Litter decomposition in the LTER
Network: gaps and bridges to synthesis
Organizers:
Grizelle González*, Whendee Silver and D. Jean Lodge (LUQ)
*Corresponding organizer (ggonzalez@fs.fed.us)
The main objectives of the
workshop were to present the "state" of decomposition research in the
LTER Network, identify the gaps in knowledge of data and study factors in sites
within the Network, and determine “bridge” questions in an effort to start the
process of synthesis given available data.
The workshop consisted of invited talks and discussion on synthesis and research
initiatives. Talks covered three main
factors affecting litter decay: climate (I. Burke, SGS), substrate quality (M.
Harmon, AND), and organisms (D. Coleman, CWT) and we considered modeling (W.J.
Parton, SGS; J. Moore, ARC/SGS ) as an integral component of the synthesis
effort (Fig. 1).
Participants represented 14
Rationale of the
workshop and its framework
The trilateral model of the controls of plant litter
decomposition sensu Swift et al.
(1979, Fig. 1) remains conceptually the most robust model of the factors
affecting decay. This is evident as the
model is almost always referenced in the first paragraphs of most studies
dealing with decomposition. The model
has been the zeal of current experimental studies that try to tease apart the
independent effects of the regulating factors.
In the quest of knowledge and advancement in the understanding of the
mechanisms of decay, some of the regulating factors have been studied more intensively
than others while some others have been greatly downplayed. Nonetheless, we can not determine and / or
quantify particular effects due to a given a single factor if we do not study
the complexity of the decomposition process in the context of the other driving
factors. Therefore, we recognize the
importance of integrating interdisciplinary fields (e.g., biology, ecology,
chemistry and modeling) in order to synthesize available data and strengthen
new research initiatives.
Highlights of
major points by topic
1) Use of LIDET Data
to Test Litter Decomposition Models
• Models work to test short-term changes in carbon
and nitrogen dynamics in litter decomposition as existing models are quite
similar (e.g., 5 carbon pools). However,
there are challenges and significant modeling uncertainties at particular sites
due to photodecomposition (JOR), N availability and organismal contributions (LUQ)
to decay.
• We need long-term
research data as base for long-term
model predictions.
• Process- and organism-oriented models incorporating soil
biological data to predict ecosystem functioning (Smith et al., 1988) are
needed. It is complicated to incorporate
faunal effects into mathematical models due: 1) confounding effects and 2)
animals are rate-limiting or rate-regulating factors in ecosystems; this is
importance may exceed their direct role in energy release (MacLean, 1974).
2) Climate and Decomposition: Decoupling temperature from
moisture effects
• Litter may not be the only or most important pool for
decomposition. In the short grass
steppe, soil C can be the most important pool (>60% of the ecosystem C is in
stable soil organic matter).
• We do not necessarily understand how temperature
influences decomposition and the stabilization of soil organic matter; are we
sure litter decomposition is what matters?
• It has been stated that climate is the dominant factor in
areas subject to unfavorable (dry, cold) weather conditions, whereas litter
quality is the dominant factor under favorable (wet, warm) conditions (Coûteaux
et al., 1995). Should we assume the
biota are completely constrained by the physicochemical environment and
substrate quality characteristics? Are we limiting predictability to lower
hierarchical levels (from global to local scales)?
3) Advances in chemical aspects
• Current wet chemistry methods do not provide a real
insight into the composition of the decomposed litter material as they are imprecise
and can have poor replicability. Other
methods like: Infra red Spectra, 13C-NMR, Microcalorimetry and Pyrolisis Mass
Spectra have other pitfalls like high cost, inadequate software or need of
trained staff for the analyses of litter and soil samples.
• Soil NMR database needs funding.
4) Soil organisms
In an attempt to determine whether we have data sets to
match or address questions related to the role of soil fauna in decomposition
studies a master list of question was presented. Some of those questions were: How do fauna interact across gradients of temperature
and moisture? What is the effect of
fauna on the comminuting vs. inoculation of litter substrates? What
biogeochemical and faunal changes will occur at high altitude and latitude sites
due to global change? Is there a relationship between soil biodiversity and
organic matter dynamics? Is the relation
confined to more labile fractions, or are more resistant ones significantly
affected? How do invasive species
impact decomposition and mineralization processes and what will be the
feedbacks of this activity on biotic composition? (Why just limit it to
decomposers?).
5) Modeling decay from an organismal
perspective: Impacts of NPP and disturbance
• Disturbance and changes in NPP
alter trophic interactions (community perspective).
• Pattern of material flow are affected by disturbance (ecosystem perspective).
• Changes in trophic
interactions and patterns of material flow affect dynamic stability.
Proposed research
initiatives
• Use
LIDET Data to 1) test long-term carbon and nitrogen dynamics of litter
decay models, and 2) to support a proposal to further study the differential
effect of climate and substrate quality at later stages of decay using new
chemistry techniques to evaluate decay (including an organismal component?).
• Study the effects of
temperature on all components of “decomposition”: CO2 evolution, stabilization
and SOM decomposition.
• Separating climate
effects into spatial and temporal components.
• A long-term
cross-site experiment (low intensity) to tease out interactions related to N deposition, carbon fractions, and faunal
effects. Could this experiment be nested
within larger temperature and moisture manipulations?
• Develop databases with food
web descriptions for all LTER Sites.
Future directions
Recommendations towards
synthesis included updating and accounting of published studies on the LTER
Bibliography. At the Network level,
there is a wealth of data related to N dynamics and organismal aspects of
litter decay. Models incorporating
long-term chemical data as well as organismal information are needed to
accurately predict ecosystem functioning, particularly C and N dynamics.
Participants agreed that
this is an important theme on which to build cross-site and interdisciplinary
synthesis and the integration process will greatly advance the science of
decomposition studies in the Network. The
organizers of the workshop will pursue funding from the LTER Network Office for
a follow up workshop. The main objective
of the follow up workshop will be to draft for publication a synthesis
manuscript on the current state of decomposition studies across the LTER and
ILTER Network. This manuscript will also
identify key research questions for future studies and new research initiatives
within the Network.
Figure 1. Regulation of decomposition by three interacting factors (modified from Swift et al., 1979).