Massey Documents by Type
Permanent URI for this communityhttps://mro.massey.ac.nz/handle/10179/294
Browse
9 results
Search Results
Item Impact of phosphate fertiliser derived fluorine on soil microbiology and white clover (Trifolium repens L) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Environmental Sciences at Massey University, Palmerston North, New Zealand(Massey University, 2019) Thangavelautham, GeretharanFluorine (F) is a significant contaminant in most phosphate fertilisers and fertiliser-derived F is accumulating in New Zealand agricultural soils as a consequent of phosphate fertiliser application. There is potential for soil fluoride (F⁻) to detrimentally affect soil biological functions such as nitrogen fixation by Rhizobium leguminosarum, and to alter soil properties. Fluorine accumulation in soil may require changes to future land use and management practices. The aim of this thesis is to investigate whether phosphate fertiliser-derived soil F has a detrimental effect on soil microorganisms. A novel analytical method for soil F analysis was developed to measure the total soil F concentration based on extraction with dilute NaOH. The relative error between a novel 4 mol L⁻¹ NaOH extraction and the conventional fusion method was < 2 for organic-matter and volcanic parent material derived soils but was > 2 for recent and pallic soils. Precision of the 4 mol L⁻¹ NaOH extraction method, measured through repeat analysis of three further soils (n = 270), was calculated as < 9% Relative Standard Deviation (RSD). To define a standard method to quantify the bioavailable F concentration in soil, samples were extracted with water, 1 mol L⁻¹ HCl, 0.01 mol L⁻¹ CaCl2, 0.01 mol L⁻¹ KCl, and 1 mol L⁻¹ NH4Cl. Compared to water, 0.01 mol L⁻¹ CaCl2 had high relative recovery (of bioavailable F) in soils which have elevated Fe and Al content. Therefore, 0.01 mol L⁻¹ CaCl2 is recommended to measure the bioavailable F concentration of New Zealand pastoral soils. There is no data available which describes the toxic effect of bioavailable F on R. leguminosarum in New Zealand soils. A laboratory incubation experiment and MicroResp 96-well format respiration-inhibition assay were conducted to investigate the effect of F on R. leguminosarum and white clover. Rhizobium leguminosarum growth was not significantly suppressed by F⁻ concentrations less than 100 mg L⁻¹. The normal rod-shaped bacterium cell of R. leguminosarum was morphologically altered when exposed to F⁻ concentrations above 100 mg L⁻¹. The IC10 values determined for F⁻ toxicity to R. leguminosarum were higher than 100 mg F⁻ L⁻¹. Pottle-based experiments showed that white clover growth was not significantly supressed at a F⁻ concentration < 70 mg L⁻¹, while healthy nodules were formed up to a F⁻ concentration of 100 mg L⁻¹. Light and TEM micrographs of nodules revealed that the Rhizobium-white clover interaction was not influenced by F⁻ concentrations up to 100 mg L⁻¹. To assess the potential effects of lime and compost amendment on the bioavailability of F, laboratory F⁻ adsorption/desorption experiments were conducted. Results revealed that at pH < 6, F⁻ adsorption significantly (p < 0.05) increased as a function of compost application. At soil pH > 6, F⁻ adsorption was not significantly (p > 0.05) influenced by compost. Lime application increased the soil pH and maximum F⁻ adsorption was recorded at soil pH between 5.5 – 6.8. These results showed that soil pH significantly influences (p < 0.05) F⁻ desorption and this should be considered in the management of pastoral soil with elevated F. A pot trial was conducted to quantify the effect of added F (equivalent to 0 - 50 years of F accumulation via the continuous application of phosphate fertiliser) on soil properties, soil microbial activity, white clover growth, and R. leguminosarum in an Allophanic soil. F addition (0 – 385 mg kg⁻¹) significantly (p < 0.05) increased soil pH and Dissolved Organic Carbon (DOC) from 5.18 to 5.53 and from 270.5 to 331.3 mg kg⁻¹, respectively. The CaCl2-extractable F concentration increased from 4.95 to 12.67 units as a function of added F. Microbial biomass carbon and soil enzyme activities, and white clover growth and interaction with R. leguminosarum, were not influenced by added F⁻ up to the highest concentration used in this study. White clover variety Merlyn and Tribute shoot F concentration was increased from 4.9 to 19.9 mg kg⁻¹ DM and 5.12 to 16.68 mg kg⁻¹ DM, respectively, however these concentrations are not expected to represent a risk to grazing livestock. This study highlights that the 4 mol L⁻¹ NaOH extraction method is a simple and accurate technique to measure the total F concentration for soils which have high Fe, Al and organic matter content. Water extractable and 0.01 mol L⁻¹ CaCl2-extractable F concentration are recommended to measure the bioavailable concentration