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

Yuan Liu,Andrew L Neal,Zhongyang Li,Feng Gao,Xiaoxian Zhang,Xiangyang Fan,Chao Hu,Erping Cui

doi:10.1007/s00374-019-01361-y

Abstract

In water-scarce regions, alternate-furrow irrigation (AFI)—alternately wetting half of the plant roots—has proven to be an effective water-saving approach without compromising yield. However, the extent to which AFI with wastewater affects N cycling genes remains poorly studied. We aimed to investigate changes in main N transformation processes, bacterial and fungal community composition, as well as relative abundance of N cycle-associated genes in soil receiving AFI with swine wastewater. The experimental plan included 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 composition, and relative abundance of genes in rhizosphere and bulk soils. Altering water use in AFI did not exert a significant effect on bacterial and fungal communities. By increasing the irrigation rate of wastewater, relative abundances of nifH, bacterial and archaeal amoA and nosZ genes decreased, whereas those of nirK and nirS genes increased in the rhizosphere soil; nitrification rate did not decrease and the denitrification rate remained unchanged in both rhizosphere and bulk soil, implying that appropriate increase of wastewater use by AFI can improve N use efficiency.

Highlights

Recycling nutrient-rich livestock wastewaters and reusing them for irrigation (Cai et al 2013) is an attractive approach to relieve water-shortage pressure, capture N and other nutrients in plant biomass and soil, and dispose of wastes in a managed manner
For exchangeable NH4+N in soil, alternate-furrow irrigation (AFI) at 50% and 65% rates using groundwater resulted in higher content than conventional furrow irrigation (CFI) in rhizosphere and bulk soils, and AFI at 65% and 80% rates using wastewater in both soil compartments and at 50% rate using wastewater in bulk soils, though not significantly
Nitrification rate in rhizosphere soil and plant N use efficiency were significantly influenced by water source but not by irrigation amount (Table S4), while water source and irrigation rate did not have significant impact on denitrification rate in either soil compartment

Summary

Introduction

Recycling nutrient-rich livestock wastewaters and reusing them for irrigation (Cai et al 2013) is an attractive approach to relieve water-shortage pressure, capture N and other nutrients in plant biomass and soil, and dispose of wastes in a managed manner. Irrigation with nutrient-rich wastewater is likely to alter N transformations in soils including nitrification, denitrification, N2-fixation, anaerobic ammonium oxidation (anammox), and complete ammonia oxidation (commamox). NH4+ is oxidized progressively to NO2− and to NO3−. The sequential reduction of NO3− and NO2− to nitric oxide (NO), or nitrous oxide (N2O) or dinitrogen gas (N2) in denitrification are anaerobic microbial processes, driven by denitrifying microorganisms that involve nitrate reductase (encoded by narG and napA), nitrite reductase (nirK and nirS), nitric oxide reductase (norB and norC), and nitrous oxide reductase (nosZ) (Li et al 2018). N2O emission is modulated by functional genes involved in nitrite reduction, such as nirS and nirK, and nosZ, which encode nitrous oxide reductase (Zehr and Kudela 2011; Hu et al 2015)

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