Nitrogenase activity associated with soil cores of grasses in Brazil

The N₂-ase activities of field-grown Brazilian grasses were measured with C₂H₂ reduction by soil cores containing the plants. C₂H₂ and C₂H₄ were observed to diffuse at similar rates through soil and equilibrated across the Brazilian soil in 3 h, but could take up to 30 h or more with some British soils. The diurnal fluctuation in the rates of N₂-ase activity by Brachiaria mutica and Sorghum vulgare were similar and the variation in rate was correlated with soil temperature. Estimates of N₂-fixation by measurement of C₂H₂ reduction by soil cores ranged from 14.7 to 51.4 g N ha^?1 day^?1 and were much lower than with “pre-incubated” excised roots from the cores or taken directly from the field. The merits of the soil core and the “pre-incubated” excised root assays are discussed. ft1|Present address: U.S. Department of Agriculture, Science and Education Administration, Agricultural Research, Northeastern Region, Room 309, Building 001, Beltsville, MD 20705, U.S.A.     

Nitrogenase activity on the roots of tropical forage grasses

Nitrogen fixation in the rhizospheres of field grown tropical forage grasses was studied by the acetylene reduction method. Values varied considerably between sites but indicate the possible economic importance of several of the species studied. Maximal nitrogenase activity measured (nmoles C₂H₄g^?1 dry roots h^?1) was 754 for Pennisetum purpureum, 750 for Brachiaria mutica, 341 for Digitaria decumbens, 299 for Panicum maximum, 283 for Paspalum notatum, 269 for Cynodon dactylon, 41 for Melinis minutiflora and 29 for Hyparrhenia rufa. Nitrogenase activity varied considerably with season and was maximal during active vegetative growth of two of the grasses. Significant differences between Paspalum notatum ecotypes and cultivars. in Azotohacter paspali occurrence and nitrogen fixation, indicate the possibility of plant breeding to enhance nitrogen fixation in grass rhizospherc associations. Other research lines of agronomic importance are fertilizer effects. In intact soil plant cores with the Paspalum system 10 parts/10⁶ NH_4⁺J-N inhibited nitrogenase activity within 2 h and 10 parts/10⁶ NO^?₃-N within 4 h. but after 1 week these effects were negligible. In the field, nitrogenase activity on roots of P. purpureum and D. decumbens, assayed 2 weeks after top dressings of 20 kg N ha^?1 as NH₄NO₃. was not affected even after eight such dressings.     

Nitrogenase activity associated with pasture grasses in Northern New South Wales

Nitrogenase activity associated with roots of grasses was initially examined at 67 sites in New South Wales using an enriched malate medium. Twenty six of the 39 grass species examined produced at least 10 nmol C₂H₄cm^?1 root h^?1—a level accepted as positive presumptive evidence of N₂-fixation: 40 of the 288 samples exceeded 100nmol C₂H₄h^?1. The seasonal N₂ase activity of up to 4 grass species collected in soil cores at 6 sites was measured over 16 months.Activity at field moisture levels, but incubated at 30°C was greatest for cores collected in summer months. Activity was increased in 33.5% of samples by raising soil moisture to field capacity. No single species of grass consistently supported higher activity than any other.Nitrogenase activity was compared in cores of Kikuyu grass (Pennisetum clandestinum) watered to in excess of field capacity and allowed to drain for between 7 and 28 days before re-watering. Activity declined rapidly in the first 7 days and although recovery was also rapid, integration of N₂ase activity over time showed a loss of 20 and 61% for 7 and 28 days drainage respectively. N₂ase activity was greatest at 30°C.Maximum N₂ase activity in field samples was only 246 nmol C₂H₄ core^?1 h^?1 indicating that fixation of N₂ would not be of agronomic significance.     

Nitrogenase activity associated with leaf,root, and rhizosphere soils in an Indian tropical forest

Summary N₂ fixation (acetylene reduction assay) by phylloplane microorganisms was measured in dominant and co-dominant plant species growing in a tropical rain forest. No significant acetylene reduction was recorded with intact leaf samples. Azotobacter sp., Beijerinckia sp., Derxia sp., and Klebsiella pneumoniae were isolated as phylloplane N₂-fixing bacteria. Azospirillum lipoferum was only isolated from soil samples containing the roots of Poaceae. Nitrogenase activity was recorded in culture derived from the roots and rhizosphere soil samples, although low acetylene reduction activity indicates that these associations did not provide large amounts of N to the systems studied.     

