Effect of Elevation and Mineralogy on the Amount and Turnover of Fractionated Organic Carbon in Paddy Soils in Nepal

The storage of soil organic matter (SOM) is essential for maintaining and improving soil fertility. To obtain basic information about the status of SOM in paddy fields in Nepal under various ecological settings, we investigated the amount and turnover rate of stored carbon (C) in fractionated SOM in the surