Abstract
Forest ecosystems are critical to global biogeochemical cycles but under pressure from harvesting and climate change. We investigated the effects of organic matter (OM) removal during forest harvesting on the genetic potential of soil communities for biomass decomposition and nitrogen cycling in five ecozones across North America. We analyzed 107 samples, representing four treatments with varied levels of OM removal, at Long-Term Soil Productivity Study sites. Samples were collected more than ten years after harvesting and replanting and were analyzed via shotgun metagenomics. High-quality short reads totaling 1.2 Tbp were compared to the Carbohydrate Active Enzyme (CAZy) database and a custom database of nitrogen cycle genes. Gene profile variation was mostly explained by ecozone and soil layer. Eleven CAZy and nine nitrogen cycle gene families were associated with particular soil layers across all ecozones. Treatment effects on gene profiles were mainly due to harvesting, and only rarely to the extent of OM removal. Harvesting generally decreased the relative abundance of CAZy genes while increasing that of nitrogen cycle genes, although these effects varied among ecozones. Our results suggest that ecozone-specific nutrient availability modulates the sensitivity of the carbon and nitrogen cycles to harvesting with possible consequences for long-term forest sustainability.
Highlights
Established in the late 1980s, the Long-Term Soil Productivity Study (LTSP) focuses on the long-term effects of organic matter (OM) removal during harvesting on forest productivity[4]
Our samples were dominated by bacterial sequences (>99.8%) according to taxonomic profiling using Metaphlan[227]
carbohydrate active enzymes (CAZy) genes had 12.8% higher relative abundances in the organic versus the mineral soil layer in unharvested reference treatments (OM0). This difference was driven by the Glycoside hydrolase (GH), polysaccharide lyase (PL), and carbohydrate esterases (CE) CAZy classes, while glycosyl transferase (GT) and auxiliary activities (AA) classes were distributed between layers
Summary
Established in the late 1980s, the Long-Term Soil Productivity Study (LTSP) focuses on the long-term effects of OM removal during harvesting on forest productivity[4]. We recently showed that harvesting reduced biomass degradation potential for more than a decade after harvesting at one LTSP site in the interior Douglas fir ecozone of British Columbia[12]. We hypothesized that OM removal during harvesting would increase the relative importance of these catabolic processes in soil communities, resulting in an increase abundance and diversity of genes involved in nitrogen cycling. We compared the diversity and relative abundance profiles (genetic potential) of biomass decomposition and nitrogen cycle genes among ecozones, soil layers, and OM removal treatments. We related environmental gradients to the metagenomic responses to OM removal during harvesting This is the most extensive metagenomic analysis of forest soil communities to date and among the first to use a field experiment replicated among different ecozones
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