232 Biosciences). Mulch thermal conductivity (km) was calculated according to km = Gm*dTsoil − Tsurface [1] where Gm was the measured heat flux through the mulch (W/m2), Tsoil was the temperature of the soil surface below the mulch, Tsurface was the mulch surface temperature, and d was mulch layer thickness (m). Conductive heat transfer (C) through each mulch type was then calculated according to Fourier’s law of conductive heat transfer using temperature data recorded in Experiment 1 during August 2004 and July 2005 according to C =−km*Tsoil − Tsurfaced [2] where km was the thermal conductivity of the mulch (W/m°C), Tsoil was the temperature of the soil surface below the mulch, Tsurface was the mulch surface temperature, and d was mulch layer thickness (m). For surface albedo, total irradiance and reflected all wave- length radiation (mV DC) were recorded in the summers of 2004 and 2005 with a LI-200S pyranometer (Li-Cor Biosciences) at- tached to a stick (approximately 1 m [3.3 ft] length) at mid- day (approximately 1400 HR) at the center of each of the land- scape plots described previously in Experiment 1. Below the pyranometer, a toothpick attached to the stick was used to de- termine the incident sun angle. Mulch surface albedo was cal- culated as the ratio of incoming all-wave radiation to reflected all-wave radiation. Experiment 3 A third experiment was conducted for 22 days (31 May to 21 June 2005) at an open sky, graded field site on the Polytechnic campus of Arizona State University (Mesa, AZ) to determine temporal patterns of evaporative water loss and moisture content of soil covered by the PPR, LTT, or DG mulches previously described. Bare soil without a mulch cover was used as a fourth control treatment. During the 22-day experiment, ambient air temperatures ranged from a minimum of 19°C (66°F) to a maxi- mum of 45°C (113°F) with a mean of 31°C (88°F); mean maxi- mum daytime irradiance was 1,203 W/m2 and there was no rainfall (local weather data courtesy of the Arizona State Uni- versity Polytechnic Photovoltaic Testing Laboratory, Mesa, AZ). Total potential evapotranspiration was 167 mm (6.68 in) (http:// ag.arizona.edu/azmet). Sixteen containers were constructed out of 30 cm (12 in) sections of 15 cm (6 in) diameter polyvinyl chloride (PVC) pipe with solid PVC bottoms. These 5.3 L (1.38 gal) PVC containers were constructed to determine temporal patterns of evaporative water loss and moisture content of mulch-covered soil. At the bottom of each container, nine, 5 mm (0.2 in) holes were drilled in a uniform pattern to allow for gravitational water loss and plastic mesh screens were inserted at the bottom of each con- tainer to prevent loss of soil. Each container was filled with either 7.5 kg (16.5 lb) (containers with mulch) or 9 kg (19.8 lb) (containers without mulch) of air-dried, sieved (5 mm [0.2 in] screen) and uniformly mixed soil (dry bulk density 1.57 g/cm3 [9.2 oz/in3]) from the research site described previously in Experiment 1. On top of the column of soil in each of the PVC containers was later placeda5cm(2in) layer of each mulch type as described subsequently except for the bare soil control containers. ©2008 International Society of Arboriculture Singer and Martin: Effect of Landscape Mulches Sixteen square plots (0.58 m2 [6.24 ft2]/plot) at an open field site that was graded and leveled were established in a four by four grid within a 100 m2 (1076 ft2) area in a randomized com- plete block design arrangement with four blocked replications. The 16 plots were equidistant from each other and separated by a 1 m (3.3 ft) buffer of bare soil. Around the perimeter of each plot was a wood border embedded in the soil such that 5 cm (2 in) of the wood border was above the plot surface grade. On 31 May 2005, the soil-filled PVC containers were placed verti- cally into the soil at the center of each plot so that the surface grade within each PVC container and surrounding plot was the same. Next,a5cm(2in) layer of one of the three surface mulch treatments was placed on the surface of each plot, including the surface of each PVC cylinder, except the bare soil control cyl- inders. The soil-filled PVC containers with or without surface mulch covers (n4) remained in place for 5 days to acclimatize to field conditions. After 5 days, the PVC containers were removed from the plots and transported to a nearby laboratory where 1.7 L (0.44 gal) (except 1.8 L [0.47 gal] in bare soil containers) of distilled water (25°C [70°F]) was added in small increments to each container. A preliminary experiment was conducted to determine the amount of distilled water needed to bring the air-dried soil in each container to field capacity with minimal gravitational water loss from the bottom of each container. The weight of each of the containers at field capacity was recorded after all water had stopped draining from the container bottom. The PVC containers were then repositioned into their respective field plot locations. At 48 hr intervals (0600 HR for 22 days), the PVC containers were removed from their plot locations and weighed. The change in container weight was assumed to be the result of soil evapo- rative water loss. After the measurements, the PVC containers were immediately repositioned into their respective field plot locations. Data Analysis An analysis of variance was calculated for each data set using a general linear model (JMP 5.0.1; SAS Institute Inc., Cary, NC). For Experiment 1, Type IV sums of squares was used because of unequal plot replication and means and standard errors of the mean were calculated for soil temperatures at 5 and 30 cm (2 and 12 in) depths, soil surface temperatures, and mulch surface tem- peratures, and integrated net radiation values by mulch treat- ment. If significant differences were found, treatment means were separated by Tukey’s multiple comparisons test at the level P0.05. Simple Pearson’s correlations of summer net radiation and pyranometer measurements were calculated. For Experiment 3, a one-way univariate model with surface mulch type as the independent variable with Type III sums of squares was used for statistical comparisons of evaporative soil water loss. RESULTS Experiment 1 Soil moisture content within each of the 14 mulch plots ranged from an average of 20% volumetric water content in March 2005 after an El Nino-enhanced 2004 to 2005 winter rainy season to an average of 6% volumetric water content during the hot, dry summer months and was not significantly affected by mulch cover type (data not shown). Over the course of two growing seasons, LTT, PPR, and DG mulches decreased in depth by 48%, 27%, and 19%, respectively.
July 2008
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