Frequent Prescribed Burning Contributes to the Sequestration of Soil Carbon in South African Mesic Rangeland Systems

Abstract

Historically, subsistence pastoralists and ranchers have used fire as a management tool in grassy rangelands for millennia. The capacity of these ecosystems to function as carbon sinks depends heavily on management interventions. Understanding optimal fire return intervals is essential for determining how prescribed burning influences long-term carbon and nitrogen cycling. This study builds on previous research at the Ukulinga Grassland Fire Experiment (UGFE), extending beyond surface soils to assess whether fire-induced changes in soil organic carbon (SOC) and total nitrogen (TotN) persist at greater depths and over multi-decadal timescales. Using data from 20 years prior as a baseline, and with grazing excluded, we examine SOC and TotN responses directly attributable to fire as a defoliant. Here we assessed the effect of frequency and seasonal timing of prescribed fires on SOC, TotN and carbon-to-nitrogen (C:N) ratios in a South African mesic grassy rangeland system. Our findings suggest that early season, frequent prescribed burns result in greater SOC concentrations and enhanced sequestration rates compared to longer burn intervals. Over the 20-year monitoring period, frequently burnt plots showed continued carbon accumulation, while longer return intervals were associated with reduced subsoil carbon sequestration below 5 cm. These results demonstrate the resilience of mesic grasslands to frequent prescribed fire as a disturbance and highlight fire as a key abiotic driver of SOC accumulation and nutrient cycling. Our findings align with local studies in temperate South African grasslands and savannas and support the potential of frequent prescribed burning as a strategy for enhancing SOC in fire-dependent grassy ecosystems. However, these carbon benefits must be weighed against potential trade-offs, particularly with grazing productivity and plant diversity, which require further consideration in the context of sustainable land management and carbon credit schemes.