What root traits determine grass resistance to phosphorus deficiency in production grassland?

Grasslands are a major form of agricultural land use worldwide. Current and future declines of phosphorus (P) inputs into production grasslands necessitate a shift towards selecting grass species based on high efficiency under suboptimal, rather than optimal 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 various grass species. Biomass production differed significantly (P < 0.1%) 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 modeling 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.