Growth of Vicia faba as affected by inoculation with vesicular-arbuscular mycorrhizae and Rhizobium leguminosarum bv. viceae in two soils

We studied the turnover of interlayer NH _inf4 ^sup+ in three soils cropped with sugar beet. The three soils chosen for this study (Carpi, Cadriano, and Ozzano) are typical of the soils in the Po Valley where sugar beet is grown. The variation in interlayer NH _inf4 ^sup+ content during the growing season was significant and very similar for the Carpi and Cadriano soils, while there was no significant variation in the NH _inf4 ^sup+ content in the Ozzano soil during the same period. The turnover of interlayer NH _inf4 ^sup+ in the Carpi and Cadriano soils was high, and appeared to cover a substantial amount of the N requirement of the crop. The turnover in these two soils showed a significant decrease during the initial phase followed by a period during which the pool of interlayer NH _inf4 ^sup+ was replenished and reached the initial levels again. The spatial distribution of the root system, the pattern of N uptake by the sugar beet crop, and the processes of immobilization-mineralization of organic N all have an influence on the mechanisms of adsorption and release of interlayer NH _inf4 ^sup+ . The release of interlayer NH _inf4 ^sup+ , and thus its availability for plant uptake, was greater at the beginning of the growing season. The physicochemical characteristics of the soils, the K concentration, and the types of clay minerals present were found to be important in determining the dynamics of interlayer NH _inf4 ^sup+ turnover of soils.     

Nitrogen transformation in arable soils of North-West Germany during the cereal growing season

In 1991, field experiments on loess (with winter wheat) and sandy soils (with summer barley) were conducted to study N dynamics in the microbial biomass and non-exchangeable NH _inf4 ^sup+ . The measurements showed a mass change in microbial N, with a maximum increase of 100 kg N ha^-1 30 cm^-1 from March to July in the loess soil, and a change for only 1 month (May) in the sandy soil. Plots treated with conventional levels of N fertilizer (213 kg N ha^-1 on a loess soil to winter wheat and 130 kg ha^-1 on the sandy soil to summer barley), reduced levels of N (83% and 62% of the conventional N application), or no N showed no consistent fertilizer N effect on microbial biomass N. From March to July, non-exchangeable NH _inf4 ^sup+ in loess soils under winter wheat decreased by 110 kg N ha^-1 30 cm^-1 in conventionally fertilized plots and by 200 kg N ha^-1 30 cm^-1 in a plot with no N fertilizer. After harvest, the pool of non-exchangeable NH _inf4 ^sup+ increased due to increasing mineral N concentrations in the soil.     

Significance of soil microorganisms for the mobilization of nonexchangeable ammonium

To estimate the availability of nonexchangeable NH _inf4 ^sup+ –N for soil microorganisms four incubation experiments were conducted under controlled conditions. The following results were obtained: Incorporating glucose as a source of readily oxidizable organic material favored the release of nonexchangeable NH _inf4 ^sup+ –N. Mobilization of NH _inf4 ^sup+ from the interlayers of the clay minerals was decreased by the application of K⁺⁺, while Ca²⁺, which is supposed to expand the lattice of the clay minerals, had no influence on the release of NH _inf4 ^sup+ . Soil temperature had no effect on microbiological mobilization of NH _inf4 ^sup+ . It is assumed that, generally, the influence of nitrifying bacteria on the mobilization of nonexchangeable NH _inf4 ^sup+ –N is negligible. However, in soils with abundant amounts of available carbon promoting the activity of heterotrophic soil microorganisms, the release of NH _inf4 ^sup+ from clay minerals is favored under fallow conditions.     

Immobilization of ammonium and nitrate and their interaction with native N in three Illinois Mollisols

Summary Three Illinois Mollisols were incubated for 2 weeks at 25°C after treatment with different amounts of glucose and/or ¹⁵N-labelled (NH₄)₂SO₄ or ¹⁵N-labelled KNO₃. The objectives were: (1) to compare the immobilization and interaction of NH _inf4 ^sup+ –N and NO _inf3 ^sup- –N with the native soil N, and (2) to study the relationship between immobilization of applied N and the added N interaction. As determined, immobilized N refers to forms not extractable with 2 MKCl (immobilized ¹⁵N+clay-fixed ¹⁵NH _inf4 ^sup+ ). In all cases, both NH _inf4 ^sup+ –N and NO _inf3 ^sup- –N were actively immobilized and transformed into organic forms in the presence of glucose. In the absence of glucose, a higher proportion of NH _inf4 ^sup+ than NO _inf3 ^sup- was recovered in organic forms. Although the three soils differed considerably in the amounts of applied N immobilized, similar trends in N immobilization were observed. A positive added N interaction occurred with all soils, the magnitude increasing with the rate of N addition. In the absence of glucose, higher added N interactions were obtained for NH _inf4 ^sup+ than NO _inf3 ^sup- , whereas there was very little difference between NH _inf4 ^sup+ and NO _inf3 ^sup- in the presence of glucose. The results indicate that under conditions of rapid immobilization (e.g., in the presence of glucose), NH _inf4 ^sup+ and NO _inf3 ^sup- will show comparable interaction with the native soil N, whereas in unamended soil, the extent of this interaction will be greater with NH _inf4 ^sup+ than with NO _inf3 ^sup- . Significant correlations were observed between applied N immobilized and the added N interaction only in one soil having a high initial mineral N content.     

Denitrification and nitrogen immobilization as affected by organic matter and different forms of nitrogen added to an anaerobic water-sediment system

The effects of wheat straw and different forms of N on denitrification and N immobilization were studied in an anaerobic water-sediment system. The water-sediment system was supplemented with various combinations of wheat straw and ¹⁵N-labelled and unlabelled (NH₄)₂SO₄ or KNO₃, and incubated anaerobically at 30°C for 10 days. ¹⁵N-labelled and unlabelled NO _inf3 ^sup- , NO _inf2 ^sup- , NH _inf4 ^sup+ , and organic N were determined in the water-sediment system. The gases evolved (N₂, CO₂, N₂O, and CH₄) were analyzed by gas chromatography at regular intervals. Larger quantities of ¹⁵N₂–N and organic ¹⁵N were formed in wheat straw-amended systems than in non-amended systems. Trends in CO₂ production were similar to those of N₂–N evolution. The evolution of N₂O and CH₄ was negligible. Denitrification processes accounted for about 22 and 71% of the added ¹⁵NO _inf3 ^sup- –N in the absence and presence of wheat straw, respectively. The corresponding denitrification rates were 3.4 and 12.4 g ¹⁵Ng^-1 dry sediment day^-1. In systems amended with ¹⁵NO _inf3 ^sup- –N and ¹⁵NO _inf3 ^sup- +NH _inf4 ^sup+ –N without wheat straw, 1.82 and 1.58%, respectively, of the added ¹⁵NH _inf3 ^sup- –N was immobilized. The corresponding figures for the same systems supplemented with wheat straw were 5.08 and 4.10%, respectively. Immobilization of ¹⁵NO _inf4 ^sup+ –N was higher than that of ¹⁵NO _inf3 ^sup- –N. The presence of NO _inf3 ^sup- –N did not stimulate NH _inf4 ^sup+ –N immobilization.     

Evaluation of denitrification losses by the acetylene inhibition technique in a permanent ryegrass field (Lolium perenne L.) fertilized with animal slurry or ammonium nitrate

In a field experiment, the effect of animal slurry, (with and without the nitrification inhibitor dicyandiamide on total denitrification losses estimated by the C₂H₂ inhibition technique was measured over 2 years (1989–1990). During this period, four different plots (each with four replicates) were fertilized six times with 150 kg N ha^-1 in the form of cattle-pig slurry or NH₄NO₃. Soil samples (0–20 cm) were analysed at regular intervals for NH _inf4 ^sup+ and NO _inf3 ^sup– concentrations. The soil water content was determined gravimetrically. During the first year (1989) total denitrification losses from unfertilized, mineral-fertilized, and animal slurry-amended plots (with or without dicyandiamide) were estimated as 0.2, 3.1, 0.7, and 0.6 kg N ha^-1, respectively. During the second year (1990) the denitrification losses were 0.4, 1.3, 0.7, and 0.7 kg N ha^-1, respectively. There was a clear relationship between the NO _inf3 ^sup– concentration or soil water content and the denitrification rate. The results are siteund experiment-specific and cannot be generalized so far.     

