Full straw incorporation into a calcareous soil increased N2O emission despite more N2O being reduced to N2 in the winter crop season

Abstract

Crop straw application in combination with fertilizer nitrogen (N) dose reduction is recommended to improve crop yields and carbon sequestration in soil. This practice may also promote soil nitrous oxide (N2O) emission, thus partially counterbalancing the expected benefits. However, the full straw return effect on the reduction of soil N2O to dinitrogen (N2) is not yet well known, owing to the methodological difficulties in quantifying soil N2 fluxes against the high atmospheric N2 concentration. This study was carried out in a long-term experimental field of a calcareous soil cultivated with summer maize and winter wheat in rotation. For the three field treatments since 2006, i.e., optimal fertilizer N with and without full straw return, and the control (without N and straw application), we conducted in- or ex-situ observations on field N2O fluxes, soil N2 emissions, crop yields, soil organic carbon (SOC) contents and soil/environmental factors. During a rotation cycle, we found that soil ammonium and nitrite concentrations, moisture and temperature jointly determined the N2O emissions, while nitrate concentration, dissolvable organic carbon to nitrate-N ratio and moisture jointly dominated the N2 emissions. Compared to straw removal, the straw return promoted reduction of more soil N2O to N2 in the wheat season than in the maize season, showing increases in the N2 emissions by 57% versus − 10% (P = 0.06). Nevertheless, it significantly enhanced the N2O emissions measured in situ by approximately 137% and 21% in the wheat and maize seasons, respectively (P < 0.05). It also significantly increased the annual crop yields by 20% on average (P < 0.05). Meanwhile it tended to enhance the SOC stock (0−20 cm) by 15‰ yr−1 (P = 0.14), showing a trend of more intensive carbon sequestration than the direct soil N2O emission (−948 versus 378 kg CO2e ha−1 yr−1 on average). We conclude that the long-term full straw return helps to sequester more carbon that offsetting the enhanced N2O emissions from the calcareous soil in the rotation system as it promotes reduction of more N2O to N2 in the wheat season.