Forestry Topics

Belowground Reponses to Global Change

A fundamental aspect of monitoring the effects of global change on ecosystem health and sustainability (especially in terms of developing indicators of change), involves assessing temporal changes in soil quality. Oftentimes, this is accomplished by measuring growing season soil nutrient dynamics within varied ecosystems and then relating these data to plant nutrient uptake and primary productivity. An added difficulty within forested ecosystems is the large microscale variability in soil properties relative to other ecosystems. Consequently, researchers historically have had a difficult time quantifying the effects of forest management practices on soil quality in both short-term and long- term contexts, let alone with the confounding influence of global change vectors.

The PRS™-probes are an effective surrogate for plant roots and have the capacity to collect biologically meaningful data under a variety of microsite environments. In addition, the PRS™-probes are now recognized as an ideal point assessment tool when investigating the impact of climate change, biophysical interactions, and past management on soil nutrient bioavailability. When assessing microsite variability, PRS™-probes can provide idiosyncratic trends of soil nutrient dynamics, which are crucial for evaluating the overall environmental impact of global change vectors. Conversely, a conventional soil analysis provides only a static determination of the extractable nutrient concentration at the time of sampling. The significance of this is evident below when comparing the soil N, P, and K analyses for different forest floor horizons using a traditional water extraction to that of a PRS™-probe in situ burial (Figure 1).

Figure 1. Comparison of PRS™-probe nutrient supply rates and water-soluble nutrient concentrations. Source: W.Z. Huang and J.J. Schoenau, 1996. Commun. Soil Sci. Plant Anal. 27: 2895-2908.

Measured nutrient levels exhibited large micro-scale variability among sampling points, which is expected given the heterogeneous nature of forest soils. However, the N supply rates measured with the PRS™-probes had a significantly narrower NH₄⁺:NO₃^- compared to the water extraction values. This was attributed to capability of the PRS™-probes to capture short-term pulses of NO₃^- passing through the profile during the two hour burial period, which was missed when a single point in time soil sample was collected and water-extracted. Similarly, fluxes of water-soluble K from the surface litter (L) horizon into the F and H horizons were measured using the PRS™-probes but not when samples were collected and water-extracted. Given the marked differences between these two analyses after a two hour burial, imagine the discrepancies after a long-term (twelve-week) PRS™-probe burial compared to a single soil extraction at the end of that period.

In a related study, Huang and Schoenau (1997) examined the spatial patterns of soil N and P availability in a boreal aspen forest by measuring nutrients (using PRS™-probes and conventional water extractions) inside and outside PVC cylinders. They used the difference between the two as an index of plant nutrient uptake (Figure 2).

Figure 2. Spatial patterns in N and P uptake by vegetation and fine root biomass in different horizons. Bars refer to one standard deviation. Source: W.Z. Huang and J.J. Schoenau, 1997. Can. J. Soil Sci. 77: 597-612.

Using this index, plant uptake of NH₄⁺-N, NO₃^--N, and P was highest in the H horizon, followed by the F and Ae horizons, with lowest uptake apparent in the L horizon. These results were consistent with plant fine root distribution: H horizon (68%); Ae and F horizons (15% each); and, the L horizon (2%). The PRS™-probe supply rates exhibited a greater sensitivity in measuring differences in plant N and P uptake among horizons compared to a traditional water extraction and also were better correlated to plant fine root distributions. This is because the PRS™-probe is used in situ and has the advantage of being mechanistically similar to a plant root in its natural environment. Collecting, handling, and analyzing a large number of soil samples is not conducive for studying soil nutrient dynamics throughout a growing season. Considering the PRS™-probes are used in situ, they are a relatively convenient and economical means of quantifying both spatial and temporal variations in nutrient supply rates for all nutrient ions simultaneously. Accurately measuring soil nutrient dynamics within forest soils is hindered by large spatial variability and temporal fluctuations. It is prudent, therefore, to employ a research tool capable of quantifying inherent micro-scale variations in soil fertility, while sensitive to the edaphic effects controlling nutrient availability over time. Compared to traditional soil testing methods, the PRS™-probe is a relatively convenient and economical means of quantifying both spatial and temporal variations in soil fertility; thereby, making it an effective tool for measuring forest soil nutrient dynamics in situ.

Sustainable Forest Management

Developing sustainable forestry relies on the continuous monitoring of specific soil quality indicators, in order to quantify any deleterious effects of timber management practices on long-term soil productivity. However, quantifying the impact of timber harvesting or management practices (i.e., site preparation, herbicides, biosolid amendments, etc.) on soil nutrient dynamics often can be problematic because an accurate assessment of nutrient availability within forest soils is hindered by high spatial variability and temporal fluctuations. In addition, considering that disturbed forest ecosystems are in a state of flux, why is the convention to use static measures of soil fertility within these systems? Not surprisingly then, conventional soil nutrient analyses, based on chemical extractions, historically have been poorly correlated with outplanted seedling nutrient uptake and growth. Herein lies the need to employ a standard monitoring tool that causes minimal disturbance to the soil, provides a convenient and cost-effective means of quantifying both spatial and temporal variations in soil nutrient dynamics, but most importantly, provides data that is biologically-meaningful to growing trees.

