Fluxes and concentrations of dissolved organic carbon in soils

Dissolved organic carbon (DOC) in soil solution plays roles in soil C storage and biogeochemical cycles. Factors regulating fluxes and concentrations of DOC still remain unclear. To identify the factors regulating fluxes and concentrations of DOC in the soil profiles, we compiled the data of site information [Country, Region or state, Coordinates, Vegetation, Mean annual air temperature (ºC), Climate type, Vegetation type, Mycorrhiza type, Soil (USDA, Soil Taxonomy)], soil properties [Litter pH (H2O), Soil pH (H2O), Soil C/N ratio, Clay (%), Alo+1/2Feo (g kg-1), O horizon C stock (Mg C ha-1), Mineral soil C stock (Mg C ha-1)], fluxes and concentrations of DOC [Throughfall DOC flux (kg C ha-1 yr-1), DOC flux at the bottom of the O horizon (kg C ha-1 yr-1), DOC flux at the bottom of the B horizon (kg C ha-1 yr-1), DOC concentration at the bottom of the O horizon (mg C L-1), DOC concentration at the bottom of the B horizon (mg C L-1), DOC/Dissolved organic N (DON) (O horizon), DOC/DON (B horizon), Precipitation (mm yr-1), Water flux at the bottom of the O horizon (mm yr-1), Water flux at the bottom of B horizon (mm yr-1)], plant litter properties [Litterfall C input (Mg C ha-1 yr-1), C/N ratio in litter, Lignin content in litter (%), Lignin/N ratio in litter, Root litter C input (Mg C ha-1 yr-1)], and DOC retention in mineral soil (%), DOC flux relative to C input (%), Contribution of DOC to C input in mineral soil (%), and Turnover time of mineral soil C (yr)].

Methods
The data were compiled from data in our study and those from published sources by searching for “dissolved organic carbon”, “solute”, “flux”, “leaching”, and “soil” in Google Scholar. We compiled the data of DOC fluxes in throughfall and soil profiles from 91 sites, of which the DOC flux data at 18 sites have been published by our group. The climate was classified into four groups [polar climate (MAT < 0 ºC), boreal climate (0 ºC < MAT < 6 ºC), temperate climate (6 ºC < MAT < 20 ºC), tropical climate (20 ºC < MAT)], based on mean annual air temperature.

The other parameters include climatic properties [mean annual precipitation and mean annual air temperature], plant litter properties [litterfall C input, C/N ratio, Klason-lignin (residue after digestion with sulfuric acid; Allen et al., 1974), lignin/N ratio, root litter production] and soil properties [soil C stocks (O horizon and mineral soil (0-30 cm depth)), pH (water extraction), clay content, short-range-order (amorphous) aluminum (Al), iron (Fe) (acid ammonium oxalate extractable Al and Fe; McKeague and Day, 1966)].

The sampling and analytical methods are concisely summarized as follows: Throughfall (canopy leaching) samples were collected by precipitation collector, while soil solution samples were collected using tension-free lysimeters for downward flux of water percolating in the soil profiles. Sample solutions were filtered through a 0.45 µm filter (e.g., PTFE syringe filter) and stored at 1°C in the dark prior to analyses. The concentrations of DOC were determined using a total organic carbon and nitrogen analyzer (TOC-VCSH, Shimadzu, Japan). The dissolved organic nitrogen (DON) concentrations were calculated by subtracting dissolved inorganic nitrogen (sum of NH4+ and NO3-) from TDN concentrations (DON = TDN - NH4+ - NO3-) to obtain DOC/DON ratios in soil solution. The DOC flux at the depth of 0 cm (the bottom of organic layers) and the bottom of B horizon (the bottom of rooting zone) was estimated by multiplying DOC concentrations in soil solution and water fluxes at each depth. Soil water fluxes were estimated by hydrological models or precipitation-evapotranspiration water budgets. Annual root production was measured by ingrowth core method, net sheet method, or sequential sampling method and estimated to be equal to annual root litter inputs.

Proportion of DOC flux from the O horizon relative to C input via both throughfall and litterfall was calculated by dividing DOC flux from the O horizon by C input via both throughfall and litterfall. DOC retention in the mineral soil was calculated as the percentage of net decrease in DOC flux between O and B horizons relative to DOC flux from the O horizon. The apparent turnover time (yr) of soil C was estimated by dividing soil C stocks (Mg C ha–1) by C inputs (net DOC inputs and root litter inputs into the mineral soil) (Mg C ha–1 yr–1).