Scientific registration n : 1744 Symposium n : 33 Presentation : poster. AROCENA Joselito M.

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1 Scientific registration n : 1744 Symposium n : 33 Presentation : poster Effects of soil compaction and organic matter retention on soil moisture retention characteristics Effets de la compaction du sol et de la rétention de matière organique sur les caractéristiques de rétention hydrique des sols AROCENA Joselito M. University of Northern British Columbia, Faculty of Natural Resources and Environmental Studies 3333 University Way, Prince George, BC Canada V2N 4Z9 e- mail: arocenaj@unbc.ca fax: (250) Introduction Forest practices such as site preparations and harvesting tend to induce soil compaction and reduce soil organic matter (SOM). Soil compaction results from the mechanized methods of tree removal (Huang et al., 1996; Hamlett et al., 1990; Worrell and Hampson, 1997). Harvesting also results to the removal of surface organic matter (Brais et al., 1995; Busse et al., 1996; Mo et al., 1995). Soil compaction and loss of OM alter soil structure and consequently the rate of nutrient and water movement from soil matrix to the plant roots. Understanding these changes is important to the management of forest for long-term sustainability Soil water retention characteristics (SWRC) are defined as the relationship between soil water content and water potential. They are often used to describe poresize distribution (Vogel and Babel, 1994). Soil compaction modifies the SWRC through the reduction in soil porosity while organic matter affects water retention through the hydrophilic functional groups and indirectly through its modification of soil structure (Klute, 1986). The main objective of this paper is to present the preliminary results in the changes in soil water retention characteristics and soil solution composition resulting from different OM retention and soil compaction treatments. Methodology Experimental design A brief description of the three field installations used in the study was given in Table 1. Each site was laid out in a factorial design comprised of three levels of both organic matter retention and soil compaction treatments. The different treatments were OM 1 = boles (tree trunk) only removed (with OM), OM 2 = boles + crowns removed, OM 3 = boles, crowns, + forest floor removed (no OM), C 0 = no compaction, C 1 = intermediate compaction and C 2 = heavy compaction. The three treatments that will be presented in this paper were OM 3 C 0, OM 3 C 2, and OM 1 C 0. These treatments were chosen because they represented the extreme cases of soil disturbances. 1

2 Table 1. Selected site characteristics of the soils used in the study (after Holcmb, 1996). Site Characteristics Site 1 Log Lake Site 2 Topley Site 3 Skulow lake Dominant Soil Orthic Humo-Ferric Orthic Gray Luvisol Orthic Gray Luvisol Podzol Location N W N W 52 N W Elevation (masl) Slope and aspect On the crest 2-12 % westerly On the crest Vegetation Dominant Douglas fir and spruce Minor Lodgepole pine and sub-alpine fir Dominant Subalpine fir, Lodgepole pine and spruce Minor aspen and cotton wood Dominant Lodgepole pine Minor spruce, Douglas fir, aspen and cotton wood The operational application of these treatments was described in Holcmb (1996). In summary, organic matter was removed when the sites were dry and manual labor was used when necessary to avoid compaction. Boles were removed prior to compaction on OM 1 plots, and then returned to the plots after compaction treatments. C 2 and C 3 treatments were defined as 40 and 80%, respectively, of the difference between a hypothetical growth-limiting maximum and pre-harvest conditions. It was measured as depth of imprint of the compactor. Core sample collection We used a steel soil sampler with a dimension of 15-cm diameter and can be used to obtain undisturbed core sample of any desired thickness. The core sample was taken by driving the sampler with the metal weight on top of the metal cutting edge until the desired depth. Sample was then taken out excavating the buried core sampler using a shovel. Our samples had thickness of 15 cm. Measurement of soil water retention characteristics (SWRC) The measurement of the SWRC was conducted from cores of about 1.5 cm thick and a diameter of 15 cm from the Ae horizon. We measured the water retention properties from saturated conditions down to a potential of 6,000 kpa using a pressure plate/membrane apparatus. Equilibration time at different water potentials ranged from three days to three weeks and normally longer at higher potential compared to lower potential. Extraction and analysis of soil solution Soil solutions were extracted from each of the water potential using the pressure membrane apparatus equilibrated under nitrogen atmosphere. The extracted solutions were analyzed for selected cations (K, Ca, Mg and Na) by inductively coupled plasma spectrometry. 2

