Elsevier

Chemosphere

Volume 103, May 2014, Pages 234-239
Chemosphere

Mustard catch crop enhances denitrification in shallow groundwater beneath a spring barley field

https://doi.org/10.1016/j.chemosphere.2013.11.072Get rights and content

Highlights

  • Mustard cover crop after spring barley enhances groundwater denitrification.

  • Mustard increases dissolved organic carbon (DOC) in shallow groundwater.

  • Denitrification in shallow groundwater is an in situ process.

  • Denitrification below mustard system results in mainly N2.

  • Without mustard, groundwater denitrification below spring barley produces only N2O.

Abstract

Over-winter green cover crops have been reported to increase dissolved organic carbon (DOC) concentrations in groundwater, which can be used as an energy source for denitrifiers. This study investigates the impact of a mustard catch crop on in situ denitrification and nitrous oxide (N2O) emissions from an aquifer overlain by arable land. Denitrification rates and N2O-N/(N2O-N + N2-N) mole fractions were measured in situ with a push–pull method in shallow groundwater under a spring barley system in experimental plots with and without a mustard cover crop. The results suggest that a mustard cover crop could substantially enhance reduction of groundwater nitrate NO3--N via denitrification without significantly increasing N2O emissions. Mean total denitrification (TDN) rates below mustard cover crop and no cover crop were 7.61 and 0.002 μg kg−1 d−1, respectively. Estimated N2O-N/(N2O-N + N2-N) ratios, being 0.001 and 1.0 below mustard cover crop and no cover crop respectively, indicate that denitrification below mustard cover crop reduces N2O to N2, unlike the plot with no cover crop. The observed enhanced denitrification under the mustard cover crop may result from the higher groundwater DOC under mustard cover crop (1.53 mg L−1) than no cover crop (0.90 mg L−1) being added by the root exudates and root masses of mustard. This study gives insights into the missing piece in agricultural nitrogen (N) balance and groundwater derived N2O emissions under arable land and thus helps minimise the uncertainty in agricultural N and N2O-N balances.

Introduction

Groundwater contamination by NO3--N is a cause of concern for the environment (Galloway et al., 2008). Aquifer discharge of NO3--N into streams, lakes, rivers and coastal transitional waters can increase the risk of eutrophication in surface waters (Stark and Richards, 2008). Excessive NO3--N leaching to groundwater below arable land in a spring barley system, where land is left fallow over winter, has been reported before by Hooker et al. (2008). In tillage farming, cover crops reduce NO3--N leaching to groundwater through the uptake of N during the fallow period between crop harvest and subsequent planting of the next crop (Shepherd et al., 1993). Over a three years period mustard sown has been found to reduce mean groundwater NO3--N concentration by c. 25% (Premrov et al., 2012). The mean DOC concentrations were found to be significantly higher by c. 32% under the mustard cover crop than under no cover crop, suggesting that mustard may help reduce groundwater NO3--N occurrence by (i) taking up soil N and/or supplying DOC in groundwater to enhance denitrification.

Nitrate reduction into un-reactive N via denitrification can be accompanied by the emissions of N2O, a potent greenhouse gas with global warming potential of 298 (IPCC, 2007). The contribution of the leached NO3--N with associated groundwater to indirect N2O-N emissions is well recognised (IPCC, 2007) but the dynamics of N2O production and reduction in situ in groundwater is not yet well understood (Clough et al., 2007). Moreover, in measuring denitrification in groundwater, it is often unclear if denitrification products are produced in situ or if they have been leached from surface soils (Groffman et al., 1998). An estimation of N2O-N/(N2O-N + N2-N)) ratios is necessary to know the potential of pollution swapping for NO3--N to N2O-N. Moreover, quantification of the end product of denitrification, N2-N, is also important to minimise uncertainty in the agricultural N balance (Galloway et al., 2004).

While previous research recognised the importance of mustard as an over winter cover crop in reducing NO3--N leaching to groundwater during the winter recharge (Hooker et al., 2008, Premrov et al., 2012), there are no reports on the effect of cover crops on groundwater denitrification and the N2O or N2 transformation rates. This information is crucial to better understand N cycling below an arable system and to improve land management. The objective of this experiment was to investigate the effect of a mustard cover crop on in situ denitrification rates and N2O-N/(N2O-N + N2-N) ratios in shallow groundwater under a spring barley cropping system.

Section snippets

Site and experimental design

The experiment was carried out during February–March, 2011 at Oak park Research Centre, Co. Carlow, Ireland (52°51′43″N, 6°54′53″W) in a shallow sand/gravel aquifer (water table <2.5 m below ground level, bgl). The top soil is a well drained sandy loam overlying inter-bedded layers of sand, gravel and silt/clay. The shallow fluvioglacial sand and gravel aquifer is underlain by a deeper Carboniferous limestone aquifer. Two over winter treatments within a spring barley system have been cultivated

Ambient hydrochemical properties

Nitrate concentrations under the mustard cover crop were significantly lower (p < 0.05) than without any cover crop (Table 1). Despite being low, DOC concentrations were 44% higher (p < 0.05) in groundwater beneath mustard cover crop cultivated plots than the no cover crop (Table 1). Other hydrochemical parameters (DO, pH, EC and SO42-) were statistically similar (p > 0.05) in groundwater below treatments, except for Eh which was significantly lower below mustard cover crop (Table 1).

In situ push–pull tracer recovery

The tracer

Impacts of mustard cover crop on groundwater geochemistry

Dissolved nutrients such as NO3--N and DOC are transported rapidly to groundwater on free draining sites with high rainfall. Our previous research on this experimental site showed that the mustard cover crop significantly reduced NO3--N and increased DOC in groundwater compared to no cover crop (Premrov et al., 2012). Groundwater NO3--N was also found to be significantly negatively correlated with groundwater DOC and temperature and positively correlated with EC, suggesting that denitrification

Conclusions

Introduction of a cover crop (mustard) into the agricultural management activities has appeared to add DOC to groundwater. The added DOC either can serve either as an energy source for denitrifiers or consume DO in the percolating water and create anaerobic sites capable to denitrification. Results from the present study suggest that the introduction of a mustard cover crop in the spring barley tillage areas can enhance denitrification rates. Although the TDN rates are low, when combined with

Acknowledgement

The study was funded by Irish Research Council and Department of Agriculture and Food through the Research Stimulus Fund Programme (Grant RSF 06383) in collaboration with the Department of Civil, Structural & Environmental Engineering, Trinity College Dublin, Ireland.

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