In simulating the uptake mechanism of plant roots, the PRS™-probes greatly improve the accuracy of measuring the bioavailability of soil nutrients. Consequently, they are a sensitive tool for measuring the effects of forest management practices within all soil types and facilitate precise monitoring of potential impacts on soil nutrient dynamics. For example, mechanical site preparation (MSP) often is used to modify a site in order to create favourable microsites for natural or artificial tree seedling regeneration. Considering that soil N and P often are limiting in the boreal forest, researchers at Laval University measured the impacts of mounding on soil nutrient supply rates in a balsam fir – yellow birch degraded forest. Specifically, PRS™-probes were buried in undisturbed, mounded, and scarified microsites (at varying distances from the mound) for three weeks following MSP (Figure 1).

Figure 1: Mounding site preparation, Measuring the effects of mounding- where a surface layer of soil (a quantity of forest floor and underlying mineral soil) is scooped up and inverted- on soil nutrient bioavailability using PRS™-probes buried in undisturbed area, in the mound, and at 0, 10, 25, 50, and 100 cm along a transect from the mound. (Photo courtesy of Pierre Gastaldello, Department of Forestry and Geomatics, Laval University).

Total N supply rates were larger in the excavated mound than in the undisturbed area, and then dropped considerably in the scarified microsites (Figure 2). Following MSP, there was an increase in NO₃^--N supply rate with a corresponding decrease in NH4+-N supply rate, which is typical following increased nitrification in a disturbed forest soil. The impact of MSP on soil P availability is apparent with the smaller P supply rates in the mound and scarified microsites compared with the undisturbed site.

Figure 2: Mean (n=3) NO₃^--N, NH₄⁺-N, and P supply rates measured at undisturbed, mounded, and scarified microsites following mechanical site preparation. Source: Pierre Gastaldello, Department of Forestry and Geomatics, Laval University.

In boreal forest soils, the principal source of available N and P is associated with organic matter; therefore, the effect of MSP on N and P availability primarily can be attributed to the displacement of the forest floor and mixing with surface mineral soil in the mounds. In addition, P is less likely to be available in mineral soil, due to increased fixation of P by Fe/Al-oxides, which presumably predominated in the mineral horizons of the acidic soil, but were less abundant or lacking in the forest floor. Despite the positive influences of MSP on short-term N availability in the mounds, decreased soil P supply is a negative impact on soil fertility and could affect long-term site productivity, especially on sites having low levels of organic material (i.e., mineralizable-P supply).

Similar with total N supply rates, the Mg and Mn supply rates were found in the mound than in the undisturbed area, and decreased in the exposed mineral soil and decreased with increasing distance from the mound (Figure 3).

Figure 3: Mean (n=4) Mg, Mn, and S supply rates measured in undisturbed and mounded microsites, and at 0, 10, 25, 50, and 100 cm along a transect from the mound, after mechanical mounding. Source: Pierre Gastaldello, Department of Forestry and Geomatics, Laval University.

Unlike all other nutrients measured in this study, the S supply rates increased in the scarified microsite farther away from the mound, indicating an increased exposure of the underlying S-enriched parent material. Contrasting trends in microsite nutrient fluxes can commonly occur after high disturbance silvicultural practices, and the PRS™-probes can capture these diverse fluxes. The ability of the PRS™-probes to simultaneously measure multiple nutrients across microsites clearly demonstrates it as an essential research tool in forest production-related research. Measuring microsite variability in nutrient bioavailability provides pertinent information for elucidating the impacts of silvicultural management on forest soil productivity. Accurately measuring soil nutrient dynamics facilitates adequate monitoring of the impact of forest management practices on soil quality. This allows for appropriate inferences to be made regarding the sustainability of current practices, while supporting effective nutrient management decisions.

PRS™-probe Related Literature:

• Johnson, D.W., Hungate, B.A., Dijkstra, P., Hymus, G., and Drake, B.J. 2001. J. Environ. Qual. 30: 501-507. Effects of elevated carbon dioxide on soils in a Florida shrub oak ecosystem.
• Johnson, D.W., Hungate, B.A., Dijkstra, P., Hymus, G., Hinkle, C.R., Stiling, P., and Drake, B.J. 2003. Ecol. Appl. 13: 1388-1399. The effects of elevated CO2 on nutrient distribution in a fire-adapted shrub oak forest.
• Johnson, D.W., Cheng, W., Joslin, J.D., Norby, R.J., Edwards, N.T., and Todd Jr., D.E. 2004. Biogeochemistry 69: 379-403. Effects of elevated CO2 on nutrient cycling in a sweetgum plantation.