Mustard catch crop enhances denitrification in shallow groundwater beneath a spring barley field
Introduction
Groundwater contamination by is a cause of concern for the environment (Galloway et al., 2008). Aquifer discharge of into streams, lakes, rivers and coastal transitional waters can increase the risk of eutrophication in surface waters (Stark and Richards, 2008). Excessive 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 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 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 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 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 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 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 ) 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 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 and increased DOC in groundwater compared to no cover crop (Premrov et al., 2012). Groundwater 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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