Vesicular-arbuscular mycorrhizae decrease zinc-toxicity to grasses growing in zinc-polluted soil

Vesicular-arbuscular mycorrhizae (VAM) are known to enhance the uptake of heavy metals in the host and could therefore increase the effects of heavy metal pollution on plant populations. The effect of mycorrhizal infection on two grasses occurring in coastal dunes downwind of a blast furnace complex and which are becoming increasingly polluted, was examined. In a 2 × 2 factorial experiment, the effects of VAM and zinc on the growth and mineral nutrition of Festuca rubra and Calamagrostis epigejos was established. It appeared that VAM infection alleviated the negative effect of zinc on growth in both species. Although both zinc and VAM influenced the uptake and translocation of various minerals, differences in mineral concentrations could not explain why a higher biomass is produced by grasses infected with mycorrhiza in zinc-polluted soil.     

Nitrogenase activity and nitrogen-fixing bacteria associated with the roots ofAtriplex spp. growing in saline sodic soils of Pakistan

Summary TwoAtriplex spp. growing in low-fertility saline sodic soils were assayed for root-associated nitrogenase activity. The excised washed and unwashed root of the two species.A. lentiformis andA. amnicola, showed high root-associated nitrogenase activity. Acetylene-reducing activity seemed to be directly influenced by moisture. The highest number of diazotrophs, enumerated using a most probable number technique was observed on the root surface. Most of the isolated diazotrophs were identified asEnterobacter agglomerans. Root-associated nitrogenase activity inAtriplex spp. may explain the high protein and biomass content of these plants growing in low-fertility saline sodic soils.     

Use of undisturbed cores of surface soil for investigating leaching losses of sulphur and phosphorus

Undisturbed soil cores 10 cm long were collected using a precisely constructed 8.5-cm diameter soil sampler. To study the loss of sulphur and phosphorus from these cores due to leaching, a technique was developed for encasing the walls of the cores in a microcrystalline wax.Distilled water was applied to the surface of the cores at the rate of 2 ml/min and the loss of native plus applied sulphur and phosphorus was determined for successive 20 ml aliquots that percolated through the soil. When water equivalent to 90 mm of rainfall was added to the soil cores, losses of 2.0 kg S/ha and 0.30 kg P/ha occurred due to leaching.     

Can tropical grasses grown as cover crops improve soil phosphorus availability?

Tropical grasses grown as cover crops can mobilize phosphorus (P) in soil and have been suggested as a tool to increase soil P cycling and bioavailability. The objective of this study was to evaluate the effect of tropical grasses on soil P dynamics, lability, desorption kinetics and bioavailability to soya bean, specifically to test the hypothesis that introducing grass species in the cropping system may affect soil P availability and soya bean development according to soil P concentration. Three grass species, ruzi grass (Urochloa ruziziensis ), palisade grass (Urochloa brizantha ) and Guinea grass (Megathyrsus maximus ), were grown in soils with contrasting P status. Soya bean was grown after grasses to assess soil P bioavailability. Hedley P fractionation, microbial biomass P, phytase‐labile P and the diffusive gradient in thin films were determined, before and after cultivation. It was found that grasses remobilized soil P, reducing the concentration of recalcitrant P forms. The effect of grasses on changing the P desorption kinetics parameters did not directly explain the observed variation on P bioavailability to soya bean. Grasses and microorganisms solubilize recalcitrant organic P (P_o) forms and tropical grasses grown as cover crops increased P bioavailability to soya bean mainly due to the supply of P by decomposition of grass residues in low‐P soil. However, no clear advantages in soya bean P nutrition were observed when in rotation with these grasses in high‐P soil. This study indicates that further advantages in soya bean P nutrition after tropical grasses may be impeded by phytate, which is not readily available to plants.     

Ionic relations in tropical pasture grasses

Ionic relations in the tops of nine tropical pasture grasses were studied by using K‐Na and K‐Mg nutrient replacement series in sand culture. Rhodes grass, green panic, and pangola grass accumulated Na when K supply was low and Na was available. Sorghum, kikuyu, paspalum, speargrass and setaria accumulated very little Na but considerable quantities of Mg when the K supply was low and Mg was available. There were smaller increases in the amounts of Mg accumulated by rhodes grass, green panic, buffel grass and pangola grass than in the other five species when grown in solutions of low K/Mg ratio.

