Effects of elevated atmospheric CO2 on soil enzyme activities at different nitrogen application treatments

It has been predicted that elevated atmospheric CO2 will increase enzyme activity as a result of CO2-induced carbon entering the soil. The objective of this study was to investigate the effects of elevated atmospheric CO2 on soil enzyme activities under a rice/wheat rotation. This experiment was conducted in Wuxi, Jiangsu, China as part of the China FACE (Free Air Carbon Dioxide Enrichment) Project. Two atmospheric CO2 concentrations (580±60) and (380±40) μmol·mol−1) and three N application treatments (low-150, normal-250 and high-350 kg N · hm−2) were included. Soil samples (0-10 cm) were collected for analysis of β-glucosidase, invertase, urease, acid phosphates and β-glucosaminidase activities. The results revealed that with elevated atmospheric CO2 β-glucosidase activity significantly decreased (P < 0.05) at low N application rates; had no significant effect with a normal N application rate; and significantly increased (P < 0.05) with a high N application rate. For urease activity, at low and normal N application rates (but not high N application rate), elevated atmospheric CO2 significantly increased (P < 0.05) it. With acid phosphatase elevated atmospheric CO2 only had significant higher effects (P < 0.05) at high N application rates. Under different CO2 concentration, effects of N fertilization are also different. Soil β-glucosidase activity at ambient CO2 concentration decreased with N fertilization, while it increased at elevated CO2 concentration. In addition, invertase and acid phosphatase activities at elevated CO2 concentration, significantly increased (P < 0.05) with N treatments, but there was no effect with the ambient CO2 concentration. For urease activity, at ambient CO2 concentration, N fertilization increased it significantly (P < 0.05), whereas at elevated CO2 concentration it was not significant. Additionally, with β-glucosaminidase activity, there were no significant effects from N application. In general, then, elevated atmospheric CO2 increased soil enzyme activity, which may be attributed to the following two factors: (1) elevated atmospheric CO2 led to more plant biomass in the soil, which in turn stimulated soil microbial biomass and activity; and (2) elevated atmospheric CO2 increased plant photosynthesis, thereby increasing plant-derived soil enzymes.