Linking Shifts in Species Composition Induced by Grazing with Root Traits for Phosphorus Acquisition in Typical Steppe of Inner Mongolia

1. Long-term overgrazing causes soil phosphorus (P) deficiency in grassland. Grazing induces shifts in species composition and associated plant functional traits, yet the functional root traits for P acquisition involved in these shifts remain unknown. Species vary in their P acquisition strategies, and we hypothesize that species that acquire P more efficiently are better adapted to grazing.

2. We measured relative biomass of six species in a field experiment conducted in Inner Mongolia with three exclosure treatments, i.e. exclosed since 1983, exclosed since 1996, and free grazing without exclosure. We also measured root physiological traits including rhizosphere acid phosphatase activity, pH, leaf manganese concentration [Mn] (as a strong indicator of root carboxylate exudation); root morphological traits, including specific root length (SRL), average root diameter (ARD), proportion of fine roots (PFR), of six common species.

3. Long-term overgrazing caused a reduction in bulk soil acid phosphatase activity and Olsen P by 48% and 26%, respectively. Efficient P acquisition species Carex duriuscula had higher rhizosphere acid phosphatase activity, higher leaf [Mn], SRL, and PFR, and lower ARD to mobilize and acquire sparingly soluble P in soils. In contrast, in inefficient P acquisition species like Stipa grandis, the rhizosphere acid phosphatase activity, leaf [Mn], SRL and PFR in both grazed and ungrazed plots were lower than those in most species. As a result, under grazing compared with ungrazed plots, the dominance of duriuscula increased by 189%, while that of S. grandis decreased by 49%.

4. We conclude that efficient P acquisition species duriuscula was well-adapted to P-impoverished soil under grazing, replacing inefficient P acquisition species like S. grandis. Our study highlights the importance of incorporating root traits into community succession induced by grazing.

keywords: bulk soil Olsen P, long-term overgrazing, P acquisition efficiency, root morphological traits, root physiological traits, species dominance, typical steppe