Can N2O emissions offset the benefits from soil organic C accumulation under long-term cassava-based conservation agriculture systems in Cambodia?

Cassava (Manihot esculenta Crantz) is an important food and energy crop; however, its cultivation impacts on soil greenhouse gas (GHG) emissions remain scarce. This study, conducted in a tropical red Oxisol in Cambodia, quantified the long-term effects of cassava-based cropping systems on nitrous oxide (N₂O) emissions. Established in 2009, the experiment comprised 3 treatments: (1) mono-cropping cassava under conventional tillage (CTM–Cs); (2) mono-cropping cassava under no-tillage with biomass retention (NTM–Cs); and (3) bi-annual rotation of cassava with maize under no-tillage with cover crops use (NTR), where cassava (NTR–Cs) and maize (NTR–Mz) are grown annually. Soil N₂O emissions were measured on crop rows and inter-rows using static chambers over 2 full cropping seasons (April 2022–April 2024). Using recent SOC stocks data from the same site, the net climate impact was calculated using the global warming potential (GWP) metric. Under cassava treatments (CTM–Cs, NTM–Cs, and NTR–Cs), short-term peaks of N₂O-N emissions were observed after each application of mineral fertilizer as top dressing in both cropping seasons. Under NTR–Mz, the N₂O-N emissions differed between the two cropping seasons. In 2022–23, N₂O-N emissions on the row sharply increased just after the successive field operations of maize cultivation along with the mineral fertilizer applied at sowing, followed by the cover crop termination (437 µg N₂O-N m^–2 hr^–1). In 2023–24, the highest peak of N₂O-N emission on the row was substantially lower (~7 times) than in 2022–23, with only 57.77 µg N₂O-N m^–2 hr^–1. The mean of the two seasons’ cumulative N₂O emissions of the cassava treatments varied (p>0.05) from 0.74 to 0.87 kg N₂O-N ha^–1 yr^–1 but was significantly (p<0.05) lower than cumulative N₂O emissions of the NTR–Mz (1.42 kg N₂O-N ha^–1 yr^–1). Cassava tuber yield was similar between NTM–Cs and CTM–Cs, while the yield of NTR–Cs was at least 17% higher than CTM–Cs. Despite these differences, yield-scaled N₂O emissions were at a similar rate among the cassava treatments. Considering the trade-off between N₂O emissions and annual SOC accumulation, conservation agriculture systems largely contributed to global warming mitigation by sequestering 2.40 to 3.58 Mg CO₂-e ha^–1 yr^–1, compared to 0.18 Mg CO₂-e ha^–1 yr^–1 under CTM–Cs. Our findings highlighted the potential of conservation agriculture systems as sustainable practices for food security and climate change mitigation.