One of the easiest approaches to increase the carbon sink is to plant trees, but 58.2% of Zhejiang Province’s area is forest, which means the afforestation potential is low. After years of development and utilization, young- or middle-aged trees account for 76.76% of Zhejiang Province’s forest area, which has a low carbon storage density but a higher growth rate than old forests ^[1][2][8,9]. If reasonable forest management is adopted, the carbon density of young- and middle-aged forests will increase gradually, thus improving the carbon fixation ability of the forest ecosystem in Zhejiang Province ^[3][10]. The Natural Forest Protection Project initiated in 1998 has contributed to the carbon sink of China. From 1998 to 2010, the projects contributed 889.1 Mt C ^[4][11]. The calculation of the forest carbon sequestration rate is mainly based on the forest carbon density ^[1][5][8,12]. One method is to divide the forest into different stages according to age and assume that the growth rate of the carbon density in each stage is constant. The forest in Zhejiang Province is mainly middle-aged, and the growth rate of the carbon density is 137 t C km⁻² yr⁻¹, which is equivalent to a carbon sequestration rate of 502.33 t CO₂ km⁻² yr^{−1 [1]}[8]. A study based on the continuous biomass expansion factor model calculated that the total forest biomass carbon stock in Zhejiang Province was 97.44 Mt from 2004 to 2008 and 120.45 Mt from 2009 to 2013 ^[5][12]. Currently, the total forest area of Zhejiang Province is 6.1 × 10⁴ km², so the average forest carbon sequestration rate of Zhejiang Province is 276.35 t CO₂ km⁻² yr⁻¹. The two studies were similar, but both were below the global average of 1192.40 t CO₂ km⁻² yr^{−1  [6]}[13].

At the same time, Zhejiang Province has a considerable number of bamboo forests, accounting for 15% of the total amount of forest land. Owing to their rapid growth rate and reproductive capacity, bamboo forests have a higher carbon sink rate than other subtropical forests ^[7][15]. The bamboo forest is especially proposed when calculating the forest carbon sequestration rate in Zhejiang Province in hereinthis review. The carbon density of bamboo forests in Zhejiang Province is the lowest (9698 t·km⁻²), lower than the average of 12,080 t·km^{−2 [3]}[10]. From 2004 to 2008, the carbon stock of bamboo forests in China was 1.99 × 10⁸ t C with an area of 5.48 × 10⁴ km², and from 1999 to 2003, it was 1.77 × 10⁸ t C with an area of 4.84 × 10⁴ km^{2 [8]}[14]. So, the average bamboo forest carbon sequestration rate of Zhejiang Province is 294.40 t CO₂ km⁻² yr⁻¹. The underground biomass of bamboo is also important plant phytolith-occluded carbon (highly refractory organic carbon occluded in the amorphous silica) storage ^[9][19]. According to the remote sensing data of bamboo forests in Zhejiang Province, aboveground carbon storage of bamboo forest in 2008 and 2014 was 1.684 × 10⁷ t C and 1.272 × 10⁷ t C, respectively ^[7][15]. The average bamboo forest carbon sequestration rate of Zhejiang Province is calculated as 277.90 t CO₂ km⁻² yr⁻¹. The carbon sequestration rates of the above two studies are similar (294.40 and 277.90 t CO₂ km⁻² yr⁻¹, respectively), but the estimates of carbon storage in Zhejiang Province differ by about an order of magnitude. The reason is that the carbon density of the two studies is different: the former is 5310–8190 t km⁻² and the latter is 1095–1907 t km^{−2 [7][8]}[14,15]. The carbon sequestration rates are similar between the whole forest and the bamboo forest. Although bamboo is a good carbon sequestration plant, intensive management strategies such as high-frequency fertilization and the removal of non-bamboo plants will not greatly improve its carbon sequestration capacity ^[10][20]. Non-native fast-growing tree species such as Pinus, Picea, Populus, and Eucalyptus are widely grown in China ^[11][21]. However, these non-native fast-growing trees may consume more soil water and nutrients and suffer a lot from pests and pathogen outbreaks ^[12][22]. More effective strategies such as reduced frequent soil reclamation and a combination of native bamboo and non-bamboo plants should be employed to manage carbon sequestration. The carbon sinks obtained by forests cannot be retained over longer time scales. The production of biochar from biomass is a useful method to solve the problem ^[13][23]. Compared with biomass, biochar is more stable and can endure longer to maintain the carbon sink ^[14][24].

