Acidic Red Soil Research Articles

Cow manure and cow manure-derived biochar application as a soil amendment for reducing cadmium availability and accumulation by Brassica chinensis L. in acidic red soil

Organic amendment is a promising, in situ phytostabilization approach to alleviate the phytotoxic effects of heavy metal contaminated soils. The aim of this study was to evaluate the feasibility of cow manure (CM) and its derived biochar (CMB) as a soil amendment on cadmium (Cd) availability and accumulation in low and high Cd-accumulating cultivars of Brassica chinensis L. grown in an acidic red soil. CM and CMB were applied to Cd-contaminated acidic red soil at the rates of 0, 3.0 and 6.0% (w/w). Application of CMB was significantly more effective than that of CM, as it reduced the availability of Cd in soil by 34.3-69.9% and its bioaccumulation in the low Cd accumulator, Aijiaoheiye 333, by 51.2 and 67.4%, respectively. The addition of CMB significantly increased the extractability and accumulation of trace metals (Zn, Mn, Fe, and Cu) by plants and improved plant biomass production. CMB application, combined with utilizing low Cd accumulating cultivars represents a new, sustainable strategy to alleviate the toxic effects on Cd and improve food safety.

Quantificational effect of reforestation to soil erosion in subtropical monsoon regions with acid red soil by sediment fingerprinting

Reforestation and afforestation have been implemented over large regions in China for decades, and this has significantly reduced dust storms, desertification and soil/water erosion in China. However, the quantificational evaluation of the reforestation program in China is still incomplete, especially in the southwest China, due to the limitation of survey data. In order to assess the effects of reforestation on soil erosion in southwest China, a subtropical monsoon region with acid red soil, a representative watershed (Dianchi, Yunnan, in southwest China), was selected to study the effect of reforestation on soil erosion over nearly five decades. The land surface soil samples and sedimentary core in this watershed were examined in 2012 to evaluate the soil erosion and sediment deposition during the period of deforestation and reforestation. The results from the multi-source indexes (nutrients, radionuclides, metallic and isotope elements) measurement from soil samples and analyses (composite fingerprinting, remote sensing and geographic information system) indicate that reforestation significantly reduced the presence of sand from surrounding basin via the sedimentary records. Natural forests had the greatest soil conservation capacity, followed by economic forests and farmland, according to a composite fingerprinting. The distribution of total phosphorus, total nitrogen (TN), total organic carbon (TOC) and isotope carbon (δ13CSOC) presents a high consistency to the results of the composite fingerprinting. The loss rate of TN and TOC in farmland is much greater than that in forest. Reforestation will thus increase the storage of TN and TOC in the long term, although this storage is lower than that in farmland during a short time due to the application of fertilizers. The logging of economic forests will significantly increase soil erosion in this region because of low grass coverage. Recommendations include limiting the period of logging in economic forests to periods outside the rainy season and intercropping other kinds of indigenous trees or grasses to reduce soil erosion.

Kinetic modeling of antimony(III) oxidation and sorption in soils

Kinetic batch and saturated column experiments were performed to study the oxidation, adsorption and transport of Sb(III) in two soils with contrasting properties. Kinetic and column experiment results clearly demonstrated the extensive oxidation of Sb(III) in soils, and this can in return influence the adsorption and transport of Sb. Both sorption capacity and kinetic oxidation rate were much higher in calcareous Huanjiang soil than in acid red Yingtan soil. The results indicate that soil serve as a catalyst in promoting oxidation of Sb(III) even under anaerobic conditions. A PHREEQC model with kinetic formulations was developed to simulate the oxidation, sorption and transport of Sb(III) in soils. The model successfully described Sb(III) oxidation and sorption data in kinetic batch experiment. It was less successful in simulating the reactive transport of Sb(III) in soil columns. Additional processes such as colloid facilitated transport need to be quantified and considered in the model.

Isolation, Identification and Characterization of Two Aluminum-Tolerant Fungi from Acidic Red Soil.

