Mechanisms for carbon stock driving and scenario modeling in typical mountainous watersheds of northeastern China.

Watershed ecosystems play a pivotal role in maintaining the global carbon cycle and reducing global warming by serving as vital carbon reservoirs for sustainable ecosystem management. In this study, we based on the "quantity-mechanism-scenario" frameworks, integrate the MCE-CA-Markov and InVEST models to evaluate the spatiotemporal variations of carbon stocks in mid- to high-latitude alpine watersheds in China under historical and future climate scenarios. Additionally, the study employs the Geographic Detector model to explore the driving mechanisms influencing the carbon storage capacity of watershed ecosystems. The results showed that the carbon stock of the watershed increased by about 15.9 Tg from 1980 to 2020. Fractional Vegetation Cover (FVC), Digital Elevation Model (DEM), and Mean Annual Temperature (MAT) had the strongest explanatory power for carbon stocks. Under different climate scenarios, it was found that the SSP2-4.5 scenario had a significant rise in carbon stock from 2020 to 2050, roughly 24.1 Tg. This increase was primarily observed in the southeastern region of the watersheds, with forest and grassland effectively protected. Conversely, according to the SSP5-8.5 scenario, the carbon stock would decrease by about 50.53 Tg with the expansion of cultivated and construction land in the watershed's southwest part. Therefore, given the vulnerability of mid- to high-latitude mountain watersheds, global warming trends continue to pose a greater threat to carbon sequestration in watersheds. Our findings carry important implications for tackling potential ecological threats in mid- to high-latitude watersheds in the Northern Hemisphere and assisting policymakers in creating carbon sequestration plans, as well as for reducing climate change.

[Effects of Canalization on the Iron Deposition in Sanjiang Plain].

Canalization is the representative process and landscape of wetland reclamation. A typical ditch system of four levels near the Honghe National Nature Reserve in Sanjiang Plain was selected. Deposition plates were set on the sediments along the ditch level and the remained natural wetland nearby was quantitatively sampled for two years as the control. The deposition fluxes, total iron concentration, iron oxides and their components, as well as biogenic elements in the sediments collected by deposition plates were measured. The results showed that the litter, mud/sand and total deposition fluxes showed no significant differences between different ditch levels, with the means of (57.00 ± 16.90) g x (m2 x a)(-1), (3 997.57 ± 798.98) g x (m2 x a)(-1) and (4054.57 ± 792.91) g x (m2 x a)(-1), respectively. The litter flux decreased with the increase of ditch level, and the flux in the natural wetland [ (120.26 ± 19.42) g x (m2 x a)(-1) ] was significantly greater than that of the ditches. The mud/sand [ (35.41 ± 11.15 ) g x (m2 x a)(-1)] and total deposition fluxes [ (155.67 ± 20.75) g x ( m2 x a](-1) ] were significantly smaller than those of the ditches. There were no significant differences in the total iron between different ditches and natural wetland, while the free iron oxide content in the ditch sediments was significantly lower than that of natural wetland sediment. Except for the main ditch, the amorphous and complex iron oxides in the other ditch and natural wetland sediments showed no significant differences. The free degree of the iron oxide in ditch sediments was 60.2% of that in the natural wetland, while the differences in the complex degree and the activated degree were insignificant. The differences in the total organic carbon, total nitrogen and total phosphorus were insignificant, and all were smaller than those of the natural wetland, with the percentages of 14.6%, 31.6% and 41.0%, respectively. It could be concluded that the effects of canalization on iron and biogenic elements were significant. Consequently, rational agricultural water managements are strongly recommended to avoid the potential environmental and ecological risks caused by canalization in Sanjiang Plain.

[DNDC model, a model of biogeochemical processes: Research progress and applications].

Denitrification-decomposition (DNDC) model can estimate the emission fluxes of soil trace gases such as carbon dioxide (CO2), methane (CH4) , and nitrous oxide (N2O) via the coupling of the denitrification and decomposition processes driven by soil environmental factors. At present, DNDC model is one of the most successful models in the world in simulating the terrestrial biogeochemical cycles. This paper mainly reviewed the development process of the DNDC model, its structure, model validation, and sensitive factor analysis, and summarized the hot fields in the applications of the model.

[Effect of reclamation on the vertical distribution of SOC and retention of DOC].

