# Early Evidence That Soil Dryness Causes Widespread Decline in Grassland Productivity in China
## 4 Discussion
### Soil Moisture Is a Key Water Constraint Controlling the Grassland Productivity in China  
Previous studies have found that soil drought is the dominant constraint on drought stress in most ecosystems worldwide [16]. We have confirmed that soil drought is indeed the main water constraint threatening GPP in Chinese grassland ecosystems from a probabilistic perspective. However, we noted that the probability difference between soil drought and atmospheric drought causing an ecosystem GPP default gradually decreases or reverses to a negative value in most regions in the future. This represents a clear increase in the extent to which atmospheric drought will affect ecosystems in the future and a decrease in the importance of soil drought in dominating vegetation deficits. This is likely due to the fact that atmospheric VPD has increased rapidly over the past century and will remain the growing trend in the future (Supplementary Figure S1), while soil moisture trends have remained stable over time (Figure 1). Continued atmospheric constraints force ecosystems to adapt to water stress by closing stomata or stopping plant growth due to the inability to coordinate water-carbon fluxes [39]. We found that the future scenario for the Tibetan Plateau is one in which the temperature limitation of alpine ecosystems is lifted by rising temperatures, possibly due to increased atmospheric pressure that promotes vegetation growth rather than inhibiting it, thus causing soil drought to remain the main moisture constraint limiting GPP accumulation in the Tibetan Plateau in the future. It is important that identifying the primary and secondary moisture constraints from the soil and atmosphere remains a challenge [40], and a probabilistic perspective provides only a possibility rather than a definitive conclusion. In the future, there is a need for further clarification of this long-standing and complex issue to open up new avenues for improved modeling and better management of drought risk.  
However, the interannual regulation of ecosystem GPP was significantly different for soil moisture at different soil depths. We found that shallow-soil water (0-50 cm) was the dominant soil layer regulating interannual variability in GPP, probably because the most accessible water to grassland plants comes from shallow soils and is the direct source [41]. In water-scarce weather, most plants preferentially draw water from the soil [12], and roots easily take up residual water from shallow soils in a variety of ways, including mass flow, diffusion, and interception [42]. Only when shallow-soil water is exhausted can deep-rooted plants draw water from deeper soils [43]. We also confirmed that the probability of ecosystem GPP loss induced by water deficit in shallow soils is much higher than that of deep-soil water, representing a greater dependence on shallow-soil water for plant growth, and that negative anomalies in shallow-soil water can cause substantial ecosystem carbon loss.  
The innovation of our study is to not only point out that soil drought outweighs atmospheric drought as the key to the restriction of ecosystem GPP but also that shallow-soil water is critical to controlling ecosystem GPP. This addresses the fact that previous studies have only detailed the effects of soil drought on ecosystems in general, without distinguishing the relative contribution of soil moisture at different soil depths [16; 44], which also provides precedent for future validation on a global scale.