Nutrient patches are transient and unpredictable in an unproductive mountain grassland

Abstract

While plant roots respond consistently to nutrient availability under experimental conditions, our understanding of the role of such response in the field is hindered by poor knowledge of size and duration of nutrient patches there. In particular, knowledge of patch duration is critically important for understanding types of root response. We determined spatial and temporal variations in phosphate-P, nitrate-N and ammonium-N concentrations, and pH in an unproductive mountain meadow for which extensive data on fine-scale root distribution exist. We sampled soil solution weekly over 2.5 growing seasons using suction cups to in a hierarchical spatial design with the smallest grain of 3.3 cm. Overall concentrations of all studied nutrients were fairly low with occasional and short-term, but large-in-magnitude peaks, with no pronounced spatial or temporal structure at any scale. Temporal variation was much stronger than spatial variation, with both interannual differences and within-season differences playing a role. Phosphate-P was consistently highest in spring, whereas ammonium-N increased during summers. The ammonium-N, the major nitrogen source at the site, was negatively correlated with phosphate-P. Our data suggest that repeated sampling of soil solution in fixed positions is necessary to cover the entire extent of nutrient variation in the field. It shows that there are no stable nutrient patches at the fine scale, and the duration and size of nutrient patches are smaller than usual growth responses of roots. This implies that under such conditions, the best rooting strategy is homogeneous space occupation linked with fast physiological response to varying nutrient concentrations.