Kinetics of field oxidation of elemental sulfur in New Zealand pastoral soils and the effects of soil temperature and moisture

The effectiveness of elemental sulfur (S°) as a fertilizer is governed by its rate of microbial oxidation in soil to the sulfate form for absorption by plants. Some 80 field oxidation rate experiments were conducted under grazing throughout New Zealand for one year by measuring the residual S° in soil at approximately two-month intervals. The S° was applied as particles 75 to 150µm in diameter at the rate of 30 kg ha^–1. The simple cubic oxidation rate model (no allowance for temperature or moisture changes) accounted for more than 80% of the variance at nearly 70% of sites.The mean annual soil temperature (10-cm depth) was the most important factor found affecting the mean annual oxidation rate constants, accounting for 38% of the variance through the Arrhenius equation. Other factors, including rainfall, soil moisture, pH and soil groups accounted for only a further 2% of variance. The factor most likely to account for the balance of variance among the sites is the oxidizing efficiency of the associated microorganisms.The rate constant versus temperature relationship for field oxidation in the five New Zealand climate regions was consistent with mean rate constants of soil groups from a similar set of 47 soils incubated at 25°C and field capacity moisture for 10 weeks or more. Field soils, therefore, had an average moisture for the oxidation rate equivalent to that at field capacity. This would explain the lack of contribution of soil moisture to the variance, and would support the use of the mean annual oxidation rate constant (from the mean annual soil temperature) for calculating the optimum particle size range of S° fertilizer. In confirmation, S° particle size recommendations from field rate constants for pastoral fertilizer were consistent with those from earlier agronomic experiments.     

Temperature effects on soil organic sulphur mineralization and elemental sulphur oxidation in subtropical soils of varying pH

The increasing sulphur (S) deficiency in soils of several parts of world has led to the use of fertilizer S, an important factor in enhancing the production and quality of crops. Very limited information is available on the use of elemental sulphur (S⁰) as a fertilizer, its oxidation into SO4^2- and transformation into organic S in semiarid subtropical soils. We studied the impact of three temperature regimes on the mineralization of soil organic S, and the oxidation and immobilization of S⁰ in acidic (pH 4.9), neutral (pH 7.1) and alkaline (pH 10.2) subtropical soils of north-western India. Repacked soil cores were incubated under aerobic conditions (60% water-filled pore space) for 0, 14, 28 and 42 d with and without incorporated S⁰ (500 g g-1 soil). Temperature had profound effects on all three soils processes, the rates of mineralization of native soil organic S, oxidation of applied S⁰ and transformation of S⁰ into soil organic S being greatest at 36 °C, irrespective of soil pH. Mineralization of native soil organic S (without added S⁰) resulted in the accumulation of 39, 66 and 47 g SO4^2-–S g-1 soil in acidic, neutral and alkaline soil in 42 d period at 36 °C. Of the total mineralization, the majority (62 – 74%) occurred during the first 14 d period. Oxidation rate of added S⁰ during initial 14 d period at 36 °C was highest in alkaline soil (292 g S cm-2 d-1), followed by neutral soil ((180 g S cm-2 d-1) and lowest in acidic soil (125 g S cm-2 d-1). Of the applied 500 g S⁰ g-1 soil, 3.2 – 10.0%, 6.8 – 15.4% and 10.0 – 23.0% oxidized to SO4^2-, and 13.4 – 28.6%, 16.0 – 29.0% and 14.6 – 29.0% were transformed into organic S in 42 d period in acidic, neutral and alkaline soil, respectively. The results of our study suggest that in order to synchronize the availability of S with plant need, elemental S may be applied well before the seeding of crops, especially in acidic soil and in regions where temperature remains low. Substantial mineralization of native soil organic S in the absence of applied S⁰ and immobilization of applied S⁰ into organic S suggest that the role of soil biomass as source and sink could be exploited in long term S management.     