Acidic red soil from a forest in Jiangxi Province was selected to isolate aluminum (Al)-resistant microbes, from which eight fungi were isolated. Two strains (S4 and S7) were found to be extremely tolerant to Al concentrations of up to 550mmolL(-1) and could grow at low pH levels (3.20-3.11). Morphological and 26S rDNA sequence analyses indicated that strain S4 belonged to Eupenicillium, while strain S7 was an unclassified Trichocomaceae. Further investigation showed that both strains were endowed with the ability to resist Al; strain S4 accumulated such a substantial amount of Al that its growth was limited to a larger extent than strain S7. The lower amounts of Al adsorbed in the mycelium and the much larger amounts of Al retained in the medium, in addition to the color change of the culture solution, implied that these two strains may resist Al by preventing Al from entering the cell and by chelating Al by secreting unique metabolites outside of the cell.

Cumulative effects of repeated chlorothalonil application on soil microbial activity and community in contrasting soils

Chlorothalonil (CTN) has received much attention due to its broad-spectrum antifungal function and repeated applications in agriculture production practice. An incubation experiment was conducted to study the accumulating effects of CTN repeated application on soil microbial activities, biomass, and community and to contrast the discrepancy of effects in contrasting soils. Different dosage CTN (5 mg kg−1, T1, and 25 mg kg−1, T5) was applied into two contrasting soils at 7-day intervals. Soil samples were taken 7 days after each application to assess soil enzyme activities and gene abundances. At the end of incubation, the soil samples were also taken to analyze microbial communities in the two test soils. Soil fluorescein diacetate hydrolysis (FDAH) and urease activities were inhibited by CTN repeated applications. After 28 days of incubation, bacterial 16S rRNA gene abundances in T1 and T5 treatments were significantly lower than those in the CK treatments (46.4 and 36.6 % of the CK treatment in acidic red soil, 53.6 and 37.9 % of the CK treatment in paddy soil). Archaeal 16S rRNA gene abundances of T1 and T5 treatments were observed the similar trends (56.1 and 40.8 % of the CK treatment in acidic red soil, 45.6 and 43.7 % of the CK treatment in paddy soil). Repeated applications at 25 mg kg−1 exerted significantly negative effects on the Shannon-Weaver, Simpson and McIntosh indices. Microbial activity, biomass, and functional diversity were significantly inhibited by repeated CTN application at the higher dosage (25 mg kg−1), but the inhibitory effects by the application at the recommended dosage (5 mg kg−1) were erratic. More emphasis needs to be placed on the soil type and cumulative toxicity from repeated CTN application when assessing environmental risk.

Effect of biochar on the growth of Poncirus trifoliata (L.) Raf. seedlings in Gannan acidic red soil

ABSTRACTSoil acidification has become a serious problem for citrus cultivation in China. As a soil amendment, biochar is expected to increase soil pH as well as soil fertility. In this study, we assessed the effect of biochar on Trifoliate orange, the most frequently used citrus rootstock, in a pot experiment using acidic red soil from the Gannan citrus production area. Plant height and shoot diameter of Poncirus trifoliata (L.) Raf. seedlings increased significantly after biochar was added to soils. This positive effect was further evidenced by the increased plant biomass and leaf net photosynthetic rate. The root system architecture (RSA) was evaluated based on root length, root surface area, root volume and root tip. Biochar amendment significantly increased the total absorptive surface area of the root system. Due to the significant role of arbuscular mycorrhizal fungi (AMF) in citrus root nutrient uptake, the AMF colonization and community in Poncirus roots were investigated. The AMF colonization rate was not significantly affected by biochar, whereas AMF diversity increased upon biochar treatment. In addition, the biochar treatment resulted in increases in soil pH, organic matter and mineral nutrients. Together, our results suggest that the positive effects of biochar on the growth performance of Poncirus seedlings can be attributed to the substantial augmentation of soil fertility, increased soil pH, optimized RSA and improved AMF species composition.

Effect of biochar amendment on soil physical, chemical and biological properties and groundnut yield in rainfed Alfisol of semi-arid tropics

