Grazingland Soil Carbon Fundamentals: From Novel Approaches to Field Studies

Abstract

Grazinglands store nearly a third of Earth’s soil organic carbon (SOC) and thus have garnered interest as a potential carbon source or sink. To that end, adaptive grazing management has emerged as a promising technique that aims to simultaneously support livestock production and ecosystem function. However, uncertainty persists regarding the extent that management can affect SOC and its constituent fractions [mainly, particulate organic carbon (POC) and mineral associated organic carbon (MAOC)] across diverse grazingland contexts. To answer these questions, we need to determine SOC stock change over time and acquire a fundamental understanding of what influences SOC storage in grazinglands. The SOC stock data reported in this dissertation represent baseline data for a large effort aiming to detect SOC stock change resulting from the adoption of adaptive versus prescriptive grazing management across four ecologically distinct grazinglands. Here this extensive dataset is used to explore (1) drivers of deep (1m) SOC within and across the four grazinglands, and (2) the influence of dominant vegetation type (C3 vs. C4 photosynthesis) on isotopic enrichment of distinct soil carbon pools relative to plant-derived inputs. Additionally, testing of a new tool – rhizotron mini-towers – was conducted for mechanistically studying the response of roots and soil to grazing treatments. We found that: (1) drivers of deep SOC vary among grazinglands with climate and vegetation important for deep POC and soil properties important for deep MAOC; (2) Across C4-dominant sites, current and historic C3 plant inputs appear to disproportionately contribute to SOC storage, as indicated by the 13C depletion of SOC pools. Additionally, precipitation emerged as an important predictor of δ13C between root inputs and SOC pools, across C3-dominat sites; and (3) the rhizotrons provide the necessary capabilities to study the response of roots and soil to treatment effects in a controlled environment. Collectively, these contributions improve fundamental understanding of drivers of deep SOC and its constituents across different grazinglands and provide a novel methodology to mechanistically study in vivo root-driven SOC dynamics.