Nitrogen mineralization and reorganization in casts of the geophagous tropical earthworm Pontoscolex corethrurus (Glossoscolecidae)

Summary Mineral N concentrations ranged from 133.1 to 167.8 g g^-1 dry soil in fresh casts of the endogeic earthworm Pontoscolex corethrurus fed on an Amazonian Ultisol; this was approximately five times the concentration in non-ingested soil. Most of this N was in the form of NH _inf4 ^sup+ . N also accumulated in microbial biomass, which increased from a control value of 10.5–11.3 to 67.5–74.1 g g^-1 in fresh casts. During a 16-day incubation, part of the NH _inf4 ^sup+ -N was nitrified and/or transferred to the microbial biomass. Total labile N (i.e., mineral+biomas N) decreased sharply at first (ca. 50% in the first 12 h), and then more slowly. The exact fate of this N (microbial metabolites, denitrification, or volatilization) is not known. After 16 days, the overall N content of the casts was still 28% higher than that of the control soil. Incubation of the soil before ingestion by the earthworms significantly increased the production of NH _inf4 ^sup+ in casts. We calculate that in a humid tropical pasture, 50–100 kg mineral N may be produced annually in earthworm casts. Part of this N may be conserved in the compact structure of the cast where the cast is not in close contact with plant roots.     

Isotope 15N enrichment of soil-ammonium N as a reference to estimate N2 fixation by stem and root-inoculated S. rostrata

Summary A pot experiment in the greenhouse was conducted to compare the contribution of N derived from the atmosphere or from biological N₂ fixation by Sesbania rostrata inoculated with Azorhizobium caulinodans, applied either to roots or to roots and stems (single or multiple stem inoculation). Two subsequent crops were grown for 50 days under flooded conditions. N derived from air was estimated by ¹⁵N dilution using ¹⁵N enrichment of soil NH _inf4 ^sup+ -N and of Echinochloa crusgalli as the non-N₂-fixing reference datum and compared with estimates obtained by the N-difference method. The first crop was grown to stabilize the ¹⁵N into the soil organic N fraction. The ¹⁵N enrichment of soil NH _inf4 ^sup+ -N in the second crop declined slowly. The extractability ratio (¹⁵N enrichment of extractable soil N to ¹⁵N enrichment of total soil N) decreased from 4.8 to 4.1 50 days after planting. The enrichment of soil NH _inf4 ^sup+ -N was comparable to that of E. crus-galli, resulting in similar estimates of N derived from air when either soil NH _inf4 ^sup+ -N or enrichment of E. crus-galli was used as a non-fixing reference. The N-difference method did not always provide reliable estimates of N derived from air; percentages ranged from 75 to more than 80 by 50 days after planting in both crops and did not differ among treatments. The study demonstrates the potential of using ¹⁵N enrichment of soil NH _inf4 ^sup+ -N as a non-N₂-fixing reference for reliable BNF estimates of crops in lowland puddled soil.     

Effects of ammonium and nitrate on mineralization of nitrogen from leguminous residues

A laboratory incubation experiment was conducted to compare the effects of NH _inf4 ^sup+ and NO _inf3 ^sup- on mineralization of N from ¹⁵N-labelled vetch (Vicia villosa Rotn) in an Illinois Mollisol, and to determine the effect of a nitrification inhibitor (nitrapyrin) on mineralization of vetch N when used with NH _inf4 ^sup+ . The addition of either NH _inf4 ^sup+ or NO _inf3 ^sup- (100 and 200 mg N kg^-1 soil) significantly increased mineralization of vetch N during incubation for 40 days. The effect was greater with NH _inf4 ^sup+ than with NO _inf3 ^sup- , and a further increase occurred in the presence of nitrapyrin (10 mg kg^-1 soil). The addition of NO _inf3 ^sup- retarded the nitrification of NH _inf4 ^sup+ -N derived from vetch.     

