Continuous farmyard manure application increases nitrogen fixation capacity of soils and relative abundance of iron-oxidizing diazotrophs in nutrient-deficient paddy fields in Madagascar

Abstract

Farmyard manure (FYM)-a mixture of animal excreta and plant residues-remains an important nutrient resource for lowland rice production in sub-Saharan Africa (SSA). However, the underlying mechanism of FYM in altering microbiological communities and supplying nutrients for rice production remains poorly understood. Hence, this study aimed to elucidate the effects of FYM application on N fixation and the diazotroph communities in the nutrient-deficient lowlands of SSA. Soil samples from three farmers' fields in Madagascar were incubated with ¹⁵N₂ gas for 1 month to assess their N fixation capacity. The plots in each field were subjected to the following fertilizer conditions: control (no fertilizer inputs) or continuous application of FYM or mineral fertilizer for previous 4 years. One field with a high N fixation capacity was selected to analyze the nitrogenase (<i>nifH</i>) and 16S rRNA genes in the rhizosphere. The continuous application of FYM significantly increased the N-fixation capacity of the soil compared with one-time or no FYM applications, particularly in fields where FYM was highly effective on rice yield improvement. Continuous FYM application also significantly increased the relative abundance of <i>nifH</i> gene sequences close to iron-oxidizing bacteria (Family <i>Gallionellaceae</i>) in the rhizosphere, whereas this change did not occur with the continuous application of mineral fertilizer. These results imply that continuous FYM application enhances N fixation capacity via changes in the diazotroph communities. The high abundance of diazotrophs related to the oxidation-reduction process of iron may be associated with the iron-rich soils in the region.