Zhejiang Province has special and extensive coastal ecosystems, the salt marshes and tidal flats areas of which rank second in China ^[15][30]. Compared to terrestrial ecosystems, marine and nearshore ecosystems are more complex. The research data on the same marine and nearshore ecosystems may vary widely. The lack of a unified field sampling and laboratory analysis method results in a large gap in the measurement of the carbon sequestration rate ^[16][31]. Some ecosystems, such as mangroves, have not been studied directly in Zhejiang Province. Therefore, this review selected data from Zhejiang, Jiangsu, Shanghai, Fujian, and Guangdong provinces were collected tto determine the range of carbon sequestration rates for these ecosystems.

As a type of blue carbon ecosystem, mangrove is located at the boundary of land and sea. The developed root system of plants reduces soil erosion, and the sediments covered by seawater are in anoxic conditions ^[16][31]. Meanwhile, the probability of forest fire among mangroves is lower than that of land forest. These features make mangroves have a higher carbon sink capacity than other terrestrial ecosystems ^[17][18][44,45]. However, Zhejiang has a small area of mangroves, i.e., 0.2 km². Despite the high carbon sink rate, mangrove does not play a vital role in the carbon neutrality of Zhejiang Province. The carbon sequestration rate of mangroves is calculated mainly based on the net primary productivity of mangroves ^[19][20][32,33], sediment core sampling ^[21][22][23][34,35,36], and the annual carbon density variation of mangroves ^[24][37]. Sediment sampling data are generally lower than the other two, possibly because carbon fixed by plants is oxidized and decomposed in soil or transported horizontally by water flow ^[20][33].

The carbon sequestration capacity of salt marsh plants is calculated similarly to that of mangroves. The carbon sink rates of Phragmites australis, Spartina alterniflora, and Scirpus mariqueter in Hangzhou Bay, Zhejiang Province were 6882, 6802, and 1005 t CO₂ km⁻² yr⁻¹, respectively ^[25][38]. The typical reed zone wetland in Changjiang Estuary has a strong carbon sink rate, ranging from 4070 to 8836.67 t CO₂ km⁻² yr^{−1 [26]}[40]. The carbon sink rate based on the organic carbon storage of the soil in Yancheng saltmarsh is 375.22 t CO₂ km⁻² yr⁻¹ on average ^[27][39]. Tidal flats have low vegetation coverage and are not generally considered to be a high carbon sink; however, they continue to receive deposition processes from estuaries and coastal zones and thus have a carbon sequestration capacity comparable to that of blue carbon ecosystems, which ranges from 84.33 to 784.67 t CO₂ km⁻² yr^{−1 [16][27]}[31,39]. The tidal flat deposits in Zhejiang Province are heavily fed by the Changjiang River. At the same time, human land reclamation and the construction of coastal defense walls have accelerated the rate of carbon sequestration in tidal flats ^[16][31].

Although the accuracy and resolution of carbon sink data for nearshore ecosystems still need to be improved, it is a clear fact that these ecosystems play an important role in achieving carbon neutrality. The other harsh reality is that climate change and human activity are also destroying them. Climate change will have a strong impact on gene expression and cellular and whole-organism physiology, leading to recombination and the migration of communities in different regions, and ultimately altering ecosystem services and functions ^[28][46]. Because of the sea-level rise caused by global warming, nearshore ecosystem areas are being destroyed. The sequestered organic carbon is re-released to water bodies or ground surfaces, where it is re-broken down into CO₂ and released into the atmosphere. Converting mangroves for agriculture or aquaculture releases carbon that has already been sequestered ^[17][44]. Human emissions of excess nutrients lead to the eutrophication of water bodies, which leads to algal blooms. Water hypoxia caused by algal blooms will further affect the respiration and nutrient transport of plant roots, resulting in plant death. These two factors are squeezing nearshore ecosystems areas, which decreases their carbon sequestration potential. In addition to the ecological significance of improving the human living environment, the protection of nearshore ecosystems can also obtain carbon credits by confirming the value of carbon sinks in the carbon trading market, which can promote the restoration of the nearshore ecosystems through market factors. China has launched the Fengxian Coastal Salt Marsh Restoration Project and the Western Jinsan Citizen Beach Consolidation and Restoration Project at the Northern Hangzhouwan Bay in Shanghai, funded by the National sea-use fees and Central islands and sea protection funds ^[29][47].

