Linking the bacterial energy channel and the soil faunal food web – a study using stable isotopes

The soil fauna are a vital component of the soil and affect C and N cycling, decomposition and mineralisation rates as well as plant growth. It has been proposed that within the soil faunal food web there are two divergent energy channels one bacterial and one fungal, which are utilised to varying extents by the taxa dependent on ecosystem and management practice, affecting decomposition and the C and N cycles. Protozoa are one of the most abundant groups of bacterivores within the soil, however they are regularly overlooked when investigating the complete soil food web. There are few methods that can be used to determine feeding preferences of the soil fauna in situ, but a technique using stable isotopes has been developed to track the feeding preferences as they occur. Indigenous flagellates and ciliates, were cultured to become highly enriched in 13C and 15N and were introduced to soil cores from two different land managements. Mesofauna were subsequently extracted, identified and separated before analysis by mass-spectrometry. Results showed that some invertebrates were highly labelled indicating consumption of the added protozoa and provided an insight into the movement of C and N through the bacterial energy channel of the soil food web. Our results show the importance of protozoa as a food source and that changes in management, affect the functioning of the soil food web. Further work is currently being developed to label fungi with stable isotopes to track this part of the soil food web in situ as well, to understand the flow of C and N through the separate energy channels.