Conversion of forest to agricultural land affects the relative contribution of bacteria and fungi to nitrification in humid subtropical soils

Land-use and management practices can affect soil nitrification. However, nitrifying microorganisms responsible for specific nitrification process under different land-use soils remains unknown. Thus, we investigated the relative contribution of bacteria and fungi to specific soil nitrification in different land-use soils (coniferous forest, upland fields planted with corn and rice paddy) in humid subtropical region in China. ¹⁵N dilution technique in combination with selective biomass inhibitors and C₂H₂ inhibition method were used to estimate the relative contribution of bacteria and fungi to heterotrophic nitrification and autotrophic nitrification in the different land-use soils in humid subtropical region. The results showed that autotrophic nitrification was the predominant nitrification process in the two agricultural soils (upland and paddy), while the nitrate production was mainly from heterotrophic nitrification in the acid forest soil. In the upland soils, streptomycin reduced autotrophic nitrification by 94%, whereas cycloheximide had no effect on autotrophic nitrification, indicating that autotrophic nitrification was mainly driven by bacteria. However, the opposite was true in another agricultural soil (paddy), indicating that fungi contributed to the oxidation of NH₄⁺ to NO₃^?. In the acid forest soil, cycloheximide, but not streptomycin, inhibited heterotrophic nitrification, demonstrating that fungi controlled the heterotrophic nitrification. The conversion of forest to agricultural soils resulted in a shift from fungi-dominated heterotrophic nitrification to bacteria- or fungi-dominated autotrophic nitrification. Our results suggest that land-use and management practices, such as the application of N fertilizer and lime, the long-term waterflooding during rice growth, straw return after harvest, and cultivation could markedly influence the relative contribution of bacteria and fungi to specific soil nitrification processes.     

Heterotrophy-coordinated diazotrophy is associated with significant changes of rare taxa in soil microbiome

Soil heterotrophic respiration during decomposition of carbon (C)-rich organic matter plays a vital role in sustaining soil fertility. However, it remains poorly understood whether dinitrogen (N₂) fixation occurs in support of soil heterotrophic respiration. In this study, ¹⁵N₂-tracing indicated that strong N₂ fixation occurred during heterotrophic respiration of carbon-rich glucose. Soil organic ¹⁵N increased from 0.37 atom% to 2.50 atom% under aerobic conditions and to 4.23 atom% under anaerobic conditions, while the concomitant CO₂ flux increased by 12.0-fold under aerobic conditions and 5.18-fold under anaerobic conditions. Soil N₂ fixation was completely absent in soils replete with inorganic N, although soil N bioavailability did not alter soil respiration. High-throughput sequencing of the 16S rRNA gene further indicated that: i) under aerobic conditions, only 15.2% of soil microbiome responded positively to glucose addition, and these responses were significantly associated with soil respiration and N₂ fixation and ii) under anaerobic conditions, the percentage of responses was even lower at 5.70%. Intriguingly, more than 95% of these responses were originally rare with < 0.5% relative abundance in background soils, including typical N₂-fixing heterotrophs such as Azotobacter and Clostridium and well-recognized non-N₂-fixing heterotrophs such as Sporosarcina, Agromyces, and Sedimentibacter. These results suggest that only a small portion of the soil microbiome could respond quickly to the amendment of readily accessible organic C in a fluvo-aquic soil and highlighted that rare phylotypes might have played more important roles than previously appreciated in catalyzing soil C and nitrogen turnovers. Our study indicates that N₂ fixation could be closely associated with microbial turnover of soil organic C when available in excess.     

