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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.
The influence of phosphorus supply on below ground interferences between browntop and white clover : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
(Massey University, 1993) Pannell, Claire Astley
A low occurrence of white clover in pastures contributes to soil nitrogen (N) deficiency and a low quality feed for stock. There is evidence to suggest that competition for soil phosphorus (P) from roots of browntop plays an important role in determining the distribution of white clover in hill country swards. However, competition for soil P between roots of browntop and white clover has not been studied separately from other factors known to affect the growth and persistence of white clover (e.g., soil moisture, grazing management, shoot interferences (shading and physical impedance), and non-competitive root interferences). In hill country pastures, P level (phosphorus fertilisers), and defoliation (grazing management), are the main factors that can be changed by farmers to alter pasture botanical composition. The high cost of superphosphate has limited the potential of farmers to manipulate pastures using fertiliser applications. Therefore, it is important to know whether roots of browntop compete with roots of white clover for soil P, and whether a low supply of soil P will contribute to more severe competition from browntop roots. The possibility of non-competitive interferences occurring between roots of browntop and white clover must also be considered. How defoliation alters the balance of P acquisition between roots of browntop and white clover needs to be determined. Three techniques were employed to examine the nature of root interferences occurring between browntop and white clover: plant strategies; dual P isotope labelling; and a more traditional competitive settings trial using aerial partitions. Plant growth and root interferences were studied at a range of levels of soil P supply. Responses of growth and phosphorus uptake of browntop and white clover to increasing soil P supply were examined first, in the glasshouse, by growing monocultures of browntop and white clover in pots. Two mini-sward trials (one at deficient soil P supply, the other at adequate to luxury soil P supply) were carried out in the glasshouse to allow examination of root interactions (without shoot interactions). The basis of the experimental design was to determine the relative amounts of phosphorus-32 and -33 absorbed by a central row of plants (either browntop or white clover) from two adjacent soil spaces, one dominated by white clover roots, the other by browntop roots. 32P was injected into the soil on one side of the central row of plants, and 33P into the other side. 32p and 33p uptake was assessed by harvesting the shoots of the central plants, and counting the two isotopes. The competitive settings type trial compared the growth and P uptake of a single central plant in a small pot (no interference with other plants) with a central plant in a larger pot grown with roots associated with roots of plants of the same species (intraspecific association), or of the other species (interspecific association). Shoots of the central plant was separated from the shoots of outer plants by an aerial partition. The growth of browntop and white clover, and the nature of root interferences occurring within and between the two species was dependent on the level of soil P supply. However, the higher root density and specific root length (SRL) of browntop compared with white clover appeared to be the most important factor determining the success of browntop at all levels of soil P supply, regardless of whether or not browntop was grown with white clover. According to the plant strategy theory of Grime, browntop was found to be a stress tolerant plant. At low levels of P supply, the lower growth rate of browntop compared with white clover would be an important factor contributing to the dominance of browntop in hill country pastures. At adequate to luxury levels of soil P supply, shoot growth of browntop was more responsive than white clover, and browntop was capable of luxury consumption of P. The high growth rate and large demand for P contributed to the competitiveness of browntop at high P supply. However, the lower demand for P by white clover, and the high P supply may have enabled white clover to avoid competition with browntop. On unamended subsoil, browntop reduced P acquisition by white clover roots, and had a greater P uptake in the presence of roots of white clover than with roots of other browntop plants. Therefore, evidence of root competition for soil P from browntop with white clover was found. The competitive effect of browntop appeared to be due to browntop decreasing the availability of P in the soil, explained by browntop's ability to acquire more radioactive P from the soil than white clover. At low P supply (subsoil), P application, but not defoliation of browntop, reduced the competitiveness of browntop. At adequate P supply, the ability of browntop to acquire P was reduced by defoliation. The effect of defoliation was rapid (four days), and browntop was able to acquire P isotope to higher concentrations in the shoots than when undefoliated. Possibly the reduction of root competitiveness of browntop may be short-lived. Some interference, other than root competition, was occurring at intermediate to luxury levels of soil P supply, and may have masked the competitive effects of browntop. White clover appeared to benefit for P acquisition from growing with browntop, due to greater local root density compared with when growing with other white clover plants. Therefore, browntop and white clover appeared to gain mutual benefit for P acquisition from the presence of roots of the other species, and the competitive effects of browntop were not of overriding importance. The possibility of autotoxicity of white clover on its own root growth was discussed in relation to rhizosphere acidity effects on the toxicity of phenolics. At adequate to luxury levels of soil P supply, neither undefoliated browntop nor undefoliated white clover benefited from defoliation of adjacently growing white clover plants. However, at lower P supply, defoliation of white clover led to an increased P isotope acquisition by nearby browntop plants. Therefore, defoliation reduced the demand for soil P by white clover. Roots of browntop were not as tolerant of defoliation as white clover. In the field, the mat forming behaviour of browntop, physically impeding the growth of white clover and shading white clover stolons, would reduce the severity of competition for soil P between roots of browntop and white clover. Overall, root competition for P from browntop with white clover was found not to be as important as previously thought. The use of several experimental techniques allowed a clearer picture of the interferences that occur between browntop and white clover to be obtained. The nature of root interference changed with increasing P supply. The responses of browntop and white clover to increasing P supply was found to be enlightening when the plant strategy theory of Grime was used to compare browntop and white clover. However, the dual P isotope technique found plant interferences that were not detected by the other methods used (P response and competitive settings trial), and allowed interferences that were occurring simultaneously to be elucidated.
