Isotopic Investigations

To verify these results with an independent method we have used δ¹⁵N and δ¹⁸O isotope ratios of nitrate in unsaturated zone water samples obtained from the lysimeters (Appendix I).

Samples for δ¹⁵N and δ¹⁸O of nitrate were analysed monthly at the outflows of the suction plates and cups (Fig. 6 and 7).

As mentioned before the application of the δ¹⁵N–NO! and δ¹⁸O–NO! isotope ratios is a useful technique to identify sources and fate of nitrate. Due to the large oxygen isotopic difference between nitrate produced in the atmosphere and that produced by microbial processes in the soil (nitrification), the oxygen isotopes in nitrate are particularly powerful for distinguishing between nitrate originating from fertilizer application and nitrate from atmospheric deposition. In addition, changes in the concentration and isotopic composition of nitrate can be used to identify processes such as denitrification [6]. The isotope ratios in the unsaturated zone will be also influenced by mixing processes from different nitrate sources. In our case, the two main sources of nitrate are from nitrification in the soil and from atmospheric deposition. During nitrification in soil, the δ¹⁸O values in the resulting nitrate normally decrease because the amount and availability of isotopically heavy atmospheric nitrate decreases with increasing depth. The mean isotopic composition of atmospheric nitrate was determined as 9.1‰ for δ¹⁵N– NO! and 37‰ for δ¹⁸O. The outflow of the suction plates and cups ranged from approximately 2‰ to 12.6‰ for δ¹⁵N– NO! and 2.2‰ to 19.1‰ for δ¹⁸O–NO!.

The lysimeter samples collected during the study period, together with groundwater and the atmospheric composition of δ¹⁵N–NO! and δ¹⁸O–NO! are plotted in Fig. 8. The unsaturated zone water samples show a clear trend of increasing nitrate concentration accompanied by decreasing δ¹⁸O values of nitrate with increasing depth. This trend could be explained by continuously admixing of nitrate originating from nitrification along the whole profile in the unsaturated zone (Fig. 9).

5. CONCLUSIONS

The isotopic composition of nitrate is not only a powerful tool to determining its sources, but can also provide information about nitrogen transformation processes such as nitrification and denitrification in the unsaturated zone. The combined use of δ¹⁵N–NO! and δ¹⁸O–NO! data confirmed that nitrate in the unsaturated zone was predominately derived from microbial nitrification and to a much lesser extent from atmospheric deposition. The δ¹⁵N–NO! and δ¹⁸O–NO! data further suggested that nitrification can occur in the whole profile of the unsaturated zone. From this study it is concluded that nitrification can occur below the plant root zone also and therefore plays a significant role in the speciation and transport of nitrogen compounds in the unsaturated zone.

ACKNOWLEDGEMENTS

Discussions with the colleagues in the CRP are acknowledged. I wish to thank my colleagues Mr. W.

Berg and M. Koinigg for assistance in soil sampling, Ms. B. Zirngast for sampling of seepage water and Mr. J. Fank and Mr. A. Stuhlbacher for providing data. I am also grateful to the Hydroisotop laboratory staff for the δ¹³C–DIC, δ¹⁵N–NO! and δ¹⁸O–NO! isotope analyses.

FIG. 4. Distribution of nitrifying bacteria in several soil profiles (0-1.5 m).

1.0 10.0 100.0 1000.0 10000.0

FIG. 5. Actual and potential nitrifying activity of soil profile STW09.

40 60 70 150 300 between May 1998 and April 1999.

40 70 110 150 300 between May 1998 and April 1999.

0

NO3 from mineral fertilizer (nitrate) NO from plants

NO3– from mineral fertilizer (ammonia) NO from soil organic matter

NO from manure and septic system effluent

δ¹⁵N-NO₃ (‰)

FIG. 9. Typical ranges for δ¹⁵N and δ¹⁸O values of nitrate from atmospheric deposition, nitrate containing mineral fertiliser and nitrification of ammonia derived from plant material, mineral fertiliser, soil organic matter, manure and septic system effluents. Also shown are the unsaturated zone water samples obtained from the lysimeters, groundwater and rain.

