Intraspecific changes in plant morphology, associated with grazing, and effects on litter quality, carbon and nutrient dynamics during decomposition

Continuous biomass removal by grazing usually changes the resource allocation pattern of plants. These changes often increase resistance to tissue removal and produce individuals with different morphometric traits, such as root to shoot or blade to sheath ratios. Shifts in morphometric traits, in turn, may alter nutrient cycling through changes in the average quality of litter that decomposes in soil. Previous work has shown that Paspalum dilatatum, a native grass from the Pampas grasslands, which inhabits a vast area and supports a wide range of grazing conditions, increases its blade to sheath ratio under continuous grazing with respect to ungrazed conditions. Here, we explored the consequences of these changes apparently associated with grazing regime on litter quality and nutrient dynamics during litter breakdown in soil. We separately analysed litter quality of blades and sheaths of P. dilatatum and determined under controlled conditions their decomposition and nutrient release kinetics over a maximum period of 1 year. We also studied the mineral nitrogen contents in soil amended with each litter type. Blade quality was significantly higher than sheath quality, nitrogen concentrations of blades and sheaths were approximately 1% and 0.6%, respectively, and lignin to nitrogen ratios were approximately 5 and 11 for blades and sheaths, respectively. Phosphorus concentration, however, was similar in both litter types. Blades decomposed 10% faster than sheaths, released 20% more nitrogen and released 15% more phosphorus than sheaths during the last half of the incubation period. During the first 3 months, the soil nitrogen content of litter-amended incubations indicated immobilization with respect to non-amended control; however, later blade incubations mineralized nitrogen, whereas sheath incubations continued immobilizing it. Results revealed that grazing potentially accelerates nutrient cycling during decomposition by increasing the blade to sheath ratio of P. dilatatum individuals, and suggest that this may be an important mechanism underlying grazing impact on nutrient cycling.