Gaseous nitrogen losses from fertilized soil during nitrification and denitrification using the acetylene blockage technique

Summary A sandy soil amended with different forms and amounts of fertilizer nitrogen (urea, ammonium sulphate and potassium nitrate) was investigated in model experiments for N₂O emission, which may be evolved during both oxidation of ammonia to nitrate and anaerobic respiration of nitrate. Since C₂H₂ inhibits both nitrification and the reduction of N₂O to N₂ during denitrification, the amount of N₂O evolved in the presence and absence of C₂H₂ represents the nitrogen released through nitrification and denitrification.Results show that amounts of N₂O-N lost from soils incubated anaerobically with 0.1% C₂H₂ and treated with potassium nitrate (23.1 µg N-NO ₃ ^– /g dry soil) exceeded those from soils incubated in the presence of 20% oxygen and treated with even larger amounts of nitrogen as urea and ammonium sulphate. This indicates that nitrogen losses by denitrification may potentially be higher than those occurring through nitrification.     

Response of denitrifying communities to successive soil freeze–thaw cycles

The effect of soil freeze–thaw cycles on the denitrification potential was examined based on the C₂H₂ inhibition method. The gross N₂O production curve of the soil sample (incubation with C₂H₂) showed minor changes between the freeze–thaw treatment and the unfrozen control. However, kinetics analysis revealed that the initial production rate, an indicator of the population density of denitrifying communities, decreased (P = 0.043) and the specific growth rate constant, an indicator of the activity of denitrifying communities, increased (P = 0.039) as a result of the freeze–thaw cycles in five of six soil samples examined. The increase in the specific growth rate constant suggested the stimulation of the activity of denitrifying communities that survived after the freeze–thaw cycles and may explain the minor suppression on the gross N₂O production in spite of decreasing the population density of denitrifying communities that was suggested by the initial production rate. The net N₂O production curve of the soil sample (incubation without C₂H₂) showed a remarkable change in one out of six soil samples, and in that one soil sample, N₂O release to the atmosphere was largely stimulated (7.6 times) by the freeze–thaw cycles. However, the stimulation of the N₂O release by the freeze–thaw cycles was even observed in two other selected soil samples (4.6 and 1.8 times), suggesting that an imbalance in the N₂O-producing and N₂O-reducing activities of denitrifying communities might complementally explain the N₂O release stimulated by the freeze–thaw cycles.     

Hydrogen oxidation in soil following rhizobial H2 production due to N2 fixation by a Vicia faba-Rhizobium leguminosarum symbiosis

Summary The rate of H₂ release from broad beans (Vicia faba) infected with Rhizobium leguminosarum Hup^- was much faster than from beans infected with the Hup⁺ strain. Acetylene reduction and H₂ release were abolished by cutting the plants down, by incubation in darkness, or after the addition of ammonium, indicating that the H₂ was released by N₂-fixing bacterial symbionts. In laboratory cultures using non-sterile soil, the bean plants released H₂ until an equilibrium between H₂ production and H₂ oxidation was reached. The H₂ equilibrium concentration was higher in Hup^--infected bean cultures (about 3 ppm H₂ in the gas phase) than in Hup⁺-infected cultures (0.3 ppm H₂) because of the higher H₂ production. The H₂ release from Hup^--infected bean cultures in sterile soil did not reach equilibrium. An equilibrium occurred, if Knallgas bacteria were added. However, the equilibrium value was higher (13 ppm H₂) than in non-sterile soil, which seemed to be more efficient at H₂ oxidation. The Knallgas bacteria exhibited a relatively high K _m for H₂ (> 1300 ppmv H₂); this activity was observed in unplanted non-sterile soil, and in nonsterile soil planted with Hup⁺-infected beans or planted with Hup^--infected beans which had been cut down before being assayed. All these soils also showed a second, low-K _m (<50 ppm) level of H₂ oxidation activity, which was presumably due to abiontic soil enzymes. In contrast, only one level of activity, which had an intermediate K _m (about 200 ppm H₂), was observed when the soil was planted with Hup^--infected beans. The origin of this activity, which was only observed in the presence of intact, H₂-producing beans, is still unknown.     

