Abstract
Fertilizer-induced changes in soil nutrients regulate nitrogen (N) fixation in the terrestrial biosphere, but the influences of N and phosphorus (P) fertilization on the diazotroph communities in successive crop seasons were unclear. In this study, we assessed the effects of N and P (high vs. low doses) on the abundance and structure of N2-fixation communities after wheat and soybean harvest in a long-term (34 and 35 years) fertilization experiment. In both seasons, long-term N addition significantly decreased the abundance of nifH genes and 16S rDNA; in addition, high doses of N and P fertilizer decreased the richness of diazotrophs, whereas low doses did not. The proportion of the dominant genus, Bradyrhizobium, in the soybean season (86.0%) was higher than that in the wheat season (47.9%). Fertilization decreased diazotroph diversity and the relative abundance of Bradyrhizobium in the wheat season, but had insignificant effects in the soybean season. The addition of N, but not P, significantly changed the communities of both diazotrophs (at the genus level) and rhizobia (at the species level) in the two seasons. Soil pH was positively associated with nifH abundance and diazotrophic richness; soil NO3− content was negatively correlated with diazotrophic richness and positively correlated with diversity. Soil pH and NO3− content were the two main drivers shaping the soil diazotrophic community. Overall, long-term inorganic N had a greater influence than P on both diazotrophic abundance and community composition, and diazotrophic diversity was more clearly affected by fertilization in the wheat season than in the soybean season.
Highlights
Nitrogen (N) is the most essential element limiting productivity in terrestrial ecosystems[1]
The diazotroph to bacteria ratio showed no clear differences among the five samples in the wheat season but was lower in N input soils (N1, N1P1, N2, and N2P2) than in unfertilized soil in the soybean season (Fig. 1C)
Fertilization was considered to have an insignificant effect on the Shannon index at the P < 0.05 level (P = 0.625) in the soybean season, whereas the index was lower with N1, N1P1, and N2P2, as compared with CK, in the wheat season (Fig. 2B)
Summary
Nitrogen (N) is the most essential element limiting productivity in terrestrial ecosystems[1]. According to the National Bureau of Statistics of China, in Heilongjiang Province in Northeast China, where the total grain output was 75.05 billion kg in 2018, accounting for more than 10% of China’s total grain output, erosion of its iconic black soil occurrs because of excessive application of chemical fertilizer (http://www.xinhuanet.com). The differences in the diazotrophic community abundance and composition between soils in two continuous crop seasons (wheat and soybean, a nonlegume and legume, respectively) under N fertilization regimes are unclear. In this study, using experimental gradients of N and P, we addressed the following specific questions: (i) Can N and P addition have different effects on the abundance and composition of the soil diazotrophic community and the dominant phyla/classes/genera in the wheat and soybean seasons? In this study, using experimental gradients of N and P, we addressed the following specific questions: (i) Can N and P addition have different effects on the abundance and composition of the soil diazotrophic community and the dominant phyla/classes/genera in the wheat and soybean seasons? (ii) would shifts in specific bacterial taxa correspond to the fertilization regimes or plants?
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