ABSTRACTThe aim of this study was to evaluate the effect of biochar and organic soil amendments on soil physicochemical and microbial load, carbon sequestration potential, nutrient uptake and yield of groundnut in acidic red soil under rainfed condition. Biochar was prepared from red gram, cotton, maize stalk and mesquite wood using pilot scale slow pyrolysis biochar unit. The above sources of biochar at the rate of 2.5 and 5 t ha−1 and enriched farmyard manure 0.75 t ha−1, composted coir pith 10 t ha−1 and arbuscular mycorrhizae 100 kg ha−1 were applied as basal with required nitrogen, phosphorous and potassium fertilizer. Biochar amendment at the rate of 5 t ha−1 reduced the bulk density from 1.41 to 1.36 g cm−3 and increased the soil moisture 2.5%. With respect to soil chemical changes, it raised soil pH from 5.7 to 6.3; increased the cation exchange capacity 1.4 cmol+ kg−1 and enhanced the carbon buildup 4.4 t ha−1. The significant differences in bacteria, fungi and actinomycetes population were observed between biochar and control. The nitrogen, phosphorous and potassium were better utilized under biochar and composted coir pith, which was 21, 5 and 20 kg ha−1 higher than control. The experimental results suggested that application of biochar to acidic red soil favoured good soil physical, chemical and biological environment, and these positive changes influenced growth and yield attributes and enhanced pod yield 29% over control.

Swine manure and quicklime have different impacts on chemical properties and composition of bacterial communities of an acidic soil

Red soil is a kind of Ferralic Cambisol. Good hydrothermal resources but usually acidic and low base saturation make it very easy to be acidified under unreasonable fertilizer managements in agricultural area. In 1990, a series of fertilizer applications were established on a homogeneous field to investigate the soil fertility changes under different fertilizations in Hunan province, among them, long-term chemical nitrogen fertilization (NPK) induced severe soil acidification, whereas long-term chemical plus swine manure fertilization (NPKM) exhibited an overwhelming advantage in terms of nutrient concentration and bacterial diversity. In 2010, to investigate the effect of short-term quicklime and swine manure applications on the acid red soil, a remediation experiment was established on the NPK treatment. Quicklime (CaO) and swine manure (the same as the swine manure in NPKM) applications on NPK soil had different effects on remediating the soil acidification: short-term quicklime application (NPK+Ca) increased soil pH and bacterial diversity more rapidly, whereas short-term swine manure application (NPK+M) improved soil nutrient concentration more effectively. Alteration of environmental conditions caused significant changes of bacterial community: oligotrophic groups were in greater relative abundances with quicklime treatment, whereas copiotrophic groups were in greater relative abundances with swine manure treatment. This study provides insights into the two major remediation approaches for an acidic red soil; these insights are important for soil remediating and improving fertility. Taken together, we recommended short-term quicklime application to increase the soil pH quickly and long-term swine manure fertilization to improve soil fertility.

Characterization of Diastereo- and Enantioselectivity in Degradation of Synthetic Pyrethroids in Soils.

Permethrin (PM), cypermethrin (CP), and cyfluthrin (CF) are three important synthetic pyrethroids, which contain two, four, and four enantiomeric pairs (diastereomers) and thus have four, eight, and eight stereoisomers, respectively. In this study, the stereo- and enantioselective degradation of PM, CP, and CF in a Shijiazhuang alkaline yellow soil and a Wuhan acidic red soil were studied in detail by a combination of achiral and chiral high-performance liquid chromatography (HPLC). The results showed that PM, CP, and CF degraded faster in Shijiazhuang soil than in Wuhan soil, and the dissipation rate followed an order of PM > CF > CP in both soils. The three pyrethroids exhibited similar diastereomer selectivity, while CP and CF showed higher enantioselectivity than PM. Moreover, the trans-diastereomers degraded faster, and showed higher enantioselectivity than the corresponding cis-diastereomers. For PM, the enantiomer 1S-trans-PM degraded most rapidly in both soils. As for CP and CF, the highest enantioselectivity was observed for diastereomer trans-3, and the insecticidally active enantiomer 1R-trans-αS degraded fastest among the 8 CP or CF stereoisomers in both soils. In addition, the Wuhan acidic soil displayed higher diastereomer and enantiomer selectivity than the Shijiazhuang alkaline soil for the three pyrethroids. Further incubation of CF in an alkaline-treated Wuhan soil showed that the dissipation rate greatly increased and the diastereo- and enantioselectivity significantly decreased after the alkaline treatment process.

