Management controls on nitrous oxide emissions from row crop agriculture

Agriculture is a significant source of the potent greenhouse gas (GHG)nitrous oxide (N2O), accounting for ~70% of total anthropic N₂O emissions in the US primarily as a result of N fertilizer application. Emissions of N₂O are the largest contributor to the global warming potential of row-crop agriculture. Management, including choice of crop type and rotation strongly impacts N₂O emissions, but continuous emissions data from row-crops over multiple rotations are lacking. Empirical quantification of these long-term emissions and the development of crop- and rotation-specific N₂O emission factors are vital for improving estimates of agricultural GHG emissions, important for informing management practices to reduce agriculture's GHG footprint, and developing mitigation protocols for environmental markets. Over 20 years we measured soil N₂O emissions and calculated crop and management specific emission factors in four continuous rotations of corn (Zea mays) -soybean (Glycine max) - wheat (Triticum aestivum) under convention altillage (CT), zero tillage (NT), low chemical input (LI), and biologically (Org) based management. Two of these systems (LI and Org)included winter cover crops, red clover (Trifolium pratense) or ray(Secale cereale). While average soil N₂O fluxes in all systems where similar (2.9±0.2 to 3.8±0.5 g N₂O-N ha-1 d-1), there was a significant interaction of total emissions with crop and phase. Surprisingly, the lowest total emissions from the corn period of the rotation were from CT, and the highest from LI, with 608±4 and983±8 g N₂O-N ha-1 crop year-1,respectively. Total emissions during the wheat period of the rotation showed the opposite trend, with total emissions of 942±7 and524±38 g N₂O-N ha-1 crop year-1,for CT ant LI, respectively. Total emissions from the soybean period of the rotation were highest under NT and lowest under CT management(526±5 and 296±2 g N₂O-N ha-1 cropyear-1, respectively). Emission efficiency, N₂O emitted for grain produced, was lowest in CT and highest in Org for corn (0.10 and 0.22 g N₂O-N kg-1 grain, respectively).For wheat the emission efficiencies were similar for CT and Org systems, despite N₂O emissions from CT being almost double those from Org, reflecting the low grain yields under biological management. For soybean, the emission factors were consistent with total emissions due to similar soybean yields across practices, and were between 0.13 and 0.20 g N₂O-N kg-1 grain, for CT and NT, respectively. Based on long-term measurements we show that management choices a) has a major effect on N₂O emissions, even under the same rotation, and b) provide different potential mitigation opportunities.