Variables controlling denitrification from earthworm casts and soil in permanent pastures

Summary Denitrification (using the acetylene block method) was determined in earthworm casts and soils from permanent, drained or undrained pasture plots fertilized with 0 or 200 kg N ha^-1 year^-1 as ammonium nitrate. Rates of N₂O production from soil cores were about three times higher from the fertilized than from the unfertilized plots while drainage had a relatively small effect. Denitrification rates from casts were 3–5 times higher than those from soil irrespective of the drainage treatment. Casts generally had higher NO _inf3 ^sup- , NH _inf4 ^sup+ , and moisture contents, and higher microbial respiration rates than soil. Rates of N₂O production were determined primarily by NO _inf3 ^sup- supply, secondarily by moisture; available C did not appear to limit denitrification in these pastures. Estimates of the potential contribution of casts to denitrification ranges from 10.1% of 29.3 kg ha^-1 year^-1 from the unfertilized, drained plot to 22% of 82.5 kg ha^-1 year^-1 from the fertilized undrained plot.     

Increase in nitrous oxide production in soil induced by ammonium and organic carbon

We observed that soil cores collected in the field containing relatively high NH _inf4 ^sup+ and C substrate levels produced relatively large quantities of N₂O. A series of laboratory experiments confirmed that the addition of NH _inf4 ^sup+ and glucose to soil increase N₂O production under aerobic conditions. Denitrifying enzyme activity was also increased by the addition of NH _inf4 ^sup+ and glucose. Furthermore, NH _inf4 ^sup+ and glocose additions increased the production of N₂O in the presence of C₂H₂. Therefore, we concluded that denitrification was the most likely source of N₂O production. Denitrification was not, however, directly affected by NH _inf4 ^sup+ in anaerobic soil slurries, although the use of C substrate increased. In the presence of a high substrate C concentration, N₂O production by denitrifiers may be affected by NO _inf3 ^sup- supplied from NH _inf4 ^sup+ through nitrification. Alternatively, N₂O may be produced during mixotrophic and heterotrophic growth of nitrifiers. The results indicated that the NH _inf4 ^sup+ concentration, in addition to NO _inf3 ^sup- , C substrate, and O₂ concentrations, is important for predicting N₂O production and denitrification under field conditions.     

Mineralization of carbon and nitrogen,and nitrification in Scots pine forest soil treated with fast- and slow-release nitrogen fertilizers

We studied the effects of fast- and slow-release organic N fertilizers (urea and urea-formaldehyde, Nitroform) on mineralization, nitrification, and N leaching in an acid, poor forest soil. We also studied the effects of a nitrification inhibitor (dicyandiamide) applied together with urea. Net nitrification, mineralization of N and C were determined by aerobic laboratory incubation of soil samples taken one and three growing seasons after N application. Numbers of autotrophic nitrifiers were estimated by a most probable number method three growing seasons after the treatment. Urea increased the CO₂ production immediately after application, but after three growing seasons, CO₂ production was the lowest in the urea-treated soils. In the nitroform-treated soils, the concentration of exchangeable NH _inf4 ^sup+ after the first and third growing seasons was of the same magnitude, in contrast to the urea-treated soils, where hydrolysis took place immediately. Three growing seasons after application, the highest amount of NH _inf4 ^sup+ accumulated during the laboratory incubation was in the nitro-form-treated soils. Unlike urea, nitroform did not increase the production of NO _inf3 ^sup- or the number of NH _inf4 ^sup+ oxidizers. In the urea+dicyandiamide-treated soils there was less NO _inf3 ^sup- and a lower number of nitrifiers than in the urea-treated soils. The results showed that a slow-release N fertilizer, such as nitroform, increases the availability of mineral N in acid forest soils without increasing nitrification and hence the risk of NO _inf3 ^sup- leaching.     