-10 0 10 20 30

FIG. 10. Mixing between two different sources of nitrate in the unsaturated zone. Source A is nitrate in atmospheric deposition and source B is nitrate originating from nitrification

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Appendix 1

δ ¹⁵N– NO! AND δ ¹⁸O– NO! DATA IN WATER SAMPLES

Sample type Date δ¹⁵N–NO! (‰) δ¹⁸O–NO! (‰) Seepage water 40 cm (crop rotation) 20.05.1998 6.3 11.6 Seepage water 40 cm (crop rotation) 23.06.1998 6.4 13.6 Seepage water 40 cm (crop rotation) 21.09.1998 8.5 7 Seepage water 40 cm (crop rotation) 20.10.1998 8.1 7.8 Seepage water 40 cm (crop rotation) 17.11.1998 8.8 7 Seepage water 40 cm (crop rotation) 16.12.1998 8.5 6.2 Seepage water 40 cm (crop rotation) 28.01.1999 5.3 11.1 Seepage water 40 cm (crop rotation) 20.04.1999 22 22.7 Seepage water 40 cm (crop rotation) 25.02.1999 4.2 11.3 Seepage water 40 cm (crop rotation) 26.03.1999 7.5 12.6 Seepage water 40 cm (crop rotation) 19.05.1999 8.6 8.1 Seepage water 40 cm (crop rotation) 21.06.1999 5.6 7.8 Seepage water 70 cm (crop rotation) 23.06.1998 4 13.2 Seepage water 70 cm (crop rotation) 21.07.1998 7.3 8.9 Seepage water 70 cm (crop rotation) 21.09.1998 6.8 10.3 Seepage water 70 cm (crop rotation) 20.10.1998 8 6.7 Seepage water 70 cm (crop rotation) 17.11.1998 8.2 7.6 Seepage water 70 cm (crop rotation) 21.12.1998 3.4 9 Seepage water 70 cm (crop rotation) 28.01.1999 3 10 Seepage water 70 cm (crop rotation) 25.02.1999 6.5 10.2 Seepage water 70 cm (crop rotation) 26.03.1999 6.6 7.4 Seepage water 70 cm (crop rotation) 19.05.1999 5 10.4 Seepage water 70 cm (crop rotation) 21.06.1999 4.5 4.6 Seepage water 110 cm (crop rotation) 05.05.1998 2 8.5 Seepage water 110 cm (crop rotation) 26.05.1998 3.2 8.6 Seepage water 110 cm (crop rotation) 23.06.1998 5.2 10.1 Seepage water 110 cm (crop rotation) 21.07.1998 5.4 7.9 Seepage water 110 cm (crop rotation) 27.09.1998 7.2 16.7 Seepage water 110 cm (crop rotation) 20.10.1998 5.1 18.7 Seepage water 110 cm (crop rotation) 20.11.1998 6.3 15.4 Seepage water 110 cm (crop rotation) 19.12.1998 3.5 7.7 Seepage water 110 cm (crop rotation) 28.01.1999 4.6 6.6 Seepage water 110 cm (crop rotation) 19.05.1999 2.8 19.1 Seepage water 110 cm (crop rotation) 25.02.1999 4.3 14.3 Seepage water 110 cm (crop rotation) 26.03.1999 3.5 16.8 Seepage water 110 cm (crop rotation) 21.06.1999 4.1 8.4 Seepage water 110 cm (crop rotation) 29.07.1999 2.8 17.5

