Soil Sustainability of Forest Bioenergy Feedstocks Across the Americas

Abstract

In addition to storing vast amounts of atmospheric carbon (C), soil C in forest plantations also improves soil health by retaining moisture and nutrients, buffering soil pH, supporting microbiota, and improving soil structure and aggregate stability. Eucalyptus plantations are used throughout the tropical/subtropical world for wood production, timber, and pulp. Eucalyptus' utility and adaptability have led to expansions throughout South America. While there is a large amount of literature that focuses on Eucalyptus and soil C in the first 1 or 2 rotations, the long-term impact of plantations on soil C quantity and quality. Here we assess the quantity and quality of soil C down to 60 cm in both the short-term and the long-term chronosequences of Eucalyptus grandis. The short-term chronosequence is represented by second rotation stands in their early, mid, and latestages of a ~10-12 year harvest cycle. The long-term chronosequence is represented by late-stage stands of rotations 1-4 with similar rotation length. We found no significant differences in soil C and nutrients across the second rotation, although the longer-term chronosequence shows a strong downward trend in soil C by the 4th rotation (p = 0.12). Total nitrogen declined across the chronosequence (p = 0.02). Other nutrients, P, K, Ca, and Mg, varied by soil depth and among rotations, showing no significant changes except at a few select depths. To assess C quality, C was separated by density into two fractions: 1) heavy fractions (HF) and 2) free light and occluded fractions (FLF+OC) at 0-15 cm and 45-60 cm. The HF reveals that recalcitrant C is declining in both surface mineral soil (0-15cm) (p ≤ 0.10) and in deeper soils (45-60 cm) (p = 0.06). This suggests that not just the quantity, but also the quality of soil C has declined in E. grandis stands over four rotations. This suggests a degradation in soil and less sustainability as biofuel.