Effect of long-term field exposure of soil to acetylene on nitrification,denitrification, and acetylene consumption

Coated CaC₂ is a newly developed product which can supply nitrification-inhibiting quantities of C₂H₂ (1–10 Pa) to the soil, throughout a cropping season. This method of applying C₂H₂ to the soil maintains C₂H₂ in the soil continuously for several months. It is not know whether these low C₂H₂ concentrations alter soil microbial processes. A field study was initiated to determine the effect of supplying C₂H₂ to a clay soil, using coated CaC₂, on soil respiration, denitrification, nitrification, and C₂H₂ consumption. The C₂H₂ consumption rate increased with length of soil exposure to C₂H₂ (r ²=0.59). The rates of CO₂ production (r ²=0.88) and denitrification (r ²=0.86) were both highly correlated with the C₂H₂ consumption rates. The nitrifier potential decreased to a minimum of 21% of the control after 3 months of C₂H₂ treatment. After this time, nitrifier activity increased to 41% of the control after 11 months of treatment. This increase was due to increased C₂H₂ consumption in the soil. After 3 months of continuous application of C₂H₂ to the soil, the C₂H₂ concentrations were generally below that necessary to inhibit nitrification. No adaptation to the C₂H₂ by nitrifiers was found. Repeating these measurements 1 year later showed that soils previously exposed to C₂H₂ retained their enhanced C₂H₂ oxidation capacity and the capacity to use C₂H₂ to increase denitrification. Nitrification potentials remained about 50% lower in soils exposed to C₂H₂ a year earlier compared to soils not previously exposed to C₂H₂.     

Air Drying Affects Extractable Sulfate in Soils of Variable Charge: Test of Two Extraction and Two Quantification Methods

Abstract

Fertilizer recommendations need to be based on reliable soil sulfate determinations. Airdrying samples changes irreversibly many properties of soils with variable charge and might affect the extractable sulfate. In this study, sulfate extracted from air‐dry and field‐moist samples was compared. Two extracting solutions [water and 00.1 M Ca(H₂PO₄) ₂] and two quantification methods (turbidimetry and ion chromatography) were assayed on A and B horizon samples of five Humic Acrisols from southeast Mexico. Air drying increased water‐extractable sulfate in Ah horizons, whereas in Bt horizons, it increased the 00.1 M Ca(H₂PO₄)₂‐extractable sulfate. Airdrying increased dissolved organic carbon contents in all samples and increased soil acidity and oxalate extractable iron in 70 and 60% of the samples, respectively. Results showed larger coefficients of variation in air‐dried samples. Turbidimetry resulted less sensible than ion chromatography. To enhance sensitivity and reproducibility, particularly organic soil samples should be analyzed field‐moist and by ion chromatography.     

The effects on N2 fixation (C2H2 reduction), bacterial population and rice plant growth of two modes of straw application to a wetland rice field

Summary The effects of incorporation and surface application of straw to a wetland rice field on nitrogen fixation (C₂H₂ reduction), bacterial population and rice plant growth were studied. Rice straw (5 t ha^–1) was chopped (10- to 15-cm pieces) and applied to the field 2 weeks before transplanting IR42, a long-duration variety, and IR50, a short-duration variety. The acetylene-reducing activity (ARA) of IR42 and IR50 measured at heading stage for 3 consecutive days showed significantly higher ARA in IR42 as a result of the 2 straw application methods. Mostly up to 20 days after straw surface application and incorporation, the dark ARA in the soil, total and N₂-fixing heterotrophs, and photoorganotrophic purple nonsulphur bacteria (POPNS) in the soil and in association with degrading straw were stimulated. Higher bacterial populations were associated with straw on the surface than with straw incorporated. The POPNS counts, in particular, were increased hundreds fold in the surface-applied straw treatment. Straw applications also increased the root, shoot and total plant biomass at heading stage and the total dry matter yield at harvest in both varieties. The data show the potentials of straw as a source of substrate for the production of microbial biomass and for the non-symbiotic N₂ fixation to improve soil fertility and plant nutrition.     