Concentrations of K, Na, Ca, Mg, P, Cl, NO , and SO. in the plant tops are presented. Although experimental treatments resulted in considerable variations in the concentrations of individual elements, cation‐anion balances (C‐A) remained relatively constant when nutrient deficiency did not depress yield in rhodes grass, green panic, pangola grass, sorghum, kikuyu, paspalum and speargrass. C‐A values in setaria and buffel grass increased as the concentration of K in the nutrient solutions increased.

These results are used as a basis for suggesting management practices that will improve the chemical composition of plant material and avoid a number of nutritional disorders that can occur in animals grazing these grasses.     

Exchange resin cores for the estimation of nutrient fluxes in highly permeable tropical soil

There is an urgent need to improve the methods to estimate solute fluxes in soil, e.g. cumulative capture of leaching ions by exchange resin cores. We compared the suitability of different resin materials, core sizes, and installation procedures to assess nutrient leaching in a highly permeable, tropical Xanthic Ferralsol. Three different cation and anion resins, respectively, and two combined anion-cation resins were tested in laboratory experiments with respect to their removal and recovery of nutrients (Mg, NH₄⁺, NO₃^—) and total dissolved organic carbon (TOC) from solution. In a field experiment, cores with three different diameters (25, 100, and 200 mm) were installed either vertically from the soil surface or laterally from a soil pit. Cumulative leaching of NO₃^— and NH₄⁺ and of applied Sr was determined after 45 days. The combined anion and cation exchange resin (MB 20) showed satisfactory recovery of NO₃^— and NH₄⁺ from solution. None of the investigated resins could be used for TOC removal from solution due to high contents of soluble carbon compounds in the resins. Wetting and drying cycles did not affect the removal of solutes from solution or subsequent recovery from the MB 20 resin. Additionally, the combined resin MB 20 was easier to handle than separate anion or cation resins and was therefore used for further field experimentation. The smallest core size (25 mm) was not suitable for nutrient leaching determination due to high preferential flow along the inner core walls. The medium diameter cores (100 mm) showed the lowest variability and the best correlation between NO₃^— and Sr capture. They were easier to install and retrieve than the large diameter cores (200 mm), which posed serious handling problems and soil disturbance. The lateral installation caused significantly lower artificial flow in comparison to a vertical installation, which was shown by the lower Sr loss and slightly lower N capture. Therefore, a lateral installation of medium sized resin cores (100 mm diameter) from a soil pit was superior to the other alternatives tested in this study.     

Factors affecting the strength of undisturbed cores from soils with low structural stability

Undisturbed core samples of two similar sandy clay loam soils (Big Ground and Plum Orchard) were equilibrated at matric potentials of ?0.01, ?0.1 and ?1 MPa and tested for indirect tensile strength (ITS) and unconfined compressive strength (UCS). Rate of deformation was found to have significant and large effects on the measured ITS and UCS. In general, both ITS and UCS increased with increasing deformation rates although, particularly for UCS tests, the effect was variable. For ITS, the effect of rate of deformation was attributed to water having insufficient time to move towards any incipient failure plane. For UCS, the effect of rate of deformation was partly attributed to loss of moisture during loading, since samples strained at lower deformation rates generally had lower moisture contents; and partly to the different modes of failure seen. ITS and UCS of Big Ground were invariably higher than those of Plum Orchard at all matric potentials. This was attributed to Big Ground having been under an intensive management system for approximately 10 years longer than Plum Orchard. Simple pretreatment effects are also considered. In a comparison of measured and theoretically predicted ITS and UCS, theory provided good estimates of measured strengths of samples equilibrated at ?0.1 MPa, although samples were considerably weaker than theoretically predicted at ?1 MPa.     