Hyphal transport by a vesicular-arbuscular mycorrhizal fungus of N applied to the soil as ammonium or nitrate

Summary Transport of N by hyphae of a vesicular-arbuscular mycorrhizal fungus was studied under controlled experimental conditions. The N source was applied to the soil as ¹⁵NH _inf4 ^sup+ or ¹⁵NO _inf3 ^sup- . Cucumis sativus was grown for 25 days, either alone or in symbiosis with Glomus intraradices, in containers with a hyphal compartment separated from the root compartment by a fine nylon mesh. Mineral N was then applied to the hyphal compartment as ¹⁵NH _inf4 ^sup+ or ¹⁵NO _inf3 ^sup- at 5 cm distance from the root compartment. Soil samples were taken from the hyphal compartment at 1, 3 and 5 cm distance from the root compartment at 7 and 12 days after labelling, and the concentration of mineral N in the samples was measured from 2 M KCl extracts. Mycorrhizal colonization did not affect plant dry weight. The recovery of ¹⁵N in mycorrhizal plants was 38 or 40%, respectively, when ¹⁵NH _inf4 ^sup+ or ¹⁵NO _inf3 ^sup- was applied. The corresponding values for non-mycorrhizal plants were 7 and 16%. The higher ¹⁵N recovery observed in mycorrhizal plants than in non-mycorrhizal plants suggests that hyphal transport of N from the applied ¹⁵N sources towards the host plant had occurred. The concentration of mineral N in the soil of hyphal compartments was considerably less in mycorrhizal treatments than in controls, indicating that the hyphae were able to deplete the soil for mineral N.     

Growth and nitrogen nutrition of maize (Zea mays L.) in soil treated with the nitrification-inhibiting insecticide Baythroid

A pot experiment was conducted to compare the uptake and dry matter production potential of NH _inf4 ^sup+ and NO _inf3 ^sup- and to study the effect of Baythroid, a contact poison for several insect pests of agricultural crops, on growth and N uptake of maize (Zea mays L.). Nitrogen was applied as (¹⁵NH₄)₂SO₄, K¹⁵NO₃, or ¹⁵NH₄NO₃ and in one treatment Baythroid was combined with ¹⁵NH₄NO₃. Source of N had, in general, a nonsignificant effect on dry matter and N yield, but uptake of NO _inf3 ^sup- was significantly higher than that of NH _inf4 ^sup+ when both N sources were applied together. Substantial loss of N occurred from both the sources, with NH _inf4 ^sup+ showing greater losses. Baythroid was found to have a significant positive effect on dry matter yield of both root and shoot; N yield also increased significantly. Uptake of N from both the applied and native sources increased significantly in the presence of Baythroid and a substantial added nitrogen interaction (ANI) was determined. The positive effect of Baythroid was attributed to: (1) a prolonged availability of NH _inf4 ^sup+ due to inhibition of nitrification, (2) an increased availability of native soil N through enhanced mineralization, and (3) an enhanced root proliferation.     

Crop growth and nitrogen transformations in wheat (Triticum aestivum L.) planted after wetland rice (Oryza sativa L.)

