Rainfall and conduit drainage combine to accelerate nitrate loss from a karst agroecosystem: Insights from stable isotope tracing and high-frequency nitrate sensing

Fu-Jun Yue,Si-Liang Li,Susan Waldron,Zhong-Jun Wang,David M Oliver,Xi Chen,Cong-Qiang Liu

doi:10.1016/j.watres.2020.116388

Abstract

Understanding where nitrate is mobilized from and under what conditions is required to reduce nitrate loss and protect water quality. Low frequency sampling may inadequately capture hydrological and biogeochemical processes that will influence nitrate behavior. We used high-frequency isotope sampling and in-situ nitrate sensing to explore nitrate export and transformation in a karst critical zone. Nitrate was mobilised during light rainfall, and transferred from soil layers to the karst matrix, where some nitrate was retained and denitrified. Nitrate isotopic composition changed rapidly during the rising limb of events and slowly during the falling limb. The main nitrate source was synthetic fertiliser (up to 80% during event flow), transported by conduit flow following high rainfall events, and this contribution increased significantly as discharge increased. Soil organic nitrogen contribution remained constant indicating at baseflow this is the primary source. Isotope source appointment of nitrate export revealed that synthetic fertilizer accounted for more than half of the total nitrate export, which is double that of the secondary source (soil organic nitrogen), providing valuable information to inform catchment management to reduce nitrate losses and fluvial loading. Careful land management and fertilizer use are necessary to avoid nitrate pollution in the karst agroecosystem, for example by timing fertilizer applications to allow for plant uptake of nitrate before rainfall can flush it from the soils into the karst and ultimately into catchment drainage.

Highlights

The critical zone (CZ) is the near-surface layer that ranges from the top of plants through to the base of the groundwater zone; it serves as the main region within which biogeochemical processes interact to sustain terrestrial ecosystems (Banwart et al, 2013; Brantley et al, 2006)
The relatively stable water level (WL) during PIII indicated that the recharge and discharge water from the catchment are balanced, nitrate is being flushed out the system
This research highlights how dual nitrate isotopes can provide depth of understanding of high resolution [NO3−–N] time series in karst aquifer system. With these tools we can identify the dominant sources of nitrate and how they change with time, we can infer how nitrate is reprocessed, stored and transported in the high heterogeneous karst critical zone (KCZ), and we can quantify the load exported of each source

Summary

Introduction

The critical zone (CZ) is the near-surface layer that ranges from the top of plants through to the base of the groundwater zone; it serves as the main region within which biogeochemical processes interact to sustain terrestrial ecosystems (Banwart et al, 2013; Brantley et al, 2006). As one of the major pathways for active N to enter the ecosystem, application of synthetic and organic N fertilisers to agricultural land is essential to support food production for a growing population (Gu et al, 2015; Zhang et al, 2015). When applied in excess of crop requirements, N can transfer from land to water and result in high concentrations of nitrate-N (NO3−–N) in aquatic environments of the CZ (Cui et al, 2013). Karst geology accounts for 20% of the ice-free global terrestrial environment and supports water sources for approximately one quarter of the world population (Ford and Williams, 2013; Sullivan et al, 2019). Hydrological response to rainfall events in karst areas is often quick, and rapid exchange between surface and underground streams promotes the transfer of

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