of F in New Zealand soils. Current New Zealand bioavailable F concentrations are orders of magnitude lower than the F⁻ concentration assessed to be toxic to R. leguminosarum and white clover, and this suggests there is no imminent risk of soil F to R. leguminosarum. Compost is not recommended as an amendment for soils which have a pH above 6.0 to minimize the bioavailable soil F⁻ concentration. Lime application is suitable in such soils to minimize the bioavailable soil F⁻ concentration through altering soil pH. The major fraction of added F is immobilised by Allophanic soil and this effectively reduces the available F concentration to plants and soil microorganisms. Future work is recommended to investigate the uptake mechanism of bioavailable F into white clover shoots and roots. However, there is no evidence to suggest that F concentrations in New Zealand soil are a risk to New Zealand’s pasture-based farming systems.Item Resistance of environmental bacteria to heavy metals and antibiotics in selected New Zealand soils : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Health Sciences at Massey University, Wellington, New Zealand(Massey University, 2020) Heydari, AliThe usage of superphosphate fertilizers, animal remedies and other material containing heavy metals (HMs) in agriculture and horticulture is a problematic issue resulting in the accumulation of HMs in the soil. The presence of these HMs in the soil leads to the induction of resistance of environmental bacteria to these heavy metals and may co-select for resistance against a broad range of antibiotics (Abs). This co-selection may increase the health risk for both humans and livestock because of resistance to the wide range of Abs. As well as direct health effects, an increase in Ab resistance may impose a significant burden on the livestock industry and primary production, leading to potential for immense social and financial losses. The current project was aimed to investigate the resistance of soil-borne bacteria sampled from selected regions of New Zealand. Genetic diversity of these bacteria and molecular aspects of horizontal transfer of HM and Ab resistance genes to other bacterial hosts was also investigated. Soil samples with a different history of usage, including pastoral and arable with high levels of HMs (e.g. cadmium (Cd) and zinc (Zn)) were collected from the Waikato region (WR), as well as soil from an area of native bush (background) as control. Waikato Region (WR) is one of the regions in New Zealand with high levels of HMs in soil, due to the regular use of HM-containing superphosphate fertilizers and animal remedies. Belmont Regional Park (BRP) airstrip soil was used as a novel site to explore bacterial communities’ resistance to HMs, and any co-selected Ab resistance. A comprehensive investigation was performed to simulate the soil environment contaminated with various levels of HMs to interrogate induced resistance to HMs and Abs in soil bacterial communities using microcosms with 6 weeks and 6 months incubation. The experiments carried out to investigate soil bacterial resistance to HMs and Abs were divided into two different categories, including physiological and molecular experiments. The physiological tests included plate culturing with a range of HMs and Abs concentrations and Pollution Induced Community Tolerance (PICT) analysis. Molecular investigations using Terminal Restricted Fragments Lengths Polymorphism (TRFLP) and Next Generation 16s rDNA were conducted to determine the probable changes in bacterial community structures induced by selection pressure of HMs presence in soil samples. Cd resistance genes were detected in individual bacterial isolates using specific oligomeric DNA primers via the polymerase chain reaction (PCR). Horizontal transfer of these genes to new bacterial recipients was investigated. Finally, Cd resistant bacterial isolates involved in Horizontal Gene Transfer (HGT) were identified using 16s rDNA Sanger Sequencing. Results clearly showed that there were significant differences between the levels of resistance to HMs and Abs in bacterial isolates from WR’s pastoral and arable soils compared to background soil (native bush). Differences between BRP soil samples with higher levels of HMs compared to those with lower HMs concentrations, and also microcosms’ with a range of HM levels showed there were significantly greater number of bacterial isolates resistant to HMs and Abs in soils with the higher initial levels of HMs. Pollution Induced Community Tolerance (PICT) analysis provided complementary results in concordance with the results of plate culturing experiments and showed the higher levels of bacterial resistance to HMs and Abs in soils with the higher initial levels of HMs. Terminal Restriction Fragment Length Polymorphism (TRFLP) and 16s rDNA Next Generation Sequencing experiments