A three-year measurement reveals that partial conversion from synthetic fertilizer to dairy manure increases cumulative nitric oxide emissions from a long-term experimental cropland

Conversion from synthetic nitrogen (N) fertilizer to organic manure has been well documented to effectively mitigate nitric oxide (NO) emissions in the current year or growing season. However, a clear understanding of NO emissions following long-term manure substitution is lacking. We hypothesize that soil conditions (particularly the accumulation of organic matter) following long-term manure substitution may stimulate NO emissions by providing more N substrates under favorable conditions. The primary aims of this study were to (1) quantify the long-term effects of manure substitution on NO emissions, and (2) identify the major regulating factors that contribute to the variations. Field measurements were conducted under summer maize-winter wheat rotations in a long-term fertilization experiment (since 1990) over three experimental years (from June 2016 to May 2019). The three treatments included an unfertilized control (CK), full synthetic urea in both crop seasons (NPK), and full synthetic urea in the maize season with 70% urea substituted by dairy manure in the winter wheat season (NPKM). Annual NO emissions varied from 0.10 to 0.22 kg N ha−1 and from 0.32 to 1.53 kg N ha−1 in CK and the fertilized treatments, respectively. We attributed the variations in annual NO emissions from the fertilized treatments to mean soil temperature by fitting a significant linear regression between the two variables (r2 = 0.681, p = 0.008). Seasonal NO emissions from NPKM treatment were lower (range, 2–66%, not always significant) and larger (range, 115–294%, not always significant) than those from NPK during the winter wheat and maize seasons, respectively. Annual NO emissions from NPKM treatment were 78% larger (range, 44–112%, not always significant) than those from NPK across the experimental years. Overall, this three-year field measurement revealed that cumulative NO emissions from NPKM treatment were significantly (p = 0.038) larger than those from NPK (2.73 versus 1.53 kg N ha−1). We concluded that long-term manure substitution for synthetic fertilizer in the winter wheat seasons did not mitigate, but rather stimulated cumulative NO emissions.