# Early Evidence That Soil Dryness Causes Widespread Decline in Grassland Productivity in China
## 3 Results
### Comparison of the Regulation of GPP by Different Soil Layers  
The impact on the GPP of the ecosystem varies across soil layers due to differences in rainwater recharge and evapotranspiration at different depths. The warm-season SM and GPP were correlated at the meta-scale to compare the bivariate structure between SM and GPP at different soil depths (Figure 4). The bivariate correlations for shallow soils (0-20 and 20-50 cm) were found to show significant interannual agreement, with highly significant positive correlations in both historical and future periods. Deeper soils (100-200 and 200-300 cm) with higher possible difficulty in water use for plants, did not show a significant consistent interannual correlation in the historical period, and the interannual relationship between the two variables tended to be consistent in the future period. As shown in Figure 4c, most of the pixel correlations between shallow-soil water and GPP are greater than 0, implying a strong positive coupling between the two, with shallow-soil water deficit significantly constraining the accumulation of GPP in the ecosystem. In contrast, about half of the pixels of the deep-soil water are on both sides of the 0 value line, showing very high uncertainty in the regulation of GPP. Overall, the correlation between ecosystem GPP and the SM of different soil layers showed a higher correlation the closer to the surface the soil layer was. Comparing different soil layers, it can be seen that soil  
Figure 3: Correlation between GPP\({}_{\text{IAV}}\) and SM\({}_{\text{IAV}}\) for Chinese grasslands during the period 1901–2099. (**a1,b1,c1,d1,e1**) is annual, and (**a2,b2,c2,d2,e2**) is the warm season. The correlation between SM\({}_{\text{IAV}}\) and GPP\({}_{\text{IAV}}\) per year in the warm season was calculated with a 41-year sliding window, i.e., by calculating the correlation between the two variables from 1861–1901 as the result in 1901, and so on, to find the correlation from 1901–2099. The red line in the figure shows the years in which the correlations passed the significance test (\(p<0.05\)). moisture at different depths has an inconsistent regulating effect on ecosystem GPP, and uncertainty has a non-negligible effect on GPP constraint. Figure 5 shows the spatial distribution of the probability of different degrees of ecosystem GPP loss under the precondition of extreme drought in soils with different soil moisture levels, and found that the probabilities show great differences between different soil levels. The probability of severe GPP loss induced by drought events at the 0-20 cm soil depth was as high as 32.49%, decreasing to 27.64%, 16.79%, 16.43%, 16.43%, and 12.61% with the increase in soil depth. We found that after drought in shallow soils, the ecosystem GPP tended to be severely deficient due to water limitations, while the probability of drought-induced vegetation deficit in deep soils was lower. In addition, we also emphasized that the probability of severe shallow-soil drought induced moderate and mild ecosystem GPP loss, while the ecosystem GPP was significantly higher than that of deep-soil drought. The locations of the spatial distribution of ecosystem GPP loss probabilities for severe, moderate, and mild were basically the same, indicating that soil drought events of different soil depths had weaker effects on the changes in probability at different locations. The spatial heterogeneity of ecosystem GPP loss induced by shallow-soil drought events was stronger. Xinjiang and the Loess Plateau are the most susceptible grasslands to soil drought, followed by Inner Mongolia grasslands and Tibetan Plateau grasslands being the least affected. The probabilities associated with deep-soil drought were more uniform and consistent in space, with less variation among regions (Figure 5(a6)). Figure 4: Correlation between warm-season SM and GPP for different soil depths. The upper column is the historical period and the lower column is the future period. (**a1,a2**) Scatter plots of SM and GPP per year.