The ocean is an important regulator of the carbon cycle, which has absorbed approximately 30% of anthropogenic CO₂ since the industrial revolution, helping to buffer climate change ^[30][48]. The ocean sequesters carbon by dissolving it in seawater and turning it into ions (the solubility pump); a small part of the organic matter fixed by plankton is deposited and sealed into the sediment through excrement and biological debris (biological pump). Biosynthetic calcium carbonate shells consume carbonate (carbonate pump). Recalcitrant dissolved organic carbon from DOC and POC is converted by microorganisms (microbial carbon pump) ^[31][32][33][41,49,50]. Although marginal seas account for 8% of the world’s oceans, the annual CO₂ uptake accounts for 10–20% of the total amount of global oceanic annual CO₂ uptake ^[34][51]. However, projections of the impact of future warming on marine productivity generally indicate a reduction in subtropical productivity ^[32][49].

The waters administered by Zhejiang Province are part of the East China Sea (ECS), and the main sea area is located in the south of the Changjiang Estuary. Because most of the current studies are focused on the whole ECS, therein,is review aims to estimate and calculate the carbon sink in the Zhejiang Sea (ZS) area are estimated and calculated by converting the area of the two sea areas to a proportional scale (5 × 10⁵ km² for ECS, 2.6 × 10⁵ km² for ZS). The carbon sequestration capacity of the ECS is affected by the east Asian monsoon, the Kuroshio, and the runoff from the Changjiang River ^[31][41]. The Changjiang plume and the coastal area of Zhejiang are sinks of atmospheric CO₂ in winter, spring, and summer while autumn is a source of atmospheric CO₂ and the shelf is an atmospheric CO₂ sink in the cold season and a source of atmospheric CO₂ in the warm season ^[35][36][43,52]. The Changjiang plume and coastal areas of Zhejiang are mainly affected by the Changjiang River while the continental shelf is mainly affected by temperature ^[36][52]. However, there are some differences between Song et al.’s analysis of the carbon sink in different regions of the East China Sea in different seasons and Liu et al.’s analysis, mainly in winter and summer, during which the nearshore area is a carbon sink or carbon source ^[36][37][52,53]. The reason for the difference may be the division of coastal and continental shelves. The following analysis was mainly based on Song et al.’s viewpoint ^[37][53].

In winter and summer, air-sea CO₂ exchange processes are relatively regular because of the relatively stable environmental conditions. In winter, due to the upwelling caused by the monsoon, pCO₂ in the Changjiang plume and the coastal areas of Zhejiang is higher, which is the carbon source. The continental shelf acts as a carbon sink by absorbing CO₂ from the surface of the ocean due to low temperatures and photosynthesis. In summer, a great amount of terrestrial discharge leads to turbidness in the Changjiang plume and the water in the coastal areas of Zhejiang, resulting in low photosynthetic efficiency. Additionally, the mineralization of the discharged organic matter produces CO₂, which is represented as a carbon source. As the distance from land increases, the body of water gradually becomes clear. The efficient photosynthesis in the continental shelf consumes a large amount of CO₂, which is represented as a carbon sink. In spring and autumn, hydrological conditions are not in a stable state, and the distribution of CO₂ sources and sinks fluctuates greatly. In spring, the carbon source and sink are similar to that in summer due to the increase in temperature and runoff in the Changjiang River. In autumn, due to the upwelling and temperature drop, the stratification degree of the water body decreases. The bottom water can be exchanged more easily with the surface water. The CO₂ accumulated in the bottom water in spring and summer returns to the air and forms the carbon source.

