The impact of ancient
fertilization practices on the
biogeochemistry of arable soils on the remote Scottish island of Hirta, St Kilda was investigated. The island was relatively unusual in that the inhabitants exploited seabird colonies for food, enabling high population
densities to be sustained on a limited, and naturally poor, soil resource. A few other Scottish islands, the Faeroes and some Icelandic Islands, had similar cultural dependence on seabirds.
Fertilization with human and
animal waste streams (mainly
peat ash and bird carcases) on Hirta over millennia has
led to over-deepened, nutrient-rich soils (plaggen). This project set out to examine if this high rate of
fertilization had adversely
impacted the soil, and if so, to determine which waste streams were responsible. Arable soils were considerably elevated in Pb and Zn compared to non-arable soils. Using Pb
isotope signatures and analysis of the waste streams, it was determined that this pollution
came from
peat and turf ash (Pb and Zn) and from bird carcases (Zn). This was also confirmed by (13)C and (15)N analysis of the profiles which showed that soil
organic matter was highly enriched in marine-derived C and N compared to non-arable soils. The pollution of such a remote island may be typical of other 'bird culture' islands, and
peat ash contamination of marginal arable soils at high latitudes may be widespread in terms of geographical area, but less intense at specific locations due to lower population
densities than on Hirta.
