Mineral nitrogen (N) fertiliser inputs are essential for achieving high crop yields in agricultural production systems and can help to drive farm profitability. However, when N fertiliser is applied to soil it can be lost to the environment and have negative consequences. Such losses include nitrous oxide (N2O) emissions, ammonia (NH3) emissions and nitrate (NO3-) leaching to waterways. This loss of N fertiliser also represents a substantial economic loss of N from the farm. The dominant N fertiliser source used on arable farms in Ireland is calcium ammonium nitrate (CAN) which, in environmental terms, principally contributes to N2O emissions and NO3- leaching. Switching from CAN to urea has the potential to reduce these N loss pathways, but can result in substantially higher NH3 emissions. Nitrogen stabilisers are compounds that can be added to N fertilisers to reduce these N losses. There are two main types of N stabilisers currently available: urease inhibitors and nitrification inhibitors. Urease inhibitors are used to regulate urea fertiliser hydrolysis and to reduce NH3 emissions and nitrification inhibitors are used to regulate the soil NO3- pool and to reduce N2O emissions and NO3- leaching. The urease inhibitor used in this study was N-(n-butyl) thiophosphoric triamide (NBPT) and the nitrification inhibitor used was dicyandiamide (DCD). An additional N fertiliser formulation, urea + the maleic – itaconic co-polymer (MICO), was added in 2014. Two field sites cropped with spring barley were established in 2013 and the overall study was conducted for three years. Nitrous oxide emissions, NH3 emissions, NO3- leaching and grain yield and N uptake were measured. The N fertilisers evaluated were CAN, urea, urea + NBPT, urea + DCD and urea + NBPT + DCD and Urea + MICO in 2014. Results showed that N2O emissions were low (over 50% lower than the IPCC default emission factor of 1%) regardless of the N fertilisers used but using the N stabilisers NBPT and DCD reduced emissions by up to 62%. There was no significant effect (P>0.05) of fertiliser formulation on NO3- leaching but there was a significant effect (P < 0.05) on NH3 emissions with urea + NBPT reducing emissions compared to urea. There was no significant effect (P > 0.05) of fertiliser formulation on spring barley grain yield but there was significantly lower N uptake with urea compared to CAN. Using urea + NBPT had similar N uptake levels to CAN. Overall this study showed that switching N fertiliser source from CAN to urea stabilised with the urease inhibitor NBPT can reduce environmental N losses and increase fertiliser N use efficiency (fNUE). This provides farmers with options to increase the environmental and economic sustainability of their arable farming systems while maintaining crop yields and quality.

The effect on copper content of herbage and accumulation of copper in soil following application to agricultural land of pig slurry containing up to 760 mg Cu/kg D.M. was investigated in field experiments. The slurry was produced by pigs receiving supplementary copper at a level of 200 mg Cu/kg in the ration. Incorporation of up to 9 kg slurry Cu/ha into soil increased the copper content of ryegrass/clover sown by a maximum of 1.3 mg Cu/kg D.M. A similar quantity of Cu later applied to the established sward increased the copper content of clover to a maximum of 24.8 and that of grass to 13.4 mg Ca,/kg D.M. Repeated applications of slurry to the soil surface in an established ryegrass sward increased herbage copper from 4 to a minimum of 16 mg kg D.M. on a sandy soil and from 8 to a maximum of 23 mgfkg D.M. on an imperfectly draining clay-loam soil. Dilution of slurry with water resulted in greater increases in herbage copper concentration than did undiluted slurry. The addition of 22.3 kg Cu,/ha to a sandy soil of 28.4 kg CuAa to a olayloam soil over a 2 year period increased A-extractable Copper from 2.7 to 19.3 mg Cu/kg on the former soil and from 11.9 to 30.3 mg Cu/kg soil on the latter soil; 89 and 88% of the quantities of slurry copper applied were extractable from topsoil using EDTA. Less than 5 mg WA were found in the aqueous phase in copper-rich slurry, while a fine particulate and colloidal fraction in the solids contained in excess of 2000 mg Cu/kg D.M. Dissolved copper in the aqueous phase when added to soil increased plant uptake of this element. Carbon mineralisation in soil, assessed, by oxygen uptake, was unaffected by addition to soil of a fraction of slurry solids containing 2 g Cu/kg D.M.4. The true availability (4.396) to sheep of copper in slurry was found to be similar to that (3.8%) of copper fed as CuSO4. The availability (5.896) to sheep of Ca in herbage grown on slurry treated land was greater than that (4.8%) in herbage from untreated land. The potential hazards to crops and livestock of disposal of high-copper pig slurries on agricultural land are discussed.

