Understanding litter decomposition in drylands: Is litter abrasion an important abiotic factor?

Drylands comprise approximately 40% of the global land cover and about 20% of the global soil organic carbon (C) pool (Lal, 2004). Changes in dryland ecosystem processes, such as litter decomposition, could significantly influence global C cycling and climate change. Current litter decomposition models underestimate rates of decomposition in drylands, and little is known about the role of abiotic drivers and their interactions in these systems. Research suggests leaf abrasion could play an important role on litter decomposition in drylands by increasing microbial activity or leaching (Throop & Archer 2009). The Scrape Site at Jornada LTER was stripped of vegetation in 1991, leaving exposed soil that could serve as a source of sand particles to promote the leaf abrasion on mesquite shrubs located downwind from site. This project examines the role that leaf abrasion, as promoted in the field by wind erosion typical of the northern Chihuahuan Desert, and induced through laboratory simulations, will play in litter decomposition. We hypothesize that abrasion on mesquite leaves will increase the rate of litter decomposition because it will facilitate microbial colonization. Mesquite leaves were collected from two locations, downwind the Scrape Site as a “field abraded” treatment, and mesquite shrubs away from the Scrape Site as an “unabraded” treatment. Abrasion of mesquite leaves that microscopically simulated the characteristics of field abraded leaves was performed by shaking leaves for 30 seconds with <0.5mm sand particles to represent “lab abraded” treatment. Laboratory abraded leaves reported an average leaf area loss of 3.267 %. Litter treatments were incubated at 22°C for 16 weeks in airtight glass jars containing 50 g of soilwet to 60% of the soil water holding capacity to test for differences in litter decomposition. Rapid colonization by fungi was apparent in all litter treatments jars, but lab abraded litter showed the most abundant growth. Consequently, Lab abraded litter treatment showed  20% and 30% times more accumulation of CO2 -C than field (P= 0.0008) and unabraded (P< 0.0001) litter treatments during the first 6 days of incubation.. Results for litter mass remaining showed an exponential decrease in mass where the strongest treatment differences were detected during the first weeks of incubation followed by an equilibrium phase and a ca. 50% mass loss for all treatments. Lab abraded litter significantly loss 4.6% and 4.8% more litter mass than field (P= 0.0005) and unabraded (P= 0.0004) litter, respectively, after one week of incubation. Our results suggest that abrasion of leaf litter enhances early stages of decomposition perhaps by creating an exposed outer tissue suitable for microbial colonization. This is reflected by a higher initial accumulation of CO₂–C and losses of litter mass in lab abraded litter. Influence of abrasion in litter decomposition was more evident on lab abraded leaves perhaps due to the faster and easier microbial colonization of recently abraded surfaces. Work is in progress to evaluate the amount of C transferred to the soil and the implications of our results on longer-term decomposition.