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1. Background & Objectives Shallow groundwater that develops on hillslopes is the main compartment in headwater catchments for flow and solute transport to rivers. Although spatial and temporal variations in its chemical composition are reported in the literature, there is no coherent description of the way these variations are organized, nor is there an accepted conceptual model for the recharge mechanisms and flows in the groundwater involved. This study is a part of ACASSYA project ANR-08-STRA-01 (http://www.inra.fr/acassya) described by Durand et al (2012, this meeting).
2. Materials & Methods We instrumented two intensive farming and subsurface dominant catchments located in Oceanic Western Europe (Kervidy-Naizin and Kerbernez, Brittany, France), two headwater catchments included in the Observatory for Research on Environment AgrHyS (Agro-HydroSystem) and a part of the French Network of catchments for environmental research (SOERE RBV focused on the Critical Zone). These systems are strongly constrained by anthropogenic pressures (agriculture) and are characterized by a clear non-equilibrium status.
- On Kervidy- Naizin, a daily monitoring of the nitrate concentrations in the stream and a three monthly monitoring of the nitrate concentrations of the shallow ground water, over 10 years, is now available. This first set of data is dedicated to the analysis of the intra and inter-annual variations of the nitrate concentrations and its relationship with those in the shallow groundwater.
- On Kerbernez, a network of 42 nested piezometers was installed along a 200 m hillslope allowing water sampling along two transects in the permanent water table as well as in what we call the “fluctuating zone”, characterized by seasonal alternance of saturated and unsaturated conditions (Legout et al., 2007). Water composition was monitored at high frequency (weekly) over a 3-year period for major anion composition and over a one year period for detailed 15N, CFC, SF6 and other dissolved gases. This second set of data is dedicated to the analysis of the recharge process.
3. Results & Discussion The results on Kervidy-Naizin shows that the concentrations in the stream increases abruptly at the beginning of the recharge period, correlatively with a hydraulic gradient, then level off during the wet and spring, despite the decrease of the hydraulic gradient. This hysteresis pattern along the water cycle can be explained by spatial variations of the nitrate concentration in the groundwater along the hillslope, which decreases from upslope to downslope, and the spatio-temporal variations of the hydraulic gradients. The results on Kerbernez which focus on the recharge processes show that local processes can also be involved to explain the spatiotemporal variations of the stream concentrations (Rouxel et al., 2011). They shows that (i) the anionic composition in water table fluctuation zone varied significantly compared to deeper portions of the aquifer on the hillslope, confirming that this layer constitutes a main compartment for the mixing of new recharge water and old
Nitrogen Workshop 2012
groundwater, (ii) seasonally, the variations of 15N and CFC are much higher during the recharge period than during the recession period, confirming the preferential flow during early recharge events, iii) variations of nitrate 15N and O18 composition was suggesting any significant denitrification process in the fluctuating zone, confirming the dominance of the mixing processes in the fluctuating zone, iv) deeper parts of the aquifer exhibited seasonal variations with structured hysteretic patterns, suggesting that mixing process also occurred at greater depths and v) these hysteretic patterns were dampered from upslope to downslope, indicating an increased influence of lateral flow downslope. These results indicate that we have to change the way we model subsurface dominant catchment, taken into account the degree of saturation of the catchment, the mixing processes varying from the surface to depth, and upslope to downslope. First modeling approaches considering a mixing process, in addition to the convection/dispersion model, are able to better fit the chemical variations of the anions in the shallow groundwater.
4. Conclusion As of now, we can deduce these results that the residence times deduced from end member approaches considering the groundwater as homogeneous lumped reservoir are likely to be highly underestimated. Instrumented observatories including spatial and temporal monitoring of the hillslope groundwater are required to understand the anthropogenic and environmental processes and their interactions, to model and predict the effect and the response time of such systems under different constraints.
References Durand, P, Ruiz, L, Vertès, F, Delaby, L, Moreau, P, Hubert-Moy, L. and Gascuel-Odoux, C. 2012. A framework for designing and evaluating nitrogen-efficient farming systems at the catchment scale by combining process studies, agro-hydrological integrated modelling and participatory approach into an iterative process. This meeting.
