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Soil organic carbon (SOC) is derived primarily from the decomposition of plant biomass. Animals that create greenfall, or green leaf litter, influence SOC dynamics by altering the phenological condition and, therefore, nutrient quality of plant litter. Animals that transport greenfall to a microsite with different microhabitat conditions from those where senesced litter would typically be found also influence SOC dynamics by altering the prevalence of various decomposition drivers. Microsite differences are particularly pronounced in arid and semi-arid ecosystems with heterogeneous vegetation cover. We investigated differences in decomposition between greenfall and senesced litter of three common Chihuahuan Desert plants from which animals frequently generate greenfall (Larrea tridentata, Sporobolus flexuosus, Yucca elata), using a litterbag study to quantify differences in mass, carbon (C), and nitrogen (N) losses between green and senesced leaves placed in shrub intercanopy and subcanopy microsites in a desert shrubland. We hypothesized that decomposition would be more rapid in 1) greenfall than naturally senesced litter, because of the higher nutrient concentration in green than senesced leaves, and 2) in intercanopy than shrub subcanopy microsites, because of increased exposure to decomposition drivers like soil-litter mixing and photodegradation in the less vegetated open area between shrub canopies. Using a litterbag study, we quantified differences in litter mass, C, and N losses between green and senesced leaves placed in shrub subcanopy and intercanopy (open) microsites. Measured variables include litter mass and litter ash, carbon, and nitrogen content. We found that there were significant differences in the nutrient concentration of green and senesced leaves of the same species, and that both litter condition and microsite affected decomposition rate. For two of the three litter species, greenfall decomposed more rapidly than senesced litter, and for all three species litter in intercanopy microsites decomposed more rapidly than in subcanopy microsites. Our results support that the creation and translocation of greenfall by animals is an important mechanism regulating the speed of decomposition and the transfer of C and nutrients from plant biomass into the soil.
At each of the three study sites (Larrea tridentata = creosotebush, Sporobolus flexuosus = mesa dropseed, Yucca eleata = soaptree yucca), eighteen experimental plots were designated: nine in shrub subcanopy microsites (‘shrub’) and nine open, shrub intercanopy microsites (‘open’). At the creosotebush study site, shrub plots were beneath creosote shrubs with a mean canopy diameter ± SE (longest axis x perpendicular axis) of 218 ± 11.1 cm x 196 ± 8.8 cm and height ± SE of 125 ± 7.1 cm. At the yucca and dropseed study sites, shrub plots were beneath mesquite shrubs with a mean canopy diameter ± SE of 210 ± 6.5 cm x 168 ± 5.9 cm and height ± SE of 85 ± 1.8 cm. Shrubs used for experimental plots were at least one canopy width away from neighboring shrubs. Open plots were designated in unvegetated microsites of bare soil, at least one shrub canopy width from the nearest shrub, in an area large enough that litterbags could be placed at least 15 cm apart from each other and the nearest herbaceous vegetation. Green and senesced leaf litter used in litterbags was collected at the Jornada Basin LTER near the study sites. Creosote litter was collected in March to May 2010. Recently senesced leaves were harvested by shaking creosotebush shrubs over a tarp to catch falling leaves. Yucca litter was collected in May 2010. Green leaves were cut from the top half of the foliar rosette in order to collect leaves that were fully expanded but still relatively young. Senesced leaves were removed from the uppermost layer of leaves from which all green coloration was absent. Dropseed green tillers were collected in October 2009, when the majority was flowering. Senesced grass tillers were collected in January 2010, when grasses had fully senesced. Litter was air dried at 30°C for at least 7 days and litter subsamples were oven dried at 60°C to determine air dry to oven dry mass conversions for each litter species and condition (green or senesced). Litter was sorted to remove damaged or diseased tissue and non-leaf material. Dropseed tiller portions with open inflorescences were discarded. Only yucca leaves with widths between 0.6 and 1.0 cm were used, and the terminal 8 cm of leaves was discarded. Remaining dropseed and yucca litter was cut into 8 cm long segments. Litterbags (10 x 10 cm^2) were constructed using fiberglass screen (New York Wire, Sun Guard 90 charcoal, 1 mm^2 mesh openings). Litterbags were filled with the air-dried litter mass equivalent of approximately 3 g of oven-dried litter. Six litterbags of each litter condition (green and senesced) were placed in every shrub and open plot; enough to collect one litterbag at 0, 1, 3, 6, 12, and 24 months (n = 3 sites x 9 plots x 2 microsites x 6 litterbags x 2 litter conditions = 648 litterbags). Litterbags in each plot were placed in a randomized order and secured to the ground with landscaping staples. Litterbags were deployed September 15-17, 2010. One randomly determined litterbag of each litter condition was retrieved from each plot immediately after placement to serve for the month 0 collection, and then another after 1, 3, 6, 12, and 24 months of being in the field. Following retrieval, litterbags were oven dried at 60°C for at least five days. Dry litter was removed from litterbags, lightly brushed to remove soil from the litter surface, and weighed to determine litter mass remaining. Litter was homogenized by being ground into a fine powder using a ball mill (8000D Mixer/Mill, Spex Certiprep, Metuchen, NJ, USA) and analyzed for total %C and %N (ECS 4010, Costech Analytical, Valencia, CA, USA). Litter %C and %N was corrected for NIST (apple leaf) standards. Litter subsamples were combusted (550°C for 4 h) to determine litter inorganic matter content (%ash).