Although there are some differences, both reviews are relatively uniform regarding the overall situation of the carbon sink and source in the ECS. The calculation of air-sea CO₂ flux shows that the ECS is a carbon sink in spring, summer, and winter but a carbon source in autumn. The average CO₂ flux of the ECS is −6.9 mmol CO₂ m⁻² d⁻¹, which is equivalent to the carbon sink rate of 80.45 t CO₂ km⁻² yr^{−1 [35]}[43]. This method is similar to the data calculated for seafloor sediments, which are 53.90 t CO₂ km⁻² yr^{−1 [38]}[42]. However, the carbon sink rate calculated from the primary productivity of the sea surface differs greatly from the first two data ^[31](Figure 3d) [41]. One of the problems with marine carbon sinks in Zhejiang Province is that there is no high-resolution carbon sink distribution map in current studies. The ECS under the jurisdiction of Zhejiang Province is mainly coastal. The estimate may be higher than the actual figure.

The problem that needs to be solved for high-resolution carbon sink maps is the division of sea areas. It is mainly divided into two aspects: law (the sea area under the jurisdiction of each country and place determined by law) and geography (the natural division of sea area determined by geography). According to the 1982 United nations convention on the law of the sea, the current division of maritime areas is the territorial sea, contiguous zone, exclusive economic zones, and high seas. Twelve nautical miles from the base of the territorial sea is the territorial sea. Twelve nautical miles from the outer edge of the territorial sea is the contiguous zone. An exclusive economic zone is an area adjacent to the territorial sea of a coastal state beyond its territorial sea and its width does not exceed 200 nautical miles from the baseline. The seas outside the exclusive economic zone are the high seas.

The land use in Zhejiang has changed since 1970, with a decrease in farmlands and grasslands. On the contrary, the area of construction land has continued to increase, which not only leads to an increase in carbon emissions but also reduces the soil organic carbon (SOC) storage ^[39][54]. Controlling the increase in construction land is necessary to reduce carbon emissions in Zhejiang Province.

Forests and grasslands are considered the most important terrestrial carbon sinks ^[40][55]. With the Grain for Green Program (GGP) and afforestation, the carbon sink of China was enhanced with the increase in forests and decrease in farmlands ^[41][56]. However, the farmland in Zhejiang Province was only 12,905 km² in 2019, which is already lower than the basic permanent cultivated land ^[42][57]. With the limit of farmlands and vacant areas, the GGP and afforestation will be not applicable in Zhejiang in the future. In addition, the age of forests also influences the carbon sink. The old forest has lower net ecosystem production, and the regenerated forest is the important carbon sink ^[43][44][58,59]. There are currently 65.45 km² of abandoned mining area in Zhejiang Province, and 7.98 km² of abandoned mining area had been rehabilitated in Zhejiang Province in 2020 ^[45][60]. At this rate, all the mining areas can be rehabilitated by 2030. Based on the global maximum carbon sequestration potential of regenerated forests of 1158 t CO₂-eq km⁻² yr^{−1 [46]}[61], the CO₂ sink of rehabilitated abandoned mines will increase by about 0.076 Mt CO₂-eq yr⁻¹ . The carbon sink rate of forests in China is lower than the global rate, indicating that forest carbon sinks in China still have a high potential to increase due to proper management. The carbon sink of forests can increase with anthropogenic N deposition even in N-rich tropical forests ^[47][62]. It was reported that new N inputs of 2.45 t N km⁻² yr⁻¹ provide an increase of 175.08 t CO₂-eq km⁻² yr⁻¹ in carbon sequestration in temperate forest ^[48][63]. It is estimated that the 60,936 km² of forest in Zhejiang will improve 10.67 Mt CO₂-eq km⁻² yr⁻¹ for carbon sequestration.

Cultivated land can be a carbon source and a carbon sink, depending on the agricultural technology and management capabilities ^[49][64] and it is still a carbon source in China ^[50][65]. An appropriate amount of nitrogen fertilizer can increase the yield and carbon sequestration efficiency of crops, but too much nitrogen fertilizer results in the release of greenhouse gases such as N₂O and CH₄ from the land. The best N fertilizer is 19 t N km⁻², which can increase net carbon sink by about 97.7 t CO₂-eq /km²/ crop season compared with the average N fertilizer for rice production in Zhejiang Province (30 t N km⁻²) at an intermediate mitigation of CO₂ emissions ^[51][66]. Calculating based on the crop season per year and the farmland area, the carbon sinks increase by 1.26 Mt CO₂-eq yr⁻¹. With proper tillage methods such as conservation tillage, the carbon sink of cultivated land can increase. For example, no tillage in the crop residue-returned farming system can increase the soil organic carbon (SOC) by 585 t km⁻² total in the 0–60 cm soil depth compared with conventional tillage after 11 years ^[52][67]. Based on the rate, the carbon sink can increase by about 2.51 Mt CO₂-eq yr⁻¹.

