Spatial distributions of plants and gross N transformation rates in a forest soil

Abstract

Summary This work demonstrates that spatial distribution of understorey vegetation and gross N transformation rates in a mixed beach‐oak forest is closely correlated within a distance of a few metres. The findings imply that plant diversity and productivity have a major influence on gross rates of N transformation and vice versa. A geostatistical analysis was used to evaluate the spatial relationships between abundance and species composition of the understorey vegetation and in situ gross N mineralization, immobilization and nitrification rates. The gross N transformation rates and the plants spatial variation were correlated within the forest, but plant distribution was more dependent on the fraction of mineralized N that was nitrified than on individual N transformation rates. The total cover of the understorey vegetation varied more in space than the species composition, and was higher in areas with high N transformation rates. Plant species benefiting from high net nitrification rates were more common in areas with a low activity of mineralizing and nitrifying microorganisms, possibly because the net and gross rates were independent of each other. In fact, those species occurred most often in areas in which a large fraction of mineralized N was nitrified. Beech and oak trees also had an effect on the spatial variation of the understorey vegetation. Beech trees provided conditions more suitable for plants benefiting from , whereas the vascular plant cover was greater under oak trees, probably in response to a higher light interception than under beech trees. Oak generally had a positive impact on gross N transformation rates compared with beech, perhaps reflecting differences in litter quality and climate caused by the two species. The influence of trees alone could not explain the full magnitude of the variation of N transformation rates or the presence of overlapping areas with high mineralization and immobilization rates. These were probably caused by other factors, such as soil moisture content. This work sheds new light on the small‐scale spatial links between above‐ground plant diversity and abundance, and below‐ground microbial N transformations.