Legout, C., Molenat, J., Aquilina, L., Gascuel-Odoux, C., Faucheux, M., Fauvel, Y. and Bariac T. 2007. Solute transfer in the unsaturated zone-groundwater continuum of a headwater catchment. Journal of Hydrology. 332 (2-4), 427-441.
Rouxel, M., Molenat, J., Ruiz, L., Legout C., Faucheux, M. and Gascuel-Odoux, C. 2011. Seasonal and spatial variation in groundwater quality at the hillslope scale: study in an agricultural headwater catchment in Brittany (France). Hydrological Processes 25, 831-841.
Nitrogen Workshop 2012
The effect of a mustard cover crop on groundwater denitrification Jahangir, M.M.R.a, b, Minet, E.a, Johnston, P.b, Coxon, C.E.c Hackett R d, Richards, K.G.a a Teagasc Environment Research Centre, Johnstown Castle, Co. Wexford, Ireland b School of Engineering; cSchool of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland d Teagasc Oak Park, Co. Carlow, Ireland
1. Background & Objectives Groundwater contamination by nitrate (NO3-) is a cause of concern for the environment. Aquifer discharge of NO3- into streams, lakes, rivers and coastal transitional waters can increase the risk of eutrophication in surface waters. Leached NO3- may also contribute to global warming via indirect nitrous oxide (N2O) emissions. Agriculture accounts for most of Ireland’s N2O emissions and mitigation techniques are required to reduce emissions. The research objective was to investigate the impact of a cover crop (mustard) on the in situ denitrification rates and the N2O mole fractions (N2O/N2O+N2) measured in shallow groundwater under spring barley cropping.
2. Materials & Methods In situ denitrification rates were measured in March 2011 using a push-pull method (Addy et al.,
2002) in a shallow sand/gravel aquifer (water table 4 m below ground level) underneath an arable field of well drained soil (sandy loam) at Oak Park Research Centre. Two treatments within a spring barley system have been cultivated since 2006: (1) mustard and (2) no cover crop, as part of a larger experiment on the effect of over-winter green cover on nitrate leaching losses (Premrov et al., 2011). Three wells (PVC pipe; 0.03 m i. d. and 1.0 m screen section) were installed in each plot.
The push-pull method consisted of collecting groundwater from each well, amending it with 15Nenriched NO3- and a conservative tracer (bromide), injecting the solution in the aquifer (“push”), incubating in situ for several hours (4 hours) and pumping out (“pull”). “Pushed” and “pulled” groundwater solutions were analysed for dissolved N gases (N2O and N2) and ions (NO3-), dissolved organic carbon (DOC), other physico-chemical parameters (SO42-, Eh, pH, electrical conductivity-EC) and stable isotope ratios (15N/14N in N2O and N2). Denitrification rates were calculated according to equations from Mosier and Klemedtsson (1994). Non-parametric Mann– Whitney U tests were performed to determine significant differences (p0.05) between both cropping systems.
3. Results & Discussion At the time of the experiment, groundwater NO3--N concentration was lower and DOC concentration higher (Table 1) in the cover compared to no cover treatments (p0.05). Other hydrochemistry (DO, Eh, pH, EC and SO42-) were statistically similar under both treatments.
Mean N2O production rates (Figure 1a) were similar (p0.05) in both treatments (2.27 and 1.97 ng N kg soil-1 d-1). In contrast, N2 production rates in the cover crop treatment were 7.61 µg N kg soil-1 d-1 whereas there was no N2 detected in the absence of a cover crop (Figure 1b). As a result, the
Figure 1 Groundwater N2O (a) and N2 (b) production rates in shallow groundwater under spring barley cropping with a mustard cover crop or no cover crop (no vegetation).
Previous results from this site showed reduced groundwater NO3--N (Premrov et al., 2011) and indicated higher groundwater DOC under the mustard cover crop (Premrov et al., 2009). Results from the present study suggest that the introduction of a mustard cover crop in spring barley tillage areas could also substantially enhance NO3- reduction via denitrification without significantly increasing N2O emissions. The observed enhanced denitrification under the cover crop may result from the higher groundwater DOC but the mechanism under highly aerobic conditions is unclear and may relate to aquifer anaerobic micro sites. Although the total groundwater denitrification rates are low, when combined with aquifer residence times (up to 5.6 years estimated by Fenton et al.