The application of phosphorus (P) fertilisers to grassland systems is a common practice to increase and sustain grassland productivity. This is requisite for satisfying the nutritional needs of grazing animals and increasing dairy and livestock output. The costs of such fertilisers are increasing and the demands for such fertiliser will also most likely rise following governmental targets set in Ireland to increase national agricultural output. However, the application of P fertiliser to grassland systems can contribute to the eutrophication of water-courses, since fertiliser applications can result in the accumulation of P at the soil surface. One potential way to facilitate plant P acquisition in grasslands may be associated with the soil biota. In particular, the soil microbial biomass is recognised as a potential P pool that can provide a source of bioavailable P to the plant community. The soil biota may also facilitate the incorporation of P from the soil surface into the soil profile, since earthworms can actively increase the transport of P-rich soil material from the surface belowground. This project thus aimed to discern how P fertilisation affects microbial biomass nutrient pools and biologically-mediated P incorporation in grassland systems, and how this relates to plant P yields. To investigate this aim, two research questions were proposed: (i) is the soil biota affected by commonly adopted P fertiliser strategies in grassland systems?; (ii) what consequence does this hold for P acquisition by the plant community? An experiment was conducted to examine how the soil biota responded to different rates of inorganic P fertilisation in two grassland sites of contrasting soil types over an 18 month period. This revealed that increasing P fertilisation did not affect microbial biomass P concentrations in the soil. However, an effect was observed upon plant P yield, in which greater plant P yields were obtained proportional to the P fertiliser rate. Two laboratory experiments were conducted to further investigate this lack of effect. These utilised soil from the same grassland sites and examined how nutrient additions to the soil affected microbial biomass nutrient pools and activity. Results from these experiments supported evidence from the field experiment, since the application of P fertiliser did not affect microbial biomass nutrient pools following fertiliser application, and supplementation of carbon (C) + P substrate to the soil did not invoke respiratory responses between P fertiliser treatments. Nevertheless,supplementation with C + nitrogen (N) and C+N+P substrates was found to suppress microbial respiration. This was attributed to greater C assimilation by the microbial community in these particular substrate-induced respiration treatments. In order to investigate biologically-mediated P incorporation, a glasshouse-based mesocosm scale experiment was carried out using two contrasting soils. Bulk soil (1 – 30 cm depth range) was derived from a nutrient poor grassland system, whereas the soil for the 0 – 1 cm depth range was taken from an intensive system that was seven times greater in labile inorganic P concentration. Three treatments were applied to mesocosms in an incomplete factorial design, involving the inclusion of earthworms, different botanical diversities (unplanted, monoculture or mixed plant community) and different fertiliser types (organic or inorganic). The absent factorial combinations involved the application of earthworms to unplanted mesocosms. With respect to the earthworm treatment, results revealed that the presence of earthworms reduced labile P concentrations in the 0 – 1 cm depth range of soil. The presence of different botanical diversities or fertiliser types did not affect microbial biomass nutrient pools, whilst the presence of mixed plant communities did increase plant P yields. However, microbial and nematode community structures were affected in an idiosyncratic manner by both botanical diversity and fertiliser type. This project demonstrated the significance of grassland management regimes in governing microbial biomass P concentrations. In particular, it was revealed that the frequent defoliation of the sward appeared to uncouple the microbial community from both fertiliser inputs and possibly plant P yields. The fact that an increase in plant P yield with increasing P fertilisation was noted in the absence of microbial responses suggests that the soil biota may not be crucial for plant P acquisition in such intensive inorganic-fertiliser based regimes. This suggestion was also supported by the mesocosm experiment, since plant P yields differed between botanical diversities but no effects were observed on microbial biomass P concentrations. Furthermore, this project showed the potential of the earthworm community to reduce P concentrations in the volume of soil which poses the greatest risk to water quality. The collective evidence highlights the need for further understanding of the consequences of inorganic-based fertiliser management systems, since current strategies may not adequately account for management effects on soil biological P cycling.