Contents and density of soil organic carbon (SOC) in soil profiles and dissolved organic carbon (DOC) of soil solution in different soil depths in wetland, soybean and paddy field reclaimed from the wetland around Xingkai Lake were determined to investigate how reclamation of wetland for soybean and rice farming impacts vertical distribution of SOC and retention of DOC. SOC contents in 0-40 cm soil layers were significantly influenced. SOC contents in 0-10, 10-20, 20-30 and 30-40 cm soil layers in soybean and paddy field were 79.07% and 82.01%, 79.01% and 82.28%, 79.86% and 92.90%, 37.49% and 78.05% respectively lower than those in wetland. Before and after reclamation, SOC contents in soil layers deeper than 40 cm were not significantly different. SOC densities in soybean and paddy field were 25.50% and 47.35% respectively lower than those in wetland. However, either in wetland or farm land, most of the SOC storage in 0-100 cm soil layer was stored in 0-50 cm soil layer. The relationships between SOC content and soil depth in wetland and two farm lands all could be described by exponential functions; cultivation did not change the variation of SOC content with soil depth. The retention of DOC was more obvious for soybean farming than wetland and rice farming, and that was roughly the same for wetland as rice farming.

[Waterborne iron migration by groundwater irrigation pumping in a typical irrigation district of Sanjiang Plain].

The iron concentration in groundwater, iron's seasonal migration from groundwater to sun-basked pools, paddy fields and drainage canals, and its distribution in the sediments/soils were observed in the Jiansanjiang Branch Bureau, Heilongjiang Agricultural Cultivation Bureau. The results suggested that the total iron mass concentration of the studied area was (1.73 +/- 0.41) mg x L(-1), ranging from 0.01 to 11.4 mg x L(-1), with the variation coefficient of 1.29%. The annual iron input mass from groundwater to paddy fields and other surface water bodies was 4 976.40 t in 2010, according to the rice planting area and rating irrigation volume. Dissolved Fe2+, Fe3+ and iron, as well as the total iron (dissolved and particle) had seasonal variation, with greater values presented in June and July. These waterborne irons in paddy field waters were greater than those in sun-basked pools and drainage canals. Obvious enrichment effect was observed in sun-basked pools and paddy fields, with their total iron mass concentrations were 6.17 and 21.65 times greater than that in groundwater. Either the total iron or iron oxides in sun-baked pool sediments were greater than that in paddy field soils, field canal and main canal sediments. The differences of the total iron and iron oxides in paddy field soils, field canal and main canal sediments were not significantly different. Considerable irons were precipitated within sun-basked pools and paddy fields during the transfer from groundwater to surface water, with a part of irons exporting into canals through drainage and then precipitated there. Not only the change of total iron mass, but the transformation of iron chemical speciation was observed during the transfer, which was affected by paddy irrigation management directly. The long-term irrigation pumping could cause the substantial enrichment of iron in paddy soils and canal sediments, resulting in the increase of potential pollution risk.

[Research progress of the coupling process of Fe and N in wetland soils].

The coupling process of the earth system is the key research content of earth surface system at present. Wetland is an important ecosystem on the earth surface. Wetland soil is under anaerobic conditions seasonally or perennially because of waterlogging, where the redox of Fe and N can be coupled by microbiology. The coupling process lies in three aspects: the microbial coupling of NO(3-)-reduction and Fe(2+)-oxidation, coupling of Fe(3+)-reduction and NH(4+)-oxidation and the interaction of NO3- and Fe3+ during reduction. Getting the knowledge of the coupling process has important significance to understand the cycles of Fe and N in wetland soil. The article reviews the research status of the three aspects. In general, we have a comparatively deep understand of the coupling process of NO(3-)-reduction and Fe(2+)-oxidation than the latter two aspects. The research of microbial mechanism of the coupling process of Fe(3+)-reduction-NH(4+)-oxidation and the comprehensive evaluation of the environmental significance of coupling process of Fe and N in wetland soil should be strengthened in the future research, and then it can provide evidence for wetland protection and management.

[Dynamic change of dissolved iron in wetland soil solutions responding to freeze-thaw cycles].

The effects of five freeze-thaw cycles on the dynamic change of dissolved iron in three typical wetland soils (humus marsh soil in Carex lasiocarpa community, meadow marsh soil in Cares meyeriarna community, and meadow albic soil in Calamagrostis angustifolia community)of Sanjiang Plain, Northeast China, was analyzed through in-situ soil column simulation. One freeze-thaw cycle was conducted as freezing at -10 degrees C for 1 d and then thawing at 5 degrees C for 7 d. The thermostatically incubated soils at 5 degrees C were controls. The results showed that most pH and Eh values increased after the first freeze-thaw cycle, and then decreased after the subsequent cycles. 84.4% of the pH values of freeze-thaw treated soils were smaller than that of control, while 82.2% of the Eh values of freeze-thaw treated soils were greater than that of control. Most of the dissolved iron in all soil solutions were Fe3+ ions and colloids, and the reduction of these Fe3+ species were inhibited. The concentrations of Fe2, Fe3+, and total dissolved iron (TFe) of the freeze-thaw treated soils were all smaller than that of controls, with the means of (0.62 +/- 0.08) mg x L(-1) and (1.25 +/- 0.16) mg x L(-1), respectively. The variation trends of pH, Eh, and dissolved iron in the humus marsh soil were significantly different from that in the meadow albic soil. The trends in the meadow marsh soil, as the transitional soil type, were more similar to the meadow albic soil for pH, while more similar to the humus marsh soil for Eh and dissolved iron. Among the three soils, the difference between freeze-thaw treated columns and controls of the second layer were all smaller than that of the third and fourth layer, which indicated that the effect of freeze-thaw cycles were more significant for the upper annular wetland soil layers than the lower layers.

