Response of apple orchard bacteria co-occurrence network pattern to long-term organic fertilizer input

Organic fertilizer substitution is frequently used for increasing the apple yield and soil fertility. However, the effects of organic fertilizer substitution on soil microbial communities and their co-occurrence patterns in apple orchard are still unclear. In the present study, we investigated the soil bacterial communities in apple orchard after 12 years of different fertilization regimes. Soil samples were collected in the four major periods of apple tree growth under four fertilization strategies: no fertilization control (CK), organic fertilizer (M), chemical fertilizer (NPK), and partial substitution of chemical fertilizer with organic fertilizer (MNPK). The soil bacterial com-munity was significantly affected by the fertilization strategy (R2 = 0.22), growth period (R2 = 0.22), and their interactions (R2 = 0.28). The & alpha;-diversity (Shannon and Richness indexes) was generally stable in the budding and setting periods of apple tree, but the & alpha;-diversity indexes in the swelling and maturing periods were 7.05 % and 7.20 % lower for NPK and MNPK compared with CK and M, respectively. The dissimilarities of the bacterial communities determined based on the Bray-Curtis distances in different growth periods were greater in MNPK than CK, M, and NPK. Network analysis showed that connections between bacterial taxa were more frequent for MNPK, where the number of total links were 130.28 %, 56.86 % and 143.82 % higher than CK, M, and NPK, respectively. The percentage of positive interaction in NPKM was 81 %, which was higher than the other treatments. These results demonstrate that long-term organic fertilizer substitution made the bacterial com-munity more sensitive to environmental changes. More keystone species were detected in the network for MNPK (module and connector hubs = 18), and the complexity and connectivity of the bacterial network increased after long-term organic fertilizer substitution. More modules in the networks for M and MNPK had significant re-lationships with the soil DON, MBC, and SOC than those in the networks for CK and NPK, indicating that more bacteria in organic fertilizer treatments participated in the C and N turnover. Overall, long-term organic fertilizer substitution made the bacterial community more sensitive to environmental changes, and might stimulated the soil C and N cycles.