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

## TL;DR

Comfrey helps reduce soil nitrogen buildup and alters bacterial communities when irrigated with treated poultry wastewater over six years.

## Contribution

Quantifies comfrey's role in mitigating nitrogen accumulation and altering soil microbiology under long-term poultry wastewater irrigation.

## Key findings

- Comfrey accounts for 66% of nitrogen input from treated poultry wastewater irrigation.
- Grassland soil pH increased by one unit and subsoil EC rose by 33.5–42.4%.
- Nitrospira, Flavobacterium, and Pseudomonas increased, while Sphingomonas decreased in grassland soil.

## Abstract

Comfrey (Symphytum officinale 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 Nitrospira and ammonifying bacterium Flavobacterium and Pseudomonas increased by 0.1–3.6-, 3.8–11.0- and 0.1–6.0-fold, respectively, while those of saline-alkali-sensitive bacteria Sphingomonas 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.