Relationships between culturable soil microbial populations and gross nitrogen transformation processes in a clay loam soil across ecosystems

The size and quality of the soil organic matter (SOM) pool can vary between ecosystems and can affect many soil properties. The objectives of this study were to examine the relationship between gross N transformation rates and microbial populations and to investigate the role that SOM plays in these factors. In our study, culturable microbial and actinomycete populations were positively correlated with gross mineralization and ammonium (NH₄⁺) consumption rates over time in both ecosystems. These correlations provide evidence that microbial plate counts could be a good representation of all microbes responsible for gross mineralization and gross NH₄⁺ consumption. Rates of gross mineralization, nitrification and NH₄⁺ consumption were significantly greater in forest soil than old-field soil. These greater rates in forest soil could be due to the presence of higher levels of readily transferable substrates in SOM. Gross nitrification rates were considerably lower than gross mineralization and NH₄⁺ consumption rates over the experimental period, indicating heterotrophic uptake of NH₄⁺ rather than use by autotrophic nitrifiers under soil and environmental conditions in this study. Additionally, microbial populations were significantly (p<0.01) greater in forest soil than in old-field soil, which could also be related to the higher level of SOM in the forest soil. Net mineralization and nitrification rates were similar between ecosystems. Results also showed that net rates were highly correlated to each other, but were not correlated with culturable microbes or gross N transformation rates, indicating the isolation of net rates in relation to fundamental controlling factors.