Ashes to Ashes, Dust to Dust: Decomposition in the Desert

Authors Aguillon, Stepfanie

Rights Copyright © is held by the author. 

Download date 09/09/2021 21:40:03

Link to Item http://hdl.handle.net/10150/197969

Ashes to Ashes, Dust to Dust: Decomposition in the Desert Stepfanie Aguillon

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

NSF Research Experience for Undergraduates Scholar

Introduction

Objectives

Study Site

Methods

Conclusions

Results

• Decomposition, the process of breaking down organic material into its physical and chemical

constituents, is a key determinant of soil fertility and an important component in the cycling

of carbon and nutrients through an ecosystem.

• Climatic variables predict decomposition rates in mesic systems reasonably well, but

underestimate the actual amount of decomposition that occurs in drylands. Photodegradation

has been proposed as an important, but overlooked, factor in this discrepancy.

• The importance of solar ultraviolet (UV) radiation in litter degradation is uncertain, some

research shows strong UV effects and others show no UV effects.

• Most decomposition studies are conducted over relatively long timescales (months to years)

and may therefore miss the finer details of mass losses during the first few weeks.

• Quantify the influence of radiant energy exposure (full sun vs. shade) and UV radiation on the

decomposition of Prosopis velutina leaf litter.

• Characterize the rates and dynamics of litter mass loss during the initial stages (first 4 weeks)

of decomposition under field conditions.

• The study was conducted on the Santa Rita

Experimental Range (SRER), a 21,513 ha shrub

savanna site in SE Arizona (~80 km south of

Tucson, AZ).

• Mean annual precipitation ranges from 275 to

400 mm (depending on elevation) and is equally

delivered as summer monsoons and winter rains.

• Litterbags were deployed in the mesquite-

grassland zone at 1,148 m elevation.

Litterbags

• 10 × 10 cm; deployed on 13 July 2011

• Constructed with 20 × 20 mesh (~0.9 mm openings) fiberglass window screen back and a

plastic front; filled with ~1.3 g of dried P. velutina leaflets

• Edges stapled; plastic front perforated (twenty-four 1.6 mm diameter holes) to allow for the

exchange of air and moisture

Sample Manipulation

• UV Environment –

• Aclar – control, UV transparent plastic (= UV present = UV+)

• Mylar – UV-B absorbing plastic (= UV absent = UV–)

• Level of Radiant Energy Exposure –

• Intercanopy Zone – bare ground between shrubs receiving full sun

• Subcanopy Zone – shaded area beneath a P. velutina canopy

• 25 litterbags in each of the experimental treatment combinations: UV+/intercanopy,

UV+/subcanopy, UV–/intercanopy, and UV–/subcanopy

• 5 litterbags from each combination collected at weeks 0, 1, 2, 3, and 4

• Soil-surface temperatures (in litterbags and on bare ground), levels of photosynthetically active

radiation, precipitation, and soil moisture were monitored during the 4-week study

Intercanopy Zone Placement Subcanopy Zone Placement

• Near-surface soil moisture

was statistically comparable

in intercanopy and

subcanopy placements

(Fig. 2; p = 0.7817)

• Litter decomposition rates in

intercanopy zones were

significantly higher than

those in subcanopy zones

(Fig. 2; p = 0.0303)

• Manipulation of UV

radiation had no significant

effect on decomposition

rates (Fig. 2; p = 0.8012)

• Temperatures in UV+ and

UV– litterbags were

statistically comparable

(p = 0.6544)

• Soil temperature (Fig. 1)

and photosynthetically

active radiation (PAR) was

greater in the intercanopy

zone (mean noon PAR =

1,816 μmol m-2 s-1) than in

the subcanopy zone

(PAR = 597 μmol m-2 s-1;

p < 0.0001)

Figure 2 – Mean ( ±1 S.E; n=5) ash-corrected mass loss

(scaled so time 0 values start at 100% and represent

initial litter mass). Numerical values associated with

each harvest date indicate mean (± S.E.) soil moisture

in intercanopy and subcanopy placements. Bars

represent the daily precipitation.

60

65

70

75

80

85

90

95

Perc

ent

ash

-co

rrec

ted

dry

mas

s re

mai

nin

g

100

0

5

10

15

20

25

30

35

40

Pre

cip

itat

ion

(m

m)

2.4±0.004

2.8±0.003

2.8±0.001

2.6±0.002 4.1±0.004

4.6±0.007

1.1±0.0009

0.8±0.0003

7/13/2011 7/17/2011 7/22/2011 7/27/2011 8/1/2011 8/6/2011 8/11/2011

23

25

27

29

31

33

35

37

Tem

per

atu

re (°C

)

7/13/2011 7/20/2011 7/27/2011 8/3/2011 8/10/2011

Figure 1 – Average daily temperature (monitored hourly)

on bare soil in intercanopy and subcanopy zones.

• Traditional decomposition studies quantify decomposition rates over months to years.

However, rapid and extensive mass losses occurred during the first four weeks of this field

trial. Future decomposition studies should thus include short- as well as long-term dynamics

of litter mass loss.

• Rates of litter mass loss were neither gradual nor linear, but rather were erratic and in

response to pulses of rainfall input.

• Levels of radiant energy interacted with precipitation to influence rates of decomposition via

temperature effects. Contrary to expectations, levels of UV radiation had no effect on short-

term mass loss.

Katharine Predick assisted with project logistics, data analysis, and litterbag collection; Eva Levi assisted with

project logistics and helped deploy and collect litterbags; Dean Keller assisted with the construction, deployment,

and collection of litterbags; Steven Archer provided guidance on experimental design, transportation to the field

site, and access to laboratory facilities and sampling equipment in the Savanna/Woodland Ecology Laboratory.

Acknowledgements