3 Results and Discussion The results that will be presented here will be limited to Site 3 Skulow Lake because the experiment is still in-progress as of date. The complete results for all the three sites will be presented during the ISSS 98 conference. Soil moisture retention characteristics The result of SWRC measurements for the Ae horizons for Site 3 Skulow Lake was given in Fig. 2. Generally, for the three treatments, water contents decreased with decreasing water potential (i.e., more negative). For instance, in treatments where OM was removed, water content at saturation was greatly reduced from 77% in no compaction treatment to 39% in heavily compacted treatment. The amount of water at field capacity (-33 kpa) was also reduced from 30% in no compaction treatment to 23% in heavy compacted soil. The effects of OM retention on water retention was evident on the highest water content at saturation for sample subjected to no compaction and with organic matter treatment. Treatment with organic matter and no compaction had the highest available soil water at 27% compared to about 3% for the compacted and without treatments. Directly, these changes on SWRC were due to increased soil bulk density from soil compaction treatments. The bulk density of the Ae horizon was 1.04 Mg m -3 and 1.25 Mg m -3 for no compaction treatments with and without OM, respectively, and was 1.56 Mg m -3 for heavily compacted and without OM treatment. These increases in bulk densities corresponded to reduction in soil porosity of 19% and 11%, for no compaction with OM and without OM treatments, respectively compared to compacted soils with no OM treatment. The results also indicated that the soil held up about 5% moisture content at potentials between and 6000 kpa (or hygroscopic water). The water held at these low potentials was not considered available to the plants but new theory indicated that mycorrhizal association maybe able to extract this water for the benefit of both the fungus and the trees. It also seemed that the amount of hygroscopic water was not affected by the different OM and compaction treatments. Moisture content (g H 2 O/100g soil) no compaction + OM no compaction + no OM heavy compaction + no OM Water potential (kpa) Fig. 1. Soil moisture contents at various water potentials. 3

4 Composition of the soil solution The effects of compaction and organic matter retention on the chemical composition of the soil solution were shown in Fig. 2. For the three treatments, the concentrations of Ca, Mg and K in the soil solution were higher in the available water (i.e., held between -33 and kpa) compared to the concentrations in hygroscopic water (i.e., held between 3000 and 6000 kpa). The concentration of Na seemed to be unaffected by the decreasing water potential. Comparison of the different treatments indicated that the concentrations of Ca, Mg and K in the available water were higher in the no compaction treatment compared to compacted treatment with and without OM. There were no clear trends observed for the concentrations of Na in the three different compaction and OM treatments. General discussion - implications to plant growth As expected, the compaction treatment imposed to the soils increased the bulk density, and therefore, reduced the pore space of the soil. Consequently, the reduced porosity decreased the total water holding capacity of the soil in general and in particular, the amount of water that can be used by the plants (Casel and Nielsen, 1986). Compaction will also reduce the aeration capacity of the soil, particularly the amount of oxygen in soil air. Dulohery et al. (1996) reported that compacted soil on skid trails formed from forest harvesting reduced the air-filled porosity by an average of 43%. The high bulk density and the reduction in water and air holding capacities of the soil can impair the normal growth of the plants. For instance, the soil penetration resistance increases with increasing bulk density (Bradford, 1986), and may impair normal root growth. Dulohery et al. (1996) also indicated that the CO 2 evolution as a measure of reduced biological activity declined by an average of 34% in soils compacted by skidder used during harvesting. Compaction also reduced the concentrations of Ca, Mg and K in the available water (Fig. 2). These results were consistent with Zabowski et al. (1996) who investigated the effects of soil OM removal and compaction in clay soil under radiata pine in New Zealand. This means that the plants may suffer deficiencies for these essential elements. Zabowski et al. (1996) also indicated that the low amounts of Ca, K (and Mg) in the soil solution were undersaturated with respect to many clay minerals. The implication of these results is enhanced breakdown of clay minerals in the compacted soils. However, the actual breakdown of the minerals in soils is the totality of the interaction of the biological and geochemical processes. If the biological activity is reduced with compaction, the weathering will be likely reduced and the supply of Ca and K to the plants will remain low until such time that the soil structure is optimal for biological activity. Structural restoration to pre-harvest conditions may take several years especially for soils that are high in clay. It is therefore, critical to minimize the compaction of clay soils during forestry operations. Summary and Conclusion The effects of forestry operations on long-term soil productivity were investigated by studying the changes in soil water retention characteristics (SWRC) and soil solution composition in the Ae horizon of selected soils of British Columbia (Canada). We used undisturbed cores from soils subjected to three levels of organic 4

