Tracing fertiliser through a grassland ecosystem using 15N-tracer approaches

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Livestock are grazed on 11.4 million hectares of grasslands in the UK and demand for their
products continues to grow worldwide. Nitrogen (N) fertilisers are critical to maximising pasture
productivity and, hence, grazing animal product yields. Excreta from grazing animals is also an
important N input and are hotspots of N-cycling in pastures. Losses of N from agriculture contribute to
freshwater pollution, reduced biodiversity and climate change, alongside reducing the economic
efficiency of the overall grazing system. With 70 to 100% more food required by 2050 to ensure global
food security, increasing emphasis is being placed on minimising negative environmental impacts and
increasing nutrient use efficiency (NUE) of grasslands. Optimisation of NUE requires detailed
understanding of the fates of N fertiliser in the grazing ecosystem. Of particular interest is the role of
the soil microbial community (SMC) in determining the fate of fertiliser N, since this provides essential
soil ecosystem services, including the potential for pollution mitigation and optimisation of plant N
supply. The research described in this thesis is focused on the application of a combination of 15N-tracer
approaches to determine the fate of N inputs to a grassland system.
Initially this involved the development of a robust GC-C-IRMS method to determine the 15N
composition of amino sugars (AS), including the first application of a two-point linear normalisation
for high precision compound-specific δ15N value determinations. This method was subsequently used
in combination with 15N-amino acid (AA) determinations to assess microbial NUE of 15N-fertilisers in
laboratory incubation experiments. Assimilation of applied 15N-fertiliser was found to increase with
microbial substrate preference (15N-nitrate < 15N-ammonium). This compound-specific 15N-stable
isotope probing (SIP) approach allowed the role of the SMC in the biochemical fate of fertiliser N to be
defined. Extending this approach to AS allowed deconvolution of N-assimilation dynamics of bacterial
and fungal pools due to source specificity of AS. Incorporation into bacterial and fungal AS showed
differing temporal responses, due to slower turnover of fungi versus bacteria.
The research then focussed on using a novel approach for evaluating 15N-fertiliser cycling in a
grassland system based on a combination of field, feeding and lysimeter experiments. N content, bulk
15N determinations and the newly developed compound-specific 15N-SIP approach enabled quantitative
mass balancing of the individual N pools in the deconstructed grassland system. This showed the
relative importance of plant uptake, animal grazing and microbial assimilation of applied N throughout
the grassland ecosystem. Tracing the fate of 15N-ammonium, applied at 70 kg N ha−1, across 18 months
in a field experiment showed high plant NUE (70 ± 6.1%), with 16.4 ± 1.0% assimilated into the soil
microbial protein pool. Sheep grazing in a feeding experiment revealed the importance of excreta in the
grassland system, accounting for 70 ± 10% of ingested forage 15N, with 15N-incorporation dynamics of
animal tissues reflecting differing turnover times. In contrast to plant NUE, N incorporation into animal
tissues was low at 24 ± 11.7%. Urine patches are hotspots for N-cycling in grasslands and a model urine
patch mesocosm experiment allowed detailed tracing of the fate of 15N-urine. As expected, the
experimental N-loadings (950 kg N ha−1) of applied urinary N was above plant and microbial demand,
resulting in high leaching losses (34.4 ± 2.6%) of applied 15N immediately after application. Subsequent
plant uptake (38.4 ± 1.6%) and microbial assimilation into the AA and AS pools (34.0 ± 2.3%) increased
across the 3-months of the experiment, moderating leaching losses. Only 1.9 ± 0.8% of urinary-15N was
lost as gaseous emissions. The three experiments were individually mass balanced, then combined, to
demonstrate the relative importance of all fates of N. Importantly, the novel compound-specific 15NSIP approaches highlight the previously neglected role of the microbial community in mediating the
fate of N inputs in grasslands. This type of detailed approach to mass balancing the fate of N in
grasslands has the potential to be extremely valuable in guiding improvements in grassland management
to increase productivity and reducing N losses to the wider environment.
Date of Award23 Jan 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SponsorsNatural Environment Research Council
SupervisorRichard P Evershed (Supervisor) & Davey Jones (Supervisor)

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