AIMS: Soil dissolved organic carbon (DOC) is a primary form of labile carbon in terrestrial ecosystems, and therefore plays a vital role in soil carbon cycling. This study aims to quantify the budgets of soil DOC at biome and global levels and to examine the variations in soil DOC and their environmental controls. LOCATION: Global. TIME PERIOD: 1981–2019. METHODS: We compiled a global dataset and analysed the concentration and distribution of DOC across 10 biomes. RESULTS: Large variations in DOC are found among biomes across space and the soil DOC concentration declines exponentially along soil depths. Tundra has the highest soil DOC concentration in 0–30 cm soils [453.75 (95% confidence interval: 324.95–633.5) mg/kg], whereas tropical and temperate forests have relatively lower DOC concentrations, ranging from 30.20 (24.78–36.80) to 54.54 (49.77–59.77) mg/kg. DOC generally accounts for < 1% of total organic carbon in soils, and DOC in 0–30 cm contributes more than half of the total DOC in the 0–100 cm soil profile. Furthermore, variations in DOC are primarily controlled by soil texture, moisture, and total organic carbon. MAIN CONCLUSIONS: A global synthesis is combined with an empirical model to extrapolate the DOC concentration along soil profiles across the globe, and global budgets of DOC are estimated as 7.20 Pg C in the top 0–30 cm and 12.97 Pg C in the 0–100 cm soil profile, respectively, with a considerable variation among biomes. The strong soil texture control but weak total organic carbon (TOC) control on DOC variations suggest that the investigation of physical protection of soil organic carbon might need to expand to consider the labile C in soils. The global maps of DOC concentration serve as a benchmark for validating land surface models in estimating carbon storage in soils.

Soil dissolved organic carbon in terrestrial ecosystems: Global budget, spatial distribution and controls

Soil organic matter decomposition under global warming has a potential to alter soil carbon and nitrogen storages in permafrost. The objectives of this study were to investigate the temperature sensitivity of greenhouse gas emissions from soil samples along a mountain wetland-forest ecotone in the continuous permafrost and determine its influencing mechanisms. The CO 2 , N 2 O and carbon, nitrogen substrates were measured at 5, 15 and 25 °C. The relation between greenhouse gas emission rates and temperature depended on substrate quality in the three ecosystems. Soil DOC, MBC, NH 4 + and NO 3 − concentrations determined the higher CO 2 and N 2 O emission rates in the thicket peatland and the surface soil layer. During the incubation period, the degrees of soil carbon and nitrogen losses in the thicket peatland were 0.6–4.7% and 1.0–14.3 (1000 × %), approximately 1.6 and 1.2 times higher than those in the forest and fen, respectively. The highest degrees of soil carbon and nitrogen losses in the thicket peatland indicated that more greenhouse gases would emit from soils when permafrost degradation induced the succession from wetlands or forest to the wetland-forest ecotone. Although the gas emission rates presented significant differences in the three ecosystems, the Q 10 values with 2.0 to 2.2 for CO 2 and 2.4 to 3.0 for N 2 O, did not change significantly, indicating that the temperature sensitivity of gas emissions would not fluctuate much in the ecosystems along the mountain wetland-forest ecotone. However, the higher Q 10 values in the deeper soil layer in our study indicated that the decomposition of soil C and N in the deeper active layer of the permafrost region is more impressionable to global warming. As laboratory results could not actually reflect the situation in the field, more field work about temperature sensitivity of soil organic matter decomposition in different ecosystems should be encouraged in the future.

Temperature sensitivity of soil carbon mineralization and nitrous oxide emission in different ecosystems along a mountain wetland-forest ecotone in the continuous permafrost of Northeast China

Nitrogen deposition and global warming each have potential to alter soil carbon storage via changing decomposition, but combined effects of N addition and temperature on CO2 production in wetland soils are less studied. In this study, we conducted laboratory incubations to examine this problem using soil from a freshwater marsh in Sanjiang Plain, China. We found that N addition and temperature had a negative and positive effect on CO2 production, respectively. Despite low decomposition rates, heavy fractions had a notable contribution to CO2 production due to greater mass proportion in soil. Short-term combined effect of N addition and temperature on CO2 production was antagonistic. Based on greater N availability as N addition and temperature increasing, extra release of soil-derived N may further suppress CO2 production. We suggest that short-term soil CO2 production under simultaneous influence of temperature and N addition may be overestimated based on their single effects in this site.

