Nitrogen fertilization of wheat residue affecting nitrous oxide and methane emission from a central Ohio Luvisol

Fertilization of wheat (Triticum aestivum, L.) residue applied to degraded soils has shown promise as an option to restoring soil organic C (SOC) stocks, but the impact of the practice on N₂O and CH₄ emissions is not clear. It was hypothesized that, in addition to the mulch-induced soil wetness conditions favorable for N₂O and CH₄ formation, emission of these gases will be stimulated due to increased availability of mineral N and interference of NH₄ ⁺ with CH₄ oxidation in soils. During the period February–November 2000, fluxes of N₂O and CH₄ were monitored in a plant-free central Ohio Crosby soil (fine, mixed, mesic Aeric Ochraqualf) amended for 4 years with wheat straw (bare, 0; low, 8 Mg ha–¹ year–¹; and high, 16 Mg ha–¹ year–¹) without and with N fertilization (244 kg N ha–¹). The average annual N₂O fluxes were 1.1 kg N₂O-N ha–¹ in the unfertilized and 4.1 kg N₂O-N ha–¹ in the fertilized treatments. Annual N₂O emission (Y, mg N₂O-N m–²) was strongly correlated to the maximum daily flux (X, mg N₂O-N m–² day–¹; Y=48.3X−58.1, R ²=0.85, P<0.001) recorded on experimental plots. These flux maxima occurred at spring thaw in the unfertilized, and 6–30 days after fertilization in the fertilized treatments. Net CH₄ uptakes were measured on some occasions; overall, however, all the treatments were net CH₄ emitters with annual rates of 3.6, 4.9 and 5.1 kg CH₄-C ha–¹ in the bare, low and high residue treatments, respectively. No significant effect of fertilization and mulch rate on CH₄ fluxes was found, but temperature and landscape position appeared as strong controllers. Regardless of treatments, the highest CH₄-emitting plots were located in a minor depressional area at the experimental site. A comparison of SOC gain and N₂O and CH₄ emission expressed as CO₂-equivalents indicates that the residue treatments have a net CO₂-mitigating effect, but since C sequestration rates are expected to decrease with time, that positive effect will likely vanish after 7 and 12 more years in the fertilized and unfertilized residue treatments, respectively.