Fate and efficiency of sulfur fertilizer applied to food crops in West Africa

Increased food production in West Africa must be linked to increased fertilizer use. However, the increased use of the high analysis sulfur-free materials currently available in the region will lead to increased incidence of sulfur (S) deficiency. In order to determine the S fertilizer requirements of major cereal crops, and compare the fate and efficiency of alternative S sources, experiments were conducted at six sites in semiarid and subhumid West Africa from 1985–1988. Sulfur fertilizers increased grain yields from 10% to 65% (200 to 2000 kg/ha) in 14 out of 20 site-years (at 5 out of 6 sites). Maximum response was generally obtained with only 5–10 kg S/ha. Both powdered (–60 mesh S⁰) and granular (S⁰-fortified TSP) elemental S sources were usually as effective as sulfate sources (gypsum or SSP). The residual effectiveness of S⁰, however, was superior to that of sulfate at most sites. Sulfur-35 balances demonstrated substantial leaching and low crop recovery (5%–10% of S applied) of fertilizer S. Up to 50% of the applied S was lost from sulfate sources, whereas <20% was lost from S⁰ sources. Although S⁰ sources were at least as effective as sulfate sources, the low S rates required suggest that S deficiencies in the region can be corrected at relatively low cost with sulfate-containing fertilizers, provided they can be supplied at more appropriate nutrient ratios.     

Influence of co-granulated nutrients and granule size on plant responses to elemental sulfur in compound fertilizers

Experiments were undertaken to determine the effect of granule size and nutrients in granulated compound fertilizers fortified with finely divided elemental sulfur (S^o) on the rate of S^o oxidation. In one experiment, S^o was banded together with or apart from triple superphosphate (TSP) while in two others, S^o was granulated with nutrient and inert carriers. A fourth experiment examined response to S in an S^o-fortified TSP from a range of granule sizes. Response and, in some cases, S^o recovery (using ³⁵S labels) by test crops (maize, wheat, upland rice) was measured. In all experiments, P mixed with S^o increased plant growth and S recovery above treatments in which P and S^o were physically separated. There was however, no effect of distance of separation on S recovery. In one experiment, N as urea and N and P as diammonium phosphate (DAP) were also found to enhance response to S^o although to a lesser degree than P alone. These observations were attributed to a nutritional requirement of S^o-oxidizing microorganisms for P and N. Granulation of S^o with carriers also influenced oxidation rate, as inferred from the fertilizer S recovery. For a given S^o concentration, the effect was inversely proportional to the mean diameter of granules. It is shown that this relationship can be explained if one assumes that S^o particles in granules collapse into a fixed number of aggregates per granule irrespective of granule size when the soluble nutrient carrier dissolves and diffuses away from the point of application.     

Enhancing the agronomic effectiveness of natural phosphate rock with poultry manure: a way forward to sustainable crop production

Phosphorus inputs are required in highly weathered tropical soils for sustainable crop production. However, high cost and limited access to mineral P fertilizers limit their use by resource-poor farmers in West Africa. Direct application of finely ground phosphate rock (PR) is a promising alternative but low solubility of PR hampers its use. Co-application of PR with manure could be a low cost means of improving the solubility of natural PR and improve their agronomic effectiveness. Our objective was to quantitatively estimate the enhancement effect of poultry manure on P availability from low reactive PR (Togo phosphate rock) applied to highly weathered soils. We utilized two highly weathered soils from Ghana and Brazil for this greenhouse study. Using ³²P isotopic tracers, the agronomic effectiveness of poultry-manure-amended Togo rock phosphate (TPR) was compared with partially acidulated Togo rock phosphate (PAPR) and triple superphosphate (TSP). Four rates of poultry manure: 0, low (30 mg P kg⁻¹ soil), high (60 mg P kg⁻¹ soil) and very high (120 mg P kg⁻¹ soil) were, respectively, added to a constant amendment (60 mg P kg⁻¹ soil) of the P sources and applied to each pot of 4 kg soil. A Randomized Complete Block Design was used for the greenhouse experiment and Maize (Zea mays L.) was used as a test crop. The plants were grown for 42 days after which the above ground biomass was harvested for analysis. Without poultry manure addition, the agronomic effectiveness, represented by the relative agronomic effectiveness (RAE) and proportion of P derived from fertilizer (% Pdff) was in the order TSP > PAPR > TPR = control (P₀). In the presence of low rate poultry manure addition, the agronomic effectiveness followed the order TSP > PAPR = PR > P₀. However, at the high and very high rates of poultry manure addition, no significant differences in agronomic effectiveness were observed among the P sources, suggesting that at this rate of poultry manure addition, PR was equally as effective as TSP. The study showed that direct application of PR co-applied with poultry manure at a 1:1 P ratio will be a viable option for P replenishment. Thus a combination of PR and poultry manure could be a cost-effective means of ensuring sustainable agricultural production in P-deficient, highly weathered tropical soils.     

