Arboriculture & Urban Forestry 36(3): March 2010 hyperthermic Typic Hapluderts, and defined as a deep, moder- ately well drained, very slowly permeable soil type (Soil Sur- vey Staff 2004). The soil is very dark gray with medium-to-fine granular structure with a clay content about 60%, silt content about 34%, and organic matter of surface horizons from 2.5% to 4%. The upper soil horizon was acidic (6–6.5), but alkalin- ity increased uniformly with depth (7.8–8.2 at 1.5 m ) or more. Treatments were applied at the beginning of the study at concen- trations of 40, 80, and 120 g/L. The ten treatments were glucose (C6 (w/w) derived from corn (Zea mays L.) each at the three con- centrations, and a water control. Treatment concentrations were chosen based on previous research using soil carbohydrate ap- plications around trees (Schmidt et al. 2000; Percival et al. 2004; Martínez-Trinidad et al. 2009a). Treatments were randomly dis- tributed among the trees with three replicates per treatment. The solutions were applied as drenches within a 0.5 m radius around the trunk using 10 L per tree on June 27, 2005. The volume of so- lution used was sufficient to saturate at least the top 15 cm of soil. Two soil cores (25 mm diameter × 150 mm long) were extracted and combined using a soil probe (AMS Inc., American Falls, ID) on the upper soil within a distance of 0.5 m from each trunk every week. Coarse and fine roots and macroscopic parts of plants were removed from the soil. Immediately after collection, samples were stored on ice and transported to the laboratory. Once in the labo- ratory, the samples were stored at 4°C (39.2°F) until processing (less than 24 hours). Soil samples for measuring soil respiration as CO2 H12 O6 ), starch (C6 H12 O6 )n evolution were collected from the nursery before treatment applications and then on a weekly basis for a period of six weeks. generally convenient, rapid, and accurate method for assessment of soil microbial activity (Anderson 1982; Bååth and Arnebrant 1994; Tate 2000). Microbial activity was estimated by measur- ing soil respiration using the alkali trap method (Anderson 1982). A subsample of 60 g of soil from each soil sample was placed into a glass jar. A small beaker with 3 mL of 1.0N NaOH was placed over the soil in the jar. Jar lids were tightened to avoid gas leakage and incubated at 27°C (80.6°F) for 24 hours. After incubation, two drops of phenolphthalein and 1 mL BaCl2 Estimate of Field Soil Respiration Previous research showed that measurement of CO2 (50% solution) were added to precipitate carbonates. Samples were then titrated with 1.0N M HCl, and CO2 repeating the same process in jars without soil. The amount of CO2 evolution was estimated based on the amount of HCl required in the titration (titration was obtained once the pink color of the solution disappeared). The amount of CO2 released by blank samples was estimated by evolved from soil was then determined following the cal- culations described by Anderson (1982). Average air tempera- ture in the field during the study period was 23.3°C (73.9°F) [34.8°C (94.5°F) maximum and 19°C (66.2°F) minimum], while the average soil temperature in the top 15 cm was 25°C (77°F). To determine the impact of temperature and season on res- piration rates, additional samples were collected at another site within the same nursery with similar soil conditions during the winter of 2006 (treatment application on January 30, 2006) and compared to the results obtained from summer 2005 following the same method previously described. The average winter tem- perature during the study period was 17.6°C (63.6°F) [23.4°C evolution is a , or a 50:50 glucose:starch mixture 67 (74.1°F) maximum and 11.8°C (53.2°F) minimum], and the average soil temperature in the top 15 cm was 16°C (60.8°F). Estimate of Laboratory Soil Respiration To study soil respiration under controlled temperature condi- tions, another experiment was performed using 30 soil samples collected from other trees at the nursery and kept separated un- der laboratory conditions during the summer period. As in the prior two experiments, samples were collected from the top 15 cm of soil at the nursery and placed in open small plastic con- tainers 18 cm long × 12 cm wide × 7 cm deep (Rubbermaid® ; Wooster, OH). The soil samples were transported to the labora- tory and kept in an incubator at 27°C (80.6°F) under dark con- ditions throughout the assay. The same carbohydrate concentra- tions as those used in the field experiment at proportional solution amounts (200 mL) were applied at the beginning of the experi- ment. Soil moisture was kept constant (25%) throughout the ex- periment by weighing the container every 48 hours and adding distilled water as needed. Soil respiration was measured weekly for nine weeks using the methodology previously described. Data Analysis Treatments were applied in a completely randomized de- sign with three replicates and including time in the analy- sis. The data were analyzed using type III sums of squares in the GLM procedure using SPSS statistical software v. 13 (SPSS Inc., Chicago, IL). When a significant (P < 0.05) treat- ment × time interaction was detected, the treatment means were compared using Dunnett’s one-tailed t-test for differ- ences from the appropriate control over time at significance level of 0.05. The normality assumptions were met by the data. RESULTS AND DISCUSSION Field Studies of Soil Respiration A significant interaction (P < 0.001) between treatment and time was found for soil respiration during the summer. Respiration rates significantly (P < 0.05) increased one week after applica- tion for all glucose concentrations (Figure 1a). Higher rates of respiration recorded soon after glucose applications suggest that the substrate was easily utilized by microbes (Schmidt et al. 1997a). The increase in respiration rates for the 120 g/L glucose concentration remained high until the third week after treatments under field conditions, while the effect by the lower glucose con- centrations (40 and 80 g/L) lasted only until the second week (Figure 1a). Respiration rates after glucose treatments had simi- lar short-term increases as those reported in previous research using soil amended with glucose (Oades and Wagner 1971; Wu et al. 1993; Jonasson et al. 1996; Illeris and Jonasson 1999). When starch was applied at 40 and 80 g/L, there was a sig- nificant increase (P < 0.05) in respiration during the second week after treatments (Figure 1b). The highest concentration (120 g/L) of starch did not significantly increase respiration until the fourth and fifth weeks. These results may have been caused by the high amounts of applied starch, which altered the carbon/nitrogen ratio in the soil, or by a low amount of starch- degrading enzymes present in the soil (Wagner and Wolf 2005). ©2010 International Society of Arboriculture
March 2010
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