Nitrogen fixation by biological soil crusts and heterotrophic bacteria in an intact Mojave Desert ecosystem with elevated CO2 and added soil carbon

Fixation of N by biological soil crusts and free-living heterotrophic soil microbes provides a significant proportion of ecosystem N in arid lands. To gain a better understanding of how elevated CO₂ may affect N₂-fixation in aridland ecosystems, we measured C₂H₂ reduction as a proxy for nitrogenase activity in biological soil crusts for 2 yr, and in soils either with or without dextrose-C additions for 1 yr, in an intact Mojave Desert ecosystem exposed to elevated CO₂. We also measured crust and soil δ¹⁵N and total N to assess changes in N sources, and δ¹³C of crusts to determine a functional shift in crust species, with elevated CO₂. The mean rate of C₂H₂ reduction by biological soil crusts was 76.9±5.6 μmol C₂H₄ m⁻² h⁻¹. There was no significant CO₂ effect, but crusts from plant interspaces showed high variability in nitrogenase activity with elevated CO₂. Additions of dextrose-C had a positive effect on rates of C₂H₂ reduction in soil. There was no elevated CO₂ effect on soil nitrogenase activity. Plant cover affected soil response to C addition, with the largest response in plant interspaces. The mean rate of C₂H₂ reduction in soils either with or without C additions were 8.5±3.6 μmol C₂H₄ m⁻² h⁻¹ and 4.8±2.1 μmol m⁻² h⁻¹, respectively. Crust and soil δ¹⁵N and δ¹³C values were not affected by CO₂ treatment, but did show an effect of cover type. Crust and soil samples in plant interspaces had the lowest values for both measurements. Analysis of soil and crust [N] and δ¹⁵N data with the Rayleigh distillation model suggests that any plant community changes with elevated CO₂ and concomitant changes in litter composition likely will overwhelm any physiological changes in N₂-fixation.     

Nitrous oxide production at different soil moisture contents in an arable soil in China

Abstract

Laboratory incubations were conducted to investigate nitrous oxide (N₂O) production from a subtropical arable soil (Typic Plinthodults) incubated at different soil moisture contents (SMC) and with different nitrogen sources using a 10% (v/v) acetylene (C₂H₂) inhibitory technique at 25°C. The production of N₂O and CO₂ was monitored during the incubations and changes in the contents of KCl-extractable NO^? ₃-N and NH⁺ ₄-N were determined. The production of N₂O increased slightly with an increase in SMC from 40% water-holding capacity (WHC) to 70% WHC, but increased dramatically at 100% WHC. After incubation the NO^? ₃-N content increased even at a SMC of 100% WHC. At a SMC of 100% WHC, the addition of NH⁺ ₄-N promoted the production of N₂O and CO₂, whereas the addition of NO^? ₃-N decreased N₂O production. Compared with the incubation without C₂H₂, the presence of C₂H₂ increased NH⁺ ₄-N content, but decreased NO^? ₃-N content, and there was no significant difference in N₂O production. These results indicate that heterotrophic nitrification contributes to N₂O production in the soil.     

Microbial activity and bacterial composition of H2-treated soils with net CO2 fixation

The effects of H₂ gas treatment of an agricultural soil cultivated previously with a mixture of clover (Trifolium pratense) and alfalfa (Medicago sativa) on CO₂ dynamics and microbial activity and composition were analyzed. The H₂ emission rate of 250 nmol H₂ g⁻¹ soil h⁻¹ was similar to the upper limit of estimated H₂ amounts emitted from N₂ fixing nodules into the surrounding soil ([Dong, Z., Layzell, D.B., 2001. H₂ oxidation, O₂ uptake and CO₂ fixation in hydrogen treated soil. Plant and Soil 229, 1-12.]). After 1 week of H₂ supply to soil samples simultaneously with H₂ uptake net CO₂ production declined continuously and this finally led to a net CO₂ fixation rate in the H₂-treated soil of 8 nmol CO₂ g⁻¹ soil h⁻¹. The time course of H₂ uptake and CO₂ fixation in the soils corresponded with an increase in microbial activity and biomass of the H₂-treated soil determined by microcalorimetric measurements, fluorescence in situ hybridization analysis (FISH) and DNA staining (DAPI). Shifts in the bacterial community structure caused by the supply of H₂ were recorded. While the H₂ treatment stimulated β-and γ-subclasses of Proteobacteria, it had no significant effect on α-Proteobacteria. In addition, FISH-detectable bacteria of the Cytophaga-Flavobacterium-Bacteroides phylum increased in numbers.     