investigated HM-induced bacterial communities' structure changes and revealed significant differences among the bacterial community structures in the selected BRP and microcosms soil samples. The HGT experiments revealed the horizontal transfer of Cd resistance genes from donor isolates (from WR, BRP, and microcosms soils) to a characterised recipient bacterial strain in vitro, suggesting these genes were carried by mobile genetic elements. Overall, the result of the current project showed that there were higher levels of bacterial resistance to HM and also to Ab occurred while different levels of HMs were present in the soil. In addition, higher levels of HM and Ab resistance induction occurred in the presence of specific concentrations of HMs in microcosms’ soils. The bacterial community structures were changed in the presence of various levels of HM in soil. The investigation of bacterial community structures changes in microcosms containing background soil samples were greater compared to the microcosms containing pastoral soils; it is concluded in higher changes in bacterial communities in soils in presence of selection pressure of HMs. Cd resistance genes located on mobile genetic elements were able to be transferred horizontally form donor bacterial strains to recipients and the transconjugants showed resistance not only to Cd, Zn and/or Hg, but also to a range of Abs; it showed the possibility of spread of these HM resistance genes to the new bacteria in soil and conferring HM and subsequent Ab resistance in recipients.Item Bioprospecting soil metagenomes for potential new antibiotics : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Genetics at Massey University, Albany, New Zealand(Massey University, 2017) Hong, KellyMany soil-dwelling microbes have the natural capacity to produce toxic compounds that inhibit growth of competing bacteria; most traditional antibiotics have been derived from small molecules made by such soil-based microorganisms, of which only a small fraction can be grown in the laboratory. Since techniques that require culturing of these microbes in the lab have been the starting point for studying them in the past, our knowledge of the uncultured majority remains limited. Functional metagenomics is a method that circumvents the need for culturing, and thus has the potential to revel a yet untapped reservoir of antibacterial compounds. Here we present a potential application of functional metagenomics using genes isolated from soil microbes that employs high throughput sequencing to identify microbial genes encoding novel compounds that inhibit bacterial growth.Item Investigation of genetic changes in inoculant strains of Rhizobium trifolii isolated from the soil : a thesis presented in fulfillment of the requirements for the degree of Master of Science in Microbiology at Massey University(Massey University, 1985) O'Hara, Michael JohnInformation about the fate of plant inoculating strains of Rhizobium trifolii entering the soil environment is incomplete. It is known that inoculating strains must compete with existing adapted strains, if such are present. It is not known whether or not the introduced strains can adapt to soil conditions. Strains of the white clover (Trifolium repens) symbiont, R. trifolii, were isolated from plants growing as a result of sowing virgin soil with bacteria-coated seed. Rhizobium bacteria were isolated from one nodule on each randomly chosen plant at two and then six months after sowing. Three different methods were used to type the isolated strains because of the importance of distinguishing between derivatives of the inoculant (R. trifolii #2668) and adapted rhizobia immigrating from adjacent pastures. Gel diffusion identification of antigens showed that all strains reacted positively to anti-2668 serum, although the response was not identical for all strains. The determination of intrinsic antibiotic resistance patterns showed that low level resistances were accumulating in a non-random manner as time progressed. Initial isolates showed the same pattern as 2668. Restriction endonuclease analysis of the isolated strains showed them all to have a high degree of similarity to 2668, with a few being identical in pattern. This was despite alterations in numbers and sizes of plasmids (as compared to those in 2668) seen in these isolates. A nif gene probe of a plasmid profile showed that several strains had alterations in the size and number of bands which would hybridize, as compared to 2668. The field isolated strains had gained the ability to produce a broad range bacteriocin-like inhibitor. Conjugation experiments between R. trifolii #0/18 and E. coli HB101 showed that this inhibitor was transferable to and expressable by the E. coli, strain. This suggests the existence of a broad host range replicon in the field isolates which either carries or mobilizes this