Non-linear Response of Microbial Activity across a Gradient of Nitrogen Addition to a Soil from the Gurbantunggut Desert

Elevated nitrogen (N) deposition has been shown to influence rates of microbial decomposition and nutrient transformation by affecting microbial properties, such as soil enzyme activity and microbial biomass and community structure. Low rates of N deposition are linked to comparatively large changes in plant community structure in desert ecosystems. But there are few published studies into N cycling in deserts, and even fewer evaluate the effects of air pollution or agricultural fertilization on arid ecosystems.

The Gurbantunggut Desert, the second largest desert in China, is surrounded by patches of farmland. In recent years, the total amount of nitrogenous fertilizer applied to these agricultural areas has increased from 7.4×10⁷ kg/y in 1980 to 4.9×10⁸ kg/y in 2004. This fertilizer is a source of N that could potentially be dispersed to adjacent desert lands by volatilization and/or leaching. This has been confirmed by studies that show N deposition into the desert has increased markedly when compared to several years ago. However, the effects of N amendment on microbial activity have yet to be elucidated. Identifying the patterns of soil microbial responses to increasing nitrogen (N) availability are important since microbial processes are related to the potential nutrient transformations.

Researchers from Xinjaing Institute of Ecology and Geography, Chinese Academy of Sciences and Macquarie University conducted over a two-year period with trials commencing at the beginning of each growing season. Soil enzyme activity, microbial biomass and microbial community level physiological profile (CLPP) were determined at 0-5 cm and 5-10 cm soil depths. Nitrogen was added to the soil at five rates plus a control, i.e. 0, 0.5, 1, 3, 6 and 24 g · N/m²·y.

They hypothesized that soil enzyme activities and microbial biomass N (MBN) would firstly increase and then decrease, and CLPP would be altered with increasing N addition, due to the deleterious effects of higher N addition upon microbial activity. Because of the relatively higher organic matter in the upper depth of soil layers, they further hypothesized that the responses of microbial activities in the 0-5 cm depth would be more marked than at 5-10 cm. In partial support of their hypothesis, soil enzyme activities, microbial biomass and nutrient concentrations responded to N addition with the most significant changes occurring in the 0-5 cm soil depth. Addition of N resulted in an increase in MBN and a decrease in urease activity. Invertase and alkaline phosphatase (AlP) activities increased at low doses of N addition and showed a decrease at higher doses. There was no evidence of change in oxidative enzyme activity at low N treatments but activity decreased at high N additions. However, the CLPP was not affected by N addition.

The results of this study suggest that N supplementation in this desert soil may affect C transformation, increase availability of N and P, and immobilize N in the microbial biomass. Responses of the enzyme activity to N supplementation occurred within the context of an apparently stable or unresponsive microbial community structure.

The main finding has been published on Soil Biology and Biochemistry, 2012, 47: 67-77. The paper is also archived at http://www.sciencedirect.com/science/article/pii/S0038071711002057.