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Coastal wetlands contribute to the mitigation of climate change through the sequestration of “blue carbon”. Microbial necromass, lignin, and glycoproteins (i.e., glomalin-related soil proteins (GRSP)), as important components of soil organic carbon (SOC), are sensitive to environmental change. However, their contributions to blue carbon formation and the underlying factors remain largely unresolved. To address this paucity of knowledge, we investigated their contributions to blue carbon formation along a salinity gradient in coastal marshes. Our results revealed decreasing contributions of microbial necromass and lignin to blue carbon as the salinity increased, while GRSP showed an opposite trend. Using random forest models, we showed that their contributions to SOC were dependent on microbial biomass and resource stoichiometry. In N-limited saline soils, contributions of microbial necromass to SOC decreased due to increased N-acquisition enzyme activity. Decreases in lignin contributions were linked to reduced mineral protection offered by short-range-ordered Fe (FeSRO). Partial least-squares path modeling (PLS-PM) further indicated that GRSP could increase microbial necromass and lignin formation by enhancing mineral protection. Our findings have implications for improving the accumulation of refractory and mineral-bound organic matter in coastal wetlands, considering the current scenario of heightened nutrient discharge and sea-level rise.

The biogeochemical cycle of Si plays an important role in maintaining the function and stability of ecosystems. Although there have been many studies on long‐term silicate weathering, there are few reports on Si fractions and cycling in early ecosystem development. Using optimized sequential chemical extraction processes, this study quantified soil Si fractions of the eluvial horizon and deposition layers in Hailuogou Glacier Retreat Area of Gongga Mountain, Southwest China. The results showed that plant‐available Si (NaAc–Si) content decreased significantly from 33.40 to 2.16 mg kg−1with the increase of soil age. There was a significantly positive correlation (p< .01) between soil pH and NaAc–Si. Within the labile Si fractions, the largest fraction was amorphous Si (Na2CO3–Si) (165.35 ± 81.50 mg kg−1), followed by pedogenic oxides/hydroxides chemisorbed Si (Oxalate–Si) (135.13 ± 66.83 mg kg−1), Si adsorbed on the surface of inorganic soil particles (Acetic–Si) (9.66 ± 4.27 mg kg−1), and soluble Si (CaCl2–Si) (4.89 ± 1.68 mg kg−1). Compared with Stage 1 (i.e., a variety of pioneer species symbiosis stage) after glacier retreat, the storage of these four labile Si fractions in Stage 3 (i.e., climax species stage) decreased by 45.95% (CaCl2–Si), 42.98% (Acetic–Si), 67.34% (Oxalate–Si), and 75.24% (Na2CO3–Si), respectively, due to the absorption and utilization of Si by plants. Our study implies that the multiple transformations of different Si fractions in soils and the absorption and return of Si by plants can keep the CaCl2–Si pool in dynamic equilibrium, which provides a basis for the stability of ecosystem functions during the vegetation evolution after glacier retreat. Plant‐available Si decreased significantly with the time of glacier retreat.Within the labile Si fractions, the largest fraction was Na2CO3–Si, followed by Oxalate–Si, Acetic–Si, and CaCl2–Si.Amorphous Si was the main source of dissolved Si in soil.Si can alleviate the biotic and abiotic stresses of plants, which may reduce soil erosion after glacier retreat.

Crop harvesting and residue removal from croplands often result in imbalanced biogeochemical cycles of carbon and nutrients in croplands, putting forward an austere challenge to sustainable agricultural production. As a beneficial element, silicon (Si) has multiple eco-physiological functions, which could help crops to acclimatize their unfavorable habitats. Although many studies have reported that the application of Si can alleviate multiple abiotic and biotic stresses and increase biomass accumulation, the effects of Si on carbon immobilization and nutrients uptake into plants in croplands have not yet been explored. This review focused on Si-associated regulation of plant carbon accumulation, lignin biosynthesis, and nutrients uptake, which are important for biogeochemical cycles of carbon and nutrients in croplands. The tradeoff analysis indicates that the supply of bioavailable Si can enhance plant net photosynthetic rate and biomass carbon production (especially root biomass input to soil organic carbon pool), but reduce shoot lignin biosynthesis. Besides, the application of Si could improve uptake of most nutrients under deficient conditions, but restricts excess uptake when they are supplied in surplus amounts. Nevertheless, Si application to crops may enhance the uptake of nitrogen and iron when they are supplied in deficient to luxurious amounts, while potassium uptake enhanced by Si application is often involved in alleviating salt stress and inhibiting excess sodium uptake in plants. More importantly, the amount of Si accumulated in plant positively correlates with nutrients release during the decay of crop biomass, but negatively correlates with straw decomposability due to the reduced lignin synthesis. The Si-mediated plant growth and litter decomposition collectively suggest that Si cycling in croplands plays important roles in biogeochemical cycles of carbon and nutrients. Hence, scientific Si management in croplands will be helpful for maintaining sustainable development of agriculture.

AbstractThe transport of river-borne dissolved and suspended substances to global oceans is significantly affected by dams. In most cases, dissolved silicon (DSi) is partly retained in reservoirs, thereby altering the community structure of phytoplankton downstream and even in their recipient estuaries. Reservoirs differ greatly in their ability to retain DSi. Factors controlling DSi retention in reservoirs have been studied for a specific reservoir or a set of reservoirs within a basin; however, the impact of trophic states, lithologic characteristics, and water exchange rates passing through reservoirs on DSi retention have not yet been compared and analyzed comprehensively across regional and continental scales. In this work, we compiled data from published literature and investigated the correlation between DSi retention efficiency and its potential controlling factors. We found that the average water depth and the hydraulic load (derived from the average water depth and the water residence time) of reservoirs were significantly and negatively correlated to DSi retention efficiency (p< 0.05). The results can improve our understanding of the retention effects of river damming on DSi and provide valuable information for ecological management of river damming.