Enzymatic activities associated with decomposition of particulate organic matter in two shallow ponds
S. Alvarez, M.C. Guerrero
Ecology Department, Universidad Autonoma de Madrid, 28049 Madrid, Spain Accepted 4 May 2000
Abstract
Particulate organic matter POM is an important source of energy in aquatic systems, where decomposition rates have traditionally been studied using the litter-bag technique. However, this method has limitations for estimating in situ decomposition rates, especially for fine
particles. In this study the litter-bag technique is combined with measurements of microbial enzyme activities to estimate POM mass loss rates in situ. Litter-bags containing benthic POM in two size ranges, coarse .1 mm and fine 0.063–0.5 mm, were placed in two
contrasting ponds Toro and Oro located in the Don˜ana area in southern Spain. Litter-bags were collected over a period of a year and analysed for mass loss and the activities of five extracellular enzymes b-glucosidase, b-xylosidase, b-N-acetylglucosamidase, phenol
oxidase, alkaline phosphatase. In situ sediment POM samples were also collected, sorted in the same size ranges, and assayed for the same enzyme activities. Using separate regression models relating mass loss to average cumulative enzyme activity, the mean turnover times
for in situ coarse particulate organic matter CPOM and fine particulate organic matter FPOM were 149 and 2778 days in Toro pond and 382 and 4347 days in Oro pond, respectively. This compared with turnover times of 578 and 2109 days for Toro and 778 and 5296 days in
Oro pond for POM confined in bags. Integrating enzyme measurements and litter-bag techniques provides in situ organic matter turnover rates and may allow the assessment of global controlling factors e.g. flooding regime, temperature and POM quality in the decomposition
process in temporally and spatially heterogeneous environments. q 2000 Elsevier Science Ltd. All rights reserved.
Keywords : Decomposition; Enzyme activities; Particulate organic matter; Enzyme decomposition models; Don˜ana
1. Introduction
Decomposition is a fundamental ecosystem process comparable in importance to primary production Moorhead
et al., 1996. It is also a composite phenomenon that can be viewed at several levels of resolution Swift et al., 1979. At
all levels, microbial decomposition plays a critical role in the macronutrient cycles and energy flows of aquatic
ecosystems Wetzel, 1992 but it has been difficult to link microbiological
variables with
ecosystem processes
Parnas, 1975; Howard and Howard, 1993. Enzyme assays, used primarily to collect descriptive
information about soils, have become useful techniques for monitoring microbial activity and uncovering the
mechanisms that underlie microbial processes Sinsabaugh et al., 2000. Recently, new types of models have been
developed to examine organic matter processing using the activities of extracellular enzymes. These models have been
used in the analysis of dissolved organic materials in aquatic environments Chro´st, 1991, mass loss of plant litter in
terrestrial systems Sinsabaugh et al., 1992; Sinsabaugh and Moorhead, 1994, processing of particulate and
dissolved organic carbon POC, DOC in streams Sinsa- baugh et al., 1994a; Sinsabaugh and Findlay, 1995; Findlay
et al., 1998 and processing of particulate organic matter POM in wetlands Jackson et al., 1995.
Non-living natural organic matter NOM comprises in most aquatic environments the largest organic matter frac-
tion and can be subdivided into dissolved organic matter DOM and POM Thurman, 1985. POM processing has
traditionally been studied using the litter-bag technique, which has some inherent problems Boulton and Boon,
1991, both for large-mesh and, especially, for fine-mesh bags, where the mesh can act as a filter restricting exchanges
with the surrounding system.
The decomposition of POM into soluble compounds is mediated by extracellular enzymes Burns, 1982; Chro´st,
1991. As microbial breakdown of detritus is directly linked to the activity of these enzymes, statistical models that
link mass loss rates of decomposing litter to the activities of selected microbial enzymes have been
Soil Biology Biochemistry 32 2000 1941–1951
0038-071700 - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 0 7 1 7 0 0 0 0 1 7 0 - X
www.elsevier.comlocatesoilbio
Corresponding author. Fax: 134-91-397-8001. E-mail address:
sergio.alvarezuam.es S. Alvarez.
developed Sinsabaugh et al., 1992, 1994b, 2000. These models may help circumvent some of the methodological
constraints that have hindered studies of in situ decomposi- tion rates, especially for fine particles.
The purpose of this study was to investigate POM proces- sing in two contrasting ponds. Enzymatic decomposition
models EDM were applied to obtain estimates of in situ processing rates. These models further our understanding of
the decomposition process at the enzyme level, and help document the extent to which the process varies among
systems. At the same time, the study provides some of the first results about decomposition processes at the microbial
community level in aquatic systems in Spain, under contrasting flooding regimes and weather conditions.
2. Materials and methods
