Browsing by Subject "Nitrogen"

Now showing 1 - 5 of 5
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    Open Access
    Can nitrogen isotopes be used to detect the effects of burning on nitrogen cycling? A study on Pinus resinosa in Minnesota, USA
    (2004) Ballantyne, Fiona; Craine, Joseph
    Pinus resinosa stands in Minnesota, USA were surveyed and fine and coarse roots from stands were analysed for nitrogen isotopes in an attempt to determine if fire effects nitrogen cycling. Fire increased the % nitrogen of coarse roots and changed the ratio of C: N. It may also decrease the depth of the A-horizon. It did not however affect the nitrogen isotope ratio of stands. Isotope values of coarse roots were affected by the depth of the A-horizon, which is consistent with other studies. A correlation between the 15N values of course and fine roots was found however fine roots were enriched proportionately to coarse roots. This suggests that they may be using a more enriched source of nitrogen such as ammonia although this is not conclusive. A PCA performed on site variables and isotope values determined the two major axes for sites were bum history and nitrogen cycle openness but that burning did not affect the second axis. A stepwise backwards regression showed the best predictors of a root's isotope value were, in decreasing order, the isotope value of coarse roots for fine roots and vice versa, the % nitrogen of roots and the age of stands. Coarse and fine roots showed opposite trends with % nitrogen. As the % nitrogen increased, fine roots were more 15 N enriched, again pointing to different nitrogen sources for coarse and fine roots. Older stands do appear to have a more open nitrogen cycle, as roots were 15N enriched in these stands. The availability of different forms of nitrogen may influence the 15N signal of roots especially if fine roots are preferentially using ammonia. Therefore, it is not possible to say if isotope values are due to stand openness, or due to different resources use by roots.
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    Open Access
    Identifying long term patterns and drivers of vegetation structure in an African savanna using stable carbon and nitrogen isotopes
    (2010) Ghaui, Mark; Gillson, Lindsey
    Savanna systems are complex and dynamic in space and time. Climate, fire, herbivory and nutrients have been identified as structuring agents of savanna form and function, but their interactions and feedbacks with one another and vegetation are poorly resolved. Increasing the spatial and temporal scope of studies will help to improve this situation, as demonstrated in recent studies in the spatial dimension in particular. This study aims to investigate vegetation and Nitrogen cycling changes over time in a diverse patch mosaic landscape in Hluhluwe-iMfolozi Park to identify drivers of vegetation structure and their dynamism over time. Sediment from a 150cm core (taken using a Russian corer) was analyzed for stable ¹³C and ¹⁵N isotope abundances, and C:N ratio of soil organic matter. The base of the core was dated at 2380±40cal.Yr.BP. δ¹³C, δ¹⁵N and C:N of soil organic matter was found to be variable over time. δ¹³C followed a pattern of stable periods of distinct abundance separated by abrupt changes; δ¹⁵N and C:N underwent changes over the same periods as δ¹³C. Vegetation follows a pattern of phase and transition as predicted by resilience theory. An aquatic vegetation phase persists around 2000cal.Yr.BP to about 500cal.Yr.BP, coinciding with a warm, wet period (including the Medieval Warm Period) with an open Nitrogen cycle. A C₄ grassland phase follows alter a transition to cool, dry conditions coinciding with the Little Ice Age, and decreasing openness of the N cycle. Recent increasing C₃ vegetation and N-openness were attributed to atmospheric CO₂ increase and Nitrogen deposition respectively. Climate is concluded to be the major driver of vegetation at this site, and a combination of climate and vegetation are responsible for changes in Nitrogen availability. Findings are discussed in relation to landscape management. Multi-proxy evidence in future studies would be useful in validating the findings of this study.
