Dissolved organic matter characteristics of deciduous and coniferous forests with variable management: different at the source, aligned in the soil

Dissolved organic matter (DOM) is part of the biogeochemical cycles of carbon and nutrients, carries pollutants and drives soil formation. The DOM concentration and properties along the water flow path through forest ecosystems depend on its sampling location and transformation processes. To improve our understanding of the effects of forest management, especially tree species selection and management intensity, on DOM concentrations and properties of samples from different ecosystem fluxes, we studied throughfall, stemflow, litter leachate and mineral soil solution at 26 forest sites in the three regions of the German Biodiversity Exploratories. We covered forest stands with three management categories (coniferous, deciduous age class and unmanaged beech forests). In water samples from these forests, we monitored DOC concentrations over 4 years and characterized the quality of DOM with UV-vis absorption, fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Additionally, we performed incubation-based biodegradation assays. Multivariate statistics revealed strong significant effects of ecosystem fluxes and smaller effects of main tree species on DOM quality. Coniferous forests differed from deciduous forests by showing larger DOC concentrations, more lignin- and protein-like molecules, and fewer tannin-like molecules in throughfall, stemflow, and litter leachate. Cluster analysis of FT-ICR-MS data indicated that DOM compositions, which varied in aboveground samples depending on tree species, become aligned in mineral soil. This alignment of DOM composition along the water flow path in mineral soil is likely caused by microbial production and consumption of DOM in combination with its interaction with the solid phase, producing a characteristic pattern of organic compounds in forest mineral soils. We found similarly pronounced effects of ecosystem fluxes on the biodegradability of DOM, but surprisingly no differences between deciduous and coniferous forests. Forest management intensity, mainly determined by biomass extraction, contribution of species, which are not site-adapted, and deadwood mass, did not influence DOC concentrations, DOM composition and properties significantly.

Dissolved organic matter characteristics of deciduous and coniferous forests with variable management: different at the source, aligned in the soil

Dissolved organic matter (DOM) is part of the biogeochemical cycles of carbon and nutrients, carries pollutants and drives soil formation. The DOM concentration and properties along the water flow path through forest ecosystems depend on its origin and transformation processes. To improve our understanding of the effects of forest management, especially tree species selection and management intensity, on DOM concentrations and properties of samples from different ecosystem fluxes, we studied throughfall, stemflow, litter leachate and mineral soil solution at 26 forest sites in the three regions of the German Biodiversity Exploratories. We covered forest stands with three management categories (coniferous and deciduous age-class, unmanaged beech forests). In water samples from these forests, we monitored DOC concentrations over four years and characterized the quality of DOM with UV-vis absorption, fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) and with Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS). Additionally, we performed incubation-based biodegradation assays. Multivariate statistics revealed strong significant effects of origin of ecosystem fluxes and smaller effects of main tree species on DOM quality. Coniferous forests differed from deciduous forests by showing larger DOC concentrations, more lignin- and protein-like molecules, and less tannin-like molecules in throughfall, stemflow, and litter leachate. Cluster analysis of FT-ICR-MS data indicated that DOM compositions, which varied in aboveground samples depending on tree species, become aligned in mineral soil. This alignment of DOM composition along the water flow path in mineral soil is likely caused by microbial production and consumption of DOM in combination with its interaction with the solid phase, producing a characteristic pattern of organic compounds in forest mineral soils. We found similarly pronounced effects of ecosystem fluxes on the biodegradability of DOM, but surprisingly no differences between deciduous and coniferous forests. Forest management intensity, mainly determined by biomass extraction, contribution of species, which are not site-adapted, and deadwood mass, did not influence DOC concentrations, DOM composition and properties.

