1 Land use/cover change (LUCC) in Qinghai Province over 30 years
The distribution of land use/cover types in Qinghai Province in 1990 and 2020 was shown in Figure S1. Grassland is one of the major land use types in the Qinghai Province with an extremely wide distribution area, and grassland converted into bare land, cropland, water bodies and marshland are more obvious over the 30-year period (Table S2 and Fig. 2 (c)). Bare land is also widely distributed and mainly located in the northwestern Desert and Gobi area and northern Sanjiangyuan Nature Reserve, and bare land converted into grassland was more obvious during the 30-year period especially in Desert and Gobi area(Fig. 2 (d)); this may be related to the change in vegetation growth conditions on the QTP due to climate change and possibly to the improvement in land desertification. The distribution of cropland and impervious land can be observed more concentrated in the Agricultural and Pastoral area, and the area of both land use types in 2020 increased compared to 1990. In addition to Qinghai Lake, the lakes and water bodies in the central and western parts of Qinghai Province are also more widely distributed and most in Sanjiangyuan Nature Reserve. The results also showed the interconversion between different land use/cover types was more significant and varies among functional areas.
The types of LUCC in Qinghai Province from 1990 to 2020 are shown in Fig. 3. Here, we showed the more obvious and typical regions of LUCC. The conversion of bare land and grassland was the most significant and mainly reflects the interconversion of grassland and bare land. The conversion of bare land to water bodies and marshland was also more obvious, considering that it was related to the increase in precipitations. Grassland was also mainly converted to shrubland, marshland and water bodies(Fig. 2(e), taking Desert and Gobi area as an example). The high proportion of area converted from cropland to grassland in 2020 indicates that it may be less difficult to reclaim grassland, and the area from cropland to grassland was also higher, mainly in Desert and Gobi area(Fig. 2(a)).
The largest converted of bare land area occurred in Desert and Gobi area, mainly to grassland and the conversion area more than 21,000 km², while to water bodies and marshland were more distributed mainly in the central part of this area(Fig. 2(d)), but only 1680.63 km² and 1424.68 km², respectively. Grassland was mainly converted to bare land, accounting for 86.93% of the conversion area of grassland in Desert and Gobi area. The marshland was mainly converted into grassland, bare land and water bodies. The conversion of cropland was not significant and mainly to grassland and impervious land.
From 1990 to 2020, grassland was converted not only into bare land(32.05%) but also mainly to cropland(15.86%) and shrubland(25.72%) in Agricultural and Pastoral area, while the new bare land was mainly from grassland, in addition to bare land(11,780km²). In 2020, almost half of the area of grassland was converted from other land use/cover types, mainly shrublands, croplands, marshlands, woodlands and water bodies. These are related to the high intensity of human activity in this area, mainly reflected in the changes in the cropland area and the fact that with the largest cropland and impervious land areas. However, the conversion of cropland to impervious land was also more evident in Agricultural and Pastoral area from 1990 to 2020(Fig. 2(a)).
Pastoral area showed relatively clear conversion between shrubland, grassland and bare land and different from the other three functional areas in that the conversion area proportion of grassland mainly to shrubland(39.62%) exceeded that of bare land(30.73%) from 1990 to 2020, the area from grassland to shrubland was in addition to bare land, and the area from bare land was significantly lower than other three functional areas. Forestland, bare land, and water bodies were also mainly converted into grassland, while in 2020 the new bare land was mainly from grassland.
Sanjiangyuan Natural Reserve had the highest percentage of conversion in bare land, and the area converted out was higher than the area converted in over 30 years. The largest area to grassland was 21288.99 km²(84.47%), followed by 2415.67 km² to water bodies, while the converted out area of grassland was lower than converted in and grassland was also mainly to bare land(1113.90 km²) and water bodies(3635.48 km²). The conversion area of grassland to water bodies in 2020 was 3468.24 km², accounting for 71.34% of the area from water bodies. The new cropland in 2020 was converted from grassland.
2 Drivers of land use/cover change(LUCC) in Qinghai Province
RF model results showed that natural drivers were the main driving factors especially climatic factors of overall Qinghai Province. The results of evapotranspiration(EVP), temperature change and elevation were all exceeding 30%(Fig. 4(a)), but slope had the lowest degree among all drivers. Elevation may have an impact on temperature and precipitation and high elevation may have limited of LUCC due to human activities, especially cropland reclamation and urban construction. The results of population increased and urbanization exceeds 20%. Soil types and soil erosion types mainly affect LUCC indirectly by influencing vegetation growth(e.g., long-term constant cover types) and agricultural production may act as potential factors affecting SOC stocks(Li et al., 2022a) but has no significant effect on Qinghai Province. NDVI can be considered a proxy for the LUCC results but did not have a high driving force on LUCC.
At the functional area level, elevation, relief degree of the land surface(RDLS), the climatic factors, including temperature change, precipitation change and EVP change all had significant effects on LUCC in four functional areas(Fig. 4(b)). In particular, the average effect of elevation in four functional areas had the highest results, and according to the spatial pattern of elevation(Figure S2), the most complex elevation change was located in Agricultural and Pastoral area. The bare land was mainly distributed in the high elevation area, while the relatively low elevation mainly showed the conversion of bare land to cropland and grassland, cropland to impervious land, and grassland to cropland. Among them, the Pastoral area was most strongly influenced by topographic factors, and it can be considered that topography was the main limiting factor for pastoral activities.