[Spatiotemporal distribution characteristics of soil iron in the annular wetland under different water regime].

The effect of water regime on the spatial distribution of total iron and the seasonal variation of dissolved iron in a typical annular wetland of Sanjiang Plain, Northeast China, was analyzed through in situ sampling of soils and soil solutions. The results showed that the average level of total iron of the wetland soil (0-60 cm) was (2.54 +/- 0.73) x 10(4) mg x kg(-1), which decreased gradually from the Calamagrostis angustifolia community in the edge of the annular wetland [(2.91 +/- 0.51) x 10(4) mg x kg(-1)], to the C. meyeriana community [(2.60 +/- 0.35) x 10(4) mg x kg(-1)], the C. lasiocarpa community [(2.48 +/- 0.31) x 10(4) mg x kg(-1)], and the of C. pseudocuraica community [(2.17 +/- 0.31) x 10(4) mg x kg(-1)] in the centre of the annular wetland. The iron solubility of perennial flooding soil was higher than seasonal flooding soil. The gross dissolved iron increased from soil thawing in the late spring [(0.35 +/- 0.086) mg x L(-1)] to freezing in the late autumn [(12.67 +/- 2.92) mg x L(-1)], because the soil iron was activated by continuous submergence. The reduced degree as shown by Fe3+/Fe2+ increased with the increment of water depth or flooding duration. Significant and extremely significant correlations were observed between dissolved Fe3+ or Fe2+ and pH, TOC, TN and PO4(3-), which suggested that the distribution of iron was influenced by the soil physical and chemical properties, and coupled with the transfer and transformation of C, N, and P elements.

[Distribution of sediment iron of the ditch system in Sanjiang Plain, northeast China].

The iron distribution of the multi-level ditch system (hair canal-field canal-lateral canal-branch canal-main canal) was studied through total iron determination of the sediments (0-60 cm). The results showed that the mean concentration was (3.02 +/- 0.10) x 10(4) mg x kg(-1). Extremely significant difference was obseved between different ditch level (F = 6.261, p << 0.001), and the highest and the lowest concentration were present in the farmland lateral canal (3.71 x 10(4) mg x kg(-1)) and wetland canal (2.43 x 10(4) mg x kg(-1)), respectively. The difference of different sediment layers was not significant (F = 0.093, p = 0.693), while the iron concentrations of 0-10 cm and 10-20 cm sediments were 51.96% and 62.22% higher than that of the natural wetland soil nearby, respectively. Iron can transfer with the runoff in a certain extent, but it was not cumulated along the ditch system with the largest cumulation location at the third level. The runoff containing iron decreased gradully because of the wetland protection and climate change nowadays. The horizontal transfer of iron along the ditch system indicated the timing and intensity of iron loss in the past since the canals were dredged.

[Characteristics of the wetland soil iron under different ages of reclamation].

The temporal-spatial trends of soil total iron concentration (Fet), free degree (Fed/Fet), activation degree (Feo/Fed) and complex degree (Fep/Fed) of soil iron oxides after reclamation were studied in Sanjiang Plain Wetlands. The result suggests that Fet in the upper tillage layers (0 - 20 cm) are influenced by reclamation more significantly than that in the lower ones (20 - 100 cm), and so does the early ages (0 - 1 years) than the late ages (1 - 25 years). Fet is negatively correlated with organic matter extremely significantly (R = - 0.62), while that with total phosphorus and pH are not significant. Fet of soil layer I (0 - 10 cm) increases obviously during the first 7 years after reclamation, and tends to become stable after 13 years, while those ages of soil layer II (10 - 20 cm) are 8 years and 15 years respectively. Soil layer I shows shorter responding time and better regularity than layer II. Fed/Fet increases rapidly after reclamation, decreases later and then increases again. Feo/Fed indicates exponential decrease with the reclamation ages as well as Fep/Fed. Feo/Fed of layer I decreases radically during the first 4 years after reclamation and tends to become stable after 13 years, while that of layer II decreases dramatically within the first year and keeps stable henceforth. The counterparts of Fep/Fed are 6 years, 14 years, and 2 years respectively. With the fitted experimental equations of Fet, Feo/Fed, and Fep/Fed, the ages of reclamation can be deduced reversely, which indicates the implication of iron on the shifts of soil environment.