function.Item Bioprospecting : the quest for novel extracellular polymers produced by soil-borne bacteria : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Microbiology at Massey University, Palmerston North, New Zealand(Massey University, 2017) Smith, JasonBacteria are ubiquitous in nature, and the surrounding environment. Bacterially produced extracellular polymers, and proteins are of particular value in the fields of medicine, food, science, and industry. Soil is an extremely rich source of bacteria with over 100 million per gram of soil, many of which produce extracellular polymers. Approximately 90% of soil-borne bacteria are yet to be cultured and classified. Here we employed an exploratory approach and culture based method for the isolation of soil-borne bacteria, and assessed their capability for extracellular polymer production. Bacteria that produced mucoid (of a mucous nature) colonies were selected for identification, imaging, and polymer production. Here we characterised three bacterial isolates that produced extracellular polymers, with a focus on one isolate that formed potentially novel proteinaceous cell surface appendages. These appendages have an unknown function, however, I suggest they may be important for bacterial communication, signalling, and nutrient transfer. They may also serve to increase the bacteria’s surface area for nutrient adsorption without compromising structural integrity of the cell. The results from this study contribute to the scientific body of knowledge and provide avenues for further research into bacterial appendage formation.Item The analysis of plasmid rearrangements observed in the soil bacterium OR168 after the introduction of transposon Tn5 : a thesis presented in fulfilment of the requirements for the degree of Master of Science in Genetics at Massey University(Massey University, 1994) Harte, David John GerardTransposon Tn5 mutagenesis has been used extensively in Escherichia coli and various other Gram-negative bacteria to produce both random and site directed mutants. The popularity of Tn5 as a mutagen stems from its apparent random insertion into the genome, leading to non-leaky polar mutations. It also confers on many bacteria resistance to aminoglycosides, providing a strong selectable marker. The site of insertion can be mapped by Southern DNA hybridisation against a specific Tn5 probe. Tn5-containing derivatives of the Rhizobia-like soil isolate, OR168, were produced using the broad host-range suicide plasmid vector pSUP1011. After the transfer of pSUP1011 to OR168 via heterogeneric bacterial conjugation, stable OR168::Tn5 exconjugants were selectively isolated at frequencies of approximately 10-4 per recipient. None of the 53 OR168::Tn5 exconjugants screened showed the parental plasmid profile. Visible alterations to the plasmid profile were common with respect to the native plasmid profile. These events generally showed large deletions from, or additions to, the native replicons of OR168. The alterations also included a low incidence of a decrease in plasmid number. Analysis of the exconjugant population shows that the insertion of Tn5 into the genome of OR168 may not be strictly random. It was shown that 66% of OR168::Tn5 exconjugants screened contain a plasmid-borne Tn5 element, with 90% of those involving Tn5 insertion in the same episome. There is evidence that events other than classical conservative transposition have occurred after the introduction of pSUP1011 into the OR168 genome. Screening of the isolated OR168::Tn5 population for pSUP1011 vector sequences revealed the presence of the pSUP1011-derived RP4-mob fragment in 33 of 35 OR168::Tn5 exconjugants containing a plasmid-borne Tn5 element. Analysis also revealed the acquisition of Tn5 alone, presumably by conservative transposition, occurred only twice in the 35 events involving a plasmid target. This suggests that another site within the RP4 fragment can act as a surrogate transposase recognition site. Alternatively, the insertion of the RP4-mob::Tn5 sequence into a plasmid target may involve a site specific recombination process peculiar to the OR168 isolate. No mechanism was elucidated for the formation of many of the alterations in plasmid mobility. Restriction fragment lengths in the immediate vicinity of the anomalous RP4-mob::Tn5 insertion arc identical in different plasmids. This may indicate sequence duplication among the OR 168 plasmids. Such duplication may precipitate, through homologous recombination processes, the plasmid instability observed.Item The effects of tillage practices on soil microbial biomass and CO2 emission : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Agricultural Engineering at Institute of Technology and Engineering, Massey University(Massey University, 1998) Aslam, TehseenConversion of permanent pasture land to forage crop rotation by conventional