Application of N2/Ar ratio method for N2 fixation studies in submerged soil

Abstract

Recently there has been developments in the measurement of N₂ fixation due mainly to the C₂H₂ reduction method (1). This method, however, has several disadvantages, especially for submerged soil, and the estimated amount of fixed N₂ on the basis of the C₂H₂ reduction activity is not very reliable. The tracer ¹⁵N₂ technique which gives a reliable estimation of the fixed N₂ is too expensive for common use. Development of an alternative method suitable for submerged soil would therefore be desirable. The present authors expected that the measurement of the ratio N₂/Ar in the soil solution might provide advantages for the estimation of the fixed N₂ in submerged soil.     

Comparison of chemical methods of assessing potentially available organic nitrogen in soil

Abstract

We recently developed two rapid and precise chemical methods of assessing potentially available organic N in soils. One method involves determination of the ammonia‐N produced by steam distillation of the soil sample with pH 11.2 phosphate‐borate buffer solution for 8 min. The other involves determination of the ammonium‐N produced by treatment of the soil sample with 2M KCl solution at 100°C for 4 hours. Studies using 33 Brazilian soils showed that the results obtained by these methods were highly correlated with those obtained by anaerobic and aerobic incubation methods of assessing potentially available organic N in soil.

The two methods were further evaluated by applying them to 30 Iowa soils and by comparing their results and those obtained by other chemical methods with the results of the incubation methods considered to be the best laboratory methods currently available for assessment of potentially available organic N in soil. The chemical methods used included the acid KMnO₄ method, the alkaline KMnO₄ method, the CaCl₂‐autoclave method, and the NaHCO₃ UV method. The incubation methods used involved determination of the ammonium‐N produced by incubation of the soil sample under anaerobic conditions for 1 week or determination of the (ammonium + nitrate + nitrite)‐N produced by incubation of the sample under aerobic conditions for 2 and 12 weeks. The data obtained showed that the results of the two chemical methods evaluated were highly correlated with those obtained by the incubation techniques used for comparison and that the correlations observed with these two methods were higher than those observed with the previously proposed chemical methods. It is concluded that these two rapid and simple methods are the best chemical methods thus far developed for laboratory assessment of potentially available organic N in soil.     

Effect of an increasing carbon: nitrate-N ratio on the reliability of acetylene in blocking the N2O-reductase activity of denitrifying bacteria in soil

Summary In model experiments with a silty loam soil the effect of different C : NO _inf3 ^sup- -N ratios on the reliability of C₂H₂ (1% v/v) in blocking N₂O-reductase activity was examined. The soil was carefully mixed with different amounts of powdered lime leaves (Tilia vulgaris) to obtain organic C contents of about 1.8, 2.3, and 2.8%, and of NO _inf3 ^sup- solution to give C : NO _inf3 ^sup- -N ratios of 84, 107, 130, 156, 200, and 243. The soil samples were incubated in specially modified anaerobic jars (22 days, 25°C, 80% water-holding capacity, He atmosphere) and the atmosphere was analysed for N₂, N₂O, CO₂, and C₂H₂ by gas chromatography at regular intervals. Destruction jars were used to analyse soil NO _inf3 ^sup- , NH ₄ ⁺ and C. The results clearly showed that N₂O-reductase activity was completely blocked by 1% (v/v) C₂H₂ only as long as NO _inf3 ^sup- was present. In the presence of C₂H₂, NO _inf3 ^sup- was apparently entirely converted into N₂O. The C₂H₂ blockage of N₂O-reductase activity ceased earlier in soils with a wide C : NO _inf3 ^sup- -N ratio (156, 200, and 243) than in those with closer C : NO _inf3 ^sup- -N ratios (84, 107, and 130). As soon as NO _inf3 ^sup- was exhausted, N₂O was reduced to N₂ in spite of C₂H₂. The wider the C : NO _inf3 ^sup- -N ratio, the earlier the production of N₂ and the less the reliability of the C₂H₂ blockage. In the untreated control complete inhibition of N₂O-reductase activity by C₂H₂ lasted for 7–12 days. In the field, estimates of total denitrification losses by the C₂H₂ inhibition technique should be considered reliable only as long as NO _inf3 ^sup- is present. Consequently, NO _inf3 ^sup- monitoring in the field is essential, particularly in soils supplied with easily decomposable organic matter.     