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.
Nitrogen relationships in grazed and cut grass-clover systems : a thesis presented in partial fulfilment for the degree of Doctor of Philosophy in Agronomy at Massey University
(Massey University, 1979) Ball, Philip Roger
Performance of a ryegrass-white clover sward subjected to a range of nitrogen treatments was recorded over three years. Herbage was removed either by periodic mob-stocking with sheep or cutting with a shearing handpiece. Nitrogen was applied as lime-ammonium nitrate in split dressings, over the period late autumn-winter-spring-early summer. A cloverless sward receiving no fertiliser nitrogen was included to gain information on the nitrogen status of the experimental site. An unbalanced design was used, with 7 nitrogen treatments represented under grazing but only 4 under cutting. The performance of pastures subjected to relatively frequent, close defoliation by mob-stocked sheep was reasonably reproduced in cut swards. The cutting technique allowed better simulation of clover herbage removal by sheep than is possible with mowing, and it was not until the third year that a difference was recorded in yearly clover dry herbage yield between grazed and cut swards. Both the cutting technique and avoidance of nutrient depletion (especially of potassium) in cut treatments are considered important in determining the pattern of results. Under these conditions, grazing increased total dry herbage (herbage nitrogen) yield each year by from 13 to 24% (18 to 22%). Annual nitrogen turnover in urine and dung averaged approximately 650 kg N/ha, with apparent recovery in herbage no more than 20% on average over the three years. Annual total dry herbage (herbage nitrogen) yield from this ryegrass-white clover association was substantial, averaging 15.6 t DM/ha (560 kg N/ha) each year without fertiliser nitrogen. The heaviest fertiliser input, 448 kg N/ha/an, increased total dry herbage (herbage nitrogen) yield each year by from 20 to 25% (30 to 43%). Quite clearly, nitrogen availability limited total herbage production from this well managed grass-clover association. On average over the three years, total herbage responses were approximately linear, and apparent recovery of fertiliser nitrogen averaged 44%. This figure would be higher if it were possible to correct for enhanced soil nitrogen uptake by clover in the nitrogen-treated swards. N2(C2H2) fixation under grazing totalled 263, 165 and 53 kg N/ha/an in grass-clover pastures receiving 0, 112 and 448 kg N/ha/an, respectively. Fertiliser nitrogen reduced symbiotic fixation through the combined influence of clover suppression and reduced fixation efficiency by clover in the nitrogen-treated swards. Main treatment (defoliation method x nitrogen treatment) interactions were largely absent from annual production data. Comparison of main treatment effects indicated that sheep-grazing and fertiliser nitrogen exerted similar but certainly not the same effects. Nitrogen removal in animal produce was estimated to have averaged about 20 kg N/ha/an, while average removal in herbage from cut grass-clover associations was almost 600 kg N/ha/an. Yet the contrast between grazing and cutting in the several parameters of nitrogen relationships measured in the soil-plant complex was unspectacular. The soil total nitrogen pool throughout the sampled profile declined over the course of the experiment. The average annual reduction under grazing (60 kg N/ha-45.7 cm) was less than under cutting (150 kg N/ha-45.7cm). Fairly complete utilisation of herbage, coupled with a close C/N ratio (c 10) for soil at the site, are considered to provide the most likely explanation for the observed pattern of change. No influence of nitrogen treatments was apparent, indicating that factors regulating soil carbon accumulation are inseparable from the size of the annual input of nitrogen, in determining nitrogen accumulation within a developed grass-clover system. It is concluded from nitrogen balances that unaccounted-for nitrogen outgoings of several hundred kg N/ha occurred each year from the grazed grass-clover systems. Results from this and associated research indicate that grazing animals cause substantial, but hitherto largely unrecognised nitrogen losses from developed grass-clover ecosystems. Most of the loss stems from aggregation of excess dietary nitrogen into urine patches.
