Fig. 4: Changes in topsoil δ15N over 2000s ~ 2010s.
From: Progressive nitrogen limitation across the Tibetan alpine permafrost region
a, b Frequency distributions of bulk soil δ15N and changes in bulk soil δ15N during the period between the 2000s and the 2010s, respectively. The change in bulk soil δ15N was examined with linear mixed-effects model, in which the fixed effect was year and the random effect was sampling site. Points in b denote mean values and error bars represent 95% confidence intervals. N, the number of sites used for analyzing bulk soil δ15N; Median, median value of bulk soil δ15N; df.resid, residual degrees of freedom.
