Now showing 1 - 3 of 3
  • Publication
    The effect of soil P on N2O emission and N-cycling in Irish grasslands
    (University College Dublin. School of Biology and Environmental Science, 2022) ;
    This research aims to address nutrient use efficiency by investigating the key nutrients; carbon, nitrogen and phosphorus relative to one another the effect that imbalanced nutrient supply has on N2O emissions. To first address if there was in fact any effect to be observed, two short-term incubation experiments were set up using soils of high and low phosphorus concentrations were taken from a long-term cut field trial. The soils were incubated in a climate controlled chamber and treated with a typical field rate of N fertiliser, further a split treatment of carbon was added such that the microcosms in the first incubation did not receive carbon and those in the second incubation received both carbon and nitrogen. The results of these incubations clearly demonstrated that carbon deficiency inhibited all activity with negligible CO2 and N2O emissions being recorded from the carbon-omitted microcosms at both phosphorus levels, whereas P deficiency had an impact on nitrogen cycling in particular. When both carbon and nitrogen were added to soils of low phosphorus there was a 70-fold increase in N2O emissions compared to the high phosphorus microcosms with no significant difference between CO2 levels. This was a very important result as it suggested a separation between activity and process-based inhibition, both of which are essential to balance for a healthy soil system. Expanding on this research led to a third incubation experiment, carried out on soils taken from the same field trial, incubated under the same climate conditions and treated with the same rates of C and N fertiliser. The nitrogen fertiliser applied however, carried an isotopic 15N label to identify the applied nitrogen from the more abundant 14N present in the atmosphere, such that the mineral nitrogen and N2O results could be used to trace the nitrogen transformation processes occurring in these soils in order to establish the dominant pathways contributing to N2O production. This experiment identified a previously unreported pathway of oxidation of labile nitrogen via heterotrophic nitrification being the dominant transformation rate occurring in both phosphorus levels but at a significantly greater rate in the low phosphorus soils. Alongside all of these incubations, subsamples from the microcosms were collected and stored for molecular analysis. The genetic abundance of key nitrogen cycling genes as well as bacterial, archaeal and fungal community identifying genes were quantified by extracting DNA from the soil samples and quantifying genetic abundance using qPCR analysis. This revealed varying abundances of different genes between phosphorus levels with nitrifier genes found to be most abundant in low phosphorus soils and denitrifier genes to be more abundant at high phosphorus soils. Furthermore, this analysis revealed a positive correlation between the fungal community and N2O emissions from the low phosphorus soils, which was the only correlation observed in the low phosphorus soils. The final experiment up-scaled this research to the field level and carried out N2O measurements from static chambers which were installed in another long-term phosphorus trial under a grazed management regime. The results of this trial confirmed those of the laboratory in that low phosphorus results in greater N2O emissions and lower nutrient use efficiency. The results of this thesis offer an immediately implementable area of nutrient management that can be used to maximise efficiency, and reduce wastage which benefits both environmental mitigation goals as well as financial savings. These results also highlight further areas to direct future research to, broadening the knowledge base on nutrient interactions can offer further mitigation and efficiency-based land management which contribute to national and international environmental improvement goals and emission-reducing targets.
  • Publication
    Optimising soil P levels reduces N2O emissions in grazing systems under different N fertilisation
    The effect of long-term soil phosphorus (P) on in situ nitrous oxide (N2O) emissions from temperate grassland soil ecosystems is not well understood. Grasslands typically receive large nitrogen (N) inputs both from animal deposition and fertiliser application, with a large proportion of this N being lost to the environment. Understanding optimum nutrient stoichiometry by applying N fertilisers in a relative balance with P will help to reduce N losses by enabling maximum N-uptake by plants and microbes. This study investigates the N2O response from soils of long-term high and low P management receiving three forms of applied N at two different rates: a nitrate-based fertiliser (KNO3) and an ammonium-based fertiliser ([NH4]2SO4) (both at 40 Kg N ha−1), and a synthetic urine (750 Kg N ha−1). Low soil P significantly increased N2O emissions from KNO3 and (NH4)2SO4 fertilisers by over 50% and numerically increased N2O from urine by over 20%, which is suggested to be representative of the lack of significant effect of N fertilisation on N-uptake observed in the low P soils. There was a significant positive effect of soil P on grass N-uptake observed in the synthetic urine and KNO3 treatments, but not in the (NH4)2SO4 treatment. Low P soils had a significantly lower pH than high P soilss and responded differently to applied synthetic urine. There was also a significant effect of P level on potential nitrification which was nearly three times that of low P, but no significant difference between potential denitrification and P level. The results from this study highlight the importance of synergy between relative nutrient applications as a deficiency of one nutrient, such as P in this case, could be detrimental to the system as a whole. Optimising soil P can result in greater N uptake (over 12, 23 and 66% in (NH4)2SO4, KNO3 and synthetic urine treatments, respectively) and in reduced emissions by up to 50% representing a win-win scenario for farmers.
  • Publication
    Linking long-term soil phosphorus management to microbial communities involved in nitrogen reactions
    The influence of soil phosphorous (P) content on the N-cycling communities and subsequent effects on N2O emissions remains unclear. Two laboratory incubation experiments were conducted on soils collected from a long-term (est. 1995) P-addition field trial sampled in summer 2018 and winter 2019. Incubations were treated with a typical field amendment rate of N as well as a C-amendment to stimulate microbial activity. Throughout both incubations, soil subsamples were collected prior to fertiliser amendment and then throughout the incubations, to quantify the abundance of bacteria (16S rRNA), fungi (ITS) and Thaumarcheota (16S rRNA) as well as functional guilds of genes involved in nitrification (bacterial and archaeal amoA, and comammox) and denitrification (nirS, nirK, nosZ clade I and II) using quantitative PCR (qPCR). We also evaluated the correlations between each gene abundance and the associated N2O emissions depending on P-treatments. Our results show that long-term P-application influenced N-cycling genes abundance differently. Except for comammox, overall nitrifiers’ genes were most abundant in low P while the opposite trend was found for denitrifiers’ genes. C and N-amendments strongly influenced the abundance of most genes with changes observed as soon as 24 h after application. ITS was the only gene correlated to N2O emissions in the low P-soils while microbes were mostly correlated to emissions in high P, suggesting possible changes in the organisms involved in N2O production depending on soil P-content. This study highlights the importance of long-term P addition on shaping the microbial community function which in turn stimulates a direct impact on the subsequent N emissions.
      60Scopus© Citations 1