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, R²=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.