What Root Traits Determine Grass Resistance to Phosphorus Deficiency in Production Grassland?.

Phosphorus (P) inputs into agricultural soils are declining as a result of restricted P application and diminishing global P resources. This means that selection criteria for grass species in high production grasslands, which hitherto focused mostly on high yields under optimal nutrient conditions, will shift toward high efficiency under suboptimal P conditions. It is therefore imperative to identify key root traits that determine P acquisition of grasses in soils with a low P status. In a 9-month greenhouse experiment we grew eight common grass species and cultivars on a soil with a low P status, and related root morphological traits to their performance under P-limiting conditions. We applied (P1) or withheld (P0) P fertilization while providing adequate amounts of all other nutrients. Omitting P fertilization greatly reduced yield and nutrient acquisition for the different grass species. Biomass production differed significantly (p<0.001) among species and P fertilization treatments, varying from 17.1 to 72.1 g pot^-1 in the P0 treatment and from 33.4 to 85.8 g pot^-1 in the P1 treatment. Root traits were species-specific and unresponsive to P fertilization, but overall we observed a trade-off between root biomass and specific root length. Structural equation modelling identified total root length as key factor with respect to resistance to P deficiency, especially when roots explored the subsoil. Optimizing root length and subsoil exploration could be the key to maintaining high productivity of production grasslands with decreasing P availability. This is relevant for both plant breeding programs and for composing seed mixtures.