Priming effects on decomposition depend on organic matter in the growing media

Abstract Background Liquid organic fertilizer (LOF) and root exudates contain easily decomposable carbon that can stimulate microbial growth. It is unknown what role the amount of organic matter (OM) in the growing media, and potential interactions between root presence and LOF, might play in enhancing the breakdown of OM. Aims The aim was to better understand how the decomposition rate of plant litter is affected by added fertilizer, plant presence, and growing media OM content. Methods A container experiment was conducted with and without tomato plants ( Solanum lycopersicum L.; Heinz 5608 variety) present to evaluate the effect of three one‐time fertilizer additions (no fertilizer, LOF, and synthetic fertilizer) on litter decomposition rate. The equivalent amount of nitrogen (N), phosphorus (P), and potassium(K) was added in both fertilizer treatments. The experiment was conducted using two growing media, one containing high OM and one with negligible OM. Results The presence of tomato roots stimulated litter decomposition in high OM media, but not in low OM media. Adding LOF did not affect decomposition in either growing medium. Adding synthetic fertilizer led to a negative priming effect in low OM media when roots were present. The rate of decomposition was not affected by root traits. Conclusions When ample OM was available, the presence of plant roots had a strong positive impact on litter decomposition. In both low and high OM media, a one‐time addition of fertilizer had minimal or negative effects on litter decomposition. We speculate that the continuous nature of root exudation leads to sustained changes in the microbial population (both community composition and size). Boosting root length growth via the one‐time addition of inorganic fertilizer when OM was negligible allowed the plant to outcompete decomposing microbes for N, possibly leading to selection for microbes that primarily feed on exudates, which resulted in retarded litter decomposition rates. In conclusion, as adding inorganic fertilizer stimulated plant and root growth more than adding the equivalent nutrients in LOF, particularly in growth media with a high OM content, it is better to add inorganic nutrients than LOF to stimulate OM breakdown when plants are present.