New PhD: Agricultural mitigation measures can have both beneficial and adverse effects

Dr. Mette Vodder Carstensen is the second PhD-student associated with BIOWATER that has achieved a PhD, and the first that is funded by NordForsk. Mette studied at the University of Aarhus, Denmark, and held her dissertation in December 2020. The final, formal approval was given at the end of February 2021, and now we are proud to present her work.

Treating water from tile drainage

Mette has focused on the types of mitigation measures that treat water from tile drainage systems, i.e. integrated buffer zones and different types of constructed wetlands. She has not just looked at single aspects of these mitigation measures, but rather approached them in a holistic manner, scrutinising both the beneficial and adverse effects of the measures. She has used a combination of international literature reviews and own research on these different mitigation measures.

Mette is the second PhD in Biowater, and the first that is funded by NordForsk.

In short, her findings show that the types of measures she has studied can remove nitrogen efficiently. For phosphorus, they acted both as sinks and sources, which means that they could remove phosphorus, but could also leak phosphorus to the waterbodies. Moreover, the measures could result in emissions of climate gases such as N2O and CH4. She suggests that these negative effects might be minimised by management and design strategies.

Of beneficial effects, she found that these measures can contribute to biomass production, as habitat providers, and in flood mitigation.

She concluded that holistic management and implementation of the measures is required to achieve cost-effective agricultural management, and that future emphasis should be put on maximising multiple functions and minimising adverse effects.


The supervisors for this thesis were Dr. Prof. Brian Kronvang, who is also leader of Module 4 in Biowater, and Dr. Carl Christian Hoffman, both at the University of Aarhus, Denmark.

Content of thesis

Mette’s thesis is titled “Multi-functionality and adverse effects of mitigation measures for water quality improvements in the agricultural landscapes.” Her work is presented in seven scientific papers, four of which she is first author:

Study I: Carstensen, M. V., F. Hashemi, C. C. Hoffmann, D. Zak, J. Audet, and B. Kronvang. 2020. Efficiency of mitigation measures targeting nutrient losses from agricultural drainage systems: a review. AMBIO 49: 1820–1837.

Study II: Carstensen, M. V., S. E. Larsen, C. Kjærgaard, and C. C. Hoffmann. 2019. Reducing adverse side effects by seasonally lowering nitrate removal in subsurface flow constructed wetlands. Journal of Environmental Management 240:190-197. doi:10.1016/j.jenvman.2019.03.081.

Mette has used a combination of international literature studies and sampling her own data through field work.

Study III: Carstensen, M. V., D. Zak, S. G. W. van’t Veen, K. Wisniewska, N. B. Ovesen, B. Kronvang, and J. Audet. 2021. Nitrogen and greenhouse gas fluxes from integrated buffer zones treating agricultural drainage water. Sci Tot Environ, Vol 774,  

Study IV: Carstensen, M. V., C. D. Børgesen, N. B. Ovesen, J. R. Poulsen, S. K. Hvid, and B. Kronvang. 2019a. Controlled drainage as a targeted mitigation measure for nitrogen and phosphorus. Journal of Environmental Quality 48:677. doi:10.2134/jeq2018.11.0393

Study V: Hoffmann, C. C., D. Zak, B. Kronvang, C. Kjaergaard, M. V. Carstensen, and J. Audet. 2020. An overview of nutrient transport mitigation measures for improvement of water quality in Denmark. Ecological Engineering 155:105863.

Study VI: Zak, D., B. Kronvang, M. V. Carstensen, C. C. Hoffmann, A. Kjeldgaard, S. E. Larsen, J. Audet, S. Egemose, C. A. Jorgensen, P. Feuerbach, F. Gertz, and H. S. Jensen. 2018. Nitrogen and phosphorus removal from agricultural runoff in integrated buffer zones. Environmental Science & Technology 52:6508-6517. doi:10.1021/acs.est.8b01036

Front page of Mette Vodder Carstensen’s PhD Thesis (2020). Photo by Mette.

Study VII: Zak, D., M. Stutter, H. S. Jensen, S. Egemose, M. V. Carstensen, J. Audet, J. A. Strand, P. Feuerbach, C. C. Hoffmann, B. Christen, S. Hille, M. Knudsen, J. Stockan, H. Watson, G. Heckrath, and B. Kronvang. 2019. An assessment of the multifunctionality of integrated buffer zones in Northwestern Europe, Journal of Environmental Quality, 48:362-375. doi:10.2134/jeq2018.05.0216

View the entire thesis: “Multi-functionality and adverse effects of mitigation measures for water quality improvements in the agricultural landscapes”.

Mette works today as a post.doc. for Biowater at the University of Aarhus, and we are lucky to still have her in our team!

Feature photo: Mette Vodder Carstensen.