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    The influence of soil parameters and denitrifiers on N₂O emissions in New Zealand dairy-grazed pasture soils : a thesis presented in fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, Massey University Palmerston North New Zealand
    (Massey University, 2015) Jha, Neha
    Denitrification is a primary source of nitrous oxide (N2O) production found globally in temperate grasslands and in New Zealand pasture soils. The various reductase enzymes coded by specific denitrifier genes, and influenced by soil and environmental factors, regulate the N2O production and reduction during the denitrification process. An understanding of the soil and environmental factors that have the potential to enhance the activity of denitrifiers reducing N2O to dinitrogen (N2) contributes to the development of novel and effective N2O mitigation technologies. This thesis attempts to address a critical gap in our understanding of the role of bacterial denitrifier communities and their abundance in denitrification under various field-moist and incubation conditions in New Zealand dairy-grazed pasture soils. This thesis consists of a literature review (Chapter 2) that describes the biochemical and molecular aspect of denitrification, and identifies key factors that affect the process and the denitrifier community structure and its abundance. It addresses the advantages and limitations of available techniques to measure denitrification, the denitrifier community structure and abundance, and also any available mitigation options to reduce denitrification losses. The review concludes by identifying gaps in our knowledge of the denitrifier communities, and their abundance and activities in New Zealand soils. The acetylene (C2H2) inhibition technique (AIT) was standardised (Chapter 3) for the measurements of denitrification enzyme activity (DEA) and denitrification rate (DR) used during the research. Similarly, terminal restriction fragment length polymorphism (T-RFLP) and quantitative polymerase chain reaction (qPCR) were standardised to assess denitrifier community structure (numbers of nirS+nirK and nosZ gene T-RFs) and abundance (nirS+nirK and nosZ gene copy numbers) in soils. Soil samples were obtained from 10 sites with known management histories, representing dairy farms across the North and South Islands of New Zealand, to determine the soil factors contributing to spatial variability in DEA, DR and denitrifier community structure and abundance within and across dairy-grazed pastures (Chapter 4). Despite the spatial variability at each site, the DEA results show large differences in potential denitrification in these soils. The outcome of this study painted an overall picture of the distribution of denitrification enzyme The bacterial denitrifier genes richness correlated significantly (P < 0.05) to Olsen P, microbial biomass C (MBC), and NH4+-N contents of soil, whereas these gene abundances correlated to MBC and NO3–-N contents in soils, thus confirming inherent soil characteristics that influence denitrifier populations in soils. NirS and nirK gene copy numbers correlated positively with N2O emissions (from nitrification and denitrification). There was no clear relationship between nosZ gene copy numbers and denitrification rate in the field-moist soils, which could be due to less anaerobic condition for denitrifiers to carry denitrification. To determine the effect of increasing soil water content (SWC) on changes in denitrification, 5 soils contrasting in DEA were incubated at field capacity (FC) and saturation SWC (Chapter 5). The measured DRs were higher in soils incubated at saturation, ranging from 21.5 to 73.9 µg N2O-N kg–1soil hr–1, than in soils incubated at FC, in which DR varied from 0.8 to 50.4 µg N2O-N kg–1soil hr–1. Although the direction of change in denitrification with the increase in SWC was similar among the soils, the magnitude of increase was variable among the five soil types. This variability was mainly driven by their inherent biochemical (NO3–-N, TC, TN, MBC, Olsen P, DEA,) and molecular characteristics (denitrifier richness and denitrifier abundance). A subsequent incubation experiment was conducted to investigate the effect of water only, cattle urine, and cattle urine with added nitrification inhibitor dicyandiamide (DCD) on denitrification and numbers of denitrifier gene T-RFs and their copy numbers in three pasture soils with contrasting DEA. The results of this incubation are described in Chapters 6 and 7. The results described in Chapter 6 explore the effect of saturation (only water addition) on DRs, denitrifier gene richness, and denitrifier gene copy numbers in three soils. At saturation soil water content in the incubated soils, the increases or decreases in DRs with incubation time were variable in three soils and depended on their TC, TN, Olsen P, MBC, and denitrifier gene richness. It is inferred from the results that, with increasing SWC, a denitrifier community of a constant size was maintained in the incubated soils. The results in Chapter 7 describe the effect of urine with and without DCD on denitrification, denitrifier