Abstract
Abstract. Seasonal variation in water table depth (WTD) determines the balance between aggradation and degradation of tropical peatlands. Longer dry seasons together with human interventions (e.g. drainage) can cause WTD drawdowns making tropical peatland C storage highly vulnerable. Better predictive capacity for effects of WTD on net CO2 exchange is thus essential to guide conservation of tropical peat deposits. Mathematical modelling of basic eco-hydrological processes under site-specific conditions can provide such predictive capacity. We hereby deploy a process-based mathematical model ecosys to study effects of seasonal variation in WTD on net ecosystem productivity (NEP) of a drainage affected tropical peat swamp forest at Palangkaraya, Indonesia. Simulated NEP suggested that the peatland was a C source (NEP ~ −2 g C m−2 d−1, where a negative sign represents a C source and a positive sign a C sink) during rainy seasons with shallow WTD, C neutral or a small sink (NEP ~ +1 g C m−2 d−1) during early dry seasons with intermediate WTD and a substantial C source (NEP ~ −4 g C m−2 d−1) during late dry seasons with deep WTD from 2002 to 2005. These values were corroborated by regressions (P < 0.0001) of hourly modelled vs. eddy covariance (EC) net ecosystem CO2 fluxes which yielded R2 > 0.8, intercepts approaching 0 and slopes approaching 1. We also simulated a gradual increase in annual NEP from 2002 (−609 g C m−2) to 2005 (−373 g C m−2) with decreasing WTD which was attributed to declines in duration and intensity of dry seasons following the El Niño event of 2002. This increase in modelled NEP was corroborated by EC-gap filled annual NEP estimates. Our modelling hypotheses suggested that (1) poor aeration in wet soils during shallow WTD caused slow nutrient (predominantly phosphorus) mineralization and consequent slow plant nutrient uptake that suppressed gross primary productivity (GPP) and hence NEP (2) better soil aeration during intermediate WTD enhanced nutrient mineralization and hence plant nutrient uptake, GPP and NEP and (3) deep WTD suppressed NEP through a combination of reduced GPP due to plant water stress and increased ecosystem respiration (Re) from enhanced deeper peat aeration. These WTD effects on NEP were modelled from basic eco-hydrological processes including microbial and root oxidation-reduction reactions driven by soil and root O2 transport and uptake which in turn drove soil and plant carbon, nitrogen and phosphorus transformations within a soil-plant-atmosphere water transfer scheme driven by water potential gradients. Including these processes in ecosystem models should therefore provide an improved predictive capacity for WTD management programs intended to reduce tropical peat degradation.
Highlights
Seasonal and interannual fluctuations in water table depth (WTD) can affect peatland net CO2 exchange through complex effects on soil oxidation-reduction reactions and on nutrient transformations
Ecosys successfully simulated the reduction of tropical peatland net ecosystem productivity (NEP) during shallow and deep WTD hydroperiods with respect to those in intermediate WTD hydroperiods for four years, i.e. 2002–2005 over Palangkaraya Drained Peat Swamp Forest (PDPSF) (Figs. 2–5) (Table 2)
Seasonal variation in NEP that was apparent in the measurements was thought to be caused by the following key responses that were modelled using following alogrithms of WTD effects on gross primary productivity (GPP) and Re from basic independent research fed by site specific inputs (Sect. 2.2.3): 1. Shallow WTD during rainy seasons reduced modelled NEP by explicitly simulating slower convectivedispersive O2 transport through soils and roots and slower root O2 uptake, slower soil nutrient transformations (A26), slower root nutrient uptake and growth (C23), and lower leaf nutrient status (C11) and slower CO2 fixation (C6) (Sect. 4.1.1)
Summary
Seasonal and interannual fluctuations in water table depth (WTD) can affect peatland net CO2 exchange through complex effects on soil oxidation-reduction reactions and on nutrient transformations. Consequent reduction in soil O2 concentrations slows O2 uptake used to drive aerobic oxidation-reduction reactions by soil microbes and roots and reduces heterotrophic and root respiration. Lower anaerobic energy yields slow microbial growth and reduce heterotrophic respiration. Root oxidation-reduction reactions driving root growth and nutrient uptake require O2 which is scarce when WTD is shallow. Reduced heterotrophic and root respiration result in reduced ecosystem respiration (Re) with shallow WTD during rainy seasons, as reported in many field studies (Couwenberg et al, 2009; Flanagan and Syed, 2011; Limpens et al, 2008; Sulman et al, 2010). Slower microbial growth reduces decomposition and nutrient mineralization, as well as root growth and nutrient uptake and gross primary productivity (GPP) (Cai et al, 2010; Flanagan and Syed, 2011; Murphy and Moore, 2010; Sulman et al, 2012)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
