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 (NO; 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 NO emissions with short-term incubations. We then identified key factors regulating NO emissions out of a series of soil physicochemical and biological factors and explored how these factors interacted to drive NO emissions. Land-use changes from primary forest to oil palm or Acacia plantation risks emitting more NO, 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 NO 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 NO emissions. In contrast to common understandings, the control of WFPS over NO emissions was largely indirect, and acted through Eh. The mineralization of LON, particularly arginine, potentially played multiple roles in NO 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 NO emissions through cascading interactions. Overall, we show that, despite identical initial soil conditions, land conversion can substantially alter the NO emission potential. Also, collectively considering soil-environmental regulators and their interactions associated with land conversion is crucial to predict and design mitigation strategies for NO emissions from land-use change.