richness, and denitrifier gene abundance. There were changes in soil pH, NO3–-N, NH4+-N, soluble C, and microbial biomass C in soils with urine application. The numbers of denitrifier gene T-RFs and copy numbers either remained unchanged or were lower under urine + DCD treatments than urine only and related to N2O productions (from both nitrification and denitrification) during the incubation. There was an overall increase in DR with application of urine and urine + DCD to soils. Comparatively higher N2O-NA (N2O productions from non- acetylene jars) were observed in urine-only treatments than in urine + DCD. The cumulative N2O-NA with the addition of urine and urine + DCD were variable among the three soils. During 4 weeks incubation the N2O-NA ranged from 20.9 to 26.7 mg N2O-N kg–1 soil in urine treatments and from 19.6 to 21.7 mg N2O-N kg–1 soil in urine + DCD treatments in three soils. The proportion of total N denitrified during the entire incubation ranged from 6.3 to 22.4 % in urine treatments and 3.6 to 5.6 % in urine + DCD treatments in three soils. Denitrifier gene richness and gene copy numbers during incubation helped identify overall changes in the denitrifier community with the application of treatments and link these changes to N2O-NA across various soils. Overall, in the urine-applied samples the N2O-NA (positively) was significantly correlated with their DEA, MBC, Olsen P, and numbers of denitrifier gene richness and denitrifier gene copy numbers. The information obtained in this research helped enhance the understanding of the variability in denitrification and the denitrifier community in the New Zealand dairy pasture soils. The molecular measurements of the soils helped identify differences in N2O productions in soils at similar incubation conditions. Soils with low denitrifier richness and, more importantly, lower abundance of complete denitrifiers had limited capacity to denitrify available inorganic N. Measurements of DEA, denitrifier richness, and denitrifier abundance in soils, along with their N2O productions (with and without acetylene), indicated the inherent potential of soils to carry denitrification. Under urine application, soil with the most abundant denitrifier population and the highest DEA responded to added treatment quickly and produced more N2O-NA and N2O-A than the other two soils did. On the other hand, soils with lower DEA and less abundant denitrifier community showed lower N2O production (with and without acetylene) even under anaerobic condition than did other soils.
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    Effect of herb-clover mixes on weaned lamb growth : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palerston North, New Zealand
    (Massey University, 2014) Somasiri, Sharini Carol
    The quality and production of ryegrass (Lolium perenne L.) /white clover (Trifolium repens) pastures are seasonal in New Zealand. Earlier research showed that a sward mix of plantain (Plantago lanceolata), chicory (Cichorium intybus L.), white- and red-clover (Trifolium pratense) resulted in greater lamb live weight gains in the late summer early autumn period. However, this has not been tested across all the seasons in New Zealand. Therefore, research was undertaken for two consecutive years (2011/2012 and 2012/2013) on three sward mixes; Pasture mix, Plantain mix and Chicory mix in early spring, late spring and early summer (late spring), summer and autumn. The Pasture mix consisted of perennial ryegrass and white clover. The Plantain mix consisted of plantain, white- and red-clover. The Chicory mix consisted of plantain, chicory, white- and red-clover. It was hypothesised that lamb performance (live weight, live weight gain (LWG) and carcass weight) and apparent carcass weight production per ha would be greatest in the Plantain and Chicory mixes in all four periods. Secondly it was hypothesised that Plantain and Chicory mixes would have lower feed conversion ratios (FCR) with higher herbage utilization efficiencies (EHU%) than the Pasture mix. In each period weaned lambs were reared in the three herbage treatments for a maximum of two months. Lambs were weighed fortnightly and they were slaughtered within 12 hours of being off the pasture at the end of the experiment. Carcass weights were obtained from the abattoir. The Plantain and Chicory mixes had a higher feeding value than the Pasture mix during early spring to autumn. Both Plantain and Chicory mixes produced heavier (P<0.05) lambs, higher (P<0.05) live weight gains (LWG) and carcass weights compared to the Pasture mix in all periods. Total apparent carcass weight production per ha were 407, 748 and 709 kg/ha in year one and 474, 607 and 642 kg/ha in year two in the Pasture mix, Plantain mix and Chicory mix, respectively. Both Plantain and Chicory mixes had lower (P<0.05) feed conversion ratios (FCR) and higher (P<0.05) herbage utilization efficiencies (EHU%) compared to the Pasture mix. This research has shown that sheep farmers in New Zealand can finish lambs at a faster rate for heavier carcasses using herb-clover mixes from spring to autumn than on ryegrass/white clover pastures.
