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    Phosphate cycling in grazed hill-country pasture : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
    (Massey University, 1987) Rowarth, Jacqueline Sara
    A detailed study of the "above-" and "below-ground" components of the phosphorus (P) cycle was carried out in the North Island hill country of New Zealand. The effect of P fertiliser rate and degree of land-slope on pasture P uptake, faecal P return and changes in soil P fractions was examined over a three year period. Plant P uptake was found to decrease with increasing slope and increase with increasing rate of P fertiliser. The changes in plant P uptake were a function of changes in both dry matter yield and pasture P concentration. Pasture on campsites which initially had a high Olsen P status (>30) showed an unexpected apparent P response in both dry matter yield and plant P uptake. This response results from an artefact of the trial design whereby P fertiliser was applied to whole paddocks. The resulting improved nitrogen status of slopes at high rates of P fertiliser created abnormally high soil nitrogen levels on campsites, leading to very high levels of production. These findings are of significance in relation to soil fertility field trials conducted on individual slopes in hill country. Over the three year period of the trial seasonal plant P uptake followed the order: spring ≥ summer > autumn > winter, irrespective of slope category, fertiliser rate or grazing regime. The distribution of faecal material was found to be markedly affected by slope and approximately 60% of the material deposited in each paddock was returned to campsites. In the remainder of the paddock, faecal P return decreased by at least 50% with each 10° increase in slope. Paddock faecal P concentration (FP%) was predicted from the pre-grazed pasture P concentration (PP%) (calculated on a paddock basis) using the relationship: FP% = 3.19 PP% - 0.09 (r = 0.94). Net P balance calculations for various slope group categories showed that deficits between plant P uptake and faecal P return increased with increasing slope but were little affected by increasing P fertiliser rate and consequent increase in stocking rate. This finding verifies the use of a single animal loss factor for a given topography, irrespective of sheep stocking rate, in the Ministry of Agriculture's (MAF's) Computerised Fertiliser Advisory Scheme (CFAS) for P. Measurements of faecal distribution in this trial suggested the use of a lower animal loss factor (0.5 kg su-1 of P) for "Easy" hill country than that used currently (0.7 kg su-1 of P) in the CFAS model. Independent studies on the rate of P cycling from faeces were conducted. The study investigating breakdown of faecal material revealed that physical disintegration of faecal material is likely to occur before chemical decomposition. In winter conditions faecal material disintegrated within a month; in summer conditions disintegration took approximately three months. In both seasons material on campsites disintegrated more rapidly than that on steeper slopes. A further study using radioisotopes in the field found that the shortterm plant availability of inorganic faecal P was approximately half that of monocalcium phosphate fertiliser over a two month period in the spring. In a study on the "below-ground" components of the P cycle total soil P was found to increase with increasing rate of P fertiliser. The magnitude of these increases decreased with increasing slope and depth. Increases in organic P were found to be higher on campsites than steeper slopes. On campsites, the extent of increase in organic P decreased with increasing rate of P fertiliser. Inorganic P increased with increasing rate of P fertiliser on all slopes; the magnitude of the increase decreased with increasing slope. At low rates of P fertiliser a decrease in inorganic P was measured on steep-slopes over time indicating that P inputs were not balancing P outputs. A change in the non-occluded P fraction accounted for the greatest proportion of the change in inorganic P on most slopes. The fact that calcium-bound P accumulated on all slopes, and that large increases were evident at high rates of P fertiliser suggested that this fraction was not playing an active part in the P cycle but was accumulating as an insoluble residue from superphosphate. The plant availability of soil P fractions was investigated in a glasshouse study. Total plant P uptake was found to be highly correlated with initial levels of total P (r = 0.92),non-occluded P (r = 0.82), inorganic P (r = 0.91), Olsen P (r = 0.93) and water-extractable P (r = 0.97). Levels of organic P, occluded P and calcium-bound P were found to be essentially unchanged by plant growth over the eleven month trial period. Changes in the size of the cycling soil P pool were examined by combining results from the field trial with those from the glasshouse study. At low rates of P fertiliser (10 kg ha-1), increases in occluded P and calcium-bound P (i.e., unavailable inorganic P) in the 0-15 cm depth were occurring at the expense of available P. At a high rate of P fertiliser (100 kg ha-1) approximately two thirds of the P applied remained in the available form. On an annual basis, Olsen P increased with an increasing rate of P fertiliser and decreased with increasing soil depth and slope. Over the period of the trial Olsen P decreased significantly at the lowest rate of P fertiliser (10 kg ha-1) and increased significantly at the highest rate (100 kg ha-1) on the two slope groups studied. This indicated that these areas were not at "equilibrium" as defined by a stable Olsen P. At moderate rates of P fertiliser (20 and 30 kg ha-1) it was not possible to determine whether or not equilibrium conditions existed as the annual variability in Olsen P was too high. An attempt was made to determine soil P losses (as defined by the CFAS model) at the trial site. Despite intensive and careful soil sampling Olsen P could not be used to determine "equilibrium" conditions which are a pre-requisite for measurement of soil P loss. This finding prevented validation of soil loss factors on this hill-country site. Data generated from the large field trial for "above-" and "below-ground" components of the P cycle enabled recommendations to be made on the location of suitable soil sampling sites and also on the location of priority areas for application of P fertiliser in grazed hill country.