Diffusion probe for gas sampling in undisturbed soil

Soil‐atmosphere fluxes of trace gases such as methane (CH₄) and nitrous oxide (N₂O) are determined by complex interactions between biological activity and soil conditions. Soil gas concentration profiles may, in combination with other information about soil conditions, help us to understand emission controls. This paper describes a simple and robust diffusion probe for soil gas sampling as part of flux monitoring programmes. It can be deployed with minimum disturbance of in‐situ conditions, and also at sites with a high or fluctuating water table. Separate probes are used for each sampling depth, in this study ranging from 5 to 100 cm. The probe has a 10‐ml diffusion cell with a 3‐mm diameter opening covered by a 0.5‐mm silicone membrane. At sampling the diffusion cell is flushed with 10 ml N₂ containing 50 µl l^?1 ethylene (C₂H₄) as a tracer; tracer recovery is used to calculate sample concentrations. Ethylene is immediately removed by flushing with unamended N₂. Equations are presented to correct for dead volumes of connecting tubing and valves. Laboratory tests evaluated recovery of CH₄, N₂O and carbon dioxide (CO₂), removal of C₂H₄ and equilibration of CH₄, N₂O and CO₂ in air and water. Field tests on peat soils used for grazing showed soil gas concentrations of CH₄ and N₂O as influenced by topography, site conditions and season. The applicability of the diffusion probe for trace gas monitoring is discussed.     

Selective sterilisation of undisturbed soil cores by gamma irradiation: Effects on free-living nematodes,microbial community and nitrogen dynamics

The techniques available for sterilisation or defaunation of soil in ecological experiments mostly have strongly unwanted effects on soil structure and the dynamics of major nutrients such as nitrogen. The potential for using gamma irradiation to prepare defaunated soil microcosms was investigated by subjecting undisturbed soil cores to a range of irradiation doses (0, 5, 10, 20 and 40 kGy). The absence of living nematodes at the lowest irradiation dose was confirmed by microscopic observation. The effects of irradiation dose on mineral nitrogen (as NO₃⁻ and NH₄⁺), microbial biomass C (C_mic), and phospholipid fatty acid (PLFA) concentration and composition were determined over a 4 week incubation period. An increase in the concentration of NO₃⁻ occurred during the incubation period after exposure to 0, 5 and 10 kGy but was barely detectable at 20 and 40 kGy. The effect of irradiation dose on NH₄⁺ release was complex and highly variable within treatments, with the 10 kGy dose resulting in the highest concentrations. Microbial biomass carbon was significantly reduced following a 20 kGy irradiation dose and below detection at 40 kGy. Most remarkably, the sum of all measured PLFAs did not differ significantly between most treatments and was not correlated with microbial biomass. In most cases the concentration of signature fatty acids only differed significantly between the control and the highest irradiation dose treatments. To ascertain the sensitivity of microbial taxa to acute gamma irradiation with accuracy, measures of microbial community structure other than PLFA analysis are needed.     

A new laboratory method to quickly determine the unsaturated hydraulic conductivity of undisturbed soil cores within a wide range of textures

A modified instantaneous profile method is described, which determines the hydraulic conductivity functions of soils with varying textures. Soil suction head as a function of time and depth, rate of outflow as a function of time and the final distribution of moisture content are monitored on undisturbed soil columns. Data is handled following a modification of the procedure of Weeks and Richards (1967). The apparatus consists of five measuring cells, a monitoring unit, five balances and equipment to section the soil columns for the gravimetric determination of soil moisture content. Each measuring cell contains a soil core of 250 cm³, into which five micro-tensiometers are inserted at 2 cm intervals. Exemplary hydraulic conductivity functions are shown for samples taken from an Arenosol, from silty and clayey-silty horizons of a Luvisol and from a clayey Vertisol. The functions showed good agreement with in situ data. Evaluations of variability show the method to be very reliable. On average, the functions of five samples can be determined in one week, making the method quick. At the same time, the method has great potential for the study of effects of soil structure on hydraulic conductivity.     

Dynamics of soil microbial biomass C, N and P in disturbed and undisturbed stands of a tropical wet-evergreen forest