Compared with land, the ocean can absorb more CO₂ with a solubility pump and biological pump. The ocean is a natural carbon reservoir and the most important buffer system for changes in the atmospheric CO₂ concentration ^[53][54][68,69]. The total amount of carbon stored in the ocean is about 50 times that of the atmosphere and the average residence time of stored carbon is hundreds of years ^[55][70]. As a coastal city, Zhejiang has a vast sea area of 2.6 × 10⁵ km², more than twice the land area. Therefore, the ocean is of great significance for Zhejiang to achieve carbon neutrality.

Current research on ocean carbon sinks mainly focuses on mangroves, seagrass beds, and salt marshes with high biodiversity and primary productivity.

Because of the limit of the high requirements for the environment and the narrow areas, increasing mangroves is an inefficient method. Instead, it was reported that the mariculture of shellfish and algae provides a carbon sink in China and the average wet weight carbon sink coefficient of shellfish and algae was 9.72% and 5.55% ^[56][72]. The data include the average carbon sink rates for different kinds of shellfish and algae calculated with Equations (1) and (2). It is predicted that the carbon sink of shellfish and algae in Zhejiang will increase in the future ^[56][57][72,73]. The area of mariculture has continued to expand, reaching 825.4 with 364.8 km² of shellfish and 173.2 km² of algae. With the expansion of the area, the production of shellfish and algae increased from 2015 to 2020 in Zhejiang Province. Algae production has increased by about 70,000 tons per year while shellfish production has increased by about 50,000 tons per year since 2017. According to the production growth rate, itwe could be ean estimated that a an increase in the carbon sink. Most shellfish and algae in China are cultivated in tidal flats and the area of tidal flat culture in Zhejiang Province is 358.15 km². If all tidal flats are changed to the cultivation of shellfish and algae, the production of shellfish will reach 4.71 Mt and the production of algae will reach 2.57 Mt according to the current culture area ratio, which can provide an increase of 1.29 and 0.40 Mt in the carbon sink: where C_s is the carbon sinks of shellfish; p_s is the production of shellfish; d_s is the dry weight ratio of shellfish; r_s is the quality weight ratio of shell; g_s is the carbon sink coefficient of shell; r_s’ is the quality weight ratio of soft tissue; and g_s’ is the carbon sink coefficient of soft tissue: where C_a is the carbon sink of algae; p_a is the production of algae; d_a is the dry weight ratio of algae (20%); and g_a is the carbon sink coefficient of algae.

Phytoplankton can adapt to various environments in the ocean, with a fast growth rate and high contribution to the primary productivity of the ocean. Phytoplankton can convert CO₂ into particulate organic carbon (POC) through photosynthesis, and then sink to the deep sea or be eaten by predators to realize carbon storage, which has the potential to improve the ocean carbon sink ^[58][59][75,76]. By fertilizing the sea (such as Fe, Al), phytoplankton in the ocean can be increased, resulting in greater absorption of CO₂. The phytoplankton particulate organic carbon (POC) flux can reach 0.77 t CO₂-eq km^–2 d^–1 when microalgae are not in bloom and 87.94 t CO₂-eq km^–2 d^–1 when they are in bloom in the East China Sea ^[60][77], resulting in about 281.05 t CO₂-eq km⁻² yr^–1 even without considering the blooming. If 10% of Zhejiang’s sea area is cultivated with phytoplankton, it can increase the carbon sink by 7.3 Mt CO₂-eq yr^–1. Although phytoplankton has great potential in carbon sequestration, the feasibility and ecological impact of fertilizing the ocean remains controversial ^[61][78]. Methods used to improve the carbon sequestration of phytoplankton need further study.