(2009)), denitrification could considerably reduce groundwater nitrate concentrations.
4. Conclusions The use of an over winter cover crop (mustard) in a spring barley cropping system can enhance groundwater denitrification thereby reducing groundwater nitrate concentrations and indirect N2O emissions. The resulting denitrification produced predominantly N2.
References Addy K., Kellogg D.Q. Gold A.J., Groffman P.M., Ferendo G. and Sawyer C. 2002. In situ push-pull method to determine groundwater denitrification in riparian zones, Journal of Environmental Quality 31, 1017-1024.
Fenton O., Coxon C., Haria A., Horan B., Humphreys J., Johnston P., Necpalova M., Premrov A. and Richards K.G.
2009. Variations in travel time and remediation potential for N loading to groundwaters in four case studies in Ireland:
Implications for policy makers and regulators, Tearmann 7, 129-142.
Mosier A.R. and Klemedtsson L. 1994. Measuring denitrification in the field, pp. 1047–1065. In: R.W. Weaver et al.
(eds.), Methods of soil analysis. Part 2. Microbiological and biochemical properties. 2nd edn. SSSA, Madison, WI.
Premrov A., Coxon C.E., Hackett and Richards K.G. 2011. Management strategies to reduce nitrate leaching from spring barley to groundwater on a vulnerable soil, In: Proc. of 31st Annual Groundw. Conference, Tullamore. IAH Irish group.
Premrov A., Coxon C.E., Hackett R., Brennan D., Sills P. and Richards, K.G. 2009. Over-winter green cover in a spring barley system: Role in exporting dissolved organic carbon to shallow groundwater and implications for denitrification, In: Bosch A., Teira M.R., & Villar J.M.. (eds.), Proc. of 16th Nitrogen Workshop. p. 357-361.
Nitrogen Workshop 2012
The effect of crop establishment system on the nitrogen use efficiency of cereal grain crops in Ireland Brennan, J.ab, McCabe, T.b, Hackett, R.a, Forristal, P.D.a a Teagasc, CELUP, Crops Research Oakpark, Co Carlow, Rep. of Ireland b School of Agriculture, Food Science and Veterinary Medicine, NUI Dublin, Ireland
1. Background & Objectives The dominant establishment system for cereal grain crops in Ireland is based on ploughing (P) followed by a secondary cultivation combined with sowing. While there has been an increased adoption of shallow, non-inversion, minimum tillage (MT) establishment systems in the past 12 years it accounts for less than 4% of the total cereal area (CAIR, 2007). Most MT established crops are winter sown on a small number of large farms. However, as the cereal grain industry becomes more specialised and the cost of production continues to rise, growers are looking for a reliable, cost-effective and sustainable method of crop establishment.
The beneficial effects of minimum tillage on a number of soil properties have been well documented (Morris et al., 2010; Rasmussen, 1999; Simon et al., 2009). However, the effect of establishment system on the Nitrogen status of a crop is not as conclusive, and appears to vary considerably with climate. A number of studies have been conducted on the nitrogen uptake of crops established with different systems (Malhi et al., 2006; Meyer-Aurich et al.,
2009) with variable results but few, if any of these studies are applicable in Ireland. The objective of these trials described here was to determine if establishment system affects the N uptake patterns of winter wheat and spring barley.
2. Materials & Methods Field experiments were carried out on winter wheat (WW) and spring barley (SB) in 2009 and 2010 on a medium textured clay loam in the south east of Ireland. A 4 x 5 factorial experimental design was used for the winter wheat trials with four establishment systems (MT; MT+S; P; P+S) where +S indicates straw incorporation and 5 N rates (0, 140, 180, 220, 260 kg/ha) with 4 replications. For the Spring Barley trials a 4 x 5 factorial design using four establishment systems (MT-Autumn; MT-Autumn+Spring; MT-Spring; Plough-Spring) and 5 N rates (0, 75, 105, 135, 165 kg ha-1) with 5 replications was used. For both wheat and barley, crop N uptake (CNU) were recorded during the growing season. Grain and straw N uptake, grain yield and grain protein were recorded at harvest. Statistical analysis was carried out by analysis of variance using Genstat.