Rates and dynamics of decomposition in drylandsAngela Duerr, Eva Levi, Steven Archer, and Katharine Predick

School of Natural Resources and the Environment, University of Arizona, Tucson, AZ 85721

HypothesisSoil deposition onto litter (by wind and water) mediates decomposition by promoting

microbial processes and simultaneously shielding litter from UV photodegradation. As a

result, rates of decomposition are a function of rates of soil deposition; which, in turn, are a

function of vegetation structure.

Objectives:

•Develop a mechanistic understanding of the processes by which soil deposition and UV

exposure interact to determine leaf litter decomposition rates.

•Gain insight into processes that affect soil organic matter formation and nutrient turnover by

explicitly linking decomposition (ecosystem science) and erosion (earth science) in a novel

framework.

•Quantify soil deposition effects on microbial activity and litter decomposition including soil

deposition x UV interactions (controlled environment and field experiments).

Materials and Methods• A total of 2160 litter chambers (Fig. 2) will be deployed at the UofA Campus Agricultural Center.

• Soil and litter (Lehmann’s Lovegrass and Mesquite leaflets) will be collected at the Santa Rita Experimental

Range field site.

• Mesquite litter will be collected by stripping senesced leaflets and grass litter will be clipped in uniform lengths

to maintain a consistent leaf:culm ratio.

• Soils (0-2 cm depth) will be collected from the intercanopy locations from >10 locations/study, homogenized,

and passed through a 0.9 mm sieve.

• Litter chambers will contain a varying amount of soil/matrix composition as shown in Fig. 3 and will be exposed

to 3 UV treatments. (Fig. 4.)

• The litter chambers will be collected at 0, 1, 3, 6, 12, and 24 months.

• Processing and analyses will include

1. Mass loss Literary Citations

2. Microbial analysis (PFLA) phospholipd fatty acid analysis testing Austin, A.T. and L. Vivanco. 2006. Plant litter decomposition in a semi-arid ecosystem controlled by photodegradation. Nature 442:555-558.

3. Litter Chemistry (Nutrient C & N analysis) Olson, J.S. 1963. Energy storage and the balance of producers and decomposers in ecological systems. Ecology 44:322-331.

Figure 1. Comparison of conventional

view and extended view of the

decomposition process in drylands

Hypothesized Results

Water availability has long been assumed to be an important driver of decomposition. While many studies have shown that

water drives decomposition in mesic systems, few studies have examined decomposition in drylands. Our expected results are

derived from recent research that has been conducted to determine the driving forces of decomposition in drylands.

Austin and Vivanco (2006) found that UV was an important driver of decomposition in drylands, challenging traditional

perceptions about water as a driving force. They showed that decomposition rates were decreased when UV was blocked.

Building on those results, Throop and Archer (2007) found that soil deposition was an important interacting factor in

determining decomposition rates. UV had a strong influence on decomposition rates on litter with little or no soil but when soil

coverage became too heavy the effects of UV was significantly declined. (Figure 6)

Therefore, we expect that both UV and soil coverage will determine rates of soil decomposition. In particular, we expect that

UV will be the major driver on decomposition rates on samples with full UV and either moderate or no soil coverage and will

decrease as a significant influence on samples with heavy soil coverage.

Implications Understanding the rates of and controls over litter decomposition is a central focus of ecosystem science

because of the importance of litter decomposition on nutrient availability and soil C pools. This research will

provide a new, mechanistic basis for understanding and elucidating linkages between decomposition and

erosion in the context of solar UV radiation. This, in turn, will provide information for improving

decomposition models in dryland systems, thus enhancing capabilities for predicting rates and dynamics of

biogeochemical cycles. Our emphasis on driver interactions will yield new insights into processes that affect

soil organic matter formation and nutrient turnover by explicitly linking decomposition and erosion.

AcknowledgementsThis project is funded by a grant from NSF and their support is greatly appreciated. We would also like to acknowledge

the AZStart program and the Science Foundation Arizona for providing additional opportunities and support . We

would like to extend our appreciation for all of the student workers who have provided their continued support

throughout this project. A special acknowledgement to the Ninja Fairies for their help with poster design.

IntroductionDecomposition, the breakdown of dead plant and animal material, is a fundamental

ecosystem process that affects long-term soil fertility and carbon storage. Most of what is

known about decomposition is from studies in high rainfall areas, but this knowledge does

not translate well to dryland ecosystems. Recent studies have suggested the UV exposure is

the driving force for decomposition in drylands however, other studies suggest soil

deposition and litter mixing to be a key factor. A series of long-term (2 year) field and

laboratory experiments will be conducted to develop a mechanistic understanding of the

processes by which soil deposition and UV exposure interact to determine leaf litter

decomposition rates.

Figure 6. Decomposition of mesquite leaflets

(represented by K, the decay constant; Olson 1963) was

strongly and positively correlated with soil deposition

into litterbags (as indicated by % ash, a highly

conservative estimate of deposition; Throop & Archer

2007).

UV Treatment

Figure 4. UV Treatments

Aclar = allows UV-B+UV-A

Llumar = blocks UV-B+UV-A

Polyester = blocks UV-B and allows UV-A

Santa Rita Experimental Range

2,160 Samples Deployed

Soil/Matrix

Composition

Figure 2 Figure 3

Figure 5. Effects of UV-B radiation and soil coverage on

mass loss (mean + 1 SE; n=5-6) of mesquite leaflets after 32

weeks of exposure in a growth chamber. Approximate

percent of leaflet area covered by sterilized, air-dry soil in the

no, moderate and heavy soil treatments were 0, 10-20 and 80-

90%, respectively. UV-B exposures simulated ambient clear-

sky summer daily doses for the SRER (PIs unpublished data).