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Soil microbial enzyme activity and nutrient availability in response to green tree retention harvesting in Coastal British Columbia Daradick, Shannon Pearl

Abstract

Green Tree Retention (GTR) was evaluated for its potential to retain soil microbial activity and nutrient availability after harvesting in the Coastal Western Hemlock biogeoclimatic zone of B.C., Canada. Soil samples were collected from four sizes (5, 10, 20, and 40 m diameter) of GTR patch at the centre, edge, and along a northerly transect to 30 m beyond the groups of live trees prior to and a few months after harvest. PRS™ Probes were used to determine the availability of nutrients; total nitrogen, nitrate (NO₃ ̄), ammonium (NH₄⁺) and phosphate (PO₄³ ̄), encountered by plant roots before and after harvest. Before harvest, total nitrogen, NO₃ ̄, and NH₄⁺ availability was similar in the organic layer and mineral layers. Phosphate availability was significantly higher in the organic layer than in the mineral layer before harvest. After harvest, nitrogen levels increased in both soil layers with NO₃ ̄ levels significantly elevated in the mineral layer and NH₄⁺ levels significantly elevated in the organic layer. There was no significant change in PO₄³ ̄after harvest. Nutrient availabilities after harvest varied little along the transects of the different sizes of retention patches. Increased availability of total nitrogen, NH₄⁺, and PO₄³ ̄was more noticeable in the smallest (5 m in diameter ) patch size when compared to the larger patch sizes (10 m, 20 m, and 40 m in diameter) after harvest. The activities of five soil enzymes important in carbon, nitrogen and phosphorus cycling - β-glucosidase, chitinase, phosphatase, phenol oxidase and peroxidase - were measured using colorimetric or fluorimetric substrates and a microplate technique. Before harvest, hydrolytic enzyme activity (β-glucosidase, chitinase, and phosphatase) was higher in the organic layer than in the mineral layer. After harvest, hydrolytic enzyme activity was still higher in the organic layer than in the mineral layer, although glucosidase activity decreased in the organic layer and increased in mineral soil after harvest, and chitinase activity decreased in the organic layer after harvest. Changes in glucosidase and chitinase activity (decrease in organic soil activity and increase in mineral soil activity) were more noticeable in the smallest (5 m in diameter ) patch size when compared to the larger patch sizes (10 m, 20 m, and 40 m in diameter) after harvest. Phosphatase activity was significantly lower in the 5 m patch size after harvest and showed a trend of declining activity with increasing distance from the GTR patches after harvest in the larger retention patches. Before harvest, oxidative enzyme activity (phenol oxidase and peroxidase) was higher in the mineral layer than in the organic layer. After harvest, oxidative enzyme activity was still higher in the mineral layer than in the organic layer, although phenol oxidase activity increased significantly in mineral soil after harvest, and peroxidase activity increased significantly in both organic and mineral soil after harvest. The stimulation of the lignin-degrading oxidative enzymes following harvest may have been caused by lignin-rich woody substrate from slash left on site. The increase in phenol oxidase and peroxidase activity after harvest was more noticeable in the smallest (5 m in diameter ) patch size when compared to the larger patch sizes (10 m, 20 m, and 40 m in diameter) after harvest. The change in enzyme activity and nutrient availability in response to harvest was greatest in 5 m retention patches for total nitrogen, NH₄⁺, PO₄³ ̄,β-glucosidase, chitinase, phosphatase, phenol oxidase and peroxidase, suggesting that a minimum diameter of 10 m for GTR plots may be useful to retain soil microbial activity and nutrient availability after harvest.

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