To understand the spatial and temporal dynamics of soil microbial biomass and its role in soil organic matter and nutrient flux in disturbed tropical wet-evergreen forests, we determined soil microbial biomass C, N and P at two soil depths (0–15 and 15–30 cm), along a disturbance gradient in Arunachal Pradesh, northeastern India. Disturbance resulted in considerable increase in air temperature and light intensity in the forest and decline in the soil nutrients concentration, which affected the growth of microbial populations and soil microbial biomass. There were significant correlations between bacterial and fungal populations and microbial biomass C, N and P. Soil microbial population was higher in the undisturbed (UD) forest stand than the disturbed forest stands during post-monsoon and less during rainy season due to heavy rainfall. Greater demand for nutrients by plants during rainy season limited the availability of nutrients to soil microbes and therefore, low microbial biomass C, N and P. Microbial biomass was negatively correlated with soil temperature and pH in all the forest stands. However, there were significant positive relationships among microbial biomass C, N and P. Percentage contribution of microbial C to soil organic C was higher in UD forest, whereas percentage contribution of microbial biomass N and P to total N and total P was higher in the moderately disturbed site than in the highly disturbed (HD) site. These results reveal that the nutrient retention by soil microbial biomass was greater in the selective logged stand and would help in the regeneration of the forest upon protection. On the other hand, the cultivated site (HD) that had the lowest labile fractions of soil organic matter may recover at a slower phase. Further, minimum and maximum microbial biomass C, N and P during rainy and winter seasons suggest the synchronization between nutrient demand for plant growth and nutrient retention in microbial biomass that would help in ecosystem recovery following disturbance.     

Fertilizer and temperature effects on urea hydrolysis in undisturbed soil

Summary Few published studies have examined the effects of a continuous fertilizer application on urea hydrolysis. In the present study we investigated the effects of 9 years of continuous application of urea and P fertilizers on urea hydrolysis in undisturbed soil samples as affected by temperature (5–45°C). Undisturbed soil samples of surface horizons (0–7 cm) were obtained with cutting rings (50 cm³) from different fertilizer-treatment plots and inserted in polyethylene bottles (with cutting rings). Each soil sample (in the cutting ring) was treated with 10 ml urea solution (0.5 mmol urea N g^-1 soil) and then broght to 90% field capacity. The samples were left to equilibrate for 30 min at a temperature of 4°C, then placed in an incubator at 37°C for 6 h. The results indicated that 9 years of continuous application of urea but not P had a significant effect on urea hydrolysis in soil. There was a good correlation between temperature and urea hydrolysis in soil. Q ₁₀ was between 1.97 and 2.08 in the temperature range 5–45°C.     

Changes in spatial distribution of fungal propagules associated with invertebrate activity in soil

Soil fungal species density and aggregation had changed 6 months after spring burning and raking in a Wisconsin prairie. Fungal species density had increased by 29% in burned relative to raked or undisturbed plots; species density was highest at the soil surface in burned and raked plots, and lowest at the surface in undisturbed plots. Fungal propagules of the same species were less aggregated in burned and raked plots than in undisturbed. These changes imply greater mixing of fungal propagules in soil of burned and raked plots than in undisturbed plots. The physical effects of burning and raking and the properties of fungal spores do not account for the changes. It is argued that soil invertebrates are responsible for the mixing because: (1) at any depth in soil, density of mites and collembola increased concurrently with fungal species density; and (2) path analysis shows that fungal species density is equally associated with microarthropod density and root biomass after burning, but after raking, fungal species density was more strongly associated with root biomass than microarthropods.     

Modelling water flow through undisturbed soil cores using a transfer function model derived from 3HOH and Cl transport

Large undisturbed soil cores (20 cm diam. × 25–30 cm long) were irrigated at rates of 0.5–4 cm h^?1 with 0.005 M CaCl₂ solution labelled with ³HOH. The cores were used at varying initial water contents and flow in all cases was unsaturated. Breakthrough curves for Cl and ³HOH were markedly asymmetric and unlike those reported for columns of packed aggregates. The data could be satisfactorily described using a density distribution function of the logarithm of cumulative drainage D. The mean and standard deviation of In D were estimated by a curve-fitting procedure from Cl and ³HOH effluent concentrations in each core. The mean pore water velocity and fraction of the soil water that participated in solute transport (the mobile volume) were also calculated. The apparent velocity of Cl movement was always greater than that of ³HOH which suggested that the mobile volume involved in convective and diffusive transport of Cl was less than that for ³HOH. We suggest that Cl and ³HOH diffused at different rates out of flowing water films in a relatively few large conducting channels into essentially immobile water within the surrounding soil matrix. The difference in mobile volume for Cl and ³HOH was used to calculate the perimeter of voids in any horizontal cross-section of the soil through which water flowed, assuming a planar interface between the mobile and immobile water.