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    Factors influencing the transformation and fate of sulphur and nitrogen in grazed hill country pastures : 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) Sakadevan, Karuppan
    The increasing cost of agricultural grade sulphur and the high leaching losses of sulphate sulphur(S) from superphosphate fertilized pastures in New Zealand create a need to develop more efficient S fertilization techniques. The objective of the present study was to identify the main origins of the sulphate being leached from superphosphate fertilized hill country pastures with soils (Typic Dystrachrepts) developed from underlying sedimentary parent materials. Origins of leached sulphate were categorized as S leached directly from fertilizer, from zones enriched in animal excreta and from the mineralization of soil organic matter. Mineralization studies, both in la??oratory and in field were conducted to establish the extent of and the relationship between sulphur and nitrogen mineralization and the fate of mineralized nutrients in pasture soils that contrasted in their superphosphate fertilizer history. In the preliminary laboratory study in which an open incubation technique was used to measure potential net mineralization, top soils (0-7.5cm) taken from sites that had received higher rates of superphosphate in the past, mineralized more soil organic sulphur and nitrogen than soils taken from sites that had received smaller amounts of superphosphate in the past. In addition top soils collected from low slope (0-12°) sites where a greater proportion of animal excreta is returned, mineralized more S and N than the soils from medium slope (13-25°) sites. The ratio of N to S mineralized was narrower (2. 0 to 3. 6 ) than the N to S ratio of the whole soil (7 .1 to 8. 9) suggesting that in these soils relatively more S remains in a mineral form in the soil and is more susceptible to leaching than N which is conserved in the soil. Cylindrical, mini-lysimeters with ion exchange resin traps for collecting solutes from drainage water were developed to measure the net mineralization of soil organic S and N under field conditions. Leaching losses of S and N, pasture uptake of S and N and changes in mineral S and N pools in the soil at the same site were measured simultaneously and the rate of mineralization calculated. A laboratory evaluation of the lysimeter showed that the resin trap was capable of removing all the sulphate from drainage water at several different flow rates. The main advantage of these lysimeters over the conventional methods of measuring the leaching losses of anions and cations in the field is that regular drainage collection was not necessary. By introducing mixtures of both anion and cation exchange resins in the trap in the lysimeter it was possible to monitor the amount of anions and cations in field drainage over long periods of time before it was necessary to change the resin mixtures. In the initial field lysimeter study the net mineralization and pasture uptake of N ( 119 to 251 kg N ha-1) was 10 times more than that of S ( 12 to 27.5 kg S ha-1) , yet approximately 10 times more sulphate S ( 2.0 to 17.3 kg S ha-1) than mineral N (0. 19 to 1.3 kg N ha-1) was lost by leaching. Previous fertilizer history had a marked effect on the leaching losses of sulphate with seven times more S lost ( 2.1 vs 1 5.3 kg S ha-1) from sites which received greater rates of superphosphate and had higher stocking rates. During the initial seven month period S leaching losses on the low and high fertility sites were equivalent to 1 5% and 3 3% of the annual fertilizer application. More sulphate was leached from areas identified as animal camping areas. The lack of any change in sulphate below the 150mm soil depth during a period of active plant growth and no leaching suggested that any sulphate that moved below 1 50mm of the soil could be considered to be effectively lost from the system. Increased leaching losses of calcium and magnesium were associated with increased sulphate losses. The amount of calcium lost by leaching ( 4.7 5 to 12. 5 ,kg Ca ha-1) was far greater than potassium (0.8 to 3 . 6 kg .K ha'1), although twice the amount of potassium ( 240 kg K ha·1 vs 120 kg Ca ha'1) was. cycled through the plant-animal system. The amount of magnesium lost by leaching was greater than the amount of potassium lost by leaching. In a second lysimeter study the direct effects of freshly applied fertilizer on the mineralization of S and N from soil organic matter, their plant availability and losses by leaching were studied under field conditions using 35S labelled superphosphate. Fertilizer application significantly increased the mineralization of both organic S and N. The recovery and measurement of 35S activity over a nine month period showed that major proportions of pasture S ( 8 5 and 8 6% of the pasture S for low and high fertility farmlets, respectively) and leached S (75 and 87% of the leached S for low the mineralization of soil organic matter and not recently applied fertilizer. The amounts of both S and N mineralized from soil organic matter depends upon the past fertilizer history of the site and the present fertilizer application rate (22 and 40 kg S ha-1 and 125 and 204 kg N ha-1 for low and high fertility farmlets, respectively). Further, when the net mineralization of S was greater a greater proportion (59%) of mineralized S was lost by leaching than removed by pasture (39%). Irrespective of the amount N mineralized virtually all was removed by pasture. The results suggested that low N availability was a major factor limiting carbon fixation and the formation of organic S in these pasture soils. In a third lysimeter study, field simulated sheep dung and urine events boosted pasture growth and S and N uptake by approximately (50%), whereas the leaching losses of S and N were not influenced by the their application. A preliminary computer simulation model describing the mineralization of soil organic S, pasture S uptake and leaching losses in grazed pasture was developed. The preliminary model gave reasonable predictions of the changes in soil sulphate concentrations in the soil up to a depth of 25cm, pasture uptake of S and leaching losses of S at four pasture sites varying in their fertilizer history. Further refinement of the model is necessary before it can provide the basis for predicting fertilizer S requirement for hill country pastures. The experimental results and model output confirm balance study predictions that large leaching losses of S occur and these are derived mainly from the mineralization of soil organic matter which accumulates in well fertilized soils. The extent of S losses appear to be a function of the general levels of soil productivity and the data suggested that only a small, probably less than 20% reduction in this loss could be achieved by changing to slow release S fertilizers.
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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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    A systems approach to research planning for North Island hill country : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy
    (Massey University, 1984) McCall, David Graeme
    The achievement of increased production from North Island Hill Country (NIHC), through an increased research input, is currently receiving considerable attention in New Zealand. The task of planning future research and setting priorities for this work requires an evaluation of the current state of knowledge relating to hill country grazing systems, and an assessment of future research needs. A number of references have been made to the potential usefulness of systems modelling in research and research planning. The objectives for this study were to apply modelling to the development of research priorities on NIHC, and to evaluate modelling in this role. A simulation model was constructed to assemble the available information on soil, pasture and sheep components of NIHC grazing systems. Evaluation of the state of knowledge on components of hill country systems was based on problems (data and conceptual) confronted during model construction. The model simulated pasture growth, senescence and decay from climate data. The sheep component was based on an energy balance using the metaholisable energy system. Parameters in major components of the model were calibrated against data set aside at the start of the study. This was done in order to obtain the 'most valid' model because there were a range of values reported in the literature for many of the parameters. Statistical goodness-of-fit tests were used as an aid to decide on the structural acceptability of the calibrated model, and some issues facing the choice of appropriate statistical lack-of-fit test for models, were discussed in detail. Model validity was established by subjective judgement. The need for subjectivity arose mainly because of uncertainty about settings of some, or all, important exogenous variables in the data available for validation. Experiments were carried out with the model where stocking rate, lambing day, length of flushing, winter and spring rotations, and the number of paddocks retired from grazing in early summer, were varied. The results were used to define decisions giving 'optimal' levels of production. Climate data from Ballantrae Hill Country Research Station were used. Five representative years were constructed to account for major variations in climate experienced at the site. Early lambing and winter and spring grazing strategies which maximised spring feed supply were clearly shown as the most important decisions in maximising system profitability. The 'optimal' system was used as a base from which to investigate possible benefits from adding feed in spring through the use of nitrogen fertiliser, and increasing ovulation rate by artificial means. Both the above were shown to be profitable, given some adjustments to management strategies. Finally, changes were made to a range of parameters influencing potential pasture and animal performance. Effecting some of these changes by physical, chemical or genetic means would be profitable, particularly where wool production was concerned. Difficulties were confronted in conceiving a means of setting objective research priorities using the model. A number of information deficiencies were noted, but the reasonableness of using sensitivity analysis to rank the importance of each deficiency in an 'invalid' model was questioned. Further the problem of choosing between obtaining information to further improve the model, and developing improved systems suggested by the model, was noted. Subjective priorities were determined based on the need to demonstrate superior systems identified by modelling, and the apparent need for a greater understanding of particular components to enable improved systems to be devised. Advantages were apparent in using modelling as an aid to making these subjective judgements. It was concluded that the process of developing a pre-research model to evaluate research needs had been valuable. The learning aspect of modelling was emphasised, though problems with validation occur where modelling is conducted in isolation from field research. The view was submitted that the modelling should be extended to become an integral part of a research programme.