Total and labile forms of soil organic carbon as affected by land use change in southwestern Iran

• POX-C, MBC, and CWEOC were higher in undisturbed forests and pastures than in agriculture.
• Agricultural land uses had relatively higher proportion of non-labile C than original forest and pasture counterparts.
• Losses of up to 30% of the preexisting SOC stocks and 60% of total N stocks occurred in cropland soils.
• Carbon pool index was more sensitive than lability index to reflect the influence of land use changes on C dynamics.
• Labile organic carbon pools were more sensitive to land use changes than TOC contents of the soils.

The labile pools of soil organic matter enable assessment of the variation in land use changes and other management practices. This study aims to: (1) examine the effect of forest (F) and pasture (P) conversion on total and labile forms of soil organic C (SOC), (2) quantify the relationship among different labile SOC, and (3) determine whether labile SOC fractions are more sensitive indicators of soil quality than total organic C (TOC) after F and P conversion to agricultural use (forests converted (F to A) and pasture converted (P to A) soils) in southwestern Iran. Accordingly, composite soil samples with eight replicates were collected from 0–15 cm of each land use in December 2014. The samples were then analyzed for TOC, microbial biomass C, permanganate oxidizable C, and cold- and hot-water extractable organic C. The results illustrated that the TOC and all labile pools of soil organic carbon followed the trend: F > P > F to A > P to A. However, from the four land uses studied, P soils showed a better and different quality of organic C than other land use systems, which was indicated by the highest proportion of microbial biomass C (3.3%), permanganate oxidizable C (4.8%), and cold- (0.55%) and hot-water extractable organic C (3.7%), but the lowest proportion of non-labile C (95.2%) to the TOC contents of the soils. The highest amounts of SOC stocks were spotted in the F soils (42.1 Mg ha− 1) followed by P (29.7 Mg ha− 1), F to A (26.7 Mg ha− 1) and P to A (19.5 Mg ha− 1) land use systems. Accordingly, organic carbon storage of the soils decreased by land use conversion from F (36.6%) and P (34.3%) to agriculture. The carbon management index, whose variation was mainly caused by carbon pool index, was higher in P to A soils than F to A soils suggesting that organic C compounds are less degraded and contain higher amounts of labile C than F to A soils. The results also showed that labile organic carbon pools are more sensitive to changes in management practices than TOC contents of the soils. However, a single carbon pool cannot be used as the most sensitive indicator of soil quality to reflect land use differences. The permanganate oxidizable C, hot water extractable organic C and microbial biomass C demonstrated the highest sensitivity to the land use induced changes in forest soils and cold water extractable organic C and microbial biomass C were realized as the most sensitive parameters in pasture conversion to arable lands. In conclusion, it seems that the quality of both F and P soils was decreased with adoption of agriculture and therefore sustainable management practices should be employed in order to achieve soil stability and biological productivity in the area.