Saccharidic compounds as soil redox effectors and their influence on potential N2O production

Cellulose, xylan, and glucose were compared in waterlogged soil as modifying factors of the redox potential (Eh), of the quantity of reducing equivalents, and of the soil capacity to produce N₂O and CO₂. During the study period (168 h) soils supplied with glucose and xylan showed a higher Eh decrease than the control soil and the soil treated with cellulose. In samples taken after 0, 24, 48, and 168 h, the soils supplied with C showed a higher number of reducing equivalents than the control soil did. These quantities were not correlated with Eh values, nor with N₂O production. N₂O production was increased compared with the control soil over the entire experimental period in the glucose-amended soils but only after 48 h in the xylan-amended soils and not until 168 h in the cellulose-treated soils. The CO₂:N₂O ratio was consistently higher than the theoretical value of 2, suggesting that denitrification and CO₂ production via fermentation occurred simultaneously. Moreover, this ratio was highly correlated with the Eh values. We conclude that more research is needed to explain the role of soil redox intensity (Eh) and capacity (quantity of redox species undergoing reduction) in the expression of soil denitrification-fermentation pathways.     

Applied Mn extracted by four methods correlated with Mn concentrations in soybean leaf tissue on a southeastern soil

Abstract

Four extractants for soil Mn were compared for their sensitivity to changes in Mn availability caused by rates and sources of added soil Mn and soil pH variations. Their ability to extract amounts of Mn correlated with plant Mn concentrations was also determined. Two field experiments were conducted on a sandy, high water table soil (Ultic Haplaquod‐Arenic Plinthaquic Paleudult) which included 5 Mn rates, 4 Mn sources and 3 soil pH levels. Soybeans [Glycine max (L.) Merr. cultivar Ransom] were grown and leaf tissue and soils sampled at the late pod‐fill stage. All four extractants separated the high‐ Mn rates, but the small exchange method did not separate the low Mn rates. Few differences were observed among extractants due to Ma sources. The DTPA method was the only procedure to correctly distinguish soil pH levels by showing decreasing extractable Ma with increasing soil pH. Including pH in multiple regressions significantly increased the plant Mn‐soil Mn correlation coefficients. The DTPA method and the 0.1N H₃PO₄ method had the highest correlation coefficients and the double acid method the lowest. The small exchange method was intermediate. Considering all the results, the DTPA was the most promising method for extracting Mn from this sandy, southern Coastal Plain soil.     

Assay system for measurement of denitrification-N loss from ornamental plants potted in peat substrate

