Evaluating storage and pool size of soil organic carbon in degraded soils: Tillage effects when crop residue is returned

Conventional tillage in Northeast China involves complete removal of crop residue and deep plowing, both of which cause significant loss of soil organic carbon (SOC). Our study aimed to evaluate SOC storage and pool sizes in degraded soils when post-harvest residue is returned to degraded soil and different tillage systems are used. We measured SOC under no-tillage (NT), ridge-tillage (RT) and moldboard plow (MP) and compared storage across treatments. We used sulfuric acid hydrolysis to separate and evaluate the size of two labile pools and one recalcitrant pool. There was no difference in crop yields (and hence C inputs) across tillage treatments over 12 years. SOC storage in the plow layer (0–20 cm) increased in all tillage systems compared to soils in the degraded state but the rate of increase was greater under NT and RT than MP. There was increased variability when we assessed deep SOC storage (0–30 cm) and SOC storage at this depth was the same across tillage systems, but still higher than in the initially degraded state. Changes observed in the size of labile and recalcitrant pools indicated these occurred mainly in the surface 0–5 cm and that RT induced slight changes in the chemical composition of SOC. Our results indicate that returning residues to degraded soils: 1) replenished SOC levels but the rate of replenishment was highest under conservation tillage, 2) had no effect on yields (C inputs) and deep SOC storage across tillage treatments, and 3) induced subtle differences in SOC pool size.