Short-term Effects of Nitrogen Additions and Increased Temperature on Wetland Soil Respiration, Sanjiang Plain, China

The freshwater marshes in northern China are heavily impacted by anthropogenic disturbances such as cultivation and fertilization and increased levels of nutrients (especially N and P) through atmospheric deposition and agricultural surface runoff. These disturbances have affected the emission of N2O from these systems. This laboratory study was conducted to determine the effects of increased inputs of inorganic N and P on N2O emission from marsh soil in response to different soil moisture conditions. The results showed that the emission of N2O increased with the enhancement of N inputs when the soil was submerged, but that the highest N treatment suppressed the emission of N2O when the soil was at 60% water holding capacity (WHC), which may have occurred due to an inadequate amount of available C. Furthermore, the results of this study indicated that a small amount of N fertilizer induced much more N2O evolution from freshwater wetland soil, while P fertilizer inputs appeared to stimulate the emission of N2O only during the first few days of the experiment. Additionally, soil that was treated with P appeared to absorb N2O when it was at 60% WHC after around 6 weeks of the incubation, which indicates that the input of P fertilizer might serve as a shift of source or N2O sink in wetland soils under non-flooded conditions. When compared to soil at 60% WHC, submerged soil had significantly higher N2O emissions, except when subjected to the medial N treatment. These findings indicate that the soil moisture condition had a significant effect on N2O emissions when the same amount of N or P was applied. Therefore, the effects of N and P fertilization in the northern temperate wetlands cannot be neglected from regional or national emissions of N2O.

/pdf/effects-of-inorganic-nitrogen-and-phosphorus-enrichment-on-3xgeu258hu.pdf

Effects of inorganic nitrogen and phosphorus enrichment on the emission of N2O from a freshwater marsh soil in Northeast China

Effect of Leucaena leucocephala on soil organic carbon conservation on slope in the purple soil area

By laboratory incubation experiment, under aerobic and submerged soil moisture conditions, we investigated the mineralization of soil organic carbon (SOC) and contents of dissolved organic carbon (DOC) with different nitrogen inputs in a freshwater marsh soil. The results showed that under aerobic condition, there were no significant effects on the mineralization of SOC and contents of DOC as the net nitrogen input was 1 mg x g(-1) (N1), however, they were significantly higher than control and N1 treatments when nitrogen input increased to 2 and 5 mg x g(-1) (N2, N3), and the amount of DOC was respectively 187.22% and 203.25% higher than control (250.62 mg x kg(-1)). Under submerged condition, all N treatments restrained the mineralization of SOC, and the content of DOC was respectively 88.34% (N1), 82.69% (N2) and 80.04% (N3) lower than control (642.52 mg x kg(-1)). There were significant positive correlations between the contents of DOC and the amounts of cumulative C by mineralization (R2 was 0.939 and 0.843, respectively), which suggested that the changes of DOC affected by N input might be one of the important reasons that arose the differences of SOC mineralization. The results also indicate that as the waterlogged environment disappeared in wetland, the supply of exogenous nitrogen might bring large loss of SOC through enhancing the mineralization of SOC and leaching of DOC.

Exogenous nitrogen enrichment impact on the carbon mineralization and DOC of the freshwater marsh soil

Plant aboveground biomass, total organic carbon( TOC), microbial biomass carbon( MBC), basal respiration( BR), microbial quotient (Cmic/Corg) and metabolic quotient (qCO2) in 0-15 cm and 15-50 cm marsh soil of Calamagrostis angustifolia under different water levels were investigated with controlled experiment. The result showed that water level exerted significant effect on plant biomass, which was the highest under 10 cm waterlogged level and of higher productivity under 0-20 cm waterlogged level. TOC, MBC, BR, Cmic/Corg and qCO2 differed significantly under different water levels. BR and TOC responded to different waterlogged levels in the same way. BR and TOC of 0-15 cm marsh soil were the highest under 0 cm waterlogged level, however, BR and TOC of 15-50 cm marsh soil decreased respectively with increasing water, which was corresponding with soil MBC and Cmic/Corg and qCO2 increased with increasing water. As a result, microbial community is altered and microbial activity is decreasing by increasing waterlogged level, and microbial activity is the lowest under 30 cm waterlogged level, which affects organic carbon accumulation and decomposition.

Influence of water level gradient on marsh soil microbial activity of Calamagrostis angustifolia

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Exogenous nitrogen enrichment impact on the carbon mineralization and DOC of the freshwater marsh soil

Influence of water level gradient on marsh soil microbial activity of Calamagrostis angustifolia