Effect of management history and temperature on the mineralization of methylene urea in soil

A soil incubation and a greenhouse study on processing tomato were used to test the effects of soil temperature and the size and activity of the soil microbial biomass (SMB) on the degradation (mineralization) rate of a slow-release N fertilizer, methylene urea (MU), a condensation product of urea and formaldehyde. The mineralization rates of three MUs: Short (S), Medium (M), and Long (L) with different water solubilities were measured at two temperatures in a soil with low (fallow, F) and high (cover crop, CC) microbial activity. In the greenhouse study, the fate of fertilizer N was followed using ¹⁵N-urea and ¹⁵N-MU. The fertilizer N efficiency calculated for urea using the ¹⁵N mass balance approach was 93 and 85% compared with 65 and 67% for MU-S in F and CC soils, respectively. During six months of incubation, 52 and 63% of MU-S N was mineralized at 20 and 30 °C, respectively. The accumulated N data suggested that the degradation of all three MU types followed first-order reaction kinetics. The reaction rates were similar for all three MUs and increased with increasing temperature. However, fitting discrete, non-accumulated data revealed that MU mineralization is more complex and cannot be modeled with simple exponential decay equations. The size and activity of SMB did not affect the mineralization rate of MU-N under laboratory or greenhouse conditions. Interestingly, Activity Index (AI), defined as the slowly available pool of MU-N, was not a reliable indicator for the mineralization rate and plant availability of MU-N.     

Re-utilization by perennial ryegrass (Lolium perenne L.) of labelled fertilizer sulphur incorporated in field-grown tops and roots of pasture plants added to soils

³⁵S-labelled gypsum fertilizer was incorporated under field conditions into pastures which were separated into white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) tops and roots. These were added to four soils from improved and unimproved pastures. The re-utilization of labelled fertilizer sulphur (S) was assessed under growth cabinet conditions (20°C day, 13°C night, daylength 16 h, light intensity 120–170 lx) by growing perennial ryegrass plants for 23 weeks.Mean recoveries of labelled fertilizer S varied from 7 to 20% depending on soil type, form amount and kind of plant residue added. Greater recovery was obtained from clover roots (9.5–16.2%) than grass roots (6.7–12.5%), and from grass tops (13.1–19.7%) than clover tops (9.7–17.9%). These results are related to contents of labelled S, total S, C/N, C/S and N/S ratios in plant residues which also accounted for their relative rates of decomposition. Ground (< 1 mm) and chopped (3 mm) roots increased labelled fertilizer S recovery by about 30% compared with whole roots. Additions of unlabelled fertilizer S influenced the recovery of labelled fertilizer S. This effect depends on the amounts of labelled grass roots and unlabelled fertilizer added.The significance of the findings is discussed with the aid of results from previous field experiments conducted on these soils.     

Powdered elemental sulphur: oxidation rate,temperature dependence and modelling

Finely-powdered elemental sulphur is a useful source of fertilizer S, being readily oxidizable in soil to plant-available sulphate yet possessing some slow release characteristics. Two mesh sizes were evaluated using four soils from northeast Scotland. Particle size analysis found that the 120 and 300 mesh S samples had specific surface areas of 1300 and 1940 cm² g^–1, respectively, with most of the surface area in particles of 10 – 20 µm diameter. The S oxidation rate was similar in all four soils but was greater for the 300 mesh than for the 120 mesh S: mean values of 51% and 18% were oxidized over 7 weeks at 14 °C, respectively. The time course of oxidation followed a sigmoidal pattern with a pronounced lag which was modelled using the logistic equation. Maximum specific oxidation rates were 11–28 µg S cm^–2 day^–2 for the 300 mesh S at 14 °C. These were significantly slower at 7 °C and the temperature response was calculated as a Q₁₀ of 4.0.A model of seasonal S oxidation was developed using a cosine function for the annual temperature, the Arrhenius equation to relate S oxidation rate to temperature and a generalization of the logistic equation to describe the time course of S oxidation. Simulations showed that the 300 mesh S would be useful for spring S applications in east Scotland and if applied in autumn could supply S during the autumn and again in the spring. The 120 mesh S would be less effective in autumn but more resistant to winter leaching. The 120 mesh S applied to the warmer soils of southwest England would behave the same as 300 mesh S applied in east Scotland.     

