Cations make a difference: Soil nutrient patches and fine‐scale root abundance of individual species in a mountain grassland

Abstract

Abstract Root densities in the field vary at the centimetre scale, but we have no information on whether this variation is linked to the variation in nutrient concentrations and availability. Roots of many species are able to proliferate in nutrient‐rich patches in controlled conditions in culture, but because data on nutrient concentrations and, in particular, on their temporal stability in the field are scarce, we do not know to what extent root distribution in the soil bears traces of such a response. Here, we linked centimetre‐scale measurements of soil nutrient concentrations over 6 weeks with estimation of root biomass and its species composition at the same points to determine whether there is any association between the two. In addition to phosphorus, nitrate and ammonium, we determined the concentrations of metal cations (magnesium, calcium and potassium). We used qPCR to determine the quantities of individual species in the root biomass samples. We found that calcium and magnesium (and to a lesser degree phosphorus and potassium) showed fine‐scale patchiness that was stable throughout the study (6 weeks) and was consistent over all these elements. Nutrient patches were associated with high root biomass and the occurrence of roots of several species. Such patches are formed primarily by elements with known low mobility in soil (cations, phosphorus). In contrast, nitrogen ions showed overwhelmingly high temporal variation with no relationship between root density and nitrogen concentration. Soil nutrient concentrations, thus, constitute a multidimensional signal. Some elements strongly vary in time, while some are much more stable and, thus, form stable patches that permit root response over periods of weeks or months. Among them, calcium and magnesium play an important role in forming soil heterogeneity at a scale comparable with the scale at which fine root densities also vary. As these cations affect a number of plant functions, their association with higher root densities confirms their role in the growth dynamics of terrestrial ecosystems. Read the free Plain Language Summary for this article on the Journal blog.