The measurement and quantification of generalized gradients of soil fertility relevant to plant community ecology

We propose an operational definition of soil “fertility” that is applicable to plant community ecology and develop a method of measuring and quantifying it, using structural equations modeling, that is generalizable to soils in different regions whose fertility has different causes. To do this, we used structural equation modeling (SEM). The measurement submodel predicts the latent “generalized fertility,” FG, of a soil using four indicator variables: the relative growth rates of Festuca rubra, Trifolium pratense, Triticum aestivum, and Arabidopsis thaliana. The direct causes of FG in this study were the supply rates of NO3？, P, and K as well as three indirect causes consisting of three physical soil properties, but these can change between studies. The model was calibrated using 76 grassland soils from southern Quebec, Canada and independently tested using aboveground net primary productivity (NPP) of the natural vegetation over two growing seasons. Both the measurement submodel and the full SEM fit the data well. The FG values predicted 51% of the variance in NPP and were a better predictor than any other single variable, including the actual nutrient flux rates. Furthermore, this model can be applied to grassland soils anywhere because of its modular nature in which the causes and effects of soil fertility are clearly separated. Trait‐based community ecology promises to predict the structure of local vegetation (McGill et al. 2006, Shipley et al. 2016) given its position along particular environmental gradients. Two models, CATS (Shipley et al. 2006, Shipley 2010, Warton et al. 2015) and TRAITSPACE (Laughlin and Laughlin 2013) have done this in several case studies (Shipley et al. 2011, 2012, Baastrup‐Spohr et al. 2015). A weakness of these models is that some important environmental gradients cannot be quantified in a way that allows for geographical generalization, a point made previously by Austin (1980). One example of this weakness is in quantifying soil “fertility.” Historically, soil fertility has been quantified by measuring its causes: typically, concentrations or fluxes of particular mineral nutrients plus certain physical properties of soils. However, plant growth in different soils is limited by different nutrients and nutrient ratios, and the conversion from soil nutrient concentrations to plant production is species dependent. Therefore, it has been impossible to date to quantify soil “fertility” at the level of entire plant communities in different regions having different species compositions using a common metric. Soil “fertility,”