Before riparian planting ask these two questions

In his last article, ’How do riparian strips fare long term?’, Bert Quin supported the use of riparian strips as an effective means of reducing environmental pollution. But he posed two questions that he said must be answered before riparian planting proceeds.

Riparian strip planting is a useful way to reduce waterway pollution, especially for its ability to filter out faecal bacteria and sediment before these get into streams. 

But there is much more to it than this. Two important questions must first be asked and answered: firstly, what management needs to be done to ensure riparian strips do not become overloaded and therefore useless? And secondly, who should pay for this management?

Let’s look at the effect of different losses, short and long term, and consider the two ‘elephants in the room’ – the continuing overuse of fertilisers N and soluble P.

Bacterial losses 

Bacteria in run-off water can indeed be filtered out by vegetation in suitably wide and planted riparian strips. The soil in the strip becomes a biologically active zone where bacteria live and die in large numbers. But unlike grazed pasture they do not have the localised huge concentrations of bacteria in excreta sitting on the soil surface. The vegetation cover just needs to be kept dense and healthy.

Sediment losses 

A limitation of riparian strips is their inability to prevent sediment losses in extreme rainfall and run-off. Accumulations of sediment from smaller run-offs can easily be lost in one heavy downpour. 

So the real answer is, in the first place, to plant the best vegetation on susceptible slopes to minimise slips and sediment loss. This requires planting trees in susceptible catchment areas to stabilise the soil.

Particulate P losses 

This goes hand in hand with sediment loss. The higher the application of soluble P, the greater the accumulation of P adsorbed onto soil particles near the soil surface. As these concentrations increase with fertiliser use, increasing amounts of particulate P are lost in sediment. This particulate P which accumulates in riparian strips is susceptible to huge losses to waterways in extreme downpours. Once this particulate P enters a waterway and concentrations re-equilibrate, much of the P adsorbed on soil particles (up to 25% in Australian studies) is desorbed back into the water. The only real way to prevent this ‘accumulation and extreme loss’ cycle again is to solve the original problem -- the overuse of fertiliser P.

Soluble P losses 

The concentration of soluble P in run-off water is almost entirely a function of the levels of readily soluble P in the near-surface soil, which in turn is driven almost entirely by the levels of soluble P fertiliser being used. Soluble P that does get filtered out by riparian strips concentrates in the soil in the strips until the point is reached where the ability of the soil to adsorb more P starts to decline markedly. This can happen in as little as 5-10 years. Extreme run-off events will remove much of this soil anyway.

Planting the strips with high P uptake vegetation and regularly harvesting it would be impractical in most situations. The real solution is very simple.  There is no need to continue using soluble P on developed pasture soils. RPR (reactive phosphate rock) maintains pasture production every bit as well, but is virtually impervious to losses in run-off, as demonstrated in many short and longer term scientific studies from the last 40 years. Unfortunately, in an indication of the extreme influence the superphosphate industry wields over exactly what aspects of the mitigation of losses of phosphate loss get funded, this fact is not even rated a mention by the Waikato Regional Council.

Nitrate losses

Quite high percentages of the nitrate that reaches the surface or shallow soil of well-maintained riparian strips (unlike fences) can be denitrified to nitrous oxide or nitrogen gases. 

It is important to provide a sufficiently high mineralisable carbon level in the soil to ensure that most of the denitrification goes all the way to nitrogen gas. 

What gets lost as nitrous oxide (the ‘halfway point’ of denitrification) is simply transferring the problem from one of water quality to one of greenhouse gas warming.

In any case, much if not most of the nitrate lost from the soil is leached vertically into groundwater and never gets anywhere near a riparian strip. Nitrate is completely soluble in drainage water and is not retained by the soil at all. Most of the leached nitrate comes from cow urine patches. 

This in turn comes from excessive fertiliser N use and the excess levels in pasture protein this helps produce. The solution to this problem is either to destock, or to develop far more efficient forms of fertiliser N use. 

• Bert Quin is the managing director of Quin Environmentals.