Microbial processes mediating Priming Effects and the coupling of C and N cycling in soil

Soil is fundamental to agricultural productivity and sustainability. Soil organic matter (SOM) is important as a “revolving nutrient fund”, which determines plant nutrient availability. Microbes are key determinants of SOM turnover and of SOM dynamics. Their activities are influenced by organic inputs from plants (Priming Effects). Understanding these interactions has the potential to reduce external N input by better predicting the soil N supply. However, which microbes are involved in primed SOM mineralization is unknown. Analytical methods utilised to investigate this were 13C isotope ratio source partitioning for quantifying C mineralised fluxes, 16S rDNA and 16S rRNA sequencing to determine growing and potentially active taxa, and metaproteomics of contrasting soil extracts for illuminating proteins involved in SOM decomposition. A sample-specific soil interference correction was developed for quantification of protein and humic substances content in contrasting soil extracts. Results showed that correction factors differed with soils and buffer extract. Further, the potential for metaproteomics to profile contrasting extracts was assessed. Results in labile extracts showed that transporter proteins for carbohydrates, amino acids and nitrogenous compounds, and arylsulfatase for enzymes were most abundant, while the humic extracts remain to be successfully profiled. Results from incubation experiments indicated, that several genera, among others from the phylum Actinobacteria (e.g. (Pseud)Arthrobacter), were implicated in mediating primed fluxes. In another incubation experiment, the magnitude of labile C additions in relation to primed SOM mineralisation responses were tested. Dynamics of specific bacterial populations showed that primarily r-strategists were implicated with the bacterially mediated SOM-C mineralisation flux at all magnitudes. Bacterial, K-strategists were only activated at higher magnitudes of labile C additions. This thesis addressed knowledge gaps on bacterial population dynamics involved in SOM and nutrient dynamics. This can inform on sustainable practices for the provision of ecosystem functions of nutrient cycling and soil C dynamics.2022-02-2