standard procedure, different fractions of Si in these soils were estimated and their correlation with physico-chemical properties of soils were worked out.&#x0D; Results: The forms of Si estimated in these soils were mobile Si, adsorbed Si, organic Si, amorphous Si, and residual Si. The percentage distributions of Si pools in these soils were mainly in the order; of residual Si &gt; amorphous Si &gt; occluded Si &gt; organic Si &gt; mobile Si &gt; adsorbed Si. Mobile and adsorbed Si were the smallest fractions and amorphous Si was the largest fraction in these soils.&#x0D; Conclusion: Mobile and occluded Si were found to be the major contributors of PAS in major rice soils of Kerala.

Sodium accumulation vs. nitrate leaching under different fertigation regimes in greenhouse soils in South Uruguay

In greenhouse conditions, soil salinity and N leaching depend on the provision of irrigation, the irrigation water quality and the application of fertilizers and organic amendments. The objective of this study was to quantify and analyze the accumulation and/or leaching process of NO3- and Na+ in greenhouse tomato production in the south region of Uruguay in fine-textured soil under different fertigation regimes. The study was conducted in four tomato crops during 2019/20/21 seasons. Three fertigation regimes were applied. Irrigation volume was the same for all treatments. Drainage was determined by using free drainage lysimeters. Concentration in soil solution and leaching of NO3- and Na+ was measured by monitoring soil solution and drainage solution. Yield, N uptake and N utilization efficiency were determined for each treatment. Soil total drainage was the main factor explaining N and Na+ leaching. The leaching of N ranges from 0 to 23.4 kg N ha-1 per tomato crop with total drainage between 0 and 46.2 % of total irrigation. Drainage necessary to avoid Na+ accumulation was 13 % of total irrigation. This drainage produced 8.4 kg of N leaching per ha-1 during tomato cropping period. Optimizing irrigation is the key factor to the salinity-nitrogen leaching paradox. Irrigation amount and timing should attempt: (1) to avoid excessive irrigation when NO3- concentration in soil solution is high, and (2) to apply leaching irrigation when Na+ concentration in soil solution is high. Soil solution monitoring with suction probes and rapid chemical analysis systems could be a useful tool to identify periods of high risk of N leaching and the right time for leaching irrigation.

Impacts of atmospheric deposition on the heavy metal mobilization and bioavailability in soils amended by lime

Atmospheric deposition is an important source of heavy metal in agricultural soils, but there is limited research on the mobility of these metals in soil and their impact on soil amendment. Here, we performed a dust incubation experiment in soils in the laboratory and a factorial transplant experiment at three field sites with a gradient of atmospheric deposition to examine the impacts of atmospherically deposited heavy metals (Cu, Cd, and Pb) on the mobility and bioavailability in soils with and without lime applications. Results showed that the atmospherically deposited heavy metals showed high mobility and were primarily presented in the soluble ionic fractions in the wet part and acid-exchangeable and reducible fractions in the dry part of atmospheric deposition. Atmospheric dust addition caused the 2p3/2 and 2p1/2 electrons of Cu atoms in uncontaminated soils to transition the 3d vacant states, resulting in similar copper absorption peaks as atmospheric particles by the observation of X-ray absorption near-edge spectroscopy (XANES). In the field, atmospheric deposition can only increase the mobile fractions in the surface soils, but not in the deeper layers. However, the deposition can increase the soluble and diffusive gradients in thin films (DGT)-measured bioavailable fractions in profile along with the soil depth. Lime applications cannot significantly reduce the mobile fractions of heavy metals in the surface soils exposed to atmospheric deposition, but significantly reduce the heavy metal concentrations in soil solutions and the DGT-measured bioavailable concentrations, particularly in the deeper layer (6–10 cm). The major implication is that atmospherically deposited heavy metals can significantly increase their bioavailable concentrations in the plough horizon of soil and constrain the effects of soil amendments on heavy metal immobilization, thereby increasing the risks of crop uptake.