Increasing livestock wastewater application in alternate-furrow irrigation reduces nitrification gene abundance but not nitrification rate in rhizosphere

Wastewater generated during livestock production is rich in nitrogen, but careless discharge could result in greenhouse gas emission, eutrophication or other problems. Using wastewater to irrigate cropland not only recovers nutrients but also partly relieves the pressure on water resources. In water-scarce regions, alternate-furrow irrigation (AFI) - alternately wetting half of the plant roots - was proven to be an effective water-saving approach without compromising yield. However, the extent to which AFI with wastewater affects nitrogen cycling genes remains poorly studied. We aimed to investigate changes in bacterial and fungal community structure, as well as relative abundance of nitrogen cycle-associated genes in soil receiving AFI with swine wastewater. We examined three irrigation rates, irrigating pepper plants with 50%, 65% and 80% of the amount of water required under conventional furrow irrigation to prevent the crop suffering water stress. Each treatment had a groundwater-irrigation control. We measured edaphic factors, microbial community structure and gene abundance in rhizosphere and bulk soils. The results showed that with decreasing irrigation rate, nifH, bacterial and archeal amoA and nosZ gene abundance increased and nirK and nirS gene abundance decreased in the rhizosphere, implying that reducing wastewater use by AFI can improve nitrogen use efficiency and reduce N2O emission via denitrification, but may increase the N2O emission via nitrification. Our findings provide an useful reference to improve water and nitrogen use efficiencies and environmental protection in agriculture in the meantime.