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    Duration-controlled grazing of dairy cows : impacts on pasture production and losses of nutrients and faecal microbes to water : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2013) Christensen, Christine Lynne
    Mitigation strategies for improved environmental sustainability of the New Zealand dairy industry need to focus on reducing the transport of nitrogen (N) from urine patches and phosphorus (P) and faecal microbes from dung patches to waterways. One strategy is Duration-controlled grazing (DC grazing), a system based upon shorter grazing periods on pasture (4 hours) and removing cows to a stand-off facility for rumination and excretion. The stored effluent is applied to pasture as a slurry at an appropriate time when nutrients are required and soil conditions are suitable. A three year field study was established in the Manawatu to compare key features of DC grazing with a standard grazed (SG) system. This thesis explores the impact of a DC grazing system on the losses of N, P, potassium (K) and faecal microbes to water through drainage and surface runoff. It also investigates the effects of such a system on pasture production and intakes of pasture by cows. Pasture accumulation was the same for both treatments in the first year, but there was a 20% and 9% decline on the DC treatment in the subsequent two years. This was due to the way that slurry applications were managed. A large amount of slurry (212 kg N/ha) was applied in the first year, and no slurry was applied in the second year. In the third year slurry was applied four times at a total rate of 115 kg N/ha. The study indicates more frequent application of all nutrients captured in the effluent from standing cows off is required to maintain pasture production. Compared to the SG plots, the reductions in N losses from DC grazed plots were large, with an average 52% reduction in NO3- and 42% reduction in total N leached. Reducing urine deposition during autumn grazings appeared to have the largest impact on reducing NO3- leaching. Runoff losses of N were small and similar between treatments. The losses of P were small through both surface runoff and drainage. There was a large variation in runoff volume, which resulted in highly variable P runoff loads across plots and between treatments. The average 32% reduction in total P load from DC grazed plots was not significantly different from SG plots. Useful predictors of P load lost from all plots were runoff depth and the time cows spent grazing. Faecal microbe losses were also similar between treatments, with the useful predictors of faecal microbe concentration across all plots being the number of days since grazing and the climate after grazing. The amount of K applied in slurry and urine had a large influence on both soil and herbage K. It was determined that in a DC grazing situation, the K-rich liquid component must be included in the applied slurry to maintain soil K levels. The OVERSEER® nutrient budgeting software was able to simulate nutrient cycling in the DC grazing system reasonably well. The total N loss from the system was predicted accurately, although the relative proportion of N in drainage and runoff was not. Several opportunities for further work arise from this research. While DC grazing is a tool that could be implemented to significantly reduce N leaching losses, the management of collected excreta needs to be further developed to ensure pasture production gains are realised, or at least maintained. The combined effects of reducing treading damage and DC grazing should be investigated. Finally, a comprehensive economic analysis of standing cows off should be undertaken.
