Effects of inorganic and organic nitrogen additions on CO2 emissions in the coastal wetlands of the Yellow River Delta, China

Abstract

Human activities have increased atmospheric nitrogen (N) deposition, in turn changing CO2 emissions. Many studies concentrated on the relationships between the amount of N deposition and CO2 emissions. However, how N forms affect CO2 emissions is still poorly understood. A two-year field experiment was conducted to investigate the effects of N forms on CO2 emissions in the coastal wetlands of the Yellow River Delta. The N forms consisted of nitrate (NaNO3), ammonium (NH4Cl) and organic N (urea-N: glycin-N = 1: 1), and the level of N addition was 8 g N m−2 yr−1. The CO2 emissions were measured in the growing season from April to October. The results showed that three types of N addition stimulated CO2 emissions (P < 0.05). Compared with the control treatment, nitrate, ammonium and organic N additions increased CO2 emissions by 23.24%, 32.92% and 49.02%, respectively. Nitrate and ammonium additions produced similar and positive effects on CO2 emissions, whereas organic N addition had larger and positive effects on CO2 emissions than those of inorganic N addition (P < 0.05). This may be attributed to the following reasons. First, three types of N addition tended to increase soil β-glucosidase activity and Q10 value of CO2 emissions, but only organic N addition significantly enhanced soil β-glucosidase activity and Q10 value (P < 0.05). Second, inorganic N addition had neutral and negative effects on the decomposition of soil organic carbon (SOC), whereas organic N addition increased SOC decomposition compared with control treatment (P < 0.05). Together, these findings highlight the significance of differentiating the influences of N forms on CO2 emissions.