tillage and reversion to pasture, for recovery of nutrients is a common practice in New Zealand. Because of their effects on soil physical, chemical and biological degradation, and the extent to which these soil management practices are sustainable is not fully known. To evaluate short- and long-term impact of tillage induced changes in soil physical, chemical and biological properties, a quad replicated field experiment was established at Massey University, Turitea campus in 1995. Permanent pasture land was converted to a double crop rotation using conventional (CT) and no-tillage (NT) practices on the Ohakea silt loam soil. The overall aim of this research programme is to develop a sustainable land use management for pasture-based arable cropping to suit local farming conditions. The present study investigated the effects of CT and NT practices on soil biological status and CO2 emission. The test crops were summer fodder maize (Zea mays L.) and winter oats (Avena sativa). An adjacent permanent pasture (PP) was used as a control. Soil samples were collected at 0-100 mm in summer, 0-50 and 50-100 mm depths in autumn and winter before or after crop harvest. The 'fresh' field moist, sieved samples were used for the measurement of microbial biomass carbon (MBC), nitrogen (MBN), phosphorus (MBP) and basal soil respiration. Earthworm population and biomass were extrusion with formaldehyde. Field CO2 emission was measured at 3-4 weeks interval for one year. After two years of continuous cropping, overall nutrients status (organic C, total N and total P) in NT remained similar to that in PP. In CT the nutrient levels were significantly lower. Earthworm population and live mass were also significantly lower in CT as compared to PP and NT treatments. However, there was no differences in plant establishment, crop dry matter yield, soil temperature and soil pH (0-100 mm depth) between the two tillage (NT and CT) systems. Higher levels of MBC, MBN and MBP were found in NT as compared with CT at 0-100 mm depth throughout the three seasons studied. When samples were analysed separately from two depths i.e. 0-50 and 50-100 mm, the microbial biomass contents were higher in surface soil (0-50 mm depth) as compared with 50-100 mm depth. Microbial biomass contents at 50-100 mm layer did not differ significantly among the three treatments. At 0-100 mm depth, MBC declined by 29%, MBN by 32% and MBP by 33% with two years (4 crops) of CT. Such a decline in microbial biomass is an early indication of future decline in soil organic matter. Soil organic matter (total C) had also declined by 22% (from 35,316 to 27,608 kg ha-1) with CT. No such decline occurred either in MBC, MBN and MBP or organic matter with NT. Basal soil respiration data indicated that microbial biomass activity in CT was 38% lower than in NT at 0-50 mm depth. However, at 50-100 mm depth, the activity was 25% higher in CT as compared with NT. Metabolic quotient (qCO2) did not differ among the three treatments at 0-50 and 50-100 mm soil depths. Field CO2 emission from PP was significantly higher as compared to NT and CT treatments. The two tillage practices did not influence the CO2 emission measured both shortly after tillage and during crop growth period. The annual estimated carbon loss through CO2 emission was 34 t C ha-1 year-1 in PP, 24 t C ha-1 year in NT and 21 t C hayear in CT treatment. Field CO emission was generally higher in summer and autumn as compared to winter and spring. Overall, this study, which spanned two cropping seasons, clearly showed that 2 years cropping with CT resulted in a decline in soil biological status and organic matter. The decline in soil biological status is likely to affect crop yields in CT over the longer period. Conversely, NT cropping was efficient in sustaining soil biological status and organic matter. NT had similar influence on soil biological status as clover based PP during a short-period. Therefore, it is concluded that NT may be used as an effective tool to enhance soil productivity while promoting agricultural sustainability.Item An investigation of some factors influencing the rate of oxidation of elemental sulphur fertilizers : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University(Massey University, 1990) Chatupote, WichienMethodologies for measuring the particle size of So in pure and compound fertilizers (sulphurized superphosphates (SSP), reactive phosphate rocks (RPR) and partially acidulated phosphate rock (PAPR)) and for determining the rate at which So in these materials oxidises in soils were evaluated and improved. Sample dispersion in 10% HC1 followed by wet sieving was the most successful method for sizing So in SSP, RPR and PAPR based fertilizers. So/bentonite fertilizers, however, were more easily dispersed in water than in acid. Acetone extraction (40 g:200 ml acetone, using a 16 h shaking period) and