Long-term nitrogen and phosphorus fertilization effects on N2 fixation rates and nifH gene community patterns in mangrove sediments

The bioavailability of nutrients in sediments is one of the most important factors controlling ecological and physiological processes in the mangrove ecosystem (Alongi et al. 2002; Feller et al. 2002; Cheeseman & Lovelock 2004; Krauss et al. 2008). Although mangroves are highly productive, they are generally nutrient-deficient, relying on microbial and plant cycling processes for nutrients (Holguin et al. 2001; Feller et al. 2002; Reef et al. 2010). Anthropogenic activity in coastal areas can largely affect mangroves by external inputs of nutrients, causing negative changes in forest development and sediment geochemistry (Reef et al. 2010). The biological fixation of nitrogen is an important source for ‘new’ nitrogen in oligotrophic mangrove forests (Capone 1988; Lee & Joye 2006; Purvaja et al. 2008). Free-living diazotrophs are widely distributed within the