Using 137Cs and 210Pb to characterise soil mixing by burrowing petrels: an exploratory study

Abstract

This exploratory study tested the hypothesis that petrels (Aves: Procellaridae) actively plough the soil of their entire breeding colonies, as implied by their well-known burrowing capabilities but contra-indicated by widespread horizonation in colony soil. Two profiles to lithic contact were excavated within a forested Westland petrel (Procellaria westlandica) colony; one, a ridgeline control without nearby petrel burrows, and the other from a steep slope among petrel burrows. On the ridgeline, 137Cs activities (estimated per volume) steadily decreased with depth as expected. At the burrowed site, a subsurface maximum at 12–16 cm depth indicated a post-1963 burrowing or landslip event. Both 210Pb profiles were successfully modelled (r 2 c. 0.9) using a simple first-order model usually applicable only to undisturbed soils. In this model, mixing is accounted for by radioactive decay and first-order, mm-scale biodiffusion. The results therefore indicated that soil mixing was not dominated by petrel burrowing; rather, petrels confine their burrowing activities to maintenance of their burrows as semi-permanent (decades to centuries) structures. However, further sampling is recommended to confirm this view.