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    Effect of sward surface height on herbage intake and performance of finishing beef cattle : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Plant Science at Massey University
    (Massey University, 1998) Realini, Carolina
    This study examined the effects of sward surface height (SSH) on the herbage intake, ingestive behaviour and performance of steers finished on ryegrass (Lolium perenne) /white clover (Trifolium repens) pastures during summer. The influence of this initial treatment contrast on subsequent cattle performance under common grazing conditions during early-autumn was also studied. Twenty six month-old steers with an initial liveweight of 522 ± 7.6 kg, 14 Angus x (Hereford x Friesian) and 10 Angus x (Hereford x Jersey), were set stocked on swards maintained at SSHs of 5 and 10 cm (L vs. H) from 18 November 1996 to 4 March 1997, with 3 replicate groups of 4 animals per treatment balanced as far as possible for "breed". Six steers from each treatment balanced for "breed" were slaughtered on 4 March and carcass and meat quality characteristics compared. The remaining animals were grazed for another 5 weeks on common pastures until the final slaughter on 8 April. Over the SSH control period, the 5 and 10 target swards averaged 4.8 ± 1.36 and 10.0 ± 3.24 cm. Herbage on the H swards contained more dead material, less crude protein, lower dry matter digestibility and live:dead tiller ratio than that on the L swards. Estimates of herbage dry matter intake were higher for steers grazing at 10 cm than for those grazing at 5 cm SSH (8.0 vs. 4.8 kg DM d-1 from 2 estimates and 2 alkane pairs, P < 0.05 for each comparison). Steers were unable to increase their grazing time in response to limiting sward conditions sufficiently to compensate for lower intake rates in short swards, resulting in reduced herbage intakes. Daily liveweight gain over the summer was higher on the 10 cm than on the 5 cm SSH (1.10 ± 0.23 vs. 0.32 ± 0.21 kg d-1, P < 0.01) and carcass weight at first slaughter was significantly higher for steers on the H swards (332 ± 10.6 vs. 287 ± 7.5 kg, P < 0.05). SSH treatment did not affect other carcass or meat quality characteristics of steers. Liveweight and carcass weight gain per hectare were 71 % and 43 % greater (318 vs. 186 kg and 166 vs. 116 kg) for steers grazing at 10 cm despite the lower stocking rate (2.86 vs. 5.80 steers ha-1) maintained by the tall swards. Over the common grazing period previously restricted steers had higher intakes, greater grazing and ruminating times, lower resting time and grew faster compared to steers previously grazed at 10 cm SSH. However, none of these parameters were significantly different between the steer groups with the exception of resting time. Increased autumn growth rates by previously restricted steers did not compensate for the differences in liveweight established during summer, and significant differences in carcass weight were still evident at the end of the compensatory period between the steer groups (335 ± 9.4 vs. 297 ± 9.4 kg, P < 0.05). There were no significant differences in meat quality characteristics with the exception of meat brightness which was higher for previously restricted steers. These results suggest that maintaining a sward height of 10 cm offers advantages in terms of individual animal output and output per hectare compared with grazing at 5 cm and that compensatory growth does not seem to be an important phenomenon in heavy (over 500 kg liveweight) finishing steers.
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    The effects of wet winters and winter management on early season milk production : this thesis is presented in partial fulfil;ment of the requirements for the degree Master of Applied Science in Agricultural Systems and Management at Massey University / John G. Smith
    (Massey University, 1997) Smith, John G.
