Nitrogen and Phosphorus Export After Flooding of Agricultural Land by Coastal Managed Realignment

Abstract

We are grateful for skillful laboratory assistance by B. Christensen, K.C. Kirkegaard and R.O. Holm, as well as several engaged students over the years. This research was funded by the Aage V. Jensen Nature Foundation.

Internal Nutrient Loading Controls Macroalgal and Cyanobacterial Succession in a Coastal Lagoon Restored by Managed Realignment of Agricultural Land

“…Since the N pools in the soil appeared labile and was rapidly exhausted, DIN concentrations in the water column inside Gyldensteen Coastal Lagoon decreased considerably during the 5 years to a level similar to or even lower than outside the lagoon. DIP concentrations, on the other hand, remained 2-3 times higher than outside and only decreased slowly due to steady supplies from large but relatively slow reacting P pools in the flooded agricultural soil (Kristensen et al, 2020). This differentiated temporal change in internal N and P supply gradually shifted the algal dominance from opportunistic annual species toward perennial species with intermediate bursts of benthic cyanobacterial cover.…”

“…This study demonstrates that the first 5 years of macrophyte succession in the restored Gyldensteen Coastal Lagoon after MR of agricultural land were primarily controlled by nutrient availability. The water column nutrient concentration was at any time a balance between internal nutrient loading from the flooded soil and loss by algal assimilation as well as rapid tidal water exchange (residence time of 2-3 days) (Kristensen et al, 2020). Based on a stable isotope approach, Thorsen et al (2019) proved the importance of fertilizer-derived nutrients from the soil for macroalgal growth in the same lagoon.…”

“…We established two sampling stations inside the lagoon, Gyldensteen Inner Eastern (GIE) and Gyldensteen Inner Western (GIW), while one station outside the eastern entrance, Gyldensteen Outer Eastern (GOE), acted as an undisturbed coastal control (Figure 1). Mapping surveys proved that GIE and GIW are representative for the major bottom types and water depths covering about 70-80% of the lagoon (Kristensen et al, 2020). Station GIE was located in a stubble field from the last grass crops in 2013, while station GIW represented bare soil disturbed by the construction of the new inland dikes in 2013.…”

“…The brief bloom of N-fixing benthic cyanobacteria that was observed in May 2014 before the green algae proliferated indicates unstable conditions and higher DIP than DIN availability probably due to fast release of DIP and "delayed" DIN release from the flooded soil (Figure 4). PO 4 3− is released instantly by desorption when the soil turns anoxic and Fe(III) is reduced to Fe(II) right after flooding (Matthiesen et al, 2001;Kraal et al, 2015;Kristensen et al, 2020), while NH 4 + is released by cation exchange on particle surfaces when excess Na + from the saline water gradually diffuses into the soil (Gardner et al, 1991;Rysgaard et al, 1999). The first field measurements of soil-water nutrient fluxes in June 2014, 2-3 months after flooding, showed a substantial release of both DIN and DIP (Kristensen et al, 2020).…”

“…Inventories of N and P nutrients in agricultural soils are usually high due to many years of fertilizing crops and/or livestock manure dropping (Rubaek et al, 2013;Sebilo et al, 2013). Flooding by MR inevitable induces an initial rapid release of the stored nutrients from the soils, when PO 4 3− is desorbed following Fe(III) reduction to Fe(II) in the flooded anoxic soil and when NH 4 + is desorbed by ion exchange with Na + from the incoming seawater (Sundby et al, 1992;Rysgaard et al, 1999;Kristensen et al, 2020). The internal nutrient loading and ratio between particularly DIN (= NH 4…”