Seasonal dynamics of soil nutrients and microbial activity due to colonization by bracken ferns (Pteridium aquilinum (L.) Kuhn) in a pine forest

Abstract

Bracken (Pteridium aquilinum) is an aggressive colonizer of disturbed and other successional habitats in temperate forests. However, little is known about how bracken colonization modifies soil microbial activity and nutrient cycling. We questioned whether bracken presence alters soil nutrient pools and soil microbial respiration throughout a growing season and whether this effect depends on the size of the bracken patch. To answer these questions, we sampled the soil in bracken patches of different sizes (large, medium and small) and in the surrounding pine forest (paired sampling design) in summer, autumn, winter and spring. We analyzed the samples to determine nutrient contents (C, N and P) and soil microbial respiration. Patch size did not influence soil microbial activity or nutrient pools. Bracken acted as nutrient sink for N pools through the growing season and as source or sink for C and P pools depending on season. Moreover, the effect on nutrient pools did not depend on bracken characteristics (area, density or height). Bracken presence enhanced soil microbial respiration proportionally to patch area. The control that bracken exerts over nutrient pools may determine the successful (or otherwise) establishment of other plant species. This contributes to making bracken a successful colonizer as no particular population threshold (area, density) has to be reached before nutrients becomes limiting to other plant species.