The overlooked role of diffuse household livestock production in nitrogen pollution at the watershed scale

Livestock production has become a primary source of nitrogen pollution in China due to the rapid socioeconomic development. While intensive production operations are the major sources, small livestock breeding units (diffuse livestock production) potentially also contribute to water quality impairments because of their dispersed nature and lack of appropriate waste treatment facilities. However, little is known about how diffuse livestock production affects the surface water quality due to the difficulty isolating its influence from other agricultural activities. In this study, the fate of diffuse livestock-derived N transported through the watershed (i.e., both amount and flow path by which diffuse livestock-derived nitrogen is delivered to the watershed mouth) across flow conditions was investigated by using a new approach that accounted for aquatic nitrogen retention during transport. Its contribution to watershed nitrogen export was also unraveled and isolated from other sources. Overall percentage of diffuse livestock-derived nitrogen that was transported to surface waters was 8.7%, 58.2% of which was delivered to the receiving water bodies (i.e., the watershed mouth). The fate of diffuse livestock-derived nitrogen that was loaded to surface waters (7.6-8.0% under baseflow, and 10.8-16.4% under stormflow) and exported to the watershed mouth differed across flow regimes (3.9-4.4% under baseflow, and 7.4-12.1% under stormflow). Further, majority of livestock nitrogen was mobilized by the direct discharge of urine (82.2%) due to the farmer's water-flushed operation, whereas, only 17.8% of livestock-derived nitrogen flux was from the loss of manure and urine during storage. The contribution of diffuse livestock to watershed nitrogen flux varied across flow conditions with higher contributions under baseflow conditions (67.9-74.9%) than under stormflow conditions (49.5-54.5%). The diffuse household livestock production can substantially degrade the stream water quality as do concentrated livestock production facilities, which was overlooked in the previous studies. A fully coupled mode that recycles both solid and liquid animal manure should be highly regarded as for the sustainability of livestock production. The findings provide important information for source identification and the management of the diffuse livestock production. (C) 2020 Elsevier Ltd. All rights reserved.