Nutrient limitation and metal toxicity have been implicated in changes of grassland communities by nitrogen (N) deposition. Below-ground processes, especially those at the soil-root interface, play important roles in determining variation in nutrient concentrations in plants. However, few studies have specifically focused on the roles of these processes in mineral-element acquisition in grassland plants in response to N enrichment.
Here we investigated the contributions of below-ground processes at the soil-root interface to the differential acquisition of phosphorus (P), calcium (Ca) and manganese (Mn) by forbs and grasses of a temperate steppe in response to N addition by combining field and glasshouse experiments.
Nitrogen addition increased the concentrations of both leaf P ([P]) and Mn ([Mn]) and decreased leaf [Ca] of forbs while it had little effects on leaf concentrations of these elements in grasses. Nitrogen addition led to a higher activity of acid phosphatase in the rhizosphere of forbs, and greater release of protons and carboxylates from forb roots than grass roots, contributing to the differential [P], [Ca] and [Mn] in the leaves of forbs and grasses. Applying oxalate to soil to simulate the release of carboxylates by N enrichment enhanced [P] and [Mn], and decreased [Ca] in the soil solution. However, addition of hydrogen-ion increased [P], [Mn] and [Ca] in the soil solution. Lime addition mitigated the N-addition-induced soil acidification while it did not abolish the stimulatory effect of short-term N addition on leaf [P] and [Mn] of forbs. Therefore, we conclude that differences in the eco-physiological processes at the soil-root interface account for changes in leaf [P], [Ca] and [Mn] under short-term N addition, and that soil acidification aggravates the responses of these elements, especially [Ca] and [Mn], to long-term N enrichment.
Synthesis. Our results highlight the contribution of below-ground processes, especially those at the soil-root interface, to variation in plant element concentrations between dominant forbs and grasses in the temperate steppe. These findings greatly enhance our mechanistic understanding of the effects of N deposition on grassland communities.