Relative Importance of Nitrous Oxide Vs. Nitric Oxide Emissions from Soils Across a Management Intensity and Biodiversity Gradient.

Soil emissions of nitrous (N₂O) and nitric (NO) oxides contribute to both greenhouse gases balance and ozone production and destruction in the atmosphere. Understanding the controls on soil fluxes of these gases is vital for the development of mitigation opportunities and for understanding their impact on atmospheric chemistry. We measured soil N₂O and NO emissions from 9 ecosystems across management intensity and biodiversity gradient using semi-continuous automatic chambers over 2 years. The fluxes were measured 4 times a day in: two continuous corn systems, a monoculture without cover crops (CC) and with cover crops (CCcc), a corn-soybean rotation with cover crops (CScc), switchgrass (SWG), miscanthus (MIS); Poplar (POP), successional community (SUC), a native grasses mix (NGM), and a native prairie (NP) ecosystem with 20 different plant species. All ecosystems except NP were fertilized.

Cumulative soil emissions of N₂O and NO from the ecosystems decreased with increasing biodiversity and reduced management. The highest fluxes were measured in the CC and the lowest fluxes in the NP ecosystems.

Within annual ecosystems the addition of cover crops reduced cumulative soil N₂O emissions by 50%: from 1140 to 620 g N ha^-1 y^-1. Fertilized perennial grasses exhibited 3 – 8 times higher cumulative N₂O fluxes then those from fertilized NGM, SUC, and POP ecosystems: 870 and 450 g N ha^-1 y^-1in SWG and MIS ecosystems, respectively. Native prairie exhibited a cumulative flux close to nil.

Cumulative NO emissions from the annual ecosystems were also reduced by 50 – 90% by cover crops: fluxes were 254, 103, and 30 g N ha^-1 y^-1 in CC, CScc, and CCcc ecosystems, respectively. In all other studied ecosystems NO emissions did not exceed 16 g N ha^-1 y^-1 and decreased with increasing biodiversity and reduced management. Overall, NO emissions contributed up to 18% to cumulative N₂O and NO emissions from the studied ecosystems.

Temporal variability of hourly N₂O fluxes was lower in perennial than in annual ecosystems, while soil emissions of NO exhibited the opposite dynamics and were more variable in perennial than in annual ecosystems. Soil temperature was a poor predictor of hourly N₂O and NO emissions from annual systems, but explained up to 20% of emissions variability in perennial ecosystems.