Fig. 4: Annual effect size of fertilization strategies on soil organic carbon contents compared to the sole mineral fertilizers application based on 6 long-term experimental sites in the dryland regions of China from 1991–2004.

NPKM combination of mineral fertilizers and manure, NPKS combination of mineral fertilizers and straw. CP Changping, ZZ Zhengzhou, GZL Gongzhuling, PL Pingliang, YL Yangling, WQ Wrumqi. NC North China, NE Northeast China, NW Northwest China. Data are presented as mean values ± standard error (n = 3).