Effects of nitrogen deposition rates and frequencies on the abundance of soil nitrogen-related functional genes in temperate grassland of northern China.

Ning, Q., Gu, Q., Shen, J., Lv, X., Yang, J., Zhang, X., He, J., Huang, J., Wang, H., Xu, Z., and Han, X. (2015). "Effects of nitrogen deposition rates and frequencies on the abundance of soil nitrogen-related functional genes in temperate grassland of northern China.", Journal of Soils and Sediments, 15(3), pp. 694-704. doi : 10.1007/s11368-015-1061-2  Access to full text

Abstract

Purpose: Microbial processes driving nitrogen (N) cycling are hot topics in terms of increasing N deposition. Abundances of N-related functional genes (NFG) can be most responsive to N deposition and commonly used to represent N transformation rates. However, empirically simulated N deposition has been exclusively conducted through large and infrequent N fertilization, which may have caused contrasting effects on NFGs. Therefore, experiments with small and frequent N additions closed to natural deposition are necessary. Materials and methods: Independently manipulated N addition rates (i.e., 0, 5, 10, 15, 20, and 50 g N m-2 year-1) and two frequencies (2 times per year addition as conventional large and infrequent N fertilization (2 N), and 12 times per year addition simulating small and frequent N deposition (12 N), respectively) were conducted in a long-term field experiment of a semiarid grassland in northern China. Quantification analysis using real-time PCR were carried out for NFGs, including nifH for N fixation, chiA for N mineralization, archaeal (AOA) and bacterial (AOB) amoA for nitrification, and narG, nirS, nirK, and nosZ for denitrification. Results and discussion: NFG abundances showed distinct sensitivities to N addition rates. The nifH, AOA-amoA, nirS, and nosZ gene abundances increased due to improved available N at low N rates, but suppressed by salt toxicity and acidification at high N rates. Large changes of chiA and AOB-amoA gene abundances highlighted their great sensitivities to the N enrichment. The abundance of AOB-amoA was more sensitive to N addition than AOA-amoA, but AOA-amoA dominated in absolute numbers and they predominated the ammonia-oxidation under different conditions. The N addition frequencies caused significant lower gene abundances of nifH, nirS, and nosZ under the 2-N frequency due to stronger suppression of acidification and salt toxicity and resulted in significant higher AOB-amoA gene abundances in response to higher N availability under the 2-N frequency. Conclusions: The NFGs abundances responded to N addition rates distinctly, highlighting that the driven processes involved in N cycling were altered by the N addition rates. The different effects of two N addition frequencies on NFG abundances demonstrated that conventional large and infrequent N fertilization cannot represent N deposition, and small and frequent N addition should be employed to project the effects of N deposition on microbial functional groups as well as on N transformations.

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