Nitrogen fertilization alters the effects of earthworms on soil physicochemical properties and bacterial community structure

Abstract

Although it is known that earthworms can improve the granular structure of the soil and enrich the nutrient content, the effects of earthworms in combination with inorganic fertilization are not well understood. In the current work, 16S rRNA high-throughput sequencing and quantitative PCR (qPCR) approaches were used to reveal the effects of earthworms (Pheretima guillelm) and N fertilization on soil nutrient availability and to clarify their interactions with soil bacterial community composition. Pot experiments involving nine treatments were conducted using a split-plot design. Earthworm throw-in density was used as the whole-plot factor and N fertilization rate as the split-plot factor. The results showed that earthworms increased the proportion of macroaggregates in the soil, and the mean weight diameter and geometric mean diameter of soil aggregates, whereas N fertilization decreased these parameters. The qPCR results revealed a significantly higher bacterial abundance in soil treated with a high earthworm density without nitrogen addition (E2N0) than in the other treatments. The microbial biomass C content (CMic) of the soil in the low earthworm density with high N fertilization treatment (E1N2) was significantly higher than in the other treatments (P < 0.05). However, no significant differences were found in the microbial biomass N (NMic) of the soil among N fertilization treatments. The hierarchically clustered heatmap analysis based on the bacterial beta diversity revealed a higher proportion of some beneficial bacteria in E1N2, including Flavobacterium (11%) and Pseudomonas (5%). Furthermore, CMic (P = 0.006), available K (P = 0.016), NMic (P = 0.018) and available N (P = 0.027) were major factors affecting the variance in the bacterial community structure according to the representational difference analysis. This study suggests that earthworms can enhance the availability of soil CMic and NMic by increasing the number of soil macroaggregates and 16S rRNA at appropriate N fertilization rates.