Initial Notes
There is some disagreement between the data and metadata in this
package and what has been published in the literature. Metadata in
this package states that litterbag deployment occurred in April
2010, and bags were collected at intervals for 30 months after that.
The publication by D. B. Hewins et. al (2013) states that litterbag
deployment occurred in April 2008 and litterbags were recovered over
12 months. It is likely that this data package includes the data in
this publication, plus additional litterbag measurements collected
as a continuation of the same study.
Additionally, this data appears to be "aggregated" such
that resolving the effects of vegetation removal treatments is
impossible.
Deployment and collection dates from prior metadata
Month interval from time of sample installation in field: 0, 1, 6,
12, 24, 30
0 = Apr 2010
1 = May 2010
6 = Oct 2010
12 = Apr 2011
24 = Apr 2012
30 = Oct 2012
Published methods
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°C for 48 h. 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°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. Transect
fetch lengths of 55, 75 and 95 m were established in response
subplots (Figure 1). Litterbags were spaced at distances
approximating the average interplant gap distance (range = 92-892
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.
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°C for 48 h, lyophilized for 48 h, 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°C for 6 h 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). A large proportion of soil that infiltrates
litterbags covers or mixes with litter, but does not adhere to
litter surface. The mass of these ‘bulk’ soils entering or exiting
litterbags is responsive to wind and water transport processes and
is thus likely highly dynamic relative to that of the soil-litter
films that form on litter surfaces. Quantifying the magnitude and
dynamics of this ‘bulk’ component of the soil-litter matrix was
beyond the scope of this study.
Correction factors are 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.
References
Hewins, Daniel B., Steven R. Archer, Gregory S. Okin, Rebecca L.
McCulley, and Heather L. Throop. "Soil–litter mixing
accelerates decomposition in a Chihuahuan desert grassland."
Ecosystems 16, no. 2 (2013): 183-195.
Throop, Heather L., and Steven R. Archer. "Interrelationships
among shrub encroachment, land management, and litter decomposition
in a semidesert grassland." Ecological Applications 17, no. 6
(2007): 1809-1823.
