Electrical conductivity as a reliable indicator for assessing land use effects on stream N2O concentration

Abstract

While it is widely acknowledged that different categories of terrestrial land use significantly affect N2O emissions from streams and rivers, a suitable indicator to comprehensively reflect such influences is still lacking. This study examined the effectiveness of stream environmental and microbial factors in reflecting the influence of land use types on stream N2O concentrations, taking into account both endogenous N2O production and exogenous N2O inputs. Our results showed that human-disturbed (urban and agricultural) streams had nearly four times higher excess N2O concentrations (ΔN2O) than forest streams (5.01 versus 1.32 nmol/L N2O). By combining our observations with previous studies, we identify electrical conductivity as a significant and even the strongest predictor of (excess) N2O concentration and flux in streams across different land uses, surpassing the predictive power of nitrogen content. Elevated ΔN2O was observed at higher conductivities (r = 0.69, p < 0.01), which coincided with higher carbon and nutrient levels across different types of land use. Also, conductivity was positively correlated with the abundance of N2O-producing microbes (p < 0.05), including both nitrifying and denitrifying microbes. Denitrifiers adapted to human disturbance and the genetic potential for net N2O production, as evidenced by the positive correlation between conductivity and nir:nosZ ratio, were enhanced under high-conductivity conditions (p < 0.05). Furthermore, electrical conductivity had great potential to qualitatively indicate the magnitude of exogenous N2O input across different land uses. Overall, our results highlight the value of electrical conductivity as a reliable indicator for assessing the effect of land use types on stream N2O concentrations. This opens opportunities for the development of simple field-based assessments of riverine N2O emissions across regional landscapes.