Processes at the soil-root interface determine the different responses of nutrient limitation and metal toxicity in forbs and grasses to nitrogen enrichment

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.