Figure 5: Sequestration quotient and its connection with C sequestration.
The sequestration quotient (SQ) in phase II (a) and in the literature21 (b), and the conceptual diagram (c) showing how to predict the impacts of earthworms on CO2 emission and net C sequestration with SQ values. In panels (a,c), means are shown±1 s.e.m., n=3 in panel (a), n=5 in panel (c); the s.e.m. is indicated by error bars. Earthworms converted the PMC into readily mineralizable C, and protected a larger proportion of readily mineralizable C. SQworm (closed squares) and SQbasal (open squares) values were calculated to quantify the balances between C mineralization and C stabilization in soil with and without earthworms, respectively. Estimation I: all litter-derived C in soil aggregates were assumed to be stabilized; estimation II: about 17% of litter-derived C in microaggregates within large macroaggregates and only 8% of litter-derived C in microaggregates were assumed to be stabilized for soil with and without earthworms, respectively21. In (c) the two dot-dash axes represent the reference x axis and y axis in control soil without earthworms; the origin of the dot-dash axes refers to a reference point of no metabolic activity. The two bold grey axes represent the x axis and y axis in soil with earthworms, namely the net effects of earthworms on C activation (Cact-w axis) and C stabilization (Csta-w axis); Zones I, II and III indicate the three major scenarios of the contribution of earthworms to C sequestration. The open white circle, that is, the origin of the bold grey axes (Cact-m, Csta-m), refers to the mean values of Cact-m and Csta-m in control soils, that is, the basal point. The closed black circle (Cact-w, Csta-w) refers to those values in soils with earthworms. Data are the same as presented in panel (a,b). Csta-m or Cmin-m and Csta-w or Cmin-w refer to the soil microbiota-induced and earthworm-induced Csta or Cmin, respectively. As part of Cmin-m may be metabolized and/or stabilized by earthworms, Cmin-m is also defined as basal C (Cbasal-m). Note that the conversion rate from PMC to Cmin-m and Csta-m may also be affected by earthworms, thus the value of Cmin-m (Cbasal-m) in a system with and without earthworms may gradually differ as earthworm incubation proceeds.