Effect of Temperature on Corrosion Behavior of X80 Pipeline Steel in Acidic Soil

Corrosion of X80 pipeline steel in an acidic red soil collected from Yingtan area of Southeast China was studied in a temperature range 20~75 ℃ by electrochemical impedance spec- troscopy (EIS). Theory concerning corrosion dynamics and transition state was applied to analyze the process and kinetics of the corrosion reaction. The results show that EIS of X80 steel in the red soil contains a capacitive arc related with soil at the high frequency region and a capacitive arc from the interface process at low frequency region. With increasing temperature, the soil resistivity and charge transfer resistance decrease, and the corrosion rate increases. Kinetics analysis shows that corrosion of X80 steel in the acidic red soil is an endothermic reaction system companied with decrease of disorder degree.

Long-term application of organic manure changes abundance and composition of ammonia-oxidizing archaea in an acidic red soil

Ammonia-oxidizing archaea (AOA) have more importance in ammonia oxidation than ammonia-oxidizing bacteria (AOB) in acidic red soils. The aim of this study was to investigate if the abundance and composition of AOA could be altered by long-term application of organic manure in an acidic red soil. The abundance and composition of AOA were evaluated by polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) targeting archaeal amoA genes after long-term (24-year) application of mineral fertilizer and/or organic manure. The treatments were: non-fertilized control, mineral nitrogen (N) fertilizer only, mineral N, phosphorus (P) and potassium (K) fertilizer only, organic manure only, and organic manure plus mineral NPK fertilizer. The abundance of archaeal amoA genes was significantly increased after the long-term application of organic manures, either with or without mineral NPK fertilizer. So were the Shannon and Richness diversity indices of AOA deduced from the DGGE patterns. Phylogenetic analyses showed that most of the AOA sequences from various fertilization treatments were affiliated with group 1.1b thaumarchaea and only one with the group 1.1a-associated thaumarchaea. Nitrification potential was significantly increased after the long-term application of organic manures in comparison with the non-fertilized control. Our results strengthened the importance of organic manure in promoting the growth of AOA and thus nitrification potential in the acidic red soils.

Analysis of the Long-Term Corrosion Behavior of X80 Pipeline Steel in Acidic Red Soil Using Electrical Resistance Test Technique

The long-term corrosion rate of X80 steel in an acidic red soil was monitored in situ by using a precise electrical resistance (ER) test system. The corrosion characteristics of X80 steel were examined via SEM, EDS, and XRD. The results indicated that the corrosion rate determined from ER test was very similar to that obtained from the mass loss test. The ER test technique made it possible to predict the long-term corrosion rate of steel in soil in situ. The corrosion rate of X80 steel in acidic red soil was about 0.0902 mm/a at 38 weeks, but the corrosion rate was dropped to 0.0226 mm/a after 5 years. The final corrosion product layer was composed mainly of FeOOH,γ-Fe2O3, and FeCO3.

Anoxic Corrosion Behavior of Pipeline Steel in Acidic Soils

Acidic red soils are highly corrosive toward carbon steel, but the corrosive nature of this particular soil has not been well documented. Electrochemical techniques including Tafel plots, electrochemical impedance spectroscopy, and weigh loss test were used to characterize the properties and nature of pipeline corrosion in a typical acidic red soil at different water content values. The results show that corrosion rate and type and intensity of corrosion attack are significantly dependent on the soil water content and hence the soil moisture droplet or film absorbed on the steel surface. The corrosion rate increases with the water content up to ca. 30% soil water content (100% water-holding capacity), and it decreases with further increases in water content in the long-term test. The evolution of a kinetic parameter and a “Fe oxide facilitating corrosion” mechanism is proposed and discussed for the anaerobic corrosion of steel in acidic soils.

Electrochemical Behavior of X80 Pipeline Steel in the Initial Stage of Corrosion in an Acidic Red Soil

Corrosion of X80 pipeline steel in an acidic red soil slurry was studied by means of electrochemical impedance spectroscopy(EIS) and polarization technique. The results show that in a water-saturated red soil, the EIS of X80 steel contains a capacitive arc at high frequency and a capacitive arc from the interface process in frequency region of 10-2~104Hz as well as an inductive loop appears at low frequency exhibiting processes related with adsorption or pitting nucleation;the electrode process was controlled by diffusion process after 30 d exposure; iron oxides residing in the soil tend to enhance the corrosion process of the steel, correspondingly a corrosion mechanism has been proposed.

