Declining total carbon stocks in carbonate-containing agricultural soils over a 62-year recultivation chronosequence under humid conditions

Abstract

Replanting of mining soils is necessary for utilizing soil resources and increasing cultivated land areas. However, limited information exists on the long-term temporal trends of carbon accrual in agricultural systems containing carbonate-rich soil material. We examined changes in soil organic carbon (SOC), soil inorganic carbon (SIC), and total carbon (TC) stocks in an agricultural soil containing carbonate over a 62-year recultivation chronosequence. The most critical differences in the SOC, SIC, and TC stocks were observed in the 0–30 cm soil layer. The results revealed that the SOC stock increased rapidly during the first 10–20 years, but only slowly thereafter. The SIC stock decreased over 62-year from approximately 40 Mg C ha−1 to 2 Mg C ha−1. According to soil δ13CTC data, the SIC to TC ratio decreased from 83% (year 0) to 7% (year 62). Overall, the average sequestration rates were 0.30 Mg C ha−1 y−1 for SOC and −0.61 Mg C ha−1 y−1 for SIC over the 62 years after recultivation. Total carbon ultimately declined by approximately 19.5 Mg C ha−1 in recultivated carbonate soils. Topsoil SOC model (Rothamsted Carbon Model) outputs predicted an equilibrium value of 38.6 Mg C ha−1 after 197 years, which was less than the SIC stock lost in the first 70 years. Therefore, an overall TC increase in these carbonate-containing agricultural soils will only occur (i) during the initial rapid SOC sequestration accumulation phase (first 20 years of recultivation); and (ii) after the soils are fully decalcified (after ∼62 years), but when SOC still slowly increases before SOC stocks reached full equilibrium (after ∼197 years). However, compared with starting TC stocks, when we consider periods over a semicentennial and beyond, we will likely lose more TC than we gain in these recultivated agricultural soils if there are no additional TC sequestration measures.