Belowground consequences of an early-colonist N-fixing shrub as revealed through a decade-long removal experiment

Background/Question/Methods   Whether and how N-fixers in early succession affect community and ecosystem properties, particularly belowground, remains poorly understood. We investigated the consequences of Carmichaelia (an N-fixing shrub) colonization in a New Zealand montane river valley for the biomass, diversity and structure of four trophic levels in this early successional ecosystem. We measured plant and belowground (bacteria, fungi, nematodes, mites) biomass and community responses to ten years of annually removing seedlings of Carmichaelia compared to controls where Carmichaelia was allowed to colonize naturally.

Results/Conclusions   Carmichaelia drove massive increases in biomass, as much as 1500%, at all trophic levels indicating severe resource limitation in its absence and strong bottom-up control on the system. Dominance of arbuscular mycorrhizal plants and improved foliar chemistry in Carmichaelia's presence indicated that the primary mechanism for this response was increased N-availability. The bacterial-based detrital food web was more stimulated by Carmichaelia than fungi and their consumers. Similarly, Carmichaelia promoted increased diversity, suggesting reduced competition, of consumer groups in the bacterial-based, but not the fungal, food web. Both of these findings indicate that Carmichaelia drives belowground characteristics to those of early successional ecosystems. Despite these effects, Carmichaelia only modestly influenced community composition both above- and below-ground (although it did strongly promote AM trees and shrubs) suggesting that it has not yet, after ten years, significantly altered the successional trajectory of this system, only accelerated it.