# 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
Red, green, blue, cyan, and pink represent soil moisture at 0–20 cm, 20–50 cm, 50–100 mm, 100–200 mm, and 200–300 mm soil depths, respectively; (**b1,b2**) correlation between SM and GPP regional averages; (**c1,c2**) probability density plot of correlation between SM and GPP. drought. For the historical period, we found that soil drought in Chinese grasslands causes a higher probability of ecosystem GPP loss than atmospheric drought, but the results vary widely with different threshold settings (Figure 6). The difference between the two was relatively large in the most severe drought scenario, with 8.16%, 10.41%, and 6.93% differences for severe, moderate, and mild ecosystem GPP losses, respectively (Figure 6a-c). The differences also showed different degrees of reduction with decreasing drought levels. From the probability perspective, we concluded that the overall impact of soil drought on Chinese grassland ecosystems is significantly higher than that of atmospheric drought, and the probability of GPP deficit caused by soil drought is on average 1-10% higher than atmospheric drought. However, the difference was found to be spatially heterogeneous, with soil drought in Inner Mongolia, the Tibetan Plateau, and the Loess Plateau apparently causing a higher probability of ecosystem GPP loss, which is the main constraint limiting GPP accumulation. In contrast, the probability of atmospheric drought causing ecosystem GPP deficit was higher in the grassland of the Tibetan Plateau, which represents a stronger constraint of atmospheric drought on the GPP of the Tibetan Plateau. For the future period, soil drought remains the main water constraint limiting the accumulation of GPP in the ecosystem (Figure 7). However, compared to the historical period, the spatial differences changed significantly. The difference in the Loess Plateau tends to be negative from the positive value in the historical period, indicating that atmospheric drought will become the most important moisture constraint in the future. Inner Mongolia and Xinjiang, although still positive, have decreased compared with the historical period, indicating that the moisture constraint from the atmosphere has increased and the soil moisture constraint has weakened. The Tibetan Plateau is very unique, and soil drought will become the main constraint on GPP accumulation in the Tibetan Plateau ecosystem in the future, and its probability difference is more than 10%. Figure 6: Difference between the probability of ecosystem GPP loss under soil drought conditions minus the probability of ecosystem GPP loss in the atmospheric drought scenario for the historical period. (**a–c**) The difference between the probability of severe, moderate, and mild ecosystem GPP loss corresponding to severe soil drought and atmospheric drought; (**d–f**) the difference between the probability of severe, moderate, and mild ecosystem GPP loss corresponding to moderate soil drought and atmospheric drought; (**g–i**) the difference between the probability of severe, moderate, and mild ecosystem GPP loss corresponding to mild soil drought and atmospheric drought; (**g–i**) the difference in the probability of severe, moderate, and mild ecosystem GPP loss corresponding to mild soil and atmospheric drought.