Modeling hydrological controls on variations in peat water content, water table depth, and surface energy exchange of a boreal western Canadian fen peatland

Sherpa Romeo green journal. Permission to archive final published versionImproved predictive capacity of hydrology and surface energy exchange is critical for conserving boreal peatland carbon sequestration under drier and warmer climates. We represented basic processes for water and O2 transport and their effects on ecosystem water, energy, carbon, and nutrient cycling in a process-based model ecosys to simulate effects of seasonal and interannual variations in hydrology on peat water content, water table depth (WTD), and surface energy exchange of a Western Canadian fen peatland. Substituting a van Genuchten model (VGM) for a modified Campbell model (MCM) in ecosys enabled a significantly better simulation of peat moisture retention as indicated by higher modeled versus measured R2 and Willmot’s index (d) with VGM (R2~0.7, d~0.8) than with MCM (R2~0.25, d~0.35) for daily peat water contents from a wetter year 2004 to a drier year 2009. With the improved peat moisture simulation, ecosys modeled hourly WTD and energy fluxes reasonably well (modeled versus measured R2: WTD ~0.6, net radiation ~0.99, sensible heat >0.8, and latent heat >0.85). Gradually declining ratios of precipitation to evapotranspiration and of lateral recharge to discharge enabled simulation of a gradual drawdown of growing season WTD and a consequent peat drying from 2004 to 2009. When WTD fell below a threshold of ~0.35m below the hollow surface, intense drying of mosses in ecosys caused a simulated reduction in evapotranspiration and an increase in Bowen ratio during late growing season that were consistent with measurements. Hence, using appropriate water desorption curve coupled with vertical-lateral hydraulic schemes is vital to accurately simulate peatland hydrology and energy balance.Ye

Modelling effects of seasonal variation in water table depth on net ecosystem CO2 exchange of a tropical peatland

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 similar to -2 gCm(-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 similar to + 1 gCm(-2) d(-1)) during early dry seasons with intermediate WTD and a substantial C source (NEP similar to - 4 gCm(-2) d(-1)) during late dry seasons with deep WTD from 2002 to 2005. These values were corroborated by regressions (P 0.8, intercepts approaching 0 and slopes approaching 1. We also simulated a gradual increase in annual NEP from 2002 (-609 gCm(-2)) to 2005 (-373 gCm(-2)) with decreasing WTD which was attributed to declines in duration and intensity of dry seasons following the El Nino event of 2002. This increase in modelled NEP was corroborated by ECgap filled annual NEP estimates. Our modelling hypotheses suggested that (1) poor aeration in wet soils during shallow WTD caused slow nutrient (predominantly phosphorus) min-eralization 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 (R-e) 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 O-2 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

Coupled eco-hydrology and biogeochemistry algorithms enable the simulation of water table depth effects on boreal peatland net CO₂ exchange

Water table depth (WTD) effects on net ecosystem CO2 exchange of boreal peatlands are largely mediated by hydrological effects on peat biogeochemistry and the ecophysiology of peatland vegetation. The lack of representation of these effects in carbon models currently limits our predictive capacity for changes in boreal peatland carbon deposits under potential future drier and warmer climates. We examined whether a process-level coupling of a prognostic WTD with (1) oxygen transport, which controls energy yields from microbial and root oxidation–reduction reactions, and (2) vascular and nonvascular plant water relations could explain mechanisms that control variations in net CO2 exchange of a boreal fen under contrasting WTD conditions, i.e., shallow vs. deep WTD. Such coupling of eco-hydrology and biogeochemistry algorithms in a process-based ecosystem model, ecosys, was tested against net ecosystem CO2 exchange measurements in a western Canadian boreal fen peatland over a period of drier-weather-driven gradual WTD drawdown. A May–October WTD drawdown of  ∼  0.25 m from 2004 to 2009 hastened oxygen transport to microbial and root surfaces, enabling greater microbial and root energy yields and peat and litter decomposition, which raised modeled ecosystem respiration (Re) by 0.26 µmol CO2 m−2 s−1 per 0.1 m of WTD drawdown. It also augmented nutrient mineralization, and hence root nutrient availability and uptake, which resulted in improved leaf nutrient (nitrogen) status that facilitated carboxylation and raised modeled vascular gross primary productivity (GPP) and plant growth. The increase in modeled vascular GPP exceeded declines in modeled nonvascular (moss) GPP due to greater shading from increased vascular plant growth and moss drying from near-surface peat desiccation, thereby causing a net increase in modeled growing season GPP by 0.39 µmol CO2 m−2 s−1 per 0.1 m of WTD drawdown. Similar increases in GPP and Re caused no significant WTD effects on modeled seasonal and interannual variations in net ecosystem productivity (NEP). These modeled trends were corroborated well by eddy covariance measured hourly net CO2 fluxes (modeled vs. measured: R2  ∼  0.8, slopes  ∼ 1 ± 0.1, intercepts  ∼ 0.05 µmol m−2 s−1), hourly measured automated chamber net CO2 fluxes (modeled vs. measured: R2  ∼ 0.7, slopes  ∼ 1 ± 0.1, intercepts  ∼ 0.4 µmol m−2 s−1), and other biometric and laboratory measurements. Modeled drainage as an analog for WTD drawdown induced by climate-change-driven drying showed that this boreal peatland would switch from a large carbon sink (NEP  ∼  160 g C m−2 yr−1) to carbon neutrality (NEP  ∼  10 g C m−2 yr−1) should the water table deepen by a further  ∼ 0.5 m. This decline in projected NEP indicated that a further WTD drawdown at this fen would eventually lead to a decline in GPP due to water limitation. Therefore, representing the effects of interactions among hydrology, biogeochemistry and plant physiological ecology on ecosystem carbon, water, and nutrient cycling in global carbon models would improve our predictive capacity for changes in boreal peatland carbon sequestration under changing climates

Effects of grazing management on spatio-temporal heterogeneity of soil carbon and greenhouse gas emissions of grasslands and rangelands: Monitoring, assessment and scaling-up

Grazing lands provide many goods and ecosystem services, such as forage, livestock, soil carbon (C) storage, biodiversity, and recreational opportunities. Ensuring the long-term sustainability of grazing lands requires optimal management to simultaneously balance livestock productivity for sustaining human food and nutritional demands while reducing environmental impacts, such as greenhouse gases (GHG) emissions and soil degradation. In this paper, we revisit grazing management in grazing lands exposed to different grazing systems. In Section 2, we briefly review parameterization and multi-faceted goals for sustainability of grazing systems considering broader sustainability from economic to environmental aspects. We also discuss the inconsistencies between grazing researchers and ranchers’ practices. In Section 3, we review the separate experimental data to examine the impacts of multi-paddock rotational grazing on soil carbon, nutrient and GHGs. In Section 4, we present status and upcoming challenges in monitoring and upscaling of grazing ecosystem research and management. In Section 5, new concepts of multiple source monitoring networks are presented that enable the analysis of scale-dependent processes. Finally, we point out future directions for monitoring and assessment of managing soil C and GHG emissions from grazing lands. The results show that the inconsistences are essentially due to (1) effects of spatiotemporal scales on both economic and ecological outcomes, and (2) simplistic representations of multi-faceted grazing systems and sustainability. The development of multi-faceted monitoring systems needs to be further parametrized and standardized to make consistent for meaningful and comparable assessment of grazing management impacts on SOC and GHGs