# Early Evidence That Soil Dryness Causes Widespread Decline in Grassland Productivity in China
## 4 Discussion  
Soil Moisture More Strongly Regulates Carbon Balance Than Atmospheric Indicators in Chinese Grasslands  
Terrestrial ecosystem models can obtain high-resolution long time series of soil moisture products, bridging the gap between remote sensing and station observations [22] and providing important information for exploring long-term soil drought evolution. We confirmed that soil moisture deficit can severely weaken the accumulation of ecosystem GPP and is an important aspect in regulating the interannual dynamics of GPP. Previous studies have shown that CO\({}_{2}\) growth rates are sensitive to observed changes in terrestrial moisture, i.e., drought years are associated with rapid increases in atmospheric CO\({}_{2}\)[7]; higher water availability promotes ecosystem productivity fixation, and water loss attenuates it [28; 29]. Our results are similar and showed that low SM corresponds to low GPP, reflecting a reduced potential for ecosystem CO\({}_{2}\) uptake in years with low soil water. Thus, a lack of soil water supply would affect the division and expansion of individual plant cells and greatly reduce the ability to obtain carbon from the atmosphere, which in turn would result in a decrease in ecosystem GPP. Considering that soil water content is closely related to precipitation, most of the precipitation infiltrates into the soil, except for a small portion that evaporates and is trapped by the canopy [30]. Most of the water available for plant uptake originates from precipitation, which affects the photosynthetic capacity of plants and is mainly regulated by the intra-annual distribution of precipitation and differences in precipitation intensity [31]. Changes in the intra-annual distribution of precipitation can lead to a mismatch between  
Figure 7: 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 future 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 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..  
water availability and plant growth requirements [32]. For example, precipitation events that occur during the germination period do not match actual water demand and supply. As a result, much of the water is likely to be inefficient and 'wasted' in the evapotranspiration process [33]. It has been suggested that increasing precipitation early in the growing season can have a positive impact on semi-arid grassland productivity [34]. This may be due to a number of factors, including the fact that the earlier the precipitation, the higher the soil moisture, and the fact that more precipitation early in the growing season promotes plant root growth. In contrast, more precipitation events later in the growing season do not significantly improve photosynthetic capacity or carbon accumulation [35]. In addition, changes in the magnitude or intensity of precipitation events can alter the vertical distribution of soil moisture, which is closely linked to the underground activity of plants [36]. Small precipitation events tend to increase the shallow-soil water content and stimulate shallow-rooted plant activity (photosynthesis and autotrophic respiration) and soil microbial activity.