Methodology
Litterbags (10 x 10 cm) were constructed using UV-resistant fiberglass window screen (0.8 x 1.0 mm openings; New York Wire Company, Mount Wolf, PA, USA) to ensure litterbag longevity under field conditions. Naturally senescing honey mesquite (P. glandulosa) litter was collected on 19 October 2007 at the JRN and ‘air dried’ at 30 degrees C for 48 hours. Drying at this temperature should not affect litter chemistry, as leaves experienced greater temperatures during the growing season. Litterbags were filled with 2 g of leaflets; this mass filled litterbags with minimal leaflet overlap. For every 10 litterbags filled, a 2 g sample was dried at 60 degrees C to establish a wet-dry mass relationship.
Litterbags were deployed on 19 and 20 April 2008, a time corresponding to the annual peak in mean monthly wind speed (Wainwright 2006). Litterbags were placed along transect lines at locations of 5, 25, and 45 m downwind from the upwind edge (hereafter ‘fetch length’) of removal subplot borders. Transects at fetch lengths of 55, 75 and 95 m were established in response subplots (Fig. 2.1). Litterbags were spaced at distances approximating the average interplant gap distance (range = 92 to 892 mm, depending on the subplot) and were fixed to the soil surface with 10 cm long steel staples. To avoid wake effects on soil transport (Okin 2008), litterbag placements were adjusted as needed to ensure that no bags were within 5 m of an upwind shrub. One litterbag from each fetch length in each subplot was randomly designated for collection at 0, 1, 3, 6, and 12 months post-deployment.
2.2.3 Laboratory Processing and Analyses
Litterbag contents (litter + accumulated soil) were separated using a 1 mm mesh sieve. Litter was then manually dusted using small brushes to remove additional soil from leaflets. The brushed litter was frozen at -80 degrees C for 48 hours, lyophilized for 48 hours, weighed, and then ground to a fine powder using a ball mill (8000D Mixer/Mill, Spex Certiprep, Metuchen, NJ, USA). Subsamples of litter were combusted at 550 degrees C for 6 hours to determine the inorganic matter content (% ash). Mass loss and litter C and N content (elemental analyzer; ECS 4010, Costech Analytical Technologies, Valencia, CA, USA) are expressed on an ash-free basis. The % ash was also used as a conservative index of soil accumulation that accounts only for soil adhering to litter surfaces after sieving and brushing (see Throop and Archer 2007).
Correction factors were used when calculating mass remaining (estimates of: transport loss, litter water content, initial ash, etc.,), so at times, especially at the 0 month collection, the % mass remaining may be a few % greater or less than 100%. Once decomposition begins in earnest these become negligible.
For more details see:
Hewins, D. B., S. R. Archer, G. S. Okin, R. L. McCulley, and H. L. Throop. 2013. Soil-litter mixing accelerates decomposition in a Chihuahuan Desert grassland. Ecosystems 16:183-195
