Soil greenhouse gas fluxes from tropical vegetable farms, using forest as a reference

Field-based quantification of soil greenhouse gas emissions from the Philippines’ agriculture sector is missing for vegetable production systems, despite its substantial contribution to agricultural production. We quantified soil N2O emission, CH4 uptake, and CO2 efflux in vegetable farms and compared these to the secondary forest. Measurements were conducted for 13 months in 10 smallholder farms and nine forest plots on Andosol soil in Leyte, Philippines using static chambers. Soil N2O and CO2 emissions were higher, whereas CH4 uptake was lower in the vegetable farms than in the forest. Vegetable farms had annual fluxes of 12.7 ± 2.6 kg N2O-N ha−1 yr−1, −1.1 ± 0.2 kg CH4-C ha−1 yr−1, and 11.7 ± 0.7 Mg CO2-C ha−1 yr−1, whereas the forest had 0.10 ± 0.02 kg N2O-N ha ha−1 yr−1, −2.0 ± 0.2 kg CH4-C ha−1 yr−1, and 8.2 ± 0.7 Mg CO2-C ha−1 yr−1. Long-term high N fertilization rates in vegetable farms resulted in large soil mineral N levels, dominated by NO3– in the topsoil and down to 1-m depth, leading to high soil N2O emissions. Increased soil bulk density in the vegetable farms probably increased anaerobic microsites during the wet season and reduced CH4 diffusion from the atmosphere into the soil, resulting in decreased soil CH4 uptake. High soil CO2 emissions from the vegetable farms suggested decomposition of labile organic matter, possibly facilitated by plowing and large N fertilization rates. The global warming potential of these vegetable farms was 31 ± 2.7 Mg CO2-eq ha−1 yr−1 (100-year time frame).