Sample type Date δ¹⁵N–NO! (‰) δ¹⁸O–NO! (‰) Seepage water 150 cm (crop rotation) 05.05.1998 3.3 5.2 Seepage water 150 cm (crop rotation) 26.05.1998 4 4.4 Seepage water 150 cm (crop rotation) 23.06.1998 3.4 9.5 Seepage water 150 cm (crop rotation) 21.07.1998 4.1 3.1 Seepage water 150 cm (crop rotation) 21.09.1998 5.3 9.7 Seepage water 150 cm (crop rotation) 20.10.1998 7.3 9.2 Seepage water 150 cm (crop rotation) 17.11.1998 6.2 5.4 Seepage water 150 cm (crop rotation) 16.12.1998 4.9 2.1 Seepage water 150 cm (crop rotation) 28.01.1999 5.6 5.3 Seepage water 150 cm (crop rotation) 25.02.1999 3.3 10.6 Seepage water 150 cm (crop rotation) 26.03.1999 5.4 2.1 Seepage water 150 cm (crop rotation) 20.04.1999 4.7 13.7 Seepage water 150 cm (crop rotation) 26.05.1999 3.8 7 Seepage water 150 cm (crop rotation) 21.06.1999 4.2 12 Seepage water 150 cm (crop rotation) 29.07.1999 3.7 7.2 Seepage water 300 cm (crop rotation) 05.05.1998 5.8 4.4 Seepage water 300 cm (crop rotation) 26.05.1998 6.6 4.9 Seepage water 300 cm (crop rotation) 23.06.1998 6 4 Seepage water 300 cm (crop rotation) 21.07.1998 5.3 6.1 Seepage water 300 cm (crop rotation) 21.09.1998 4.9 2.7 Seepage water 300 cm (crop rotation) 20.10.1998 6.3 3.4 Seepage water 300 cm (crop rotation) 17.11.1998 6.9 5.9 Seepage water 300 cm (crop rotation) 16.12.1998 4.2 4.6 Seepage water 300 cm (crop rotation) 28.01.1999 5.4 3 Seepage water 300 cm (crop rotation) 25.02.1999 5.5 5.2 Seepage water 300 cm (crop rotation) 26.03.1999 5.7 3.5 Seepage water 300 cm (crop rotation) 20.04.1999 4.7 1 Seepage water 300 cm (crop rotation) 21.06.1999 3.4 5.6 Seepage water 300 cm (crop rotation) 29.07.1999 3.2 7

Seepage water 40 cm (maize) 21.09.1998 13.8 10.3 Seepage water 40 cm (maize) 25.10.1998 12.1 11 Seepage water 60 cm (maize) 05.05.1998 5.9 11.9 Seepage water 60 cm (maize) 26.05.1998 6.3 11.2 Seepage water 60 cm (maize) 23.06.1998 5.9 13

Seepage water 60 cm (maize) 21.09.1998 8.4 13.2 Seepage water 60 cm (maize) 20.10.1998 5.2 8.1

Seepage water 60 cm (maize) 25.11.1998 12.6 18.8 Seepage water 60 cm (maize) 28.12.1998 10.3 19.1 Seepage water 60 cm (maize) 25.02.1999 5.5 13.5 Seepage water 60 cm (maize) 19.05.1999 5.9 15.1 Seepage water 60 cm (maize) 21.06.1999 3.9 13.8

Sample type Date δ¹⁵N–NO! (‰) δ¹⁸O–NO! (‰) Seepage water 70 cm (maize) 05.05.1998 6.9 4.6 Seepage water 70 cm (maize) 26.05.1998 6.9 5.5 Seepage water 70 cm (maize) 23.06.1998 7.1 8.3

Seepage water 70 cm (maize) 21.07.1998 8.6 17.2 Seepage water 70 cm (maize) 21.09.1998 7.2 16

Seepage water 70 cm (maize) 20.10.1998 7 5.5 Seepage water 70 cm (maize) 17.11.1998 7.4 7.1

Seepage water 70 cm (maize) 21.12.1998 4.1 12.4 Seepage water 70 cm (maize) 28.01.1999 5.3 12.2 Seepage water 70 cm (maize) 20.04.1999 4.3 13.1 Seepage water 70 cm (maize) 25.02.1999 4.8 11.8 Seepage water 70 cm (maize) 26.03.1999 4.7 16.4 Seepage water 70 cm (maize) 19.05.1999 4.7 6.3

