Nutrient dynamics and development of soil fauna in vegetated roofs with the focus on biochar amendment

Abstract

The impacts of biochar amendment on the quality and quantity of runoff, substrate properties and soil fauna in two types of experimental vegetated roofs (‘plantings’ and ‘mats’) were examined during a 7-year period in southern Finland. Biochar reduced the leaching of phosphorus (P) in both plantings, which were established with seeds of meadow plants in a 100-mm-thick crushed brick mixture as growth medium, and mats comprising a 40-mm-thick layer of pregrown vegetation on a 60-mm-thick crushed brick mixture. Biochar reduced nitrogen (N) leaching only in plantings, where it positively impacted the plant cover. Nutrient loads declined with ageing of the roofs, but elevated P loads were measured following fertilization 6 years after establishment, with no mitigating impact of biochar on P leaching. Fertilization did not affect plant cover. Although nutrient loads from nonvegetated roofs were lower than from vegetated roofs in individual rain events, the difference in cumulative annual loads between vegetated and nonvegetated roofs was narrowed down by the high rainfall retention by vegetated roofs. Neither biochar amendment nor substrate organic matter content had any consistent impacts on the numbers and stability of soil fauna. Despite the extreme abiotic conditions in the vegetated roofs, the soil animal communities were unexpectedly diverse, with high population densities.