Microbial carbon concentration in samples of seabird and non-seabird forest soil: Implications for leaf litter cycling

The paucity of leaf litter in seabird forest is usually explained by litter burial in burrows, but burial by itself fails to address the processes controlling decomposition. We measured soil microbial C in samples from a Westland petrel (Procellaria westlandica) colony both within and outside the breeding season, and compared the results with two non-seabird forests. From the few studies of seabird soil microbial C, we initially hypothesised a soil microbial C concentration sequence of occupied burrows > unoccupied burrows > adjacent forest floor > non-seabird forest. Instead, the highest values came from non-seabird forest, a pattern consistent with published meta-analyses on the effects of N addition. Within the colony, highest concentrations were in forest floor soil and there was no burrow occupation effect. However, seabird forest soil microbial C followed a strong inverse relationship with soil δ13C (r = −0.58; P < 0.001) as well as the expected relationship with total soil C (r = 0.75); the relationship with soil δ13C in non-seabird forest was not significant (P = 0.29). We propose that soil microbes in seabird forest repeatedly process a single pool of increasingly refractory terrestrial soil C, facilitated by seabird guano priming of organic matter mineralisation. In this context, the paucity of leaf litter in seabird forest can be seen as a consequence of microbial C limitation in a nutrient-saturated system, an explanation consistent with recent theory.