Terrestrial Carbon Storage Estimation

The carbon storage and sequestration of the InVEST model (integrated valuation of ecosystem services and trade-offs) is an effective tool for estimating terrestrial carbon storage [18 ]. It divides terrestrial carbon storage into four principal carbon pools, which are AGC (above-ground carbon storage), BGC (below-ground carbon storage), SOC (soil carbon storage), and dead organism carbon storage (Equation (3)) [33 (link)].
$$C_i=C_{i-above}+C_{i-below}+C_{i-soil}+C_{i-dead}$$
$$C_{total}={{\displaystyle \sum }}_{i=1}^n{C}_i{S}_i$$
In Equations (3) and (4), C_total is the total terrestrial carbon storage, C_i represents the total carbon storage of land category i, C_above refers to above-ground biotic carbon storage, and C_below represents below-ground biological carbon storage. C_soil means soil carbon storage with soil depth of 10 cm. C_dead indicates the carbon storage of dead organisms. S_i is the total area of land category i [17 ].
Due to the low carbon density content of dead organisms and the difficulty of obtaining data, only the carbon storage of three major carbon pools was studied in this work. To ensure the accuracy of the study results, carbon density data within Jilin Province were used as much as possible, and carbon density values obtained by the same scholar or the same method were used due to the difference in research methods that may have caused deviations in the results [34 ]. The data collected through previous studies were all carbon density data from the 2010s. However, carbon density has the characteristics of changing with time and climate change [13 ]. To enhance the accuracy of carbon storage simulation results, it is necessary to average the carbon density values of secondary land categories with little difference and correct the carbon density by using meteorological factors.
The results of domestic and international studies show that both biological carbon density and soil carbon density are positively correlated with annual average precipitation [35 (link),36 (link)], and most of the established studies have obtained correction coefficients by comparing other regions with clear historical carbon density with the study area [2 ]. Combining the established studies with the needs of this work, the following equations were selected for the correction of carbon density.
$$C_{sp}=3.3968\times P+3996.1\left(R^2=0.11\right)$$
$$C_{bp}=6.7981e^{0.00541P}\left(R^2=0.7\right)$$
$$K_{sp}=C_{sp1}/C_{sp2}$$
$$K_{bp}=C_{bp1}/C_{bp2}$$
where C_sp is the annual precipitation-corrected soil carbon density data (unit: Mg/hm⁻²); C_bp is the annual precipitation-corrected biological carbon density data (unit: Mg/hm⁻²); and P is the annual average precipitation (unit: mm). K_sp is the annual average precipitation correction factor of soil carbon density. K_bp is the annual average precipitation correction factor of biological carbon density; C_sp1 and C_bp1 are the carbon density correction factors of Jilin Province. C_sp1 and C_bp1 are the correction coefficients for carbon density in Jilin Province. C_sp2 and C_bp2 are the correction coefficients for historical carbon density in Jilin Province. The values of the average annual precipitation are 766.6 mm and 811.2 mm, which are the average annual precipitation of Jilin Province in 2020 and of Jilin Province in 2010, respectively. The correction coefficients were substituted into the correction equation and revised to obtain the missing carbon density values for each land category. Considering the difficulty of obtaining carbon density data and the deviation in predicted values, the constant carbon density values selected were used to calculate the changes in carbon storage in historical and future periods in this work (Table 3). The partial carbon density data of 2020 obtained via correction were compared with the carbon density data collected in the field for verification. The measured above-ground carbon density of grassland in western Jilin Province is 19.8 Mg·hm⁻², and the soil carbon density is 330.2 Mg·hm⁻²; the soil carbon density of wetland is 141.7 Mg·hm⁻², and the soil carbon density of other lands is 248.1 Mg·hm⁻². In a reasonable range, the results can be used to input model parameters.