Description:
<jats:title>Abstract</jats:title><jats:p>Perennial grasses can sequester soil organic carbon (<jats:styled-content style="fixed-case">SOC</jats:styled-content>) in sustainably managed biofuel systems, directly mitigating atmospheric <jats:styled-content style="fixed-case">CO</jats:styled-content><jats:sub>2</jats:sub> concentrations while simultaneously generating biomass for renewable energy. The objective of this study was to quantify <jats:styled-content style="fixed-case">SOC</jats:styled-content> accumulation and identify the primary drivers of belowground C dynamics in a zero‐tillage production system of tropical perennial C4 grasses grown for biofuel feedstock in Hawaii. Specifically, the quantity, quality, and fate of soil C inputs were determined for eight grass accessions – four varieties each of napier grass and guinea grass. Carbon fluxes (soil <jats:styled-content style="fixed-case">CO</jats:styled-content><jats:sub>2</jats:sub> efflux, aboveground net primary productivity, litterfall, total belowground carbon flux, root decay constant), C pools (<jats:styled-content style="fixed-case">SOC</jats:styled-content> pool and root biomass), and C quality (root chemistry, C and nitrogen concentrations, and ratios) were measured through three harvest cycles following conversion of a fallow field to cultivated perennial grasses. A wide range of <jats:styled-content style="fixed-case">SOC</jats:styled-content> accumulation occurred, with both significant species and accession effects. Aboveground biomass yield was greater, and root lignin concentration was lower for napier grass than guinea grass. Structural equation modeling revealed that root lignin concentration was the most important driver of <jats:styled-content style="fixed-case">SOC</jats:styled-content> pool: varieties with low root lignin concentration, which was significantly related to rapid root decomposition, accumulated the greatest amount of <jats:styled-content style="fixed-case">SOC</jats:styled-content>. Roots with low lignin concentration decomposed rapidly, but the residue and associated microbial biomass/by‐products accumulated as <jats:styled-content style="fixed-case">SOC</jats:styled-content>. In general, napier grass was better suited for promoting soil C sequestration in this system. Further, high‐yielding varieties with low root lignin concentration provided the greatest climate change mitigation potential in a ratoon system. Understanding the factors affecting <jats:styled-content style="fixed-case">SOC</jats:styled-content> accumulation and the net greenhouse gas trade‐offs within a biofuel production system will aid in crop selection to meet multiple goals toward environmental and economic sustainability.</jats:p>