2.2. Nitrogen and phosphorus supply rate in surface soil
Nitrate and P supply rate in the entire experimental design were
determined using Plant Root Simulator (PRS™-Probes) technology
from Western Ag Innovations Inc. Saskatoon, Canada. The PRSprobes
consist of ion exchange membranes encapsulated in thin
plastic probes, which were inserted vertically in the surface soil
(0e10 cm) with little disturbance of soil structure. The membrane
surface exhibits surface and sorption characteristics similar to
those of a plant root. Nutrient supply rates measured with this
method are generally significantly correlated with conventional
soil extraction methods over a wide range of soil types (Qian et al.,
1992). PRS-probes have also been recently useful in detecting
mineral-N and P hot spots in forest soils (Johnson et al., 2010).
In August 2011, four pairs of probes (an anion and a cation probe
in each pair) were buried in the A horizon of each plot (n ¼ 80) for a
20-day period. After probes were removed from the soil, they were
washed in the field with deionised water, and returned toWestern
Ag Labs for NO3 and P analysis. Composite samples were made in
each plot by combining the four pairs of probes. Probe supply rates
are reported as mg of nutrient 10 cm2/20 d.
2.3. Aboveground biomass sampling
In the hybrid poplar buffers, stem and branch biomass were
calculated for each individual tree using diameter at breast height
(DBH) measurements (made in Fall 2011) and allometric relationships
developed at the end of the 9th growing season in the same