CIE Spotlight: N-fixing trees in restoration plantings: Effects on nitrogen supply and soil microbial communities

Shaun C.

Shaun C.

Title: N-fixing trees in restoration plantings: Effects on nitrogen supply and soil microbial communities.

Authors: M. Hoogmoed; S.C. Cunningham; P. Baker; J. Beringer; T.R. Cavagnaro.

Source: Soil Biology and Biochemistry, Volume 77, October 2014, Pages 203–212.

Brief summary of the paper: Mixed-species restoration tree plantings are being established increasingly, contributing to mitigate climate change and restore ecosystems.

Including nitrogen (N)-fixing tree species may increase carbon (C) sequestration in mixed-species plantings, as these species may substantially increase soil C beneath them.

We need to better understand the role of N-fixers in mixed-species plantings to potentially maximize soil C sequestration in these systems. Here, we present a field-based study that asked two specific questions related to the inclusion of N-fixing trees in a mixed-species planting:

  1. Do non-N-fixing trees have access to N derived from fixation of atmospheric N2 by neighbouring N-fixing trees?
  2. Do soil microbial communities differ under N-fixing trees and non-N-fixing trees in a mixed-species restoration planting?

We sampled leaves from the crowns, and litter and soils beneath the crowns of two N-fixing and two non-N-fixing tree species that dominated the planting. Using the 15N natural abundance method, we found indications that fixed atmospheric N was utilized by the non-N-fixing trees, most likely through tight root connections or organic forms of N from the litter layer, rather than through the decomposition of N-fixers litter.

While the two N-fixing tree species that were studied appeared to fix atmospheric N, they were substantially different in terms of C and N addition to the soil, as well as microbial community composition beneath them. This shows that the effect of N-fixing tree species on soil carbon sequestration is species-specific, cannot be generalized and requires planting trails to determine if there will be benefits to carbon sequestration.