Deciphering nitrous oxide emissions from tropical soils of different land uses

Tropical regions are hotspots of increasing greenhouse gas emissions associated with land-use change. Although many field studies have quantified soil fluxes of nitrous oxide (N₂O; a potent greenhouse gas) from various land uses, the driving mechanisms remain uncertain. Here, we used tropical soils of diverse land uses and actively manipulated the soil moisture (35%, 60%, and 95% water-filled pore space [WFPS]) and substrate supply (control, nitrate, and nitrate plus glucose) to investigate the responses of N₂O emissions with short-term incubations. We then identified key factors regulating N₂O emissions out of a series of soil physicochemical and biological factors and explored how these factors interacted to drive N₂O emissions. Land-use changes from primary forest to oil palm or Acacia plantation risks emitting more N₂O, whereas low emissions could be maintained by conversion to Macaranga forest or Imperata grassland; these laboratory observations were corroborated by a literature synthesis of field N₂O measurements across tropical regions. Soil redox potential (Eh) and labile organic nitrogen (LON; amino acid mixture, arginine, and urea) mineralization were among the factors with greatest influence on N₂O emissions. In contrast to common understandings, the control of WFPS over N₂O emissions was largely indirect, and acted through Eh. The mineralization of LON, particularly arginine, potentially played multiple roles in N₂O production (e.g., bottlenecks of nitrifier-denitrification or simultaneous nitrification-denitrification versus substrate competition for co-denitrification). Structural equation models suggest that soil-environmental factors of different levels (from distal including land use, soil moisture, and pH to proximal such as LON mineralization) drive N₂O emissions through cascading interactions. Overall, we show that, despite identical initial soil conditions, land conversion can substantially alter the N₂O emission potential. Also, collectively considering soil-environmental regulators and their interactions associated with land conversion is crucial to predict and design mitigation strategies for N₂O emissions from land-use change.