Temperature, precipitation change and EVP change have a relatively high effect on the LUCC of the Sanjiangyuan Natural Reserve while the 3 climatic factors have strong effects on the four functional areas but have higher impact on the LUCC of functional areas with low intensity of human activities, especially for Sanjiangyuan Natural Reserve located in the hinterland of the QTP, which focuses more on ecological and environmental protection rather than the main production activities gathered in. NDVI showed a more significant effect only in Desert and Gobi area, considering the increase in vegetation cover over 30 years due to the high area of bare land and the influence of climate and other factors.
The extent of soil erosion on the overall LUCC in Qinghai Province was not high. However, the impact on the LUCC of the Desert and Gobi area was more obvious where mainly wind erosion occurs, indicating that such a situation as the conversion of bare land to grassland may be limited by soil conditions in addition to the influence of precipitation.
The most significant population growth was in Desert and Gobi area and Agricultural and Pastoral area(Figure S2), with cities having a population growth rate of over 30%, while in the central part of the Sanjiangyuan Natural Reserve and some cities in the southern part of the Pastoral area, the population growth was not significant, but the change in urbanization rate seems to have the opposite trend, especially in eastern part, where there was a decrease in urbanization rate. Human intervention under the process of returning farmland to forest/grass was also influenced by changes in population dynamics. Although the Agricultural and Pastoral did not yield significant results for changes in population and urbanization rate, we still believe that this may be influenced by more complex anthropogenic factors.
3 Soil organic carbon(SOC) stocks under different land use cover types in Qinghai
We obtained the results by sampling sites(Figure S3) of SOCD at 0–30 cm(Fig. 5(a)(c)) and 0-300 cm(Fig. 5(c)) in Qinghai Province, which corrected the accuracy of the SOCD dataset(Fig. 5(a)(c)) of the TPDC "3 m depth soil organic carbon pool dataset". Based on the equation of SOC stocks, the average SOCD and the SOC stocks of corresponding land use/cover types in each functional area were calculated in both 1990 and 2020.
The changes in SOC stocks in 0-30cm layer of the four functional areas over 30 years was not obvious(Fig. 6(b)). Sanjiangyuan Natural Reserve has the highest SOC stocks in 0-30cm layer and increase of 48.16 Tg C in 2020(1312.23 Tg C) compared to 1990(1264.07 Tg C). The lowest SOC stock in 0-30cm layer was in Desert and Gobi area and decreased in 2020(732.62 Tg C) compared to 1990(729.84 Tg C) and was the only functional area where SOC stocks showed a decreasing result in 0-30cm layer. We found that the SOC stocks of the Desert and Gobi area and Sanjiangyuan Natural Reserve mainly stored in grassland and bare land(Fig. 6(a)), and the SOC stocks of bare land accumulated in Desert and Gobi area are higher(although we have marked the range of possible values of carbon stocks in water bodies, the SOC stocks we calculated do not take into account water bodies and ice/snow), considering that the SOC stocks decrease in Desert and Gobi area of 0-30cm layer was due to a decrease in grassland and bare land(Table S2). Agricultural and Pastoral area as well as Pastoral area showed a total increase in 0-30cm layer SOC stocks from 1990 to 2020 with a decrease in bare land in Agricultural and Pastoral area, which was a land cover type with lower SOCD density in this area. However, the increasing areas of grassland and forestland with higher average SOCD since the area of cropland encroached on part of the grassland, which also decreased part of the SOC stocks. The SOC stocks in the 0-30cm layer of the Pastoral area are mainly in the grassland, and the proportion of SOC stocks in bare land also decreased in 0-30cm layer, replaced by the increase area of grassland and shrubland accumulated more SOC stocks.
The SOC stocks within the 0-300cm range differed from 0-30cm layer SOC stocks, with the highest content in both 1990 and 2020 being the Pastoral area, and the Agricultural and Pastoral area also had higher SOC stocks than Sanjiangyuan Natural Reserve within the deep range, suggesting that Sanjiangyuan Natural Reserve and Agricultural and Pastoral area may have accumulated more SOC in deep soil layers than the other two functional area, especially in some key SOC stock land use/cover types with differences in SOCD between 0-30cm layer and deep ranges(0-300cm), for example, the average SOCD of 0-300cm layer grassland in Desert and Gobi area was 2.05 times higher than 0-30cm layer, while more than 4 times higher in Pastoral area. In addition, the land use/cover type with higher SOC stocks was also a key factor affecting SOC stocks. However, the Sanjiangyuan Natural Reserve was the only area where SOC stocks increased in 0-300cm layer. Desert and Gobi area had lower SOC stocks in both surface and deep range layers, especially in 0-300cm SOC stocks that are mainly contributed by bare land, and the total change in 0-300cm stocks was decrease, with this functional area decreasing by 2.78 Tg C, over the 30 years and was the only functional area where both surface and deep range layers SOC stocks decrease, which can indicate that the potential of carbon sequestration here was indeed low and mainly effected by LUCC driven by climate, soil erosion.
