Nitrous and nitric oxide emissions from a cornfield and managed grassland: 11 years of continuous measurement with manure and fertilizer applications, and land-use change

<p>Changes in weather and management practices such as manure and fertilizer applications have a major effect on nitrous oxide (N<sub>2</sub>O) and nitric oxide (NO) emissions from soils. N<sub>2</sub>O and NO emissions exhibit high intra- and inter-annual fluctuations, which are also highly influenced by land-use change. In this study we investigated how land-use change between grassland and cornfield affects soil N<sub>2</sub>O and NO emissions using long-term field measurements in a mollic andosol soil in Southern Hokkaido, Japan. Soil N<sub>2</sub>O and NO emissions were monitored for 5 years in a 30-year old grassland (OG), which was then plowed and converted to a cornfield for 3 years and then converted back to grassland (new grassland, NG) for another 3 years. We established four treatment plots: control, without any nitrogen (N) input (CT plot); chemical fertilizer only (F plot); chemical fertilizer and manure (MF plot); and manure only (M plot).</p> <p>Changing land use from OG to cornfield increased annual N<sub>2</sub>O emissions by 6–7 times, while the change from cornfield to NG resulted in a 0.3–0.6 times reduction in annual N<sub>2</sub>O emissions. N<sub>2</sub>O emissions in the newly established grassland were 2–5 times higher than those in the 30-year old grassland. Soil mineral N (NO<sub>3</sub><sup>−</sup> and NH<sub>4</sub><sup>+</sup>) was higher in cornfield, followed by NG and lowest in OG, while water extractable organic carbon (WEOC) did not significantly change with changing land use but tended to be higher in OG and NG than in cornfield. The ratio of WEOC to soil NO<sub>3</sub><sup>−</sup> was the most important explanatory variable for differences in N<sub>2</sub>O emissions as land use changed. High N input, surplus soil N, and precipitation and low soil pH led to increased N<sub>2</sub>O emissions. N<sub>2</sub>O emissions in fertilizer- and/or manure-amended plots were 3–4, 2–5 and 1.4–2 times higher than those in the control treatment in OG, cornfield and NG, respectively. NO emissions were largely influenced by soil mineral N and N addition, and showed less response to changing land use. There were high inter-annual variations in both NO and N<sub>2</sub>O emissions in all plots, including the control treatment, highlighting the need for long-term measurements when determining local emission rates.</p>