There is a stigma attached to conventional soil testing methodologies within the forest industry and for good reason; the nutrient availability data provided are poorly correlated with seedling nutrient uptake and growth during the early establishment phase (Pritchett and Fisher, 1987). A single ‘point-in-time’ extraction only represents a snapshot of readily available soil nutrients and, therefore, may not adequately reflect the soil nutrient supply throughout the growing season (Stark and Hart, 1997; Driscoll et al., 1999). Conversely, an index of soil N supplying power is biologically more meaningful, because it integrates all of the factors affecting nutrient availability over time (Pritchett and Fisher, 1987). Unlike conventional soil extractions, in situ burials of PRS™-probes integrate all of the principal edaphic factors affecting nutrient uptake by plants (i.e., soil moisture and temperature, mineralization and immobilization, buffer power, dissolution, ion diffusion from greater distances, free ion activities, etc.) independent of soil type (Qian and Schoenau, 2002). Consequently, PRS™-probes are an effective surrogate for bio-mimicking nutrient absorption by plant roots as they remove soil nutrients through ion-exchange; therefore, providing the most reliable index of nutrient bioavailability (Yang et al., 1991; Qian et al., 1992; van Raij, 1998).

Unfortunately, adoption of the ion-exchange resin technology for routine monitoring of forest soil nutrient dynamics has been limited. Primarily, this is due to the practical limitations of using mesh bags containing ion-exchange resin beads that often rupture and/or are difficult to wash free of fine roots or soil particles. Advantages of the PRS™-probes include: easy insertion with minimal soil disturbance; flat structure ensuring a constant (i.e., quantifiable) adsorptive surface area; direct contact exchange with no secondary ion diffusion through mesh required; mechanistically similar to a plant root in its natural environment; easy removal and cleaning; and, reusable.

Researchers often will determine a cumulative measure of nutrient supply rate by removing buried PRS™-probes after some weeks and re-inserting fresh PRS™-probes in the same soil slot. Given that the PRS™-probe measurement accounts for the principal edaphic factors controlling nutrient flux to plant roots, it is not surprising that these cumulative nutrient supply rates correlate well with outplanted seedling nutrient concentration (Figure 1). Recently, PRS™-probes were used to measure the effects of shelterwood (SW) vs. clearcut (CC) harvesting systems and different vegetation management practices (control vs. herbicide) on growing season soil N supply rates and its relationship with conifer seedling N uptake (Figure 1) and growth (r2= 0.88, P<0.001; data not shown).

Figure 1. Relationship between PRS™-probe measured cumulative N supply rate and outplanted western hemlock N uptake. Source: Dr. Doug Maynard, Canadian Forest Service, Pacific Forestry Research Centre, Victoria, BC.; dmaynard@pfc.cfs.nrcan.gc.ca

The effect of site disturbance on biologically-mediated soil processes, such as N mineralization, is complex and difficult to quantify. However, the results of this study illustrate the effectiveness of PRS™-probes to integrate all of the principal edaphic factors affecting growing season soil N bioavailability; therefore, making it a reliable method for use in accounting for temporal variations in forest soil N mineralization. Accurately measuring soil N dynamics following a disturbance facilitates adequate monitoring of the impact of forest management practices on soil quality and long-term site productivity. This allows for appropriate inferences to be made regarding the sustainability of current silvicultural practices, while supporting effective soil nutrient management decisions.

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2. Pritchett, W.L., and Fisher, R.H. 1987. Properties and management of forest soils. J. Wiley and Sons, New York. 494 pp.
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