Nitrogen fertilization and irrigation types do not affect the overall N2O production potential of a sandy soil, but the microbial community structure and the quantity of functional genes related to the N cycle

Nitrous oxide (N2O), one of the major greenhouse gases, features a 273-fold higher global warming potential compared to carbon dioxide (CO2). Soil-born N2O emissions are based on microbial processes, while the microbial processes are affected by environmental factors and agronomic management measures. The objective of this study was the exploration of the effects of different irrigation and nitrogen (N) fertilization regimes on N2O emissions in relation to the microbial community structure and the quantities of genes involved in the N cycle. The following four treatments were investigated in a sandy soil during the potato cropping season in 2019 in Brandenburg, Germany: drip irrigation without N fertilization (DI-ZN), drip irrigation with broadcasted N fertilization (DI-N), simultaneous application of dissolved N in irrigation water, the so-called fertigation (F), and fertigation without crops (F-ZC). N2O fluxes were measured with the closed chamber method. Microbial biomass was analyzed using phospholipid fatty acids (PLFAs). The bacterial functional genes amoA (ammonium monooxygenase), nxrB (nitrite oxidoreductase), narG (nitrate reductase), nirS/nirK (nitrite reductase), and nosZ (N2O reductase) were determined using quantitative real-time polymerase chain reaction (qPCR). N2O fluxes varied over the season and could mainly be explained by temperature changes. The fertilized treatments showed slightly higher area-related cumulative N2O emissions (DI-ZN: 1.13 kg N2O-N ha−1, DI-N: 1.28 N2O-N ha−1, F: 1.26 N2O-N ha−1, median values). Additionally, N2O flux rates as well as the area- and yield-related N2O emissions of DI-N and F were approximately at the same level, even though the quantities and correlation patterns of the investigated functional genes differed. Highest seasonal median flux rates were detected under DI-N (15.0–100.1 μg N2O-N m−2 h−1), where a greater influence of a nirS/nosZ harboring microbial community was found. In contrast, under F more diverse correlation patterns were found, especially regarding a predominance of nirK and nirS/nosZ harboring microbial community. F-ZC showed the lowest N2O flux rates (−8.7–44.2 μg N2O-N m−2 h−1), which might be explained by the lack of plant carbon sources, particularly root exudates. To conclude, the applied treatments affected the microbial community structure and their genetically determined potential for N2O production, whereby the overall N2O flux rates and hence N2O emissions remained unaffected. Although fertigation is considered to be more demand-based particularly in terms of N fertilization, this study indicates that agronomic management measures according to good faming practice might feature an oversupply of N, neglecting the real N demand of cultivated crops and hence hindering the mitigation of N2O emissions.