Effect of Biochar Addition on Maize Growth and Nitrogen Use Efficiency in Acidic Red Soils

Biochar added to soil can improve crop growth through both direct and indirect effects, particularly in acidic, highly weathered soils in subtropical and tropical regions. However, the mechanisms of biochar improving crop growth are not well understood. The objectives of this study were i) to determine the crop responses to biochar addition and ii) to understand the effect of biochar addition on N use efficiency. Seven acidic red soils varying in texture, pH, and soil nutrient were taken from southern China and subjected to four treatments: zero biochar and fertilizer as a control (CK), 10 g kg−4 biochar (BC), NPK fertilizers (NPK), and 10 g kg−1 biochar plus NPK fertilizers (BC+NPK). 15N-labeled fertilizer was used as a tracer to assess N use efficiency. After a 46-d pot experiment, biochar addition increased soil pH and available P, and decreased soil exchangable Al3+, but did not impact soil availabe N and cation exchange capacity (P > 0.05). The N use efficiency and N retained in the soil were not significantly affected by biochar application except for the soil with the lowest available P (3.81 mg kg−1) and highest exchanageable Al3+ (4.54 cmol kg−1). Greater maize biomass was observed in all soils amended with biochar compared to soils without biochar (BC vs. CK, BC+NPK vs. NPK). This agronomic effect was negatively related to the concentration of soil exchangeable Al3+ (P < 0.1). The results of this study implied that the liming effect of biochar improved plant growth through alleviating Al toxicity and P deficiency, especially in poor acidic red soils.

Activity of earthworm in Latosol under simulated acid rain stress.

Acid rain is still an issue of environmental concerns. This study investigated the impacts of simulated acid rain (SAR) upon earthworm activity from the Latosol (acidic red soil). Laboratory experiment was performed by leaching the soil columns grown with earthworms (Eisenia fetida) at the SAR pH levels ranged from 2.0 to 6.5 over a 34-day period. Results showed that earthworms tended to escape from the soil and eventually died for the SAR at pH = 2.0 as a result of acid toxicity. The catalase activity in the earthworms decreased with the SAR pH levels, whereas the superoxide dismutases activity in the earthworms showed a fluctuate pattern: decreasing from pH 6.5 to 5.0 and increasing from pH 5.0 to 4.0. Results implied that the growth of earthworms was retarded at the SAR pH ≤ 3.0.

Effects of inorganic and organic amendments on the uptake of lead and trace elements by Brassica chinensis grown in an acidic red soil

A greenhouse study was conducted to investigate the effects of inorganic (phosphate rock, single superphosphate and calcium magnesium phosphate) and organic amendments (peat, straw manure and pig manure) on the uptake of lead (Pb) and trace elements by Chinese Cabbage (Brassica chinensis) grown in an acidic red soil. The application of all organic amendments increased the soil pH while inorganic amendments such as single superphosphate did not. Both inorganic and organic amendments decreased the availability and uptake of Pb while the organic amendments were superior to the inorganic (phosphate) amendments in reducing the availability of the more labile (soluble and exchangeable Pb) forms of soil Pb. More Pb was taken up by roots than shoots with all soil amendments. Among the organic amendments, straw manure and pig manure caused the largest decrease in Pb availability at 456.5 and 457.3mgkg−1, respectively, when a high level of 30g organic amendments kg−1 was applied. The organic amendments greatly increased the fraction D targeted to Fe–Mn oxides bound Pb, and decreased the fraction A (water-soluble), B (exchangeable), and C (carbonate-bound), thereby decreasing the solubility and mobility of Pb in soil. The organic amendments also significantly improved the concentrations of Fe, Mn, Cu and Zn in the soil and shoots (except Fe in shoots and/or roots), which are essential for plant nutrition. The organic amendments of straw and pig manure lowered the availability and uptake of Pb but not that of other trace metals. Thus, these amendments have the potential to remediate Pb-contaminated soils in situ.

