Long-term fertilization changes bacterial diversity and bacterial communities in the maize rhizosphere of Chinese Mollisols

The rhizosphere is a dynamic interface in which interactions among a myriad of microorganisms affect plants growth and tolerance to biotic and abiotic stress. Although rhizosphere effects on soil microbial communities have been widely investigated, few studies have evaluated such impacts of long-term fertilization on rhizosphere microbial communities in black soils common to northeast China. Here, we applied quantitative real-time polymerase chain reaction and high-throughput pyrosequencing to characterize rhizosphere and bulk soil bacterial communities in a long-term (36-year) fertilizer experiment. Soils were subjected to six treatments: CK (no fertilizer), N1 (150 kg urea ha−1 y−1), N2 (300 kg urea ha−1 y−1), M (18,600 kg horse manure ha−1 y−1), NPK (150 kg urea plus 33 kg P plus 62 kg K ha−1 y−1), and MNPK (M plus NPK). Inorganic fertilizer, especially N, decreased the 16S rRNA gene copy numbers and bacterial diversity in the rhizosphere and bulk soil, while manure fertilizer increased these values. Moreover, 16S rRNA gene copy numbers were higher and bacterial diversity was lower in the rhizosphere than the bulk soil, indicating that the maize rhizosphere had significant effects on bacterial diversity. The bacterial communities were predominantly composed of Proteobacteria and Acidobacteria in both the rhizosphere and bulk soil, but the rhizosphere and bulk soil communities were distinguished by principal coordinates analysis. Soil pH correlated with bacterial community composition and diversity in both rhizosphere and bulk soil. However, bacterial community composition in rhizosphere was more correlated with soil nutrient concentrations than in bulk soil under long-term fertilization. A redundancy analysis also indicated that soil pH, organic matter and available phosphorus concentrations were the most important factors in shaping bacterial communities in the maize rhizosphere. Our results revealed that long-term fertilization with increasing nutrients availability increased bacterial abundance, decreased biodiversity and changed bacterial composition in the rhizosphere.