Cropland age from grassland conversion to cropland affects nitrous oxide emissions

The effects of soil properties and cropland age on atmospheric nitrous oxide (N₂O) emissions following the conversion of grassland to cropland in temperate grassland ecosystems are uncertain. In this study, N₂O emissions were compared among grassland and cropland soils in the agro-pastoral ecotone of Inner Mongolia over three growing seasons. Four adjacent sites with different land-use histories were selected, including grassland and croplands cultivated for 5, 10, and 50 years after conversion. N₂O flux measurements were obtained using a closed-chamber method and were performed continuously during vegetation periods. After the conversion of grassland to cropland, N₂O emission initially decreased and thereafter increased in the study sites. The cumulative N₂O emissions of the cropland soils 5 and 10 years in age were 10–50% less than those of the grassland, and the N₂O emissions from the cropland soil 50 years in age were 10–30% greater than the grassland. When the seasonal emissions were correlated against single soil parameter, the key soil parameter that affected N₂O emissions over the entire growing season was the soil moisture content. When the interactions among soil parameters were considered, the amount of N₂O emissions could be quantitatively described by a linear combination of two soil variables, the soil ammonium nitrogen (NH₄⁺-N) and moisture concentrations. This study demonstrates how the time of land use conversion from grassland to cropland can positively or negatively affect N₂O emission.