Symphytum officinale L. Plays a Dominant Role in Mitigating Nitrogen Accumulation in Soil Under Long-Term Irrigation with Treated Poultry Wastewater

Comfrey (<i>Symphytum officinale</i> L.) was a promising crop in the integrated crop-livestock pattern. However, the impact of long-term irrigation with treated poultry wastewater (TPW) on soil chemical properties and bacterial community, as well as the contribution of comfrey to mitigate N accumulation in soil, remain unclear. This study investigated the changes in chemical and microbiological characteristics of the comfrey soil under six-year TPW irrigation (grassland) in comparison with the adjacent conventional crop soil irrigated with freshwater (farmland). Results showed that N accumulation in comfrey accounted for 66.0% of the N input from TPW irrigation. In grassland, soil pH at all depths increased by one unit and EC in the subsoil increased by 33.5-42.4%, while TN and NO₃^--N in the surface soil decreased by 26.7% and 64.5%, respectively. The composition and structure of the bacterial community in the grassland dramatically changed, and the relative abundances of nitrite-oxidizing bacteria <i>Nitrospira</i> and ammonifying bacterium <i>Flavobacterium</i> and <i>Pseudomonas</i> increased by 0.1-3.6-, 3.8-11.0- and 0.1-6.0-fold, respectively, while those of saline-alkali-sensitive bacteria <i>Sphingomonas</i> decreased by 72.3-83.2% in the subsoil. Soil pH and NO₃^--N were the primary factors influencing changes in bacterial communities. These findings revealed that there was no nitrogen accumulation, but alkalization occurred in the comfrey field under long-term TPW irrigation, which highlighted the prospective application of comfrey in the crop-livestock system.