Herd concentration areas create greenhouse gas hotspots

Herd concentration areas (HCAs) (e.g. laneways, water troughs, shaded areas), where cattle spend a larger proportion of their time relative to other farm areas, have been identified as ‘hotspots’ for soil-borne greenhouse gas (GHG) losses due to high carbon and nitrogen loading from animal excreta and compacting by animal treading. The existence of these GHG ‘hotspots’ is clear, but we lack the information to verify or quantify these claims in subtropical biomes, where emissions may deviate substantially from temperate zones due to heavy rainfall events (> 100 mm day−1) and high temperatures all year round. This study measured nitrous oxide (N2O) and methane (CH4) emissions from different farm management areas (herd concentration areas, pasture and riparian zones) over two years in three dairy systems located in the subtropics to determine farm-scale GHG emissions. Nitrous oxide emissions in HCAs were significantly greater than pasture soils (nine times greater on average), while CH4 emissions were only significantly greater at the warmest and wettest site. The key finding for intensively grazed pasture systems, is to acknowledge that it is a small proportion of the farm area (~ 3%) that is responsible for a large proportion of the farm scale GHG budget (~ 28%). This significant ratio indicates that for farm scale GHG mitigation to be effective, both in monetary and environmental terms, management measures should target these emission hotspots.