Direct and residual fertilizer phosphorus effects on yield and phosphorus efficiency of upland rice in an Ultisol

Phosphorus (P) deficiency is one of the major limiting factors for crop production in highly weathered soils in the humid zone of West Africa. Very few studies have evaluated the residual value of fertilizer P to rice in these soils. Field experiments were conducted for three years (1993–1995) to determine the response of four upland rice cultivars to fertilizer P applied at 0, 45, 90, 145 and 180 kg P ha^-1 only once in 1993, and to fertilizer P residues in 1994 and 1995. The soil at the experimental site, in the humid forest zone of Ivory Coast (West Africa), was an Ultisol, low in available P. Grain yields of the rice cultivars were significantly increased by fertilizer P in 1993, and by the fertilizer P residues in 1994 and 1995 although the magnitude of response decreased with time since the fertilizer was applied. The cultivars differed in cumulative agronomic and physiological efficiencies, and the efficiencies were higher at the lower rates of P. The amounts of total P removed in three successive crops were similar for all the four rice cultivars although P harvest index was 10–12% higher in the P efficient than inefficient cultivars. The results suggest that the differences observed in the P efficiency of rice cultivars are due to differences in the internal efficiency of P. This revised version was published online in August 2006 with corrections to the Cover Date.     

A model for the release of sulfur from elemental S and superphosphate

A mathematical model was developed to predict the release of sulfate from elemental S (S⁰) and gypsum in single superphosphate. The release algorithm is based on the observation that release is linearly related to particle surface area. Release rates under various conditions could then be described by the change in radius for each time increment, which allows easier comparison of release rates between different particle sizes. A model based on spherical particles was found to be adequate in accounting for the range of particle shapes found in crushed agricultural S⁰. Release rates calculated from experimental data range from 0.07 to 0.45µm/d depending on environmental conditions.Equations for incorporating the effects of environmental variables and the release of S from S⁰ and from the gypsum component of single superphosphate (SSP) were developed from the literature, and were incorporated within a larger model of S cycling. The model predicted that after 72 days, 99% of the S in SSP would have been released, compared to a release after one year of 54% of the S in sulfur-fortified superphosphate, and 23% of that in crushed agricultural grade S⁰. The model provides a means of assessing the effect of the particle size of S⁰ on release rates and should allow the formulation of fertilizers that supply S at a rate closer to the rate of plant uptake.     

Comparison of greenhouse and 32P isotopic laboratory methods for evaluating the agronomic effectiveness of natural and modified rock phosphates in some acid soils of Ghana