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    Open Access
    Nitrogen uptake by phytoplankton in the Atlantic sector of the Southern Ocean during late austral summer
    (2011) Joubert, W R; Thomalla, S J; Waldron, H N; Lucas, M I; Boye, M; Le Moigne, F A C; Planchon, F; Speich, S
    As part of the Bonus-Good Hope (BGH) campaign, 15N-labelled nitrate, ammonium and urea uptake measurements were made along the BGH transect from Cape Town to ~ 60° S in late austral summer, 2008. Our results are categorised according to distinct hydrographic regions defined by oceanic fronts and open ocean zones. Nitrogen uptake (ρN) in the oligotrophic Subtropical Zone (STZ) was dominated by ρ urea, which contributed up to 70 % of ρN. High regenerated ρN in the STZ resulted in low f-ratios (f = 0.2). Size fractionated chlorophyll data showed that the greatest contribution (>50 %) of picophytoplankton (
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    Open Access
    A size-based model of carbon and nitrogen flows in plankton communities
    (1988) Moloney, Coleen Lyn; Field, John G; Lucas, Michael I
    A generic, size-based simulation model is developed to investigate the dynamics of carbon and nitrogen flows in plankton communities. All parameters in the model are determined by body size using empirically-determined relationships calculated from published data. The model is robust with respect to most parameters and assumptions. Because the model is based on general ecological principles, it can be used to simulate microplankton community interactions in any planktonic ecosystem. Two coastal ecosystems from the southern Benguela region in South Africa are simulated; one typical of the relatively stable surface waters on the Agulhas Bank and one typical of upwelling plumes, usually found off the west coast of South Africa. Simulated communities compare well with field observations in terms of standing stocks and size composition, and simulation results indicate that the small-scale structure of the two ecosystems and the processes occurring within them are relatively well understood. Consequently, the dynamic functioning of the two systems is investigated at the ecosystem level, using the simulation results. Hypothetical carbon flow networks are constructed, and the average importance of different flow pathways at different times is assessed. In both ecosystems, the vast majority of carbon flows pass through short, efficient-transfer pathways, although longer pathways are potentially possible. Simulation analyses are extended from coastal to oceanic food webs, and the model results are consistent with the hypothesis that oceanic phytoplankton have rapid rates of primary production. At-sea sampling of a phytoplankton bloom is mimicked by "sampling" from simulation output, and interpretation of the data using standard techniques is compared with the model output. The dangers of extrapolating from snapshot measurements is highlighted, and the experiment emphasizes the importance of size-fractionated sampling of phytoplankton. A hypothetical pelagic food web is described, consisting of at least five different trophic pathways from phytoplankton to pelagic fish. It is suggested that coastal waters probably have all the different pathways, and the relative importance and efficiency of the different pathways will determine the total fish production in an ecosystem.
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    The distribution of tree and grass roots in savannas in relation to soil nitrogen and water
    (2010) February, Edmund C; Higgins, S I
    Here we describe the fine root distribution of trees and grasses relative to soil nitrogen and water profiles. The primary objective is to improve our understanding of edaphic processes influencing the relative abundance of trees and grasses in savanna systems. We do this at both a mesic (737 mm MAP) site on sandy-loam soils and at an arid (547 mm MAP) site on clay rich soils in the Kruger National Park in South Africa. The proportion of tree and grass fine roots at each soil depth were estimated using the δ13C values of fine roots and the δ13C end members of the fine roots of the dominant trees and grasses at our study sites. Changes in soil nitrogen concentrations with depth were indexed using total soil nitrogen concentrations and soil δ15N values. Soil water content was measured at different depths using capacitance probes. We show that most tree and grass roots are located in the upper layers of the soil and that both tree and grass roots are present at the bottom of the profile. We demonstrate that root density is positively related to the distribution of soil nitrogen and negatively related to soil moisture. We attribute the negative correlation with soil moisture to evaporation from the soil surface and uptake by roots. Our data is a snapshot of a dynamic process, here the picture it provides is potentially misleading. To understand whether roots in this system are primarily foraging for water or for nitrogen future studies need to include a dynamic component.