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

Abstract

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.

Keywords: Global Nutrient Cycle; Microbiology; Plant Ecology.

Graphical abstract

Figure 1

Nitrogen cycle Schematic representation of major pathways in the N cycle occurring in the atmosphere-soil interface. The microbial genes involved in each step are indicated. The values in the figure represent the fluxes in the N budget measured for the cerrado sensu stricto described in a review by Bustamante et al. (2006). Anammox, anaerobic ammonia oxidation; DNRA, dissimilative nitrate reduction to ammonium; N_org, organic nitrogen. Assimilative processes are indicated by gray arrows. Dissimilative processes are indicated by solid dark arrows. Decomposition is indicated by colored solid arrows. Reduction and oxidation reactions are represented by solid and dashed arrows, respectively.

Figure 2

Proteobacteria community Hierarchical taxonomy of Proteobacteria (A) and principal coordinate analysis (PCoA) plot with Bray-Curtis dissimilarity (B). Members of Proteobacteria were recovered from the 16S rRNA gene in the soil samples (0–10 cm depth) collected in the rainy and dry seasons of 2018. Soils were collected from a typical cerrado area where the long-term fertilization experiment was installed, in the Reserva Ecológica do IBGE, Brasília, Brazil. The gray tree on the right represents a key for the unlabeled trees. Each of the smaller trees represents a comparison between the treatments in the columns and rows. Node size represents the relative proportions for that taxon. A taxon colored brown is more abundant in the treatment in the column, and a taxon colored green is more abundant in the treatment of the row. Control, untreated control; Ca, liming; N, nitrogen addition; NP, nitrogen and phosphorus addition; P, phosphorus addition.

Figure 3

nifH gene abundance Relative abundance of nifH gene in the cerrado' soil samples based on calibrated normalized relative quantities (CNRQ values) generated in the qbase + software. The relative abundance of nifH gene was measured from soils (0–10 cm depth) collected in the rainy and dry seasons of 2018 in a long-term fertilization experiment in a typical cerrado area, located at Reserva Ecológica do IBGE, Brasília, Brazil. Control, untreated control; Ca, liming; N, nitrogen addition; NP, nitrogen and phosphorus addition; P, phosphorus addition. Bar plots and error bars represent the average and standard errors between technical replicates, respectively. Red asterisks indicate significant differences in nifH relative abundance between rainy and dry seasons for the same treatment. Black asterisks indicate significant differences in nifH relative abundance among treatments. ∗∗∗ = p < 0.01; ∗ = p < 0.05.

Figure 4

Soil parameters and nifH relative abundance Spearman's correlation between soil parameters and nifH gene relative abundance in soils (0–10 cm depth) from a typical cerrado area where the long-term fertilization experiment was installed in the Reserva Ecológica do IBGE, Brasília, Brazil. Significant correlations and their respective p values are highlighted in red. Control, untreated control; Ca, liming; N, nitrogen addition; NP, nitrogen and phosphorus addition; P, phosphorus addition.