    The effect of a wet winter on milksolid production in early lactation was measured for the years 1994 and 1995, against the 1993 year which had a dry winter. Data for milksolid production to the end of December, and for the whole season, were collected from 70 farms in the Manawatu region that had remained relatively unchanged in area and cow numbers over the three years. Management information was also collected from 18 selected farmers, whose milksolid production was either affected or unaffected by the wet winters. These farmers were also visited to obtain supplementary information about their management. For the 70 farms, the effect of the wet winters on production in early lactation was a significant reduction (P<1%) in milksolids for both wet years studied. (-24 KgMS/ha;-9 KgMS/cow in 1994, and -66 KgMS/ha;-26 KgMS/cow in 1995). These effects could have been caused by 0.2tDM/ha and 0.5tDM/ha less pasture being eaten in the wet springs of 1994 and 1995 respectively. However the effect on the total lactation milksolid production was not significant, with increases in late lactation compensating for the decreases apparent in early lactation. The effect of the wet winters was probably to increase the incidence of pugging damage and to lower the pasture growth rates and pasture cover through calving and into early spring, reducing the ability of the farmers to feed lactating cows in early lactation. For the 18 farms selected in the management survey, the effect of the wet winter (1995) on production in early lactation was larger on the affected farms (-133 KgMS/ha; -45 KgMS/cow), than for the unaffected farms (-4 KgMS/ha; -11 KgMS/cow). There was a significant decrease in the total seasons production for the affected farmers with lower production (-93 KgMS/ha; -25 KgMS/cow) in 1995 than 1993. The total seasons production for the unaffected farmers was a significant increase of 80 KgMS/ha, and an increase of 13 KgMS/cow for 1995 compared to 1993. There was no one single management strategy that was used during the wet winter of 1995 by unaffected farmers that was not used by the affected farmers. Instead, more unaffected farmers used off farm grazing, on-off grazing, and had better overall farm drainage. This decreased the risk of pugging damage during the winter, and early spring to enable more pasture to be converted to milksolids. There were differences in goals between the groups, because the affected farmers had more goals associated with lifestyle, while the unaffected farmers had more production orientated goals, with the unaffected farmers constantly monitoring the system to ensure the achievement of their production goals.
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    The effect of herbage availability and species choice on grazing preference of dairy cattle : a thesis presented in partial fulfilment of the requirements for the degree of Masterate in Applied Science at Massey University
    (Massey University, 1997) Torres-Rodríguez, Alberto
    Herbage intake is one of the factors determining animal performance. Intake and quality of the diet consumed by animals are both determined by selective grazing. The motivation to graze selectively is in part a function of dietary preferences. The study of diet selection requires knowledge of what animals prefer to eat when there are no or minimal constraints to them obtaining their diet. This experiment aimed to investigate the effect of relative availability of a preferred species on dairy cattle response at grazing, and to evaluate the feasibility of the use of monocultures of pasture species for studies of preferences of dairy cattle. Three species-contrasts each composed of two adjacent 1-ha monocultures of either White clover:Ryegrass (W_Rye), Lotus corniculatus:Ryegrass (L_Rye) or Lotus corniculatus:Red clover (L_Red) were used. White clover (W) and lotus (L) had been previously determined as preferred over ryegrass (Rye) and red clover (Red). Each species-contrast was subdivided into four plots and the height of the preferred species was set at 4, 6, 8 and 10 cm, whereas that of the less preferred species was set at 10 cm across plots. Groups of yearling Holstein heifers grazed the plots, and observations on grazing behaviour were made by recording grazing activity and species location at 10-minute intervals during daylight hours for three consecutive days, twice in summer and twice in autumn during 95/96 at the AgResearch Flock House Research Centre, near Bulls. During summer, a second week of grazing followed each period of observations for grazing activity, where attempts to estimate herbage dry matter intake and diet composition using the alkane technique were made. From the species-location information, total grazing time (GTt), expressed in hours, and distribution of GTt between preferred (GTp) and less preferred (GTI) species was obtained. The proportion of GTt allocated to grazing the preferred species was considered as a measure of preference. Statistical analysis was performed by GLM procedures of SAS. Regression analyses were carried out for grazing activity parameters on actual height of the preferred species. Animals showed preference for a mixed diet with partial preference for the legume component (W, 67 %, and L, 70 %) over grass, whereas partial preference in the L_Red species-contrast was close to indifference (L, 55 %). However, this partial preference differed between seasons, being in general stronger in summer than in autumn. Partial preference decreased with decreases in height of the preferred species. However, herbage bulk density (BD) appeared to be important also in influencing preference since more marked responses to height were observed in autumn when sward had lower BD compared with summer. Botanical composition of the sward upper stratum was also considered to influence animal preferences. Diet composition estimation from herbage and faecal alkanes suggested that animals consumed the preferred species at higher proportions than indicated by the proportion of GTt allocated to the preferred species. This was possibly due to differences in rate of intake between herbage species. However, more research is needed in this area in order to establish more accurately the relationship between these two techniques. It is concluded that animals respond to changes in herbage availability of a preferred species and to species choice by adjusting grazing time between preferred and less preferred species. It would be appropriate to research the potential animal performance benefits of increasing the availability of a preferred species in proportion to that preferred by the animals. This works confirms the use of monocultures of pasture species to be useful in the evaluation of preference of dairy cattle. Inclusion of a wider arrangement of species-contrasts is recommended.
