Burning poop: carbon dynamics in herbivore dung during southern-African savanna fires

Herbivores play a vital role in the functioning of savanna ecosystems. They ingest plants, modifying the vegetation cover, and disperse nutrients across the landscape in the form of dung. Fire in savanna is also a key nutrient recycling pathway, making elements readily available through the resultant ash and smoke. Wildfire ash, known for its susceptibility to be transported by wind and water, plays a key role in redistributing pyrogenic organic matter and nutrients across the landscape. However, our level of understanding of ash characteristics from burnt dung is very low. In addition, and due to its high carbon content, dung also adds to the wildland fuels for fires, alongside vegetation. Given that savannas are the dominant source of global Cemissions from fires, assessing the role of burnt dung in C dynamics is, therefore, also crucial for more accurate estimations of the overall C released during savanna fires.

We quantified C losses from dung combustion during fire in four savanna sites burnt by experimental fires in Kruger National Park (South Africa). We also analysed chemical properties, including major nutrients and metals, of dung and dung-derived ash. The studied dung came from large herbivores (zebra, elephant, giraffe, buffalo and wildebeest). The concentration of carbon and nitrogen in burnt dung was significantly lower than unburnt dung (carbon: 41 and 4.1%, nitrogen: 1.1 and 0.3% in unburnt and burnt dung, respectively). The carbon released from dung burning accounted for up to 6% of the carbon released from vegetation burning, emphasizing the substantial role of dung in carbon emissions during savanna fires. Our results also highlight burnt dung as a hotspot for minerals and nutrients with chemical characteristics different to those found in vegetation ash (e.g., phosphorus: 9,195 and 6,158 mg kg^-1, copper: 55.8 and 28.1 mg kg^-1 in dung-derived and vegetation ash respectively). This is likely to affect local soil physical and chemical properties and hence enhance ecosystem diversity.