Kinetic modeling of antimony(V) adsorption–desorption and transport in soils

Antimonate [Sb(V)] adsorption–desorption and transport in an acidic red soil (Yingtan) and a calcareous soil (Huanjiang) was investigated using kinetic batch and miscible displacement experiments. Different formulations of a multi-reaction model (MRM) were evaluated for their capabilities of describing the retention and transport mechanisms of Sb(V) in soils. The experimental results showed that adsorption of Sb(V) by two soils was kinetically controlled and largely irreversible. The Sb(V) adsorption capacity and kinetic rate of the acidic red soil was much higher than that of the calcareous soil. The asymmetrical breakthrough curves indicated the strong dominance of non-equilibrium retention of Sb(V). A four step sequential extraction procedure provided evidence that majority of applied Sb(V) was irreversibly retained. A formulation of MRM with two kinetic sorption sites (reversible and irreversible) successfully described Sb(V) adsorption–desorption data. The use of kinetic batch rate coefficients for predictions of breakthrough curves (BTCs) underestimated Sb(V) retention and overestimated its mobility. In an inverse mode with optimized rate coefficients, the MRM formulation was capable of simulating Sb(V) transport in soil columns.

Nitrification and acidification from urea application in red soil (Ferralic Cambisol) after different long-term fertilization treatments

Long-term manure applications can prevent or reverse soil acidification by chemical nitrogen (N) fertilizer. However, the resistance to re-acidification from further chemical fertilization is unknown. The aim of this study was to examine the effect of urea application on nitrification and acidification processes in an acid red soil (Ferralic Cambisol) after long-term different field fertilization treatments. Soils were collected from six treatments of a 19-year field trial: (1) non-fertilization control, (2) chemical phosphorus and potassium (PK), (3) chemical N only (N), (4) chemical N, P, and K (NPK), (5) pig manure only (M), and (6) NPK plus M (NPKM; 70 % N from M). In a 35-day laboratory incubation experiment, the soils were incubated and examined for changes in pH, NH4 +, and NO3 −, and their correlations from urea application at 80 mg N kg−1(−80) compared to 0 rate (−0). From urea addition, manure-treated soils exhibited the highest acidification and nitrification rates due to high soil pH (5.75–6.38) and the lowest in the chemical N treated soils due to low soil pH (3.83–3.90) with no N-treated soils (pH 4.98–5.12) fell between. By day 35, soil pH decreased to 5.21 and 5.81 (0.54 and 0.57 unit decrease) in the NPKM-80 and M-80 treatments, respectively, and to 4.69 and 4.53 (0.43 and 0.45 unit decrease) in the control-80 and PK-80 treatments, respectively, with no changes in the N-80 and NPK-80 treatments. The soil pH decrease was highly correlated with nitrification potential, and the estimated net proton released. The maximum nitrification rates (K max) of NPKM and M soils (14.7 and 21.6 mg N kg−1 day−1, respectively) were significantly higher than other treatments (2.86–3.48 mg N kg−1 day−1). The priming effect on mineralization of organic N was high in manure treated soils. Field data have shown clearly that manure amendment can prevent or reverse the acidification of the red soil. When a chemical fertilizer such as urea is applied to the soil again, however, soil acidification will occur at possibly high rates. Thus, the strategy in soil N management is continuous incorporation of manure to prevent acidification to maintain soil productivity. Further studies under field conditions are needed to provide more accurate assessments on acidification rate from chemical N fertilizer applications.

Ammonia oxidation-dependent growth of group I.1b Thaumarchaeota in acidic red soil microcosms.

Accumulating evidence suggests that Thaumarchaeota may control nitrification in acidic soils. However, the composition of the thaumarchaeotal communities and their functioning is not well known. Therefore, we studied nitrification activity in relation to abundance and composition of Thaumarchaeota in an acidic red soil from China, using microcosms incubated with and without cellulose amendment. Cellulose was selected to simulate the input of crop residues used to increase soil fertility by local farming. Accumulation of NO3-(-N) was correlated with the growth of Thaumarchaeota as determined by qPCR of 16S rRNA and ammonia monooxygenase (amoA) genes. Both nitrification activity and thaumarchaeotal growth were inhibited by acetylene. They were also inhibited by cellulose amendment, possibly due to the depletion of ammonium by enhanced heterotrophic assimilation. These results indicated that growth of Thaumarchaeota was dependent on ammonia oxidation. The thaumarchaeotal 16S rRNA gene sequences in the red soil were dominated by a clade related to soil fosmid clone 29i4 within the group I.1b, which is widely distributed but so far uncultured. The archaeal amoA sequences were mainly related to the Nitrososphaera sister cluster. These observations suggest that fosmid clone 29i4 and Nitrososphaera sister cluster represent the same group of Thaumarchaeota and dominate ammonia oxidation in acidic red soil.