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    The production of volatile fatty acids in sheep on different pasture types : a thesis presented in partial fulfilment of the requiremenrts for the degree of Master of Agricultural Science at Massey University
    (Massey University, 1970) Mackintosh, James Bruce
    The New Zealand economy depends largely on the efficient conversion of pasture to exportable animal products. The current emphasis on higher stocking rates increases the need for pastures best suited to animal production. Pasture species used in New Zealand are recognised as differing in their effects on animal production, these differences being loosely attributed to variation in "pasture quality". If the level of output of saleable product is the accepted measure of pasture quality, then the principal factors governing this are the quantity of feed consumed and its subsequent utilisation. A number of studies have shown that differences in food intake alone cannot account for the observed differences in animal performance. It has thus been considered important to investigate the factors affecting the utilisation of pasture by the animal. Volatile fatty acids (VFA) produced in the rumen are generally considered to account for 70 to 60 percent of the net energy requirements of ruminants, and their production must be a major determinant of feed utilisation. Differences in the ruminal concentrations and proportions of VFA have often been observed with pasture feeding and have been cited as possible reasons for differences in pasture quality. The investigation described here was undertaken as a preliminary study of the role of VFA production in determining the quality of New Zealand pasture species.
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    A comparison of the fate of elemental sulphur and sulphate sulphur based fertilizers in pasture soils : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
    (Massey University, 1991) Phimsarn, Sathien
    Nitrogen fixation by legumes has a particular requirement for adequate soil sulphur status. Sulphur (S) is a mobile nutrient and is easily leached from aquic soil environments, therefore regular topdressing with S fertilizer is required to maintain legume vigor and pasture production in most New Zealand pasture soils. Escalating fertilizer costs have focused attention on the efficiency of use of S fertilizers, particularly superphosphate (SSP) and alternative elemental S (S0) based fertilizers less liable to leaching loss in this aquic environment. Field and glasshouse trials, using the resident clover/ryegrass sward on undisturbed soil cores (150 mm diameter, 100 mm depth) , were undertaken to determine the comparative short-term fate of SSP and different particle sizes of S0. Methods for manufacturing radioactively labelled (35s) fertilizers were developed. In addition, the effect of sheep dung on the short-term immobilization of soil and fertilizer S was also investigated. A simple computer simulation model explaining the observed transformation of soil sulphur and 35s labelled fertilizer was developed. Initially, the effect of sheep dung on the short-term immobilization of soil and fertilizer S was investigated. Very small amounts (about 2-5%) of plant (clover/ryegrass pasture) S and P, within 1 00 mm of the area surrounding the dung pellet, were derived from the dung. Under the experimental conditions that prevailed, dung S behaved as a slow release S form causing neither greater immobilization of soil or fertilizer S nor mineralization of soil organic S. It was concluded that the impact of dung return on short-term (< one year) S fertilizer fate need not be considered. An initial field trial comparing the fate of microfine S0 (< 0.010 mm) relative to sulphate-based SSP was undertaken on Tokomaru silt loam, a New Zealand yellow-grey earth (Fragiaqualf). The microfine S0 oxidized within 30 days of application but initially (up to 60 days) was slightly less effective than SSP in terms of plant uptake. Over longer periods of time (150 days) their performances were comparable. Final cumulative plant uptake at 150 days accounted for 13.6% of microfine S0 and 16.3% of the SSP-sulphate. The major transformation of 35s from microfine S0 and 35s belied gypsum In SSP to soil organic 35s forms occurred in the first 30 days after application. The organic 35s activity formed from 35s0 was twice that formed from sulphate-based fertilizer and was mainly carbon- bonded 35s in the top 33mm of the pasture soil profile. The amount of organic 35s remaining as