Plant phenology modulates and undersown cover crops mitigate N2O emissions

Abstract

Mitigation of N₂O emissions, a potent greenhouse gas, remain challenging due to knowledge gaps in plant-mediated nitrogen (N) transformation pathways, which limits ability to identify optimal approaches for efficient N utilization. We set up mesocosms with barley, Italian ryegrass, and barley in combination with Italian ryegrass to assess role of cover crop in N₂O emission mitigation. Soil emitted N₂O was collected simultaneously from the pots with plants at three growth stages: namely, vegetative, canopy expansion, and grain filling. The gas sample N₂O contents, N in microbial biomass (MBN), mineral N content, and phospholipid fatty acid (PLFA) analysis in soils were determined at the three growth stages. Cumulatively, highest N₂O was emitted from soil under Italian ryegrass (0.056 mg N g⁻¹ soil) followed by barley (0.0051 mg N g⁻¹ soil) and the least under barley and Italian ryegrass combination (0.0014 mg N g⁻¹ soil). The high emissions under Italian ryegrass occurred at vegetative stage due to high reactive N availability. Strong emissions were observed at canopy expansion stage under barley and were linked to access to the large mineral N proportion redistributed to the lower depth as depicted by highest MBN (0.025 mg N g⁻¹ soil) and decreased extractable N (0.0068 mg N g⁻¹ soil). The high emissions under barley correlated with high fungal/bacterial ratio, pointing towards a fungal role in the emissions. The least soil N₂O emissions under barley and Italian ryegrass combination were accompanied by elimination of variations induced by the plant growth stages. Absence of 18:2ω6,9 fungal PLFA biomarker under barley and Italian ryegrass combination indicates a potential inhibition and corresponds with reduced N₂O emissions. Together, these results broaden our understanding on how plant-soil interactions drives N₂O emissions processes and improves our ability to identify optimal plant-based emission mitigation approaches.