Summary A field study was undertaken to examine the effects of various management strategies on wheat (Triticum aestivum L.) performance and N cycling in an intensively cropped soil. Microplots receiving 100 kg N ha^–1 as¹⁵NH₄ ^{+ 15}NO₃ ^– at sowing, tillering or stem elongation were compared with unfertilized microplots. Stubble from the previous rice crop was either incorporated, burnt without tillage, burnt then tilled or retained on the surface of untilled soil. Wheat grain yield ranged from 1.5 to 5.1 t ha^– and was closely related to N uptake. Plant accumulation of soil N averaged 36 kg N ha^–1 (LSD 5% = 10) on stubble-incorporation plots and 54 kg N ha^–1 on stubble-retention plots. Fertilizer N accumulation averaged 18 kg N ha^–1 (LSD 51% = 6) on stubble-incorporation plots and 50 kg N ha^–1 on stubble-retention plots. Tillage had little effect on burnt plots. Delaying N application from sowing until stem elongation increased average fertilizer N uptake from 26 to 39 kg N ha^–1 (LSD 5% = 6), but reduced soil N uptake from 50 to 37 kg N ha^– (LSD 5% = 10).Immobilization and leaching did not vary greatly between treatments and approximately one-third of the fertilizer was immobilized. Less than 1% of the fertilizer was found below a depth of 300 mm. Incorporating 9 t ha^–1 of rice stubble 13 days before wheat sowing reduced net apparent mineralization of native soil N from 37 to 3 kg ha^–1 between tillering and maturity. It also increased apparent denitrification of fertilizer N from an average 34 to 53 kg N ha^–1 (LSD 5% = 6). N loss occurred over several months, suggesting that denitrification was maintained by continued release of metabolizable carbohydrate from the decaying rice stubble. The results demonstrate that no-till systems increase crop yield and use of both fertilizer and soil N in intensive rice-based rotations.     

Near-infrared characteristics of forest humus are correlated with soil respiration and microbial biomass in burnt soil

Near-infrared spectroscopy and soil physicochemical determinations (pH_H2O, organic matter content, total C content, NH _inf4 ^sup+ , total N content, cation-exchange capacity, and base saturation) were used to characterize fire-or wood ash-treated humus samples. The spectroscopic and the soil physicochemical analysis data from the humus samples were used separately to explain observed variations in soil respiration and microbial biomass C by partial least-square regression. The first regression component obtained from the physicochemical and spectroscopic characterization explained 10–12% and 60–80% of the biological variation, respectively. This suggests that information on organic material collected from near-infrared spectra is very useful for explaining biological variations in forest humus.     

Impact of faunal complexity on microbial biomass and N turnover in field mesocosms from a spruce forest soil

In a field study using soil mesocosms in an acid spruce forest soil we investigated the effects of mesofauna and macrofauna on microbial biomass, dissolved organic matter, and N cycling. Intact soil monoliths were taken from the ground, defaunated by deep-freezing, and wrapped in nets of various mesh-sizes to control re-immigration of different faunal size-classes. The monoliths were then replanted in the field. Three treatments of mesocosms were prepared: (1) with only microbiota, (2) microbiota and mesofauna, and (3) microbiota, mesofauna, and macrofauna (= complex fauna). After 8 months of exposure the mesocosms and the unmanipulated control plots (treatment 4) were destructively sampled. We estimated microbial biomass by substrate-induced respiration and the chloroform fumigation-extraction method. N cycling was measured by monitoring microbial N mineralization, the NH _inf4 ^sup+ content, and selected amino acids and the activities of protease, urease, and deaminase. The results from the L/F layer showed that the pool of the microbial biomass was not changed by the activity of the mesofauna. However, the mesofauna and macrofauna together enhanced SIR. An increase in microbial N mineralization was only observed in treatment 3 (microbiota + complex fauna). Protease activity and NH _inf4 ^sup+ content increased in treatments 2 (microbiota + mesofauna) and 3 (microbiota + complex fauna). The complex fauna induced a soil pH increase in treatment 3 as opposed to treatment 1 and the control. This increase was presumably due to excretory NH _inf4 ^sup+ . Principal component analysis revealed that the complex fauna in treatment 3 caused a significantly higher N turnover per unit of microbial biomass.     

