3 Floodplain meadows as a tool to reduce nutrients

In the past, swathes of floodplain meadows alongside watercourses played a role in managing water quality. By taking a hay cut, we can remove phosphorus from the freshwater system by using meadow management as a ‘phosphorus pump’.

The timing of haymaking affects the nutrient content. The optimal time is when the plants allocate nutrients to their shoots for flowering and seed production. After that, nutrients are taken back below ground. Hay harvest was traditionally around midsummer (24 June) in lowland England. This optimal date may become earlier if climate change results in the faster development of the crop. A timely first cut allows a second harvest in September. Two harvests have substantial benefits for nutrient removal (see Figure 4); ~60% greater than a single cut (Bowskill et al., 2023).

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Figure 4 Effect of cutting date on nutrient offtake.

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The image shows two graphs side by side. The first graph shows yield with different cutting times (late June, mid September re-cut, mid September single cut and combined double cut. The results show that cutting twice (late June and mid September) produces a yield of 4225 kg/ha - 1000 kg/ha more than cutting once. The second graph shows the phosphorus off take in the same cutting scenarios. Again the combined double cut produces the most phosphorus removal, with 7.8 kg/ha compared to 5.1 kg/ha in a single cut scenario.

Figure 4 Effect of cutting date on nutrient offtake.

The Floodplain Meadows Partnership has compiled a database of phosphorus budgets over the past 20 years. They suggest meadows are net exporters of phosphorus from river systems. The mean export of phosphorus is 7.9 kg ha^-1 yr^-1 to the wider environment (assuming a single hay crop and three floods per year.).

Watch this video showing how hay makes meadows from an ancient floodplain meadow in Oxfordshire.

Activity 2

Timing: Allow about 5 minutes

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Figure 3 (repeated) The nutrient cycle on a floodplain Meadow. Numbers represent amount of phosphorus in kg ha^-1 y^-1. Bold arrows denote the major flux through the system.

Examine Figure 3.

1. Calculate the percentage of the phosphorus entering the site that is removed from the site by a) hay and b) grazing.
2. What could an additional land management decision be to remove further nutrients?

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Discussion

1. a) 50% b) 6.25%
2. An additional cut in autumn

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Figure 5 Poster showing nutrient flows through a floodplain meadow.

Study this poster and list the two main ways floodplain meadows can be managed to tackle water quality in floodplains.

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Discussion

The two ways are:

1. cut twice
2. cut at the right time of year