2.3. Estimation of Plant Nutrient Uptake

Nutrient retention is only partially controlled by plant nutrient uptake. In order to estimate plant nutrient uptake and compare it with the overall nutrient retention in the ditches, plant biomass (leaves and stems combined) was sampled in June 2015 and May 2016 from five 3-m sections of the ditches at successive sampling positions in the covered (C) and uncovered (U) parts. Width of the vegetation cover across the nominal 0.5-m wetted width of ditch was noted. Above-ground dry matter content of watercress and other vegetation was determined by weighing samples before and after drying in an oven at 60 °C. The mean and standard deviation of dry matter for the covered and uncovered sections of the ditches were determined. For 2015 only, sub-samples of the vegetation samples were analysed for total N using a Thermo Finnigan Elemental Analyser, Thermo Fisher Scientific, Hemel Hempstead, UK (FlashEA 1112 Series).

Total P determination was done using inductively coupled plasma optical emission spectrometry (ICP-OES) analysis of a nitric acid digest of plant material. Total Plant uptake of N and P from seeding to the sampling date was calculated using Equation (4):

where: UPT_N,P = N,P uptake (mg/m² stream surface); X_N,P = %N,P in dry matter of plant material (-); DM% = %DM in samples of fresh plant material (-); FW = weight of fresh plant material harvested per m² of stream surface area (kg/m²).

Note that for 2016, mean X_N,P data from 2015 were used to estimate uptake, as there are no measurements of nutrient content of plant material. In addition, as the ditches are unreplicated, it is not possible to draw firm statistical inference from dry matter and nutrient uptake data, and no statistical test has been conducted on this data.

Temperature of the air and ditch water in each covered and uncovered section was noted on the sampling dates. In June 2016, the percent cover associated with watercress, grasses, other vegetation, and open water was determined by a quadrat on five 3 m lengths for the covered and uncovered sections in each ditch.