Mineral nitrogen dynamics in poorly drained blanket peat

Summary Mineral-N dynamics have been measured over a period of 3 years in PK- and NPK-treated plots (4 m²) laid out on an area of poorly drained, reseeded, blanket peat in the north of Scotland. Mineral-N, present in the peat almost entirely as NH _in4 ^sup+ , accumulated in winter, reaching 42 kg N ha^–1 in the surface 10 cm in April before the application of 112.5 kg N ha^–1 as NH₄NO₃ or urea. In situ incubation of peat cores isolated to prevent leaching, and with grass tops removed, confirmed that net mineralization occurred between November and April, with the greatest rate, 1.2 kg N ha^–1 day^–1, recorded between March and April. During the period May to early June, immobilization of N predominated and rates of net immobilization ranged between 0.2 and 0.8 kg N ha^–1 day^–1. This coincided with a poor uptake into herbage, less than 16% of soil mineral N and fertilizer NH₄NO₃ in June of the first 2 years. The largest counts (most probable number) of ammonifying bacteria in the surface 5 cm were recorded in July for aerobes (27.1×10⁹ litre^–1) and August for anaerorbes (7.1×10⁹ litre^–1). N fertilizer increased these counts significantly (P<0.05) to 56×10⁹ aerobes and 13×10⁹ anaerobes. During July and August, in 2 out of the 3 years, mineralization predominated over immobilization and mean net rates of up to 0.9 kg N ha^–1 were recorded.     

Effect of nitrogen fertiliser on temporal and spatial variation of mineral nitrogen and microbial biomass in a silvopastoral system

This paper describes a field study to assess the effect of increasing the frequency of split applications of N fertiliser on the pattern of plant uptake, soil N availability, and microbial biomass C and N. Measurements were taken during the growing season in different positions relative to young trees (Prunus avium L.) in an upland silvopastoral system in its first year after establishment. At fertiliser rates of 72 and 144 kg ha^-1 N applied as NH₄NO₃, increasing the number of split applications increased N uptake by the pasture. Mineral forms of soil N measured 2 weeks after application indicated that residual NH _inf4 ^sup+ -N and total mineral N were also greater in this treatment on certain dates. Soil NO _inf3 ^sup- -N was positively correlated with the soil moisture content, and nitrification reached a maximum in early May and declined rapidly thereafter except within the herbicide-treated areas around the trees where soil moisture had been conserved. Results of the study suggest that high NO _inf3 ^sup- -N in herbicide-treated areas was probably caused by mineralisation of grass residues and low uptake by the tree rather than by preferential urine excretion by sheep sheltering beside the trees. Mean microbial biomass C and N values of 894 and 213 kg ha^-1, respectively, were obtained. Microbial C was slightly increased by the higher frequency of split applications at 144 kg ha^-1 N and was probably related to the greater herbage production with this treatment. Microbial N was not significantly affected by the N treatments. Both microbial biomass C and N increased during the growing season, resulting in the net immobilisation of at least 45 kg ha^-1 N which was later released during the autumn.     

Effects of biotechnology byproducts and organic acids on nitrification in soils

Summary Recent developments in biotechnology industries produce increasing amounts of byproducts with potential uses in agriculture. The present research focused on the nitrification of NH _inf4 ^sup+ -N in biotechnology byproducts added to soils, and on the effects of 29 naturally occurring organic acids (19 aliphatic and 10 aromatic) on nitrification in soils. A 10-g soil sample was incubated for 10 days at 30°C with 2.0 mg NH _inf4 ^sup+ -N in a byproduct or with 10 or 50 mol organic acid and 2.0 mg reagent-grade NH _inf4 ^sup+ -N. In condensed molasses-fermentation solubles, produced during the microbial fermentation of sugar derived from corn (Zea mays L.) and molasses derived from beets (Beta sp.), in the production of lysine as a supplement in animal food, the nitrification of NH _inf4 ^sup+ -N was similar to that of byproduct or reagent-grade (NH₄)₂SO₄. Nitrite accumulated when either of these materials was added to a calcareous Canisteo soil. The NH _inf4 ^sup+ -N in slops (produced during microbial fermentation processes occurring in the production of citric acid) was not nitrified in soils. Some organic acids inhibited, whereas others activated, nitrification in soils. Formic, acetic, and fumaric acids enhanced the production of NO _inf2 ^sup- -N in a calcareous Canisteo soil, whereas all other aliphatic and aromatic acids studied decreased the accumulation of NO _inf2 ^sup- -N. It is concluded that the addition or production of organic acids in soils affects the microbial dynamics, leading to significant changes in rates of nitrification and possibly in other N-transformation processes in soils.