Trade-off between ‘new’ SOC stabilisation from above-ground inputs and priming of native C as determined by soil type and residue placement

Abstract

Due to the geographical expanse of grasslands with depleted organic matter stocks, there has been growing interest in the management of these ecosystems for C sequestration to help mitigate climate change. It is generally accepted that management practices intending to increase forage production (e.g. decreasing grazing density) result in increased soil C stocks by increasing the return of biomass inputs to the soil organic carbon (SOC) pool. However, the contribution of C inputs to stable SOC versus GHG losses, and how this is affected by soil properties, remains largely unknown, particularly within subtropical biomes. To investigate the role of soil texture and mineralogy on SOC stabilisation, we identified three different soil types with varying physical properties in close proximity (< 2 km²) to each other. We used isotopically labelled plant material (¹³C), placed on the soil surface versus incorporated within the mineral soil, to trace the fate of fresh residue inputs into SOM fractions that differed in their degree of protection and mechanistic interactions with the soil matrix. Weekly GHG measurements (CO₂, N₂O and CH₄) were taken to understand the overall GHG balance resulting from C inputs (i.e. SOC accrual versus GHG losses in CO₂ equivalents). In finer textured soils with a greater smectite content, SOC accrual was greater but was significantly outweighed by GHG losses, primarily from native SOC priming. The incorporation of residue within the soil increased residue-derived SOC accrual by 4- to 5-fold, whilst also suppressing the priming of native SOC. This improved understanding of how soil texture and residue placement affect the global warming mitigation potential of subtropical grassland soils will be important in determining identifiable regions that should be targeted for SOC restoration efforts by increasing C inputs.