Harvesting trees from forests, even when replanting efforts are made, has a huge impact on the long-term functioning of the soil microbiome. The unique microbial community in the soil performs essential tasks like decomposing plant material, which recycles nutrients for new plants to use. Plus, these microbes play vital roles in nutrient cycling such as the carbon and nitrogen cycles. To better understand the effects that removing organic matter (harvesting) has on the capacity of the soil microbiome to perform these duties, researchers from the University of British Colombia and the Georgia Institute of Technology assessed the genetic potential of soil communities for biomass decomposition and nitrogen cycling in harvested sites across North America, each representing a unique ecozone.

Using study sites and designs from the Long Term Soil Productivity Study, established during the 1980s, the researchers used shotgun metagenomic sequencing to quantify the diversity and abundance of genes essential to the microbial community’s decomposition and nutrient cycling functions. Harvesting and replanting occurred roughly ten years prior, with three different levels of organic material being taken at each site: stem-only harvesting, whole-tree harvesting, and whole-tree harvesting plus forest floor removal.

Harvesting overall played a role in altering the soil gene profiles, but the level of organic matter harvested did not. Researchers observed a reduced relative abundance of carbohydrate active enzymes genes—which are important for decomposition—and an increase in the abundance of nitrogen cycling genes. However, the increase in nitrogen cycling genes did vary by ecozone, suggesting ecozone-specific nutrient availability plays a role in the sensitivity of the carbon and nitrogen cycles to harvesting.

This was the first large-scale metagenomics study looking at the effects of harvesting on the potential for soil communities to perform some of their natural functions. The team believes that these changes could have an affect on forest productivity as trees grow and their nutrient demand increases, and may also alter a forest’s ability to resist future perturbations. According to the researchers, “our results suggest a mechanism by which harvesting can exacerbate nitrogen losses at sites predisposed to such losses, potentially lowering plant productivity and increasing greenhouse gas emissions.”

Cardenas E, Orellana LH, Konstantinidis KT, Mohn W. Effects of timber harvesting on the genetic potential for carbon and nitrogen cycling in five North American forest ecozones. Sci Rep. 2018; 8: 3142.