carbon-bonded 35s decreased with soil depth and the reverse occurred for the estersulphate 35s. By 1 50 days, greater activity from the microfine 35s0 remained in the soil organics fraction than from the sulphate-35s fertilizer, indicating that more soil organicS reserves may be formed through the use of fine S0 fertilizer than from the sulphate-based fertilizer. This also indicated the advantage of using S0 in minimizing the S leaching losses in this aquic environment. An inverse dilution technique using an isotope injector developed at Massey University to uniformly label undisturbed soil cores with carrier-free 35so4= solution was used to measure the impact of S0 and sulphate-based fertilizers on the fate of soil S. Results were consistent with the labelled fertilizer technique and both techniques indicated rapid incorporation of 35s into soil organic S and that the carbon-bonded S formed was likely to be a subsequent source of mineralized S available to plants. Soil samples from the preliminary field study were used to evaluate soil preparation and extraction techniques. Soil sampling and preparation techniques were evaluated on the basis that an extract sampling the plant available S pool in soil should have the same 35s specific activity as plant growing on that soil. The average 35s specific activity in a calcium dihydrogen phosphate (CaP-S) (0.04 M) extract from a freeze-dried sample of the top 60 mm of a pasture soil was most closely related to the 35s specific activity of plants growing on that soil. CaP-S extracts from field-moist soil or 0.01 M CaCI2 extracts from field-moist or freeze-dried soils had higher specific activities than plants. lt was concluded that plants were able to extract soil S from soils which was not exchangeable with added 35so4= fertilizers during either the field experiment or extraction with 0.01 M CaCI2. The second series of field and glasshouse trials were conducted to investigate the fate of 35s labelled SSP, gypsum and S0 of varying particle sizes (<0 . 1 50 mm, 0.1 50-0.250 m m and 0.250-0.500 mm, in granulated and non-granulated forms) in two pasture soils contrasting in mineralogy and fertility status. Under glasshouse conditions, 50 mm of simulated rainfall was applied to each of the undisturbed soil cores during the first 56 days after fertilizer application. For the remainder of the period, cores were watered from below using a saucer. Field cores remained subject to the local climate. Both the rate of oxidation in soil and the efficiency of plant use of S0 was improved by decreasing its particle size. Relative to soluble so4=-s applied as gypsum or SSP, the plant utilization of oxidized SJ was similar. Granulation of finer S0 with or without finely ground phosphate rock had little effect on the long-term ( 180 days) oxidation rate or the efficiency with which, after oxidation, finely ground S0 was taken up plants. Apart from S0 of large particle size (>0 . 1 50 m m) which had not oxidized, the major fate of fertilizer 35s, either under glasshouse or field conditions, was again in soil organic matter mostly formed in the top 33 mm of the soil. Applications of gypsum and SSP caused 35s to move to the 33-1 00 mm soil depths but there was no additional influence of P on the depth to which so4= was leached. A preliminary computer simulation model describing the fate of 35so4 =-s fertilizer was developed. The model provided a very accurate method of predicting plant uptake of S from both SSP fertilized and u nfertilized soil cores. The model also indicated that, at any particular soil depth, on average, actual rates of mineralizatio n a nd i m mobilization may exceed root uptake of S by 1.5 to 2 fold (mg S turned over per unit of S taken up by plants). The accuracy of the estimated turnover rate could not be validated because the model gave relatively inaccurate predictions of the measured movement and transformations of 35s tracer added to the soil as SSP. There was, however, relative similarity between the observed and predicted proportional distribution of 35s between soil and plant S forms. Such a distribution supported the concept of using root activity as a modifier of mineralization and immobilization rates in order to describe the extent of these processes at different soil depths. The study has emphasized the greater importance of the surface few millimeters of pasture soil in S transformations, important in the fate of fertilizer and pasture plant nutrition. There appears to be scope in manipulating S0 particle size to improve the efficiency of the S fertilizer used.