Seepage water 70 cm (maize) 21.06.1999 3.4 9.5 Seepage water 150 cm (maize) 05.05.1998 4.9 5.7 Seepage water 150 cm (maize) 26.05.1998 5.3 6.7 Seepage water 150 cm (maize) 04.07.1998 6.3 7.13 Seepage water 150 cm (maize) 21.07.1998 7.5 12.6 Seepage water 150 cm (maize) 21.09.1998 4.6 13.5 Seepage water 150 cm (maize) 20.10.1998 7.7 16.1 Seepage water 150 cm (maize) 16.12.1998 4.8 7 Seepage water 150 cm (maize) 28.01.1999 4.2 11.3 Seepage water 150 cm (maize) 25.02.1999 5.1 7.4 Seepage water 150 cm (maize) 26.03.1999 4.4 9.2 Seepage water 150 cm (maize) 20.04.1999 3.8 13.1 Seepage water 150 cm (maize) 19.05.1999 4.2 10.1 Seepage water 150 cm (maize) 21.06.1999 4.7 2.8 Seepage water 150 cm (maize) 29.07.1999 5.7 7.1 Seepage water 300 cm (maize) 05.05.1998 5.1 5 Seepage water 300 cm (maize) 26.05.1998 6.2 4.9 Seepage water 300 cm (maize) 23.06.1998 4.7 10.7 Seepage water 300 cm (maize) 21.07.1998 5.4 14 Seepage water 300 cm (maize) 21.09.1998 5.4 7.6 Seepage water 300 cm (maize) 20.10.1998 4.9 3.6 Seepage water 300 cm (maize) 17.11.1998 5.1 2.2 Seepage water 300 cm (maize) 16.12.1998 4.3 5.7 Seepage water 300 cm (maize) 28.01.1999 4.6 9.9 Seepage water 300 cm (maize) 25.02.1999 4 5.9 Seepage water 300 cm (maize) 26.03.1999 5.8 8.6 Seepage water 300 cm (maize) 20.04.1999 5.6 7.2 Seepage water 300 cm (maize) 19.05.1999 3.4 5.5

Sample type Date δ¹⁵N–NO! (‰) δ¹⁸O–NO! (‰) Seepage water 300 cm (maize) 21.06.1999 3.6 9.9 Seepage water 300 cm (maize) 29.07.1999 4.2 7 Ground water 21.09.1998 6 10.5

Ground water 16.12.1998 5.5 7

Ground water 05.05.1998 7.3 17.7 Ground water 23.06.1998 5.1 11.8 Ground water 07.07.1998 5.1 12.5 Ground water 25.02.1999 3.8 12.5 Ground water 20.04.1999 4.3 13.7 Ground water 21.06.1999 3.1 13.5

Appendix II

δ¹³C–DIC DATA IN WATER SAMPLES

Sample type Date δ¹³C–DIC (‰) Seepage water 40 cm (crop rotation) 14.09.1998 –9.9 Seepage water 40 cm (crop rotation) 09.11.1998 –12.5 Seepage water 40 cm (crop rotation) 13.01.1999 –12.1 Seepage water 40 cm (crop rotation) 17.03.1999 –11.5 Seepage water 40 cm (crop rotation) 12.05.1999 –11.6 Seepage water 40 cm (crop rotation) 12.07.1999 –6.9 Seepage water 70 cm (crop rotation) 30.06.1998 –11.7 Seepage water 70 cm (crop rotation) 14.09.1998 –8.5 Seepage water 70 cm (crop rotation) 09.11.1998 –10.7 Seepage water 70 cm (crop rotation) 13.01.1999 –11.5 Seepage water 70 cm (crop rotation) 17.03.1999 –8.8 Seepage water 70 cm (crop rotation) 12.05.1999 –10.5 Seepage water 70 cm (crop rotation) 12.07.1999 –7.9 Seepage water 110 cm (crop rotation) 09.06.1998 –13.2 Seepage water 110 cm (crop rotation) 14.09.1998 –10.4 Seepage water 110 cm (crop rotation) 09.11.1998 –11.7 Seepage water 110 cm (crop rotation) 13.01.1999 –12.4 Seepage water 110 cm (crop rotation) 17.03.1999 –10.5 Seepage water 110 cm (crop rotation) 12.05.1999 –13.6 Seepage water 110 cm (crop rotation) 12.07.1999 –9.1 Seepage water 150 cm (crop rotation) 20.09.1998 –6.3 Seepage water 150 cm (crop rotation) 08.06.1998 –12.7 Seepage water 150 cm (crop rotation) 30.06.1998 –12.3 Seepage water 150 cm (crop rotation) 03.08.1998 –9.3 Seepage water 150 cm (crop rotation) 14.09.1998 –10.4 Seepage water 150 cm (crop rotation) 09.11.1998 –10.8 Seepage water 150 cm (crop rotation) 13.01.1999 –10.6 Seepage water 150 cm (crop rotation) 17.03.1999 –10.2 Seepage water 150 cm (crop rotation) 12.05.1999 –12.6 Seepage water 300 cm (crop rotation) 03.08.1998 –10.9 Seepage water 300 cm (crop rotation) 14.09.1998 –10.7 Seepage water 300 cm (crop rotation) 09.11.1998 –10.5 Seepage water 300 cm (crop rotation) 13.01.1999 –10.1

