Soil carbon dioxide emission from intensively cultivated black soil in Northeast China: nitrogen fertilization effect

The aim of this study was to understand the effect of nitrogen fertilization on soil respiration and native soil organic carbon (SOC) decomposition and to identify the key factor affecting soil respiration in a cultivated black soil. A field experiment was conducted at the Harbin State Key Agroecological Experimental Station, China. The study consisted of four treatments: unplanted and N-unfertilized soil (U0), unplanted soil treated with 225 kg N ha(-1) (UN), maize planted and N-unfertilized soil (P0), and planted soil fertilized with 225 kg N ha(-1) (PN). Soil CO2 and N2O fluxes were measured using the static closed chamber method. Cumulative CO2 emissions during the maize growing season with the U0, UN, P0, and PN treatments were 1.29, 1.04, 2.30 and 2.27 Mg C ha(-1), respectively, indicating that N fertilization significantly reduced the decomposition of native SOC. However, no marked effect on soil respiration in planted soil was observed because the increase of rhizosphere respiration caused by N addition was counteracted by the reduction of native SOC decomposition. Soil CO2 fluxes were significantly affected by soil temperature but not by soil moisture. The temperature sensitivity (Q (10)) of soil respiration was 2.16-2.47 for unplanted soil but increased to 3.16-3.44 in planted soil. N addition reduced the Q (10) of native SOC decomposition possibly due to low labile organic C but increased the Q (10) of soil respiration due to the stimulation of maize growth. The estimated annual CO2 emission in N-fertilized soil was 1.28 Mg C ha(-1) and was replenished by the residual stubble, roots, and exudates. In contrast, the lost C (1.53 Mg C ha(-1)) in N-unfertilized soil was not completely supplemented by maize residues, resulting in a reduction of SOC. Although N fertilization significantly increased N2O emissions, the global warming potential of N2O and CO2 emissions in N-fertilized soil was significantly lower than in N-unfertilized soil. The stimulatory or inhibitory effect of N fertilization on soil respiration and basal respiration may depend on labile organic C concentration in soil. The inhibitory effect of N fertilization on native SOC decomposition was mainly associated with low labile organic C in tested black soil. N application could reduce the global warming potential of CO2 and N2O emissions in black soil.