Phosphorus deficiency is one of the major constraints for normal plant growth and crop yields in the acid soils of Ghana and therefore addition of P inputs is required for sustainable crop production. This is often difficult, if not impossible for small-scale farmers due to the high cost of mineral P fertilizers and limited access to fertilizer supplies. Direct application of finely ground phosphate rocks (PRs) and their modified forms have been recommended as alternatives for P fertilization. The direct application of the natural and modified PRs to these acid soils implies the need to predict their agronomic effectiveness of the PRs in the simplest and most cost-effective manner. In this study the classical greenhouse pot experiment was compared to the ³²P isotopic kinetics laboratory method for evaluating the agronomic effectiveness of natural and modified Togo PR in six highly weathered Oxisols from southwest Ghana. In the ³²P isotopic kinetics laboratory experiment the six soil samples were each fertilised at the rate of 50 mg P kg^–1 soil in the form of triple superphosphate (TSP), Togo PAPR-50%, and Togo PR, respectively. Controls without P amendment were also included. Isotopic exchange kinetics experiments were carried out on two sets of samples, immediately after P fertilizer additions (without incubation) and after 6 weeks of incubation under wet conditions and at a room temperature of 25 °C. In the greenhouse pot experiment, P fertilizers in the form of Togo PR, Togo PAPR, Mali PR and TSP were each applied to the six soils at rates equivalent to 0, 30, 60, and 120 kg P ha^–1, respectively. The P fertilizers were mixed with the soils and maize (Zea mays L.) variety Obatanpa was grown for 42 days before harvest. The isotopic kinetics data of the control samples indicated that 5 of the studied soils had very low P fertility status as reflected by their low P concentrations in solution (C_P<0.02 mg P l^–1) and low exchangeable P (E₁min < 5 mg P kg^–1). The capacity factor and the fixation index of the soils were variable. Application of water-soluble P as TSP increased both the C_P and E₁ values of all the soils above the critical levels. Togo PR was least effective among the fertilizers tested for all soil soils, except in Boi soil. Acidulation of Togo PR (Togo PAPR-50%) was an effective means to increase its agronomic effectiveness. Direct application of natural Togo PR would be only feasible in the Boi soil series as reflected by its high Pdff% value in soil solution. Incubation with the P fertilizers caused an increase in the soil pH and a decline in the effectiveness of the applied P fertilizers, irrespective of the soil and the fertilizer utilized. Based upon the results of the greenhouse pot experiment, the relative crop response index (RCRI) in terms of increasing dry matter yield and P uptake followed the order of TSP > PAPR = Mali PR >Togo PR = Control. Both the laboratory index, Pdff% in soil solution derived from the isotopic method and the RCRI values obtained from the pot experiment produced similar results in ranking the P fertilizers tested according to their agronomic effectiveness. The isotopic kinetic method may be considered as an alternative to both greenhouse and field methods in the evaluation of agronomic effectiveness of P fertilizers in tropical acid soils when it offers comparative advantages in assessing the soil P status and its changes. But trained staff and adequate laboratory facilities are needed to perform this technique. Also the method can be used as a reference for comparison purposes as in this case. Further research is needed to assess the overall agronomic effectiveness (immediate and residual effects) of PR sources in predominant cropping systems of this region of Ghana.     

32P isotopic techniques for evaluating the agronomic effectiveness of rock phosphate materials

Tropical soils are often low in available P and therefore require inputs of P fertilizer for optimum plant growth and production of food and fiber. The cost of applying imported or locally produced, water-soluble, P fertilizers is often greater than utilizing indigenous phosphate rock. Therefore quantifying the P availability of soils amended with phosphate rock-based products in a variety of crop management and environmental conditions in developing countries is desirable for making recommendations on best type and rate of fertilizers to use to obtain maximum agronomic and economic benefits. One adequate approach for evaluating the agronomic effectiveness of rock phosphate materials is through the use of³²P/³³P isotopic tracers. The present paper describes the principles and assumptions of the³²P isotopic techniques commonly used in the field and greenhouse for the agronomic evaluation of rock phosphate materials. An overview of the applications of these techniques is also given.     

Towards understanding factors that govern fertilizer response in cassava: lessons from East Africa

Information on fertilizer response in cassava in Africa is scarce. We conducted a series of on-farm and on-station trials in two consecutive years to quantify yield responses of cassava to mineral fertilizer in Kenya and Uganda and to evaluate factors governing the responses. Average unfertilized yields ranged from 4.2 to 25.7 t ha⁻¹ between sites and years. Mineral fertilizer use increased yields significantly, but response to fertilizer was highly variable (−0.2 to 15.3 t ha⁻¹). Average yield response per kg applied nutrient was 37, 168 and 45 and 106, 482 and 128 kg fresh yield per kg of applied N, P and K, respectively in 2004 and 2005. Fertilizer response was governed by soil fertility, rainfall and weed management, but was not influenced by variety, pest and disease pressure and harvest age. Relative N and K yields were positively correlated to SOC and exchangeable K, while response to fertilizer decreased on more fertile soils. Still, fertilizer response varied widely on low fertility soils (e.g. on soils with <10 g kg⁻¹ SOC, responses ranged from −8.6 to 24.4 t ha⁻¹), indicating strong interactions between factors governing fertilizer response. Response to fertilizer was reduced if total rainfall <1,500 mm or rainfall from 0 to 3 months after planting <400 mm. Fertilizer application promoted plant growth and resulted in a better soil coverage and reduced weed competition. Yields in fertilized fields were independent of weed management, unless growing conditions were unfavourable.     