determination of So in the extract proved to be a suitable method for measuring amounts of So in finely ground fertilizers and soils at concentrations above 5 μg S g-1 soil and below 200 μg S ml-1 acetone. The rate of So oxidation in soil was determined by regularly measuring residual amounts of So. The influence of soil type and fertilizer history on the potential of soils to oxidise So was examined in incubation studies. On average, soils that had previously received So applications had higher initial rhodanese enzyme activities (RA) and higher So oxidation rates but there was no simple relationship between fertilizer history or RA and initial So oxidation rate. Different sources of So, namely Rotokawa So (geothermal So), dark So, Damman So, and agricultural grade So had similar oxidation rates per unit surface area. Granules or prills oxidised slowly in incubated soil because they did not disintegrate when placed in soil and had small specific surface area. On average, the oxidation rate of So was increased when mixed or granulated with reactive phosphate rocks and incorporated in soil but this effect was not consistently reproducible. Further incubations of So in the presence of various combinations, CaHPO4, CaCl2 and CaCO3, demonstrated that the presence of CaHPO4 and CaCO3 could elevate So oxidation rates. Granulation of RPR and PAPR with So did not significantly increase (p >0.05) the oxidation rate of So surface applied to undisturbed pasture soils (glasshouse and field trials). Under surface application conditions granulated So had similar oxidation rates to finely divided So forms. An iterative computer program was developed to calculate specific oxidation rates (K, μg So cm-2 day-1) from the amounts of acetone extractable So remaining in soils at different times. On average, K for <150 So μm was significantly lower (p <0.05) when surface applied to undisturbed soil cores than when incorporated into incubated soils. Specific oxidation rates of different particle sizes (<150, 150-250 and 250-500 μm) of surface applied So were similar (ranging form 11-19 μg So cm-2 day-1) but were different (P <0.05) for the two soil types used in glasshouse trials (means of 17 and 13 μg So cm-2 day-1 for Ramiha and Tokomaru soil, respectively). Corrections for the effects of soil moisture on oxidation rates provided evidence that all So could have similar maximum potential K values (Kmax = 18 μg S cm-2 day-1) in both soils. This suggested, with other evidence from the literature, that So oxidation in soil could be effectively modelled by knowing So particle size and the effects of soil moisture and temperature on So oxidation. A So oxidation simulation model was constructed using a value for Kmax determined in the glasshouse trials. Within experimental error, the simulation model predicted So oxidation in field soil well (explaining between 76 and 97% of data variance at 3 field sites) and provides a useful basis for designing future research projects.Item A quantitative analysis of the variability in the activity of nitrifying organisms in a soil under pasture : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University(Massey University, 1989) Bramley, Robert G. V.Variability in the inputs, outputs and transformations of mineral N under field conditions makes the predictive modelling of the leaching of soil nitrate very difficult. In an attempt to understand and quantify this variability, the activity of nitrifying organisms in the Tokomaru silt loam (a Typic fragiaqualf) under pasture was measured using a short-term nitrification assay (SNA). spatial dependence of the variability in SNA was examined using geostatistical methods, and the effect on SNA of soil pH change through liming, and of seasonal changes in soil temperature and moisture were investigated. Nitrifier activity and associated soil properties such as the amount of exchangeable ammonium arid the soil nitrate concentration, were found to decrease in value with depth between 0-24 cm. The greatest decrease in SNA was observed between 0-9 cm depth, but due to the need for sufficient quantities of soil to make SNA measurements, and the desire to avoid the possibility of inhibitory effects of grass roots on nitrification, soil was sampled from the 3-9 cm depth range for the bulk of the work reported here. Results indicated that the technique of sieving and mixing samples was satisfactory for removing depth-dependence from the results for spatial variability and other analyses. The spatial variability of SNA, soil NO3-, soil moisture content and the pH of the SNA incubation, which was assumed to approximate the field soil pH, was investigated over areas of 9 m2 and 625 m2 using a regular 11 x 11 square grid sampling design with minimum sample separations of 30 cm and 2.5 m respectively. However, the results of these analyses proved inconclusive, apparently