depth. The average R2 value in deeper layer was twice as large as
that in the surface layer (0.49–0.81). The effect of the accumulation
of fertilizer in the top layer was obvious here. The abundance of
SOC in the surface soil was typical of the area, and the original SOC
depth was approximately 30–40 cm. This SOC layer dropped to 10–
20 cm after three decades of tillage. Under the layer of top soil,
there was mainly Baijiang soil in the second layer (Argialboll), and
the SOC content was much lower.