Soil pH regulates soil biogeochemical processes and has cascading effects on terrestrial ecosystem structure and functions. Afforestation has been widely adopted to increase terrestrial carbon sequestration and enhance water and soil preservation. However, the effect of afforestation on soil pH is still poorly understood and inconclusive. Here we investigate the afforestation-caused soil pH changes with pairwise samplings from 549 afforested and 148 control plots in northern China. We find significant soil pH neutralization by afforestation—afforestation lowers pH in relatively alkaline soil but raises pH in relatively acid soil. The soil pH thresholds (TpH), the point when afforestation changes from increasing to decreasing soil pH, are species-specific, ranging from 5.5 (Pinus koraiensis) to 7.3 (Populus spp.) with a mean of 6.3. These findings indicate that afforestation can modify soil pH if tree species and initial pH are properly matched, which may potentially improve soil fertility and promote ecosystem productivity.

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Global vegetation greening has been widely confirmed in previous studies, yet the changes in the velocity of green-up in each month of green-up period (GUP) remains unclear. Here, we defined the velocity of vegetation green-up as V NDVI (the monthly increase of Normalized Difference Vegetation Index [NDVI] during GUP) and further explored its response to climate change in middle-high-latitude Northern Hemisphere.We found that in early GUP, V NDVI generally showed positive trends from 1982 to 2015, whereas in late GUP, it showed negative trends in most areas. Such contrasting trends were mainly due to a positive temperature effect on V NDVI in early GUP, but this effect turned negative in late GUP. The increase of soil moisture also in part explained the accelerated vegetation green-up, especially in the arid and semi-arid ecosystems of inland areas. Our analyses also indicate that the first month of the GUP was the key stage impacting vegetation greenness in summer. Future warming may continuously speed up the early growth of vegetation, altering the seasonal trajectory of vegetation and its feedbacks to the Earth system.

Globally about 950 petagrams (Pg) carbon is stored in soils in inorganic form (Schlesinger, 1990;Schlesinger & Andrews, 2000). The size of this global soil inorganic carbon (SIC) pool is smaller than that of soil organic carbon (SOC, 1,550 Pg, Lal, 2004) but more than that of vegetation carbon (450-650 Pg, Friedlingstein et al., 2020). This large SIC pool is usually considered stable and thus has received little attention, especially when compared with SOC (Zamanian et al., 2018). However, SIC can regulate global C cycle both directly through absorbing and releasing CO 2 (Emmerich, 2003) and indirectly through affecting soil physical and chemical properties (Bowman et al., 2008). Moreover, several recent local-scale studies showed considerable SIC responses to agricultural management and land use changes (e.g., fertilization, afforestation, and grassland restoration), therefore challenging the conventional notion treating SIC as an inert carbon component (