Although the application of the commercial slurry additive SB to slurry resulted in significantly higher concentration of TP compared to the control slurry, no analysis was performed on likely changes in the microbial communities present within the slurry following the treatment of slurry with SB, in order to account for the lack of significant difference in other nutrient elements. The mechanism responsible for the difference in TP content for SB-amended versus control slurry was not identified. However, it was hypothesised that the SB additive resulted in significantly reduced
emissions of PH3 from slurry during storage. Further experimental work is required to
understand whether changes in the magnitude of PH3 emissions are generated
following the use of additives, such as SB, during longer-term storage (more than the 9-week storage undertaken in Chapter 3), alongside the specific mechanism
responsible for any change in PH3 emissions from slurry following the application of
an additive. Similarly, further research is needed to determine whether changes in the microbial community within slurry during storage are also transferred to changes in soil microbial communities following slurry application, in order to use slurry
additives such as SB to ultimately influence PH3 emissions from soils to the
atmosphere.
No significant changes in the NH4-N or TN content of livestock slurry were
observed between control and SB-amended slurries. However, additional work is required using different slurry types and different additives to more broadly assess the impact of slurry additives on the N content of slurry, alongside the microbial and/or physicochemical mechanisms that are responsible for any impact. A higher TS content was observed in SB-amended slurry compared to the control slurry, potentially reflecting greater microbial biomass accumulation following application of the SB
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additive to slurry. In order to test this hypothesis, microbial biomass within slurry should be determined, alongside further experimental work to understand the mechanism responsible for higher TS content in slurry as governed by the use of additives such as SB.
With respect to changes in soil nutrient concentrations following application of slurry and inorganic fertiliser, in this thesis a significant increase in SOM content was observed only in O soil as a result of SB-amended slurry application, compared to the inorganic fertiliser treatment. In addition, this short-term study was designed with only one application of an individual treatment. Therefore, longer-term studies (more than 85 days) with repetead additions of organic amendments, such as slurry or FYM, would be useful to establish the SOM dynamics across different soil types in response to the application of organic substrates. In Chapter 3, the potential for accumulation of organic N in soils was also observed following the application of organic
amendments, alongside a slow remineralisation of the immobilised NH4-N pool, in
turn contributing to an increase in soil NO3-N in both slurry treatments during the later
stages of the incubation. Thus, longer-term studies are needed with different types of organic amendments, such as slurry or FYM, in order to advise farmers on the importance of the application of these amendments to soil for accumulation of organic N and longer term changes in available N from these soil pools.
With regard to the quality of C substrates affecting the function of the soil microbial community, suppression of soil microbial respiration and assimilation of the added carbohydrate was observed in Chapter 4. It was hypothesised that this suppression is linked with the specific chemical characteristics of the slurry used in the experiments, for example due to particularly low VFA concentrations, or the release of suppressing factors from slurry microorganisms in response to the
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competition with native soil microorganisms. However, further research is needed to establish whether variations in the composition of organic compounds in slurry, such as carbohydrates and lignocellulosic materials, or microbial suppression following slurry application to soils is the potential mechanism through which to reduce the microbial respiration of the carbohydrate by native soil microorganisms and to allow accumulation of substrate-C into the cells of the slurry microorganisms.
In Chapter 4 it was observed that, in contrast to previous studies in which PSU
normally occurs when the amount of substrate C added to soil is greater than the Cmic
existing within the soil, the microbial metabolic switch occurred when the added
substrate C to soil was lower than Cmic. Therefore, further investigation is required to
verify whether PSU continues to be evident with relatively low substrate C added to
soil as a proportion of Cmic across different temperate grassland soils, with and without
slurry application. Further research is also needed to test whether longer-term incubation experiments with the same amount of substrate C added to soil ultimately stimulate greater SOM decomposition compared to an 18-day incubation experiment and, therefore, promote a positive PE. Furthermore, a microbial demand for C was suggested to drive a parallel release of P into the bioavailable soil pool, following the application of a phosphorylated C compound to soil. However, further experimental work is needed to confirm the mechanistic basis to a possible coupling between C and P mineralisation in grassland soils.
In Chapter 5 it was hypothesised that the increase in Cmic that was observed
following the addition of slurry to soil, including when slurry was applied to soil alongside a carbohydrate, was due to the microbial community that switched from the carbohydrates to the labile C in slurry. However, further research is required to test the hypothesis of this switch from the carbohydrate to the labile C in slurry following
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slurry application. In addition, although a dominance of bacteria over fungi, and of G –ve over G +ve bacteria, was observed in all treatments, further analyses are needed to determine whether shifts in the balance between fungi and bacteria, and in the G +ve:G –ve bacterial community, can be induced in temperate grassland soils over longer-term incubation experiments (>49 days) under treatments with different quantities of carbohydrates, as well as with amended and unamended slurry. Finally, a limited taxonomic resolution has been observed with PLFA analysis. Therefore, molecular approaches using PCR-based methods and metagenomics analysis are expected to achieve higher resolutions at species- or strain-level for a complete analysis of the microbial community in grassland soil before and after slurry treatments.