Impacts of nitrogen fertilization on CO2 efflux with and without organic amendments in a high‐pH soil

Abstract Organic amendments and nitrogen fertilization are common practices in agriculture, which have complex influences on the carbon cycle. To evaluate the effects of nitrogen‐organic carbon combination on CO 2 efflux, we conducted a field incubation experiment with control, N fertilization without and with straw/biochar amendments for 20 months in a typical soil of North China Plain. CO 2 efflux was measured every ~3–4 weeks, and water‐extractable organic carbon and soil microbial biomass carbon were analyzed in spring, summer, and autumn. Our results showed an asymmetric seasonality (slow increase in spring but rapid decrease in fall) in the control and biochar treatments, but a symmetric seasonality under straw treatments. Organic amendments with N fertilization caused an increase of CO 2 efflux in most seasons (comparing without N fertilization), with a much greater increase in spring–summer of the first year (22%–35%) than the second year (1%–3%). Nitrogen fertilization caused a much greater increase in cumulative CO 2 efflux with biochar (7%–13%) and with straw treatment (20%) than without organic amendments (3%) over the period of 20 months. SIC content showed an increase under organic amendments, with a greater increase under biochar amendments than straw amendments. The increases of CO 2 efflux under nitrogen‐biochar and nitrogen‐straw combinations could be explained mainly by enhanced decomposition of SOC, biochar, and straw rather than SIC dissolution. Our study indicated that biochar amendments were more effective for carbon sequestration than straw amendments and nitrogen fertilization with organic amendments could cause changes in various processes of CO 2 production in the cropland of north China.