Seasonal and long-term effects of nutrient additions and liming on the nifH gene in Cerrado soils under native vegetation

Summary Biological nitrogen-fixation (BNF) represents the main input source of N in tropical savannas. BNF could be particularly important for Brazilian savannas (known as Cerrado) that show a highly conservative N cycle. We evaluated the effects of seasonal precipitation and nutrient additions on the nifH gene abundance in soils from a long-term fertilization experiment in a Cerrado´s native area. The experiment consists of five treatments: 1) control, 2) liming, 3) nitrogen (N), 4) nitrogen + phosphorus (NP) and 5) phosphorus (P) additions. The nifH gene sequence was related to Bradyrhizobium members. Seasonal effects on N-fixing potential were observed by a decrease in the nifH relative abundance from rainy to dry season in control, N, and NP treatments. A significant reduction in nifH abundance was found in the liming treatment in both seasons. The findings evidenced the multiple factors controlling the potential N-fixing by free-living diazotrophs in these nutrient-limited and seasonally-dry ecosystems.