Effects of elevated atmospheric CO2 concentrations on carbon and nitrogen fluxes in a grazed pasture : a thesis presented in partial fulfilment of the requirements for the degree of Ph. D. in Plant Science at Massey University and the degree of Docteur en Sciences, speciality Sciences Agronomiques at the Institut National Polytechnique de Lorraine
(Massey University, 2003) Allard, Vincent
Predicting the response of grazed grasslands to elevated CO2 is of central importance in global change research as grasslands represent 20% of the worlds' land area and grassland soils are a major sink for carbon (C). Grasslands responses to elevated CO2 are strongly controlled by the availability of other nutrients and nitrogen (N) in particular. There have been many previous studies of N cycling in grasslands exposed to elevated CO2 but none of these experiments were grazed. In this thesis I present data on CO2 effects on N cycling from an experimental system (FACE: Free Air Carbon dioxide Enrichment) that enabled grazing to be included. The thesis focuses on the effects of elevated CO2 on the different processes involved in organic matter (OM) returns from the plant to the soil and the consequences for N availability. In Chapter 1, it was shown that elevated CO2 modified N returns by grazing animals by altering the partitioning of N between faeces and urine creating a potential for enhanced N losses at elevated CO2. Plant litter decomposition rates were, at the ecosystem scale, not affected by elevated CO2 (Chapter 3), but a marked increase in the organic matter fluxes, from roots, led to an accumulation of coarse OM in the soil (Chapter 4). In Chapter 5, using 14C and 15N labelling, I compared short-term (plant mediated) and long-term (soil mediated) effects of elevated CO2 on soil OM dynamics and concluded that soil OM accumulation under elevated CO2 was not caused by C or N limitation but probably by the availability of other nutrients. The thesis demonstrates that the inclusion of grazing animals can strongly modify N cycling under elevated CO2. As most grasslands are grazed, the prediction of grassland responses to elevated CO2 must be derived from systems in which animals are an integral part.
Dynamics of nitrogen in three contrasting pastures grazed by sheep : 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) Ruz Jerez, Belarmino Emilio
The dynamics of nitrogen (N) were studied during two years (March 1989 - May 1991) in three contrasting pastures grazed by sheep. The pastures were: ryegrass-white clover, herbal ley (a legume-based pasture of interest for "organic" agriculture) and pure ryegrass receiving 400 kg fertiliser N/ha/yr. This study was undertaken on a recent alluvial soil at DSIR Grasslands. Palmerston North. Treatments were replicated as small paddocks, and periodically mob-grazed with sheep. Frequent soil measurements provided estimates for leaching and denitrification. Herbage yields and botanical composition were recorded, and symbiotic N2 fixation was measured in swards of the two treatments containing forage legumes. Soil total N and carbon were measured annually, providing estimates of the partial mass balance for N. The soil mineral N pool was dominated (especially in the systems receiving no fertiliser N) by the highly concentrated pulse of N returned in the excreta of grazing animals to a small proportion of the grazed area. In the pure grass sward the large inputs of fertiliser N had a significant effect in increasing the amount of mineral N available in the top 45 cm of soil. On average, in composite samples including urine-affected and non-affected areas, about 30 kg/ha-45 cm more mineral N was available throughout the year in the ryegrass fertilised with N than in the legume-based pastures. This consistently high level of soil mineral N in the ryegrass+N sward was responsible for the greatest annual herbage yield; however annual losses by leaching and denitrification were 5 to 6 times greater than in the legueme-based pastures. A common feature of the three pastures was the small amount of N recovered in animal products, with most of the N that circulated through the plant to the sheep being returned to the soil in urine. This concentrated input was localised in about 10% of the area, which provided the major avenues for N escape from the pastures receiving no fertiliser N. It was estimated that a little more than half the nitrate leached (total, about 6 kg NO3-N/ha/yr) arose from this restricted area, but in the grass+N pasture the contribution of animal-induced losses was proportionally smaller than in the legume-based pastures. Fertiliser N. by increasing soil mineral N, offered more site opportunities for N leaching and denitrification, in addition to that from urine. Here, only one-quarter of leached nitrate (total, 41 kg NO3-N/ha/yr) arose from urine patches. Denitrification accounted for 4-5 kg N/ha/yr from the legume-based pastures, but 20 kg N/ha/yr from swards receiving fertiliser N. Ammonia volatilisation, which was estimated using data from previous studies at this site, was enhanced by direct emission from the fertiliser N (urea) as it is hydrolysed on the soil surface. Calculation of N inputs and outputs for these three pastures indicated that the two legume-based systems were more or less in balance, but in the pasture receiving fertiliser N some 180 kg N/ha/yr was unaccounted for. This difference may reflect incorporation of N into soil organic matter, as indicated by a small increase in soil total N during the second year. Pasture production (average of two years) from the herbal ley was about 15 t DM/ha/yr, or about 90% of the yield from pasture receiving fertiliser N, and some 25-30% more than from ryegrass-clover. Symbiotic N2 fixation, estimated by the acetylene reduction assay to have been 140-150 kg N/ha/yr, was similar in both systems based on forage legumes. The herbal ley utilised soil N more efficiently than the ryegrass-clover and ryegrass+N pastures, hence achieving an outstanding yield of herbage. It is argued that this apparently better exploitation of soil N was brought about largely by stimulation of microbial biomass in the rhizosphere around chicory roots, with the additional N that was scavenged by bacteria being made available to this herb after protozoan digestion of the bacteria. A herbal ley offers the possibility of sustaining a high level of forage production, but with reduced N emissions to the environment.
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.
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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