5 matter removal and compaction treatments. The SWRC and soil solution were determined and extracted, respectively, using pressure plate/membrane apparatus. Our results showed that there was an increase in bulk density with soil compaction. The results also indicated that the available water (water held between -33 kpa and 1500 kpa) decreased by about % with soil compaction and organic matter removal treatments. The concentrations of Ca and K in the available water also decreased with soil compaction. Our results implied that soils subjected to compaction and organic matter removal during forestry operations may experience water and nutritional deficiencies. Plants may also encounter resistance to normal root growth due to increased bulk density. Our results stressed the need to minimize compaction and organic matter removal during forest operations to ensure long-term physical and chemical fertility Ca Mg K Na kpa Concentrations (mg L -1 ) No Comp + OM No Comp + no OM Heavy Comp + no OM kpa No Comp + OM No Comp + no OM Heavy Comp + no OM kpa 10 0 No Comp + OM No Comp + no OM Heavy Comp + no OM Treatments Fig. 2. Concentrations of Ca, Mg, K and Na in soil solution extracted at various potentials. 5

6 Acknowledgement I wish to acknowledge the Forest Renewal BC for providing the financial support, R. Carter for field and laboratory support, J. Craig for the ICP analyses of the soil solution and the LTSP research team of BC Ministry of Forests for providing me an access to their experimental plots. References Bradford, J.M p in Klute, A. et al. (ed) Agronomy No. 9 (Part 1). Soil Sci. Soc. Amer. Inc., Wisconsin, USA. Brais, S., C. Camiré, and D. Paré Impacts of whole-tree harvesting and winter windrowing on soil ph and base status of clayey sites of northwestern Quebec. Can. J. For. Res. 25: Busse, M.D., P.H. Cochran, and J.W. Barrett. Changes in ponderosa pine site productivity following removal of understory vegetation. Soil Sci. Soc. Am. J. 60: Casel, D.K. and D.R. Nielsen p in Klute, A. et al. (ed) Agronomy No. 9 (Part 1). Soil Sci. Soc. Amer. Inc., Wisconsin, USA. Dulohery, C.J., L.A. Morris and R. Lowrance Assessing forest soil disturbance through biogenic gas fluxes. Soil Sci. Soc. Am. J. 60: Hamlett, J.M., S.W. Melvin, and R. Horton Traffic and soil amendment effects on infiltration and compaction. Transactions of the ASAE. 33: Holcmb, R.W The long-term soil productivity study in British Columbia. FRDA report, ISSN ; 256. Huang, J., S.T. Lacey, and P.J. Ryan Impact of forest harvesting on the hydraulic properties of surface soil. Soil Sci. 161: Klute, A p in Klute, A. et al. (ed) Agronomy No. 9 (Part 1). Soil Sci. Soc. Amer. Inc., Wisconsin, USA. Mo, J., S. Brown, and M. Lenart Nutrient dynamics of a human-impacted pine forest in a MAB reserve of subtropical China. Biotropica. 27: Vogel, H.J. and U. Babel p in Ringrose-Voase, A.J. and G.S. Humphreys (ed) Dev. in Soil Sci. No. 22. Elsevier Science, B.V., Amsterdam. Worrell, R., and A. Hampson The influence of some forest operations on the sustainable management of forest soils-a review. Forestry. 70: Zabowski, D., P.T. Rygiewicz and M.F. Skinner Site disturbance on clay soil under a radiata pine. Plant and Soil 186: Keywords : soil moisture retention characteristics; compaction Mots clés : caractéristiques de rétention hydrique, compaction 6