4 Analysis of soil organic carbon(SOC) stocks changes under the influence of land use/land cover change(LUCC)
According to the changes in SOC stocks in different land use/cover types(Fig. 6 and Table 2), grassland that mainly affected the increase in SOC stocks, and the SOC stocks of grassland in Sanjiangyuan Natural Reserve have increased the most over 30 years, with an increase of 294.55 Tg C in 0-300cm and was also the main contributing factor to the increase in total SOC stocks in Qinghai Province in both 0-30cm and the deep range layer. In contrast, the SOC stocks of bare land were all decreased, which may be directly related to the decreased in bare land areas. We also found that the cropland of Desert and Gobi area as well as Agricultural and Pastoral area had increased SOC stocks in both 0-30cm and deep range layers. On the one hand, the improvement of soil quality when the Desert and Gobi area was converted from bare land to cropland can increase certain carbon sequestration capacity by human activities, and second, since the cropland of the Agricultural and Pastoral area was larger, which may also contribute to the increase of SOC stocks in cropland, but both ways cannot cause the total SOC stocks impact because the account of SOC stocks increase was slight. The same situation was also shown for impervious land. Forestland and shrubland showed a weaker change but undeniable that they have a higher average SOCD with a higher carbon sequestration potential. In contrast, the average SOCD of marshland was highest in Sanjiangyuan Natural Reserve area and most of the marshland was converted from bare land due to climate change, which may become the area with higher carbon sequestration potential in the future.
Table 2
SOC stocks and changes in different land use/cover types within 0-300cm in Qinghai. (Tg C)
Land use/cover types | 1990 | 2020 | Total Changes |
Ⅰ | II | III | IV | Total | Ⅰ | II | III | IV | Total |
Cropland | 8.30 | 155.89 | 6.59 | 0.00 | 170.78 | 11.30 | 161.11 | 7.49 | 0.05 | 179.95 | 9.17 |
Forestland | 11.96 | 110.51 | 62.29 | 1.11 | 185.87 | 11.81 | 109.48 | 63.50 | 1.14 | 185.92 | 0.05 |
Grassland | 697.81 | 2131.11 | 3153.90 | 2003.68 | 7986.50 | 696.32 | 2283.70 | 3242.67 | 2298.24 | 8520.92 | 534.42 |
Impervious land | 2.53 | 10.47 | 1.01 | 0.11 | 14.11 | 4.30 | 19.13 | 2.17 | 0.32 | 25.92 | 11.81 |
Bare land | 1329.32 | 630.25 | 282.66 | 744.56 | 2986.79 | 1313.57 | 441.55 | 190.11 | 614.37 | 2559.59 | -427.20 |
Shrubland | 15.49 | 317.01 | 185.30 | 3.86 | 521.66 | 15.35 | 312.54 | 183.39 | 2.89 | 514.16 | -7.50 |
Marshland | 57.04 | 151.86 | 83.74 | 41.45 | 334.09 | 48.22 | 147.65 | 83.50 | 41.15 | 320.52 | -13.57 |
An increase in SOC stocks effected by LUCC was mainly caused by converting into grassland(Fig. 7 and figure S4) in all four functional areas. Desert and Gobi area also shows an increase SOC stocks due to the conversion to wasteland and, especially in the deep(300 cm) range, exceeded the increase caused by the conversion to grassland. The same result was evident in Sanjiangyuan Natural Reserve, but the proportion was much lower in 0-30cm(10.34%) and 0-300cm(18.12%) layers than conversion to grassland. The increase in SOC stocks in Agricultural and Pastoral area was also mainly contributed by conversion to grassland(89.54%), with the largest increase in SOC stocks from the impervious land to grassland. In addition, the conversion to bare land, cropland and shrubland in Agricultural and Pastoral area also caused increase in SOC stocks and in Pastoral area also caused a certain proportion of SOC stock increase by conversion to grassland in both 0-30cm and 0-300cm layers.
The decrease in SOC stocks due to LUCC(Fig. 8 and figure S5) was more evident in all four functional areas caused by the grassland to other land use/cover types and was most obvious in Pastoral, while the conversion of bare land to other LULC types in Desert and Gobi area was the main reason for the decrease in SOC stocks in this functional area, indicating that the large area of bare land in Desert and Gobi area has accumulated a certain amount of SOC over a long period of time, and the decrease in SOC stocks was mainly caused by the conversion to cropland and impervious land. Similarly, in Agricultural and Pastoral area, the proportion of SOC stocks decreasing due to the conversion of grassland to impervious land(23.32% in 0-30cm and 21.86% in 0-300cm layer) was the highest, and the proportion of SOC stocks decreasing due to the conversion to forestland was higher than to cropland. In Sanjiangyuan Natural Reserve, although the converted out of the bare land caused a slight decrease in SOC stocks, the highest proportion of SOC stock decrease was caused by grassland and bare land to shrubland.