Effects of elevated nitrogen deposition on soil organic carbon mineralization and soil enzyme activities in a Chinese fir plantation

Abstract

Human activities have dramatically increased the quantity of nitrogen fixed in terrestrial ecosystems, due to fossil fuel combustion, production and use of chemical fertilizers, and livestock ranching. Increased N supply can have a fertilization effect on forest ecosystems, but in the long-term, excess N can negatively impact biogeochemical cycling, soil chemistry, and productivity. Most research to date has focused on aggregate effects of N deposition on soil chemistry and N cycling. However, very little is known about the response of the microbial activities that are responsible for soil nutrient cycling and decomposition to these environmental changes. To investigate the response of soil organic carbon mineralization and soil enzyme activities to elevated nitrogen deposition, a field experiment was conducted in a Chinese fir plantation at Shaxian State Forest Farm of Fujian Province, China. Nitrogen loadings were designed at 4 levels as N0 (control), N1, N2 and N3 at the doses of 0, 60, 120 and 240 kg·hm-2· a-1 N, respectively. Each treatment comprised three replicate plots of 20m×20m which were sprayed with CO(NH2)2 solutions on the forest floor at the beginning of each month, lasting from January 2004 to the sampling time, March 2010. Soil samples were incubated in the laboratory at 28 °C for 28 days, and the alkali absorption method was applied to measure soil respiration. The carbon that mineralized as CO2 evolved was measured on the third day and once every week thereafter, and determined on the 28th day for the last time. Soil invertase, cellulose and amylase were detected by 3, 5-dinitrosalicylic acid assay, β-glucosidase by nitrophenol colorimetric method, and polyphenol oxidase and peroxidase by iodometric titration. During the incubation period, the daily mineralization of soil organic carbon decreased, but the cumulative mineralization of soil organic carbon increased with increasing time. Generally, the cumulative mineralization of soil organic carbon decreased with increasing doses of nitrogen deposition. The daily mineralization of soil organic carbon decreased in the sequence of N1>N0>N2>N3. At the same level of nitrogen deposition, the activities of the six soil enzymes involving carbon cycle (invertase, cellulose, amylase, β-glucosidase, polyphenol oxidase and peroxidase),the daily mineralization of soil organic carbon and the cumulative mineralization of soil organic carbon all decreased with the increasing soil depth. The activities of six soil enzymes responded differently to nitrogen treatments. At the soil depth of 0-20 cm, nitrogen additions promoted cellulose and polyphenol oxidase activities, but inhibited amylase and peroxidase to some extents. High level of nitrogen loading (N3) showed significant positive impact on invertase, but negative on β-glucosidase. Significant correlation was established between some soil enzymes, with cellulose being positively related with invertase and amylase, polyphenol oxidase with amylase and cellulose, peroxidase with invertase, amylase, cellulose and polyphenol oxidase. Soil organic carbon mineralization was also found to be positively correlated with β-glucosidase and peroxidase, but negatively with invertase and cellulose at the surface soil (0-20 cm). Hence, nitrogen deposition in this experiment accelerated the activities of surface soil cellulose, polyphenol oxidase and invertase, but inhibited amylase and peroxide to some extents, and suppressed soil organic carbon mineralization.