Microbial desulfurization of three different coals from Indonesia, China and Korea in varying growth medium

Shake flask studies on microbial desulfurization of three different coal samples (Indonesian lignite, Chinese lignite and Korean anthracite) were performed to optimize the best suitable growth medium. Among the three different growth mediums (basal salt medium, basal salt medium supplemented with 9 g/L Fe and basal salt medium supplemented with 2.5% S⁰) tested, the basal salt medium was found to be the best, considering process dynamics and economical factors. The extent of pyrite oxidation was highest with 95% in the experiments with Korean anthracite in basal salt medium supplemented with 9 g/L Fe, while the lowest pyrite oxidation of 70–71% was observed in the experiments with Indonesian and Chinese Lignite’s in only basal salt medium. The microbial sulfur removal in the experiments with basal salt medium supplemented with 9 g/L Fe for all the three coal samples was between 94–97%, while the experiments on basal salt medium supplemented with 2.5% S⁰ for all the coal samples were relatively much lower ranging between 27–48%. However, the overall study resulted with promising directions for further scaling up of microbial desulphurization in a best growth medium devoid of iron and sulfur supplement.     

Modelling the oxidation of elemental sulfur in soils

Direct and recursive estimation models for the oxidation rate of elemental sulfur (S°) in soil have been proposed, both essentially based on a constant oxidation rate per unit area of exposed surface. Fertilizer S° is taken to consist largely of blocky shaped particles, i.e. having similar dimensions along three axes, which can be treated as equivalent spheres. The most important implication in applying the rate assumption to these shaped particles is that the mass at any time is related to the cube of the time. This has been verified experimentally for oxidation by thiobacilli. Although the assumption is less likely for heterotrophs, experiments involving four soils conformed to the cubic relation. Implications for the particle variables of size and size distribution have been given more limited testing. The data are generally consistent with theory, such as independence of the rate constant with particle size. Assuming an activation energy for the oxidation process implies, in addition to the above, an exponential relation of rate constant with temperature. This is supported by experiment. Values for the activation energy are approximately 85 kJ mol⁻¹, and therefore consistent with the rate limiting step for the oxidation being a chemical or biochemical reaction, rather than a diffusion process. Because absolute rate constants are generated by the models, they are useful for examining the effects of environmental variables not hitherto included. Empirical relationships, once established, can then be included in the model, such as the quadratic relation between rate constant and soil moisture, with the maximum at approximately field capacity. The delay time (the time to reach maximum oxidation rate) was useful, together with the rate constant, for distinguishing species of oxidizing microorganisms. Typically, under optimum conditions at 25°C, thiobacilli have a delay time of several days and a rate constant of 50μg cm⁻² day⁻¹ S, while heterotrophs have a negligible delay time but a rate constant of only 5μg cm⁻² day⁻¹ S. The cubic model with a single rate constant gave a surprisingly good fit to the oxidation rate over 12 months in New Zealand pastoral soils under field conditions of varying temperature and moisture. This was attributed to the balancing effect of moisture and temperature on the rate constant under the cool temperate climate. A knowledge of the annual average soil temperature is sufficient to provide advice on the optimum particle size for S° fertilizer.     