Hydrologically transported dissolved organic carbon influences soil respiration in a tropical rainforest

To better understand the effect of dissolved organic carbon (DOC) transported by hydrological processes (rainfall, throughfall, litter leachate, and surface soil water; 0–20 cm) on soil respiration in tropical rainforests, we detected the DOC flux in rainfall, throughfall, litter leachate, and surface soil water (0–20 cm), compared the seasonality of δ13CDOC in each hydrological process, and δ13C in leaves, litter, and surface soil, and analysed the throughfall, litter leachate, and surface soil water (0–20 cm) effect on soil respiration in a tropical rainforest in Xishuangbanna, south-west China. Results showed that the surface soil intercepted 94.4 ± 1.2 % of the annual litter leachate DOC flux and is a sink for DOC. The throughfall and litter leachate DOC fluxes amounted to 6.81 and 7.23 % of the net ecosystem exchange respectively, indicating that the DOC flux through hydrological processes is an important component of the carbon budget, and may be an important link between hydrological processes and soil respiration in a tropical rainforest. Even the variability in soil respiration is more dependent on the hydrologically transported water than DOC flux insignificantly, soil temperature, and soil-water content (at 0–20 cm). The difference in δ13C between the soil, soil water (at 0–20 cm), throughfall, and litter leachate indicated that DOC is transformed in the surface soil and decreased the sensitivity indices of soil respiration of DOC flux to water flux, which suggests that soil respiration is more sensitive to the DOC flux in hydrological processes, especially the soil-water DOC flux, than to soil temperature or soil moisture.

Fast-freezing with liquid nitrogen preserves bulk dissolved organic matter concentrations, but not its composition

Freezing can affect concentrations and spectroscopic properties of dissolved organic matter (DOM) in water samples. Nevertheless, water samples are regularly frozen for sample preservation. In this study we tested the effect of different freezing methods (standard freezing at −18 °C and fast-freezing with liquid nitrogen) on DOM concentrations measured as organic carbon (DOC) concentrations and on spectroscopic properties of DOM from different terrestrial ecosystems (forest and grassland). Fresh and differently frozen throughfall, stemflow, litter leachate and soil solution samples were analyzed for DOC concentrations, UV-vis absorption and fluorescence excitation–emission matrices combined with parallel factor analysis (PARAFAC). Fast-freezing with liquid nitrogen prevented a significant decrease of DOC concentrations observed after freezing at −18 °C. Nonetheless, the share of PARAFAC components 1 (EXmax < 250 nm (340 nm), EXmax: 480 nm) and 2 (EXmax: 335 nm, EXmax: 408 nm) to total fluorescence and the humification index (HIX) decreased after both freezing treatments, while the shares of component 3 (EXmax: < 250 nm (305 nm), EXmax: 438 nm) as well as SUVA254 increased. The contribution of PARAFAC component 4 (EXmax: 280 nm, EXmax: 328 nm) to total fluorescence was not affected by freezing. We recommend fast-freezing with liquid nitrogen for preservation of bulk DOC concentrations of samples from terrestrial sources, whereas immediate measuring is preferable to preserve spectroscopic properties of DOM.

Hydrologically transported dissolved organic carbon influences soil respiration in a tropical rainforest

To better understand the role of the dissolved organic carbon (DOC) transported by hydrological processes in soil respiration in tropical rainforests, we measured: (1) the DOC flux in rainfall, throughfall, litter leachate, and surface soil water (0–20 cm), (2) the seasonality of δ13CDOC in each hydrological process, and δ13C in leaves, litter, and surface soil, and (3) soil respiration in a tropical rainforest in Xishuangbanna, southwest China. The results showed: the surface soil intercepted 94.4 ± 1.2 % of the annual litter leachate DOC flux and is a sink for DOC. The throughfall and litter leachate DOC fluxes amounted to 6.81 % and 7.23 % of the net ecosystem exchange, respectively, indicating that the DOC flux through hydrological processes is a key component of the carbon budget, and may be a key link between hydrological processes and soil respiration in the tropical rainforest. The difference in δ13C among the soil, soil water (at 0–20 cm), throughfall, and litter leachate indicated that DOC is transformed in the surface soil. The variability in soil respiration is more dependent on the hydrologically transported DOC flux than on the soil water content (at 0–20 cm), and is more sensitive to the soil water DOC flux (at 0–20 cm) than to the soil temperature, which suggests that soil respiration is more sensitive to the DOC flux in hydrological processes, especially the soil water DOC flux, than to soil temperature or soil moisture.