Defoliation induces root exudation and triggers positive rhizospheric feedbacks in a temperate grassland

• Published in: Soil biology & biochemistry 2008-11, Vol.40 (11), p.2865-2873
• Main Authors: Hamilton, E. William, Frank, Douglas A., Hinchey, Paul M., Murray, Tanya R.
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Biochemistry and biology; biogeochemical cycles; Biological and medical sciences; carbon; Chemical, physicochemical, biochemical and biological properties; Decomposition; Defoliation; Fundamental and applied biological sciences. Psychology; grasses; grassland soils; grasslands; herbivores; mineralization; nitrogen; nitrogen content; nutrient availability; nutrient uptake; Organic matter; Physics, chemistry, biochemistry and biology of agricultural and forest soils; plant nutrition; Poa pratensis; Rhizosphere; root exudates; soil microorganisms; Soil science
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/j.soilbio.2008.08.007

The facilitating effect of the exudation of carbon (C) compounds from roots on rhizospheric processes has been shown in controlled experiments; however, it still remains unclear how important this pathway of C from plants to the soil may be in energy and nutrient processes in grazed grasslands under natural conditions. Root exudation may be a particularly important C pathway in grazed grasslands and help promote positive feedbacks between large herbivores and plants. In this study we performed a 13C pulse-chase experiment on plots that were clipped or left unclipped in a mesic grassland in Yellowstone National Park. The dominant grass species in the plots was Poa pratensis and it was used to measure the effect of defoliation on root C exudation, the rhizospheric microbial community, and feedbacks on plant nutrient uptake over a time period of 24–72 h. Defoliation stimulated C exudation from roots by 1.5-fold, which concomitantly increased rhizospheric microbial biomass by the same factor. The facilitating effects of defoliation on rhizospheric processes resulted in positive feedback on soil inorganic N pools and leaf N content, which increased by 1.2- and 1.5-fold respectively. Changes in soil inorganic N pools during the experiment indicated that the effect of the C flush on the rhizospheric decomposer community of defoliated plants resulted in a 5-fold increase in rhizospheric daily net N mineralization rate. These findings demonstrate that in a natural grassland community defoliation-induced stimulation of C exudation stimulates rhizospheric N-mineralization which ultimately benefits defoliated plants. The results also indicate the important role that short-term root–rhizospheric microbe interactions play in the C and N processes in grazed grasslands.