Seepage water 40 cm (maize) 18.09.1998 –11.3 Seepage water 40 cm (maize) 17.03.1999 –10.7 Seepage water 40 cm (maize) 12.05.1999 –12.7 Seepage water 60 cm (maize) 09.06.1998 –10.2 Seepage water 60 cm (maize) 14.09.1998 –8.4 Seepage water 60 cm (maize) 09.11.1998 –11.3

Sample type Date δ¹³C–DIC (‰)

Seepage water 60 cm (maize) 13.01.1999 –12.5 Seepage water 60 cm (maize) 17.03.1999 –11.5 Seepage water 60 cm (maize) 12.05.1999 –9.9 Seepage water 60 cm (maize) 12.07.1999 –7.1 Seepage water 70 cm (maize) 09.06.1998 –11.3 Seepage water 70 cm (maize) 14.09.1998 –7.1 Seepage water 70 cm (maize) 09.11.1998 –11.5 Seepage water 70 cm (maize) 13.01.1999 –13.7 Seepage water 70 cm (maize) 17.03.1999 –15.4 Seepage water 70 cm (maize) 12.05.1999 –13.1 Seepage water 70 cm (maize) 12.07.1999 –6.6 Seepage water 150 cm (maize) 04.07.1998 –5.6 Seepage water 150 cm (maize) 03.08.1998 –7 Seepage water 150 cm (maize) 15.09.1998 –7.1 Seepage water 150 cm (maize) 09.11.1998 –7.7 Seepage water 150 cm (maize) 13.01.1999 –8.3 Seepage water 300 cm (maize) 27.07.1998 –5.9 Seepage water 300 cm (maize) 03.08.1998 –5.8 Seepage water 300 cm (maize) 14.09.1998 –3.9 Seepage water 300 cm (maize) 09.11.1998 –4.8 Ground water 09.06.1998 –11.8

Ground water 03.08.1998 –11.1 Ground water 14.09.1998 –10.8 Ground water 09.11.1998 –11.2 Ground water 13.01.1999 –12.5 Ground water 17.03.1999 –11.2 Ground water 12.05.1999 –11.4 Ground water 12.07.1999 –10

Appendix III

δ³⁴S-SO4 AND δ ¹⁸O-SO4 DATA IN WATER SAMPLES

Sample type Date ³⁴S–SO4 (‰) ¹⁸O–

SO4 (‰) Seepage water 110 cm (crop rotation) 03.07.1998 9

Seepage water 150 cm (crop rotation) 03.07.1998 6.2 Seepage water 300 cm (crop rotation) 12.07.1998 8.7

Seepage water 60 cm (maize) 07.07.1998 5.2 Seepage water 70 cm (maize) 03.07.1998 3.5 Seepage water 150 cm (maize) 05.07.1998 3.7 4.2



NITRATE IN GROUNDWATER AND THE UNSATURATED ZONE,