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Experiments were conducted at the University of Aberdeen under controlled laboratory conditions to examine the interactive effect of combining crop residues barley (<i>Hordeum vulgare</i>), clover <i>(Trifolium pretense)</i>, cowpea (<i>Vigna unguiculata</i>), <i>Leucaena (Leucena leucocephala</i>) and <i>Mucuna (Mucuna pruriens)</i> and inorganic N fertiliser on N₂O emission and mineral N dynamics. Emissions of N₂O increased following sole application of the residues dependent on their utility or chemical composition. In general emissions were significantly higher from soils amended with low C:N ratio clover, cowpea, <i>Leucena </i>and <i>Mucuna </i>residues compared to the high C:N ratio barley species. N₂O emission was further increased following combined application of crop residues and inorganic N fertiliser but the magnitude of emission was influenced by the proportional ratios at which the residue-N and the fertiliser-N were combined, and the C:N ratio, lignin and polyphenols contents of the residues, with the low lignin and low polyphenols cowpea treatment emitting higher N₂O over 30 days compared to the high lignin <i>Mucuna </i>and the  high polyphenols <i>Leucena </i>treatments. Results from experiments demonstrated that whilst there is the potential for N₂O emission to be controlled through varying ratios of residue:fertiliser input, the magnitude and direction of interactions between these N sources varies between different species as a result of their differing qualities. Based on our applications of <i>Leucaena, Mucuna </i>and cowpea residues, the 75:25 residue: fertiliser ratio at 100 mg N kg^-1 soil is recommended from this study as offering the best compromise between release of N for crop uptake and management of N₂O emission, but this requires further investigation at field scale.

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Solving climate change needs new renewable technologies that can be deployed at scale economically and sustainably. These technologies must also help stabilise the grid and follow demand rather than simply generate when nature provides. Anaerobic digestion could be part of the solution. It produces gas that can be stored and fed to engines as needed to generate electricity. However currently it is too expensive, the feedstock resource is limited, and much of the feedstock comes from energy crops that displace food. This is clearly not sustainable long term. To fulfil its potential, anaerobic digestion needs a new resource base to add to the present ones, and it needs to be much lower cost. Part 1 of this thesis examines the costs and resource base. It concludes that the resource base could be very substantially increased by growing hyper-water-efficient plants that use the crassulacean acid metabolism on degraded and semi-arid land globally. Between 5% and 15% of this land could provide as much electrical power as natural gas. The resource could be further increased by hybridisation with solar PV. Food production need not reduce, and may even increase. To become economic, however, needs a major reduction in capital costs. This can be achieved by increasing fourfold the volumetric power density of an anaerobic digester, principally by speeding up the rate of reaction. Part 2 of the thesis compares and contrasts digestion in ruminants with conventional anaerobic digestion technology. It concludes that ruminants perform the rate limiting step 20-30 times faster than a commercial anaerobic digester, partly as a result of adopting different mechanical and chemical strategies. The thesis identifies and explores these differences, and proposes future work to understand and quantify them, to facilitate development of a new generation of low cost Advanced Anaerobic Digestion that meets the challenging cost reduction targets set in Part 1.

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