due to the lack of samples separated by lags that were sufficiently short in relation to the overall dimensions of the sampling area. Accordingly, spatial analysis of the above properties, together with exchangeable ammonium, was carried out over 625 m2 using a nested sampling design that permitted an adequate number of observation points at lags ranging from 12.5 cm to 25 m. This design was a considerable improvement on the regular square design, although it had a number of shortcomings, notably bias caused in the estimation of the sample variance due to the nesting of a large number of data points within a small area, and bias caused in the estimation of values of the semivariance at some lags due to missing sampling points at some positions in the sampling grid. The values of SNA, NO3- and exchangeable ammonium were all highly variable and conformed to lognormal distributions. The range of spatial dependence in the variability of SNA, soil NO3- and incubation pH was 2.4, 5.4 and 6.1 m respectively. Exchangeable ammonium, SNA, soil NO3- and incubation pH varied isotropically within the sampling area but Ex- NH4+ showed no spatial dependence. Soil moisture content was strongly anisotropic, and showed no spatial dependence in one direction, but clear evidence of drift in a perpendicular direction. These results are discussed in relation to the most efficient sampling strategy for estimation of the mean field NO3- concentration. It was concluded that sufficient small localized clusters of samples should be taken to give a low standard error of the mean, with each cluster separated by at least 5 m. In the case of the Tokomaru silt loam, 20 clusters, each comprising 5 samples (bulked), would be required for estimation of the mean field nitrate concentration with 95% probability of being within ± 5% of μ, the true mean. This represents a large sampling effort. The activity of nitrifiers was studied in relation to soil pH and seasonal changes in soil moisture and temperature over two consecutive years in an attempt to explain the spatial variability in SNA values. The pH optimum for nitrifier activity (pHopt) was defined for four variates of the Tokomaru silt loam with different liming histories. Values of pHopt which varied between the four soils in the range 5.92-6.45 did not vary markedly with season, and it was found that the form of the relationship between SNA and pH remained constant with time. It was further observed that the addition of lime in 1987 had the effect of raising the mean soil pH and pHopt in previously unlimed soil, but had negligible effect on either the soil pH or pHopt in soil that had been limed in 1982. The significance of heterotrophic relative to autotrophic nitrification could not be discerned. No significant relationships could be found for the four soils between soil pH, pHopt, SNA, soil moisture content and soil temperature at 30 cm depth. Values of SNA (μmol N g-1 soil h-1) at pHopt (SNAopt) were calculated from equations fitted to plots of SNA vs. the pH of SNA incubation, and these show a more obvious seasonal trend. SNA values calculated for the prevailing soil pH (SNApH) were never very different from values of SNAopt and follow a 1:1 relationship over a range of values from 0.015-0.110 μmol g-1 h-1; that is, the nitrifier activity in the soil, irrespective of variations that were random (unknown influences) or associated with seasonal variables (temperature and moisture), was near the optimum with respect to the soil pH at the time of sampling. The effect of soil moisture variation on nitrifier activity was further investigated in an experiment in which soil samples were stored for 124 days at different soil moisture tensions. The optimum moisture conditions for nitrifier activity in the Tokomaru silt loam prevailed at pF 3.39. However, this optimum was less clearly defined than was the pHopt. Since the soil moisture status changes considerably with season, whilst soil pH does not, it was concluded that nitrifiers were more tolerant of changes in pF than changes in pH. Comparison of these with published results indicates that not only is the soil nitrifier population dynamic, and changes in response to changes in its environment, but the degree to which nitrifier activity is affected by various soil properties is soil-specific. It is therefore concluded that the spatial variability of nitrifier activity will also be soil-specific, and that different soils are likely to have different ranges of spatial dependence for the parameters of mineral N. Furthermore, the fact that SNA is not the only factor governing the soil NO3- concentration, and that other factors such as plant uptake and leaching are also important, indicates that SNA variability is not necessarily a good estimator of soil NO3- variability. This conclusion is certainly supported by the geostatistical aspects of this work.