Variations in microbial populations in soils with different methane uptake rates

The rate of methane oxidation has been measured from under a variety of land uses and management practices on Rothamsted Experimental Station. All these sites have their management histories well documented, in many cases over centuries, and experience the same atmospheric inputs. We have found consistent patterns in methane oxidation rates associated with land use, where the rate of methane oxidation in unfertilized arable soil (c. 30 μg CH₄ m^-2 d^-1) is only 15 % that in undisturbed grassland and woodland soils (c. 200 μg CH₄ m^-2 d^-1). Investigation of the mechanisms regulating these differences have shown that they are microbially mediated. The microbial basis for differences in methane uptake rates are unclear, but probably involve three groups of microorganism, methanogens, methanotrophs and ammonia oxidisers. Using traditional enumeration techniques we show that soils under grassland and woodland have similar numbers of bacteria and also similar numbers of putative methane oxidisers (organisms of unknown identity, that can oxidise methane), but that an unfertilized arable soil has significant lower total bacterial numbers and also putative methane oxidisers. This study is extended to compare the capacities of the soils under the different land uses to metabolise multi-carbon compounds in addition to methane. Using a modification of the Biolog<reg> technique we demonstrate that the microbial populations in the soil under the woodland and grassland can metabolise a greater range of carbon compounds compared to the arable soil, as well as having higher methane oxidising capabilities. This revised version was published online in August 2006 with corrections to the Cover Date.     

Short-term fates of high sulfur inputs in Northern California vineyard soils

The widespread application of elemental sulfur (S⁰) to vineyards may have ecosystem effects at multiple scales. We evaluated the short-term fates of applied S⁰ in a Napa Valley vineyard; we determined changes in soil sulfur (S) speciation (measured by X-ray absorption near-edge structure (XANES) spectroscopy), soil pH, extractable sulfate (SO₄ ²⁻), and total S to evaluate changes in acidity and soil S within the vineyard over time. Surface soil samples were collected immediately prior to and following two applications of S⁰ (6.7 kg S⁰ ha⁻¹), with weekly collections in the 2 weeks between applications and following the last application. XANES spectra indicated that the majority of soil S persists in the +6 oxidation state and that S⁰ oxidizes within 7 days following application. Soil pH and extractable SO₄ ²⁻ measurements taken at 30 min after S⁰ application revealed generation of acidity and an increase in extractable SO₄ ²⁻, but by 12 days after application, soil pH increased to approximately pre-application levels. These data suggest that the major consequence of reactive S applications in vineyards may be the accumulation of soil SO₄ ²⁻ and organic S during the growing season, which can be mobilized during storm events during the dormant (wet) season. In spatially-extensive winegrowing regions where these applications are made by hundreds of individual farmers each year, it will be important to understand the long-term implications of this perturbation to the regional S cycle.     

Recent research on problems in the use of urea as a nitrogen fertilizer

Recent research on the NH₃ volatilization, NO ₂ ^- accumulation, and phytotoxicity problems encountered in the use of urea fertilizer is reviewed. This research has shown that the adverse effects of urea fertilizers on seed germination and seedling growth in soil are due to NH₃ produced through hydrolysis of urea by soil urease and can be eliminated by addition of a urease inhibitor to these fertilizers. It also has shown that the leaf burn commonly observed after foliar fertilization of soybean with urea results from accumulation of toxic amounts of urea in soybean leaves rather than formation of toxic amounts of NH3 through hydrolysis of urea by leaf urease. It further showed that this leaf burn is accordingly increased rather than decreased by addition of a urease inhibitor to the urea fertilizer applied. N-(n-butyl)thiophosphoric triamide (NBPT) is the most effective compound currently available for retarding hydrolysis of urea fertilizer in soil, decreasing NH3 volatilization and NO ₂ ^- accumulation in soils treated with urea, and eliminating the adverse effects of urea fertilizer on seed germination and seedling growth in soil. NBPT is a poor inhibitor of plant or microbial urease, but it decomposes quite rapidly in soil with formation of its oxon analog N-(n-butyl) phosphoric triamide, which is a potent inhibitor of urease activity. It is not as effective as phenylphosphorodiamidate (PPD) for retarding urea hydrolysis and ammonia volatilization in soils under waterlogged conditions, presumably because these conditions retard formation of its oxon analog. PPD is a potent inhibitor of urease activity but it decomposes quite rapidly in soils with formation of phenol, which is a relatively weak inhibitor of urease activity. Recent studies of the effects of pesticides on transformations of urea N in soil indicate that fungicides have greater potential than herbicides or insecticides for retarding hydrolysis of urea and nitrification of urea N in soil.     

