Global pattern of soil carbon losses due to the conversion of forests to agricultural land

Abstract

Several reviews have analyzed the factors that affect the change in soil organic C (SOC) when forest is converted to agricultural land; however, the effects of forest type and cultivation stage on these changes have generally been overlooked. We collated observations from 453 paired or chronosequential sites where forests have been converted to agricultural land and then assessed the effects of forest type, cultivation stage, climate factors, and soil properties on the change in the SOC stock and the SOC turnover rate constant (k). The percent decrease in SOC stocks and the turnover rate constants both varied significantly according to forest type and cultivation stage. The largest decrease in SOC stocks was observed in temperate regions (52% decrease), followed by tropical regions (41% decrease) and boreal regions (31% decrease). Climate and soil factors affected the decrease in SOC stocks. The SOC turnover rate constant after the conversion of forests to agricultural land increased with the mean annual precipitation and temperature. To our knowledge, this is the first time that original forest type was considered when evaluating changes in SOC after being converted to agricultural land. The differences between forest types should be considered when calculating global changes in SOC stocks.

Figure 1. Distribution of the study sites in which forests have been converted to agricultural land (created in ArcGis 10.0).

Figure 2. Percent decrease in SOC after the conversion of forests to agricultural land.

Negative values indicate that SOC stocks increased after the land-use change.

Figure 3. The effects of cultivation age and forest type on the percent decrease in SOC stocks (A) and turnover rate constants (k) (B) after the conversion of forests to agricultural land.

Negative values indicate that SOC stocks increased after the land-use change. The error bars represent the standard errors for each group. The boreal region had 0, 12, and 10 sites in the early (≤10 yr), middle (11–50 yr), and late (>50) stages of cultivation, respectively. The temperate region had 19, 58, and 34 sites in the early, middle, and late stages of cultivation, respectively. The tropical region had 128, 162, and 30 sites in the early, middle, and late stages of cultivation, respectively. Values with different lowercase letters are different at a significance level of P < 0.05 for each cultivation stage. Values with different uppercase letters are different at a significance level of P < 0.05 for each forest type. The interaction between forest type and cultivation stage had no significant effect on the turnover rate constant (P = 0.3073) or the percent decrease in SOC (P = 0.0793).

Figure 4. The effects of initial soil clay content, bulk density, initial SOC concentration, and initial N concentration on the percent decrease in SOC stock after the conversion of forests to agricultural land.

Negative values indicate that SOC stocks increased after the land-use change.

Figure 5. The effects of mean annual precipitation and mean annual temperature on the SOC turnover rate constant (k) after the conversion of forests to agricultural land.