An assay system was evaluated for denitrification measurement with potted ornamental plants cultivated in peat substrate (Pelargonium zonale, Euphorbia pulcherrima). Flow-through chambers only enclosing the pot of the plants were considered best for denitrification measurement. Loss of N₂O from the chambers by transport through the plant shoot was negligible with both species. To determine (N₂O + N₂)-N loss, C₂H₂ was applied to inhibit reduction of N₂O. Experiments were conducted with unplanted substrate in closed incubation systems to determine optimum C₂H₂ concentration and pre-treatment duration. Complete inhibition of N₂O reduction in peat substrate was achieved using 1 vol% C₂H₂. However, a concentration of 5 vol% C₂H₂ was chosen for further experiments because C₂H₂ concentrations in flow-through chambers varied. The duration of C₂H₂ pre-treatment (0, 2, 12, 24 h) showed no clear effect on (N₂O + N₂)-N accumulation. However, a pre-treatment duration of 2 h was chosen to guarantee immediate inhibition of N₂O reductase at the start of experiments. Exposure to C₂H₂ gas proved to affect plants of both species. During C₂H₂ exposition in flow-through chambers, the leaves of P. zonale became chlorotic (48 h) and necrotic (72 h). E. pulcherrima showed no chlorosis but did exhibit leaf epinasty (24 h) and wilting (96 h). Transpiration of P. zonale and C availability in the growing medium of both species were not affected by 52 h and 24 h treatments with C₂H₂, respectively. As N emissions usually ended within 38 h of C₂H₂ treatment, it was concluded that side effects of C₂H₂ did not affect denitrification measurements.     

Nitrogen fixation in paddy soils

The 4 long-term experimental plots (Umbric haplaquept) with different fertilizer treatment at Cent. Agric. Exp. Sta. in Konosu City, Saitama Prefecture, were used for the sites of investigation. The 4 plots were NF (applied with no fertilizer), IF (applied with inorganic fertilizers), GM (applied with green manure and CaCO₂), and OM (applied with manure and inorganic fertilizers). Flooded water, floating weed, upper (0-2cm) and lower (2-10cm) parts of Apg horizon and rhizosphere were collected from each plot before flooding, during flooding, and after drainage. These samples were analyzed for N₂-fixing activity by acetylene reduction method, pH, Eh, and contents of Fe²⁺, NH₄ ⁺, chlorophyll-type compounds, and water-soluble carbohydrates.

The N₂-fixing activity of all samples showed almost the same pattern of change with time: very low before flooding, rapidly increased after flooding, the maximum value at the maximum tillering stage of rice plant, declined afterwards and reached a very low value after drainage.

Rough estimation of the “N₂-fixing capacity” of each part of the paddy field revealed that the most important site of the N₂ fixation was the reduced Apg horizon, that the importance of flooded water and/or the oxidized layer in the N₂ fixation was rather low except in infertile soil, and that the role of rhizosphere in the N₂ fixation could not be neglected also in Japan.

Reduced condition and content of easily decomposable organic substances were judged to be main factors which control the N₂-fixing activity in the flooded soil on the basis of correlations between the Nt-fixing activity and several analytical data of the paddy soils.     

Availability and extractability of soil manganese in a liming experiment

Abstract

The availability of soil Mn to corn in relation to extractability of soil Mn by EDTA, Mg(NO₃)₂, CH₃COONH₄, hydroquinone, H₃PO₄, and NH₄H₂PO₄ as affected by liming was evaluated under field conditions on a single soil type. EDTA, Mg(NO₃)₂ and CH₃COONH₄‐extractable Mn were related inversely to available Mn. No useful relationships were found between hydroquinone, H₃PO₄, and NH₄H₂PO₄‐extractable soil Mn and Mn uptake by sweet corn.     

Indirect determination of micrograms of sulfate by barium absorption spectroscopy

Abstract

The indirect procedure for sulfate determination by Ba absorption spectroscopy was modified so that low concentrations of SO₄ in small volumes of solutions could be determined rapidly and precisely. Major modifications consisted of seeding the sample with BaSO₄, precipitating in ethanol solutions to lower BaSO₄ solubility, and determining Ba in a N₂O‐acetylene flame using the absorption mode.

The results showed: complete SO₄ precipitation as BaSO₄ after 15 min of shaking, little or no effect from solution Al on SO₄ determination, quantitative recovery of SO₄ from 0.01 M Ca(H₂PO₄)₂ soil extracts, and greater precision of SO₄ measurement with indirect method than with turbidimetric method.