Extreme weather dominates farm management effects on long-term trends in soil carbon

Classical reductionist experimentation tends to conceptually compartmentalise mitigation and adaptation into binary categories, shielding insight into how greenhouse gas (GHG) emissions and climate change interact. Here, our primary aim was to examine how a key tenant of the global climate crisis – drought – is likely to influence soil organic carbon (SOC). We deconstruct these paradigms using case study farms in Tasmania, Australia, using state of the art models to simulate pasture production and SOC under historical and 2050 climates, the latter encapsulating more frequent extreme weather events. We show that longitudinal changes in SOC stocks correlate positively with standard precipitation evapotranspiration index (SPEI) via the mediating effects of seasonal pasture growth. Drought elicited notable SOC losses, particularly when antecedent SOC stocks were high, whereas high rainfall years amplified SOC sequestration. Renovating pastures with perennial legumes enhanced sequestration under 2050 climates, as did introducing irrigation and increasing soil fertility. In most cases however, the influence of aridity on SOC dominated over that of farm management, suggesting that climate change - and by extension, seasonal rainfall distribution - are likely to engender greater influence on SOC stocks compared with farm management or practice change. As such, aspirations to maintain SOC stocks at ceiling levels over the long-term are likely to be challenged by the changing climate and particularly drought. Even so, we contend that adoption of practices aimed at improving soil organic matter can benefit productivity through enhancement of soil fertility, water-holding capacity and health, and as such should be encouraged, particularly where status quo agro-ecosystems are degraded.