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    Interactions of molybdate and phosphate with soils : a thesis presented in partial fulfilment of the requirement for the degree of Doctor of Philosophy in Soil Science
    (Massey University, 1977) Hope, Graeme d'Egville
    Using four New Zealand soils, it was found that pH, extractable Al, citrate-dithionite-bicarbonate-Al, oxalate Fe, and crystalline Fe appeared to be important soil properties in both P and Mo sorption. Allophane appeared less important in the sorption of Mo than in the sorption of P. For the sorption of Mo, the initial, rapid removal of Mo was followed by a slow, continuing removal of Mo from solution. An estimate of equilibrium Mo concentration was obtained by extrapolation of the relationship between solution Mo and 1/t to 1/t = 0, i.e. t = ∞. The effect of ionic strength on Mo sorption appeared to be kinetically controlled at low final Mo concentrations (< 5 µmol l-1), but appeared to be absolute at high final concentrations (> 10 µmol l-1). Isotherms for the sorption of Mo by both topsoils and subsoils, at equilibrium and 40 hr, and by synthetic hydrous ferric oxide gel (Fe gel) and allophane at 40 hr, could be described by three Langmuir equations. Values for the free energies of sorption for each region of sorption, which were remarkably similar for the different sorbents, indicated that sorption in regions I and II corresponded to chemisorption reactions, whereas sorption in region III involved a more-physical type of sorption. Fe gel appeared to be a satisfactory model for Mo sorption by soils. Isotherms for the sorption of P by the four soils, Fe gel, and allophane during 40 hr were described by three Langmuir equations. Because the free energies of sorption for each region, for both Mo and P, were very similar, the sites for sorption and types of sorption reaction for both anions are probably similar. Synthetic allophane chemisorbed much less Mo than P, relative to Fe gel, and this was attributed to kinetic charge effects. Sorption of Mo by Fe gel in each region was affected differently by changes in pH and ionic strength, and the charge relationships for each region were also different. These data, along with the three distinct free energies of sorption obtained for Mo, suggested that three distinct sorption reactions were involved. The data suggested that sorption of Mo in regions I and II involved ligand-exchange chemisorption of MoO42- for -OH2+ and -OH, respectively, resulting in the formation of a bidentate complex. Sorption in region III was considered to involve sorption at a plane distant from the sorbing surface. The Langmuir equation developed to describe competitive sorption was not obeyed for Mo and P, but the sorption of Mo, in the presence of P, could be described by three simple Langmuir equations. It appeared that Mo and P competed for similar surface sites. Solution P increased the amounts of sorbed Mo that could be desorbed, relative to Cl. The amounts of Mo desorbed by both Cl and P decreased with time after addition of Mo to soils. This was attributed Mo suggested that the absorption of adsorbed Mo was also occurring. For several soils to which Mo had been added in the field, no Mo was desorbed by P solutions. Solution:soil ratio affected only the rate at which P was removed from solution, not the final equilibrium concentrations. Incubating soil with P prior to the addition of Mo reduced both chemisorption and more-physical sorption of Mo. For a soil that had received annual additions of phosphate and lime for 22 yr, the chemisorption of added Mo was reduced by both fertilizer P and lime, whereas the more-physical sorption maxima were only reduced by lime additions. The results were discussed in terms of both the persistence and plant-availability of Mo added in the field situation.