Relationship between electroultrafiltration (EUF) extractable nitrogen,grain yield,and optimum nitrogen fertilizer rates for winter wheat

The objective of the investigation was to examine whether there exist relationships between the optimum nitrogen fertilizer rate for winter wheat and soil nitrogen fractions extracted by electroultrafiltration (EUF) from autumn samples of the upper soil layer (0–30 cm). Optimum nitrogen fertilizer rates were derived from grain yield curves of field trials carried out with increasing nitrogen fertilizer rates on 19 different sites in 1985/86 and 1986/87. Most soils were luvisols derived from loess, two soils were brown earths and one a pararendzina. Total Nitrogen fertilizer rates were 0, 40, 80, and 120 kg N/ha applied twice before ear emergence. The final nitrogen rate at ear emergence was the same for all treatments, namely 60 kg N/ha.Optimum nitrogen fertilizer rates were derived from the grain yield curve fitted to a modified Mitscherlich equation. The optimum nitrogen fertilizer rates were correlated with the nitrogen fractions extracted by EUF. The regression equation thus obtained showed that NO ₃ ^- , the organic N fraction (EUF Norg), and the EUF Norg-quotient each had a highly significant impact on the optimum nitrogen fertilizer rate. The higher the amounts of EUF-N extracted the lower the optimum nitrogen rate. Substituting the EUF Norg-fraction for total nitrogen concentration in the upper soil layer gave a poorer relationship between the optimum nitrogen fertilizer rate and the soil data. In absolute terms the EUF Norg-fraction had by far the greatest impact on calculating the optimum nitrogen fertilizer rate. The investigation shows that the EUF method is a suitable technique for the determination of available soil nitrogen from which optimum nitrogen fertilizer rates can be derived for winter wheat cultivated under soil and climatic conditions typical for cereal growing areas in central Europe.     

Transformations of residual fertilizer P in a semi-arid tropical soil under eight-year peanut-wheat rotation

Long-term transformations of residual phosphorus (P) governs the availability of phosphorus to crops. Very limited information is available on the transformations of residual fertilizer P in semi-arid tropical soils under long-term crop rotations. Therefore, using sequential phosphorus fractionation procedure, we studied changes in labile and stable forms of inorganic and organic P in a semi-arid alluvial soil (Typic Ustisamments) after eight years of annual fertilizer P application either to one crop (alternate) or to both crops (cumulative) in a peanut (Arachis hypogaea) — wheat (Triticum aestivum) rotation.Total residual fertilizer P in soil (P recovered from P-fertilized minus control plots) ranged from 62 to 176 mg P kg^–1. In the alternate P treatments (P applied to peanut or wheat only), on an average of 3 rates of applied P (13, 26 and 39 kg P ha^–1), in surface (0–15 cm) and subsurface (15 to 30 cm) soil, respectively, residual fertilizer P consisted of 14.8 and 2.2% resin-P, 8.6 and 2.8% NaHCO₃-P, 6.3 and 0% microbial-P, 31.4 and 4.2% NaOH-P, 7.8 and 3.0% aggregate protected-P, 12.5 and 3.0% HCl-P, 3.4 and 0% H₂SO₄-P. The corresponding values for surface and subsurface soils of cumulative P treatments (P applied to both peanut and wheat) were: 12.8 and 1.6% resin-P, 6.9 and 2.3% NaHCO₃-P, 4.7 and 0% microbial-P, 32.5 and 4.2% NaOH-P, 5.6 and 2.0% aggregate protected-P, 14.8 and 3.8% HCl-P, 6.7 and 2.1% H₂SO₄-P. Considerable lower values for the 15–30 cm depth indicate only a very small movement of residual P to the subsoil.Significantly lower amount of fertilizer P (28% and 44%) found in labile (resin, NaHCO₃ and microbial P) and semi-labile (NaOH and sonicated NaOH P) fractions for the cumulative P treatment than alternate P treatment (35 and 46%, respectively) suggests that increased rates and frequency of applied P tend to enhance the conversion of residual P to stable forms which are less available to plants. About 12 to 19% of residual fertilizer P found as organic P in labile and semi-labile forms confirmed that organic P increased with long-term fertilizer management. In conclusion, the results of our study suggest that the alternate application of fertilizer P to a crop, as is shown for wheat, helps reduce the transformations of residual P to stable P forms.