Deciphering the biotic and abiotic drivers of coalescence asymmetry between soil and manure microbiomes.

Manure application improves soil fertility, yet its implications on the success of invasion of manure-borne microorganisms in the soil are poorly understood. Here, we assessed the importance of abiotic and biotic factors in modulating the extent to which manure-borne fungal and bacterial communities can invade resident soil microbial communities. For this purpose, we applied varying frequencies of two different amounts of manure to nine soils differing in physico-chemical properties, as well as in land-use history, over 180 days and monitored changes in bacterial and fungal communities. Variance partitioning revealed differential contributions of abiotic and biotic factors to invasion success, which together accounted for up to 82 % of the variance explained. We showed that the effects of interactions between biotic and abiotic factors increased with coalescence frequency and manure amount for the bacterial and fungal communities, respectively. Both abiotic and biotic factors were important for modulating coalescence asymmetry for the bacterial community, whereas abiotic factors had a greater effect on the fungal community. These results provide new insights into the drivers of coalescence events between manure and resident soil microbial communities. Moreover, our findings highlight the roles of the mixing ratio and frequency of coalescence events in modulating the survival of manure-borne microorganisms.