Fig. 2. Geographical distribution of field sites and nonlinear responses of individual soil functions and multifunctionality to increasing aridity.

a Map of the 130 field sites in drylands across northern China. The sites represent the four different dryland subtypes (i.e., dry-subhumid, semiarid, arid, and hyperarid regions), which are defined with different aridity levels, as demonstrated in the legend. Inset map shows China with the study area colored in light blue. b–i Nonlinear responses of DNA concentration (b), soil organic carbon (c), total nitrogen (d), ammonium (e), nitrate (f), total phosphorus (g), available phosphorus (h), and soil multifunctionality (i) to aridity, and their respective aridity thresholds. The orange and red dashed lines indicate the nonlinear trends fitted by quadratic or generalized additive models (GAMs). The gray dashed lines and inset numbers in red represent the aridity thresholds identified. The blue solid lines denote the linear fits at both sides of each aridity threshold.