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    An investigation of the spatial distribution of N2O emissions from sheep grazed hill country pastures in New Zealand :|ba thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Environmental Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2012) Letica, Selai Ahovelo
    New Zealand’s (NZ) greenhouse gas (GHG) profile is unique amongst developed countries as almost 50% of GHG emissions are derived from agriculture. In contrast, agricultural sectors of other developed countries typically contribute <10% to the national total GHG profile. In NZ, agricultural GHG emissions are dominated by methane (CH4) from enteric fermentation and nitrous oxide (N2O) from excreta deposition and nitrogen (N) fertiliser application. Nitrous oxide emissions from agricultural soils are largely affected by N inputs and soil moisture conditions, and contribute 33% of agricultural GHG emissions. In pastoral hill country these factors are inherently more variable than in flat land pastures due to topographydriven differences in excretal N returns and in soil moisture. This limits the application of N2O emission data collected from trials conducted on flat land to hill country situations. The objective of this thesis was to determine the influence of topography and fertiliser N inputs to soil on N2O emissions in hill country. Small scale trials were conducted to measure these aspects of N cycling. Three trials were conducted to measure the effect of slope and fertiliser N input on nitrification potential (NP) and N2O emissions. The results of these short term trials suggested that slope class and fertiliser N rates significantly affected nitrification rates and N2O emissions in hill country due to differences in N inputs and moisture status, as affected by slope. Both NP and N2O emissions were highly spatially variable during the measurement periods and the results presented in this thesis suggest that the majority of N2O emissions in sheep grazed hill country are produced from low slope/stock camping areas. Based on our findings it is recommended that mitigation options to reduce the risk of N loss from sheep grazed hill country should be targeted at low slope/stock campsite areas. Due to the significant relationship between slope class and N2O emissions, slope class may be a suitable parameter for up-scaling estimates of N2O emissions from sheep grazed hill country.
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    An assessment of the nitrogen fertilizer requirements of winter cabbages (Brassica oleracea var. capitata L.): 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) Bonoan, Roberto Reginaldo
    The increasing costs of N fertilizers and the danger of creating environmental pollution due to excessive N fertilisation practices create a need for more efficient N fertilisation of vegetable crops. This present study was conducted with the main objective of assessing the N fertilizer requirements of winter cabbages on a coarse loamy mixed mesic Dystric Eutrochrept soil and consequently developing a model which would assist in predicting N fertilizer requirements over a wider area. Glasshouse and field experiments were conducted to assess the utility of soil and plant (sap) tests for assisting in determining the N fertilizer requirements of winter cabbages. The concentrations of NO3-N and NH4-N in either the xylem or petiole sap of cabbages were found to be influenced by several factors such as leaf position, time of day, sample storage time, plant age and form of fertilizer N. A large field trial indicated that at 4 sampling dates (50, 60, 80 and 90 days after transplanting; DAT) and prior to sidedressing, xylem (R2 = 0.73**) and petiole (R2 = 0.86**) sap were strongly correlated to extractable NO3-N and NH4-N in the soil to a depth of 30 cm. Nitrate-N levels in xylem sap at 60 and 80 DAT and petiole sap at 50, 60 and 80 DAT were good predictors of harvestable fresh head yield. Maximum marketable fresh head yield (55 t/ha) was achieved with an initial N application of 300 kg N/ha over a growing period of 150 days in which 448 mm of drainage was estimated. At heading, on the 300 kg N ha-1, soil mineral N levels were 75 kg N ha-1, xylem sap concentration was 333 μg NO3-N ml-1 and 1651 μg NO3-N ml-1 in the petiole sap. This critical value for petiole sap is higher than that reported in the literature for cabbages. At petiole sap levels below the critical value, sidedressing with 100 kg N/ha as urea was required to achieve a similar yield as found with an initial application of 300 kg N ha-1 as calcium ammonium nitrate. In a small scale field experiment, plant recovery (62-65%) of sidedressed 15N labelled urea N did not differ between sidedressing rates (100 and 200 kg N). Total recovery of 15N in the plant and soil was considered high (114 ± 0.9% and 90 ± 1.1%) for the respective rates. Using the data obtained from the field trials, a simple model termed a "sidedressing model" was developed. The model specifically determines the amount of N fertilizer needed to be applied as a sidedressing at a critical time (heading) to obtain maximum yield. The model was validated, using the data from another N fertilizer field trial conducted in the following year. The model successfully predicted whether N sidedressing is required or not but only a limited validation could be made of the prediction rates. The limitation of the sidedressing model of being site and season specific can be reduced by using simple submodels to predict the measured component which assessed N in cabbages at heading (Nh). One submodel used (the heat unit model) was modified by including data from 2-year trial results, to predict Nh and also provided a prediction of N uptake at maturity (Ny). Although not able to be validated in this study, the model shows potential for use by environmental administrators in predicting the likely effects of various growers practices in relation to identifying problems associated with NO3-N in drinking water and in edible cabbage heads.
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    Dry matter and nitrogen partitioning in sweet corn (Zea mays L.) for processing : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University
    (Massey University, 2000) Hansen, Luke N.; Hansen, Luke N.
    Increasing land values without comparable increases in yield or reduced input costs have reduced the attractiveness to growers of processing sweet corn (Zea mays L.) as a cropping enterprise at Gisborne, New Zealand. As a consequence, the problem of consistently sourcing adequate volumes of raw material has been one factor leading the region's major sweet corn processor to consider withdrawing from the region. Hence, the development of agronomic practices which reduce crop production costs, improve marketable yields, or both, will be important for maintaining the viability of the sweet corn processing industry in Gisborne for both growers and processors alike. Two of the most important factors influencing yield of sweet corn are plant density and nitrogen (N) nutrition. The density range which maximised marketable yield of cobs and kernels for Jubilee and SS42, the two prominent cultivars grown at Gisborne, was 69-77,000 plants per hectare. Although yield response to fertiliser N was also investigated in the same study, the yield response was either negligible (SS42) or did not follow a trend consistent with incremental increases in N rate (Jubilee). The limited response was attributed to high background levels of soil available N (269 kg/ha). A second experiment was designed to investigate the yield response, to fertiliser N on a soil with a low available N level. Although only 92 kg N/ha was available from the soil, yield response in this experiment was also negligible with N rates greater than 73 kg/ha. Combining the two years' results indicated that yield response to N fertiliser will be limited when soil available N levels are > 213 kg/ha. The rate of yield improvement could be enhanced by greater understanding of the physiological processes limiting yield in maize and sweet corn. The study of source-sink relationships can provide useful insights into yield determinants. A field experiment was established with Jubilee and SS42 to study how variables influencing weight of primary and secondary ears (e.g., silk delay, tiller number per plant) adjust to plant density and N nutrition. Path analysis and canonical discriminant analysis indicated that tillers were important for supplying the secondary ears of both cultivars with photoassimilate at low densities (e.g., 40,000 plants per hectare) and were important for Jubilee, but not SS42, at high densities (e.g., 100,000 plants per hectare). A short silk delay for both the primary and secondary ear was important for both cultivars at low densities to establish a large ear sink. Thus, at low densities, the presence of a secondary ear at low densities appeared to enhance kernel development on the primary ear. To further understand the partitioning of DM and N to kernels, further experiments quantified sink strength (or source strength) of an organ (i.e., leaves, stems, roots, kernels, rachis, husk, or shank) between defined ontogenetic stages. Sweet corn grown at 70,000 plants per hectare with rates of applied N ranging from 0 to 230 kg/ha were harvested throughout ontogeny until R4. Although N rate generally did not influence partitioning of N or dry matter (DM) to any organ, significant cultivar differences were detected. Kernel sink strength of Jubilee was two-fold greater for DM than SS42 and three-fold greater for N between R1 and R3. As a consequence, kernels of Jubilee contained 34% more DM than those of SS42 at R4 and were significantly more efficient than SS42 kernels at translating endogenous N into kernel DM. The observation that DM was partitioned to vegetative organs during reproductive growth while N was being remobilised from these organs indicated that both Jubilee and SS42 were source limited for N, yet sink limited for current photoassimilate. No published studies have been sighted which have identified a link between the source limitation for N and the sink limitation for DM in Z. mays. Investigating source-sink, relationships indicated that the two events are linked and initiated by low kernel sink strength during early grain filling. SS42 partitioned large proportions of DM to both husks and stems between R1 and R3, in contrast to Jubilee which partitioned most DM directly to kernels. As partitioning DM to vegetative tissue and husks reflects photoassimilate not consumed in reproductive growth, excess photoassimilate resulting from limited sink strength may have decreased photosynthetic rates through 'feedback' inhibition. Consequently, the ability of Jubilee to partition DM to roots for N assimilation between R3 and R4 may reflect less inhibited photosynthesis than for SS42. A subsequent experiment provided further evidence that kernel sink strength influences N and DM partitioning. This experiment also indicated that low kernel sink strength during early grain filling may actually initiate an inhibitory cycle. When maximum leaf area in maize and sweet corn is reached around R1, the ear is a relatively weak sink and unable to accumulate all the photoassimilate being produced. Although the excess is partitioned to stems and husks, these organs can only accumulate a limited quantity before they become saturated. When the stem and husks become saturated, photoassimilate may accumulate in leaves causing feedback inhibition of photosynthetic enzymes to reduce the supply of photoassimilate. However, as N assimilation rate is dependent on the rate of photoassimilate supply to roots, the inhibited photosynthesis reduces N uptake and as a consequence, remobilisation of N is stimulated. Excessive remobilisation of N from leaves may further impair photosynthetic activity to further restrict the photoassimilate supply for root and shoot functions including grain filling. Hence, an inhibitory cycle may evolve from the limited capacity of kernels and rachis to accumulate photoassimilate. Since SS42 (sh2 mutant) had a significantly lower kernel sink strength than Jubilee (su1 mutant) during early grain filling, SS42 was apparently more influenced by the inhibitory cycle than Jubilee. To add support to the theory that limited kernel sink strength during early grain filling may lead to an inhibitory cycle, a final experiment investigated the association of the endosperm storage protein, zein, with kernel sink strength. A high correlation (r=0.91) was observed between kernel DM and the level of zein. Further, the wild type (Furio) contained 25 and 49% more zein at R4, and accumulated 18 and 49% more DM, respectively than the su1 (Jubilee) and sh2 (SS42) mutants. Similarly, kernels of Jubilee, which contained 31% more zein than those of SS42., accumulated 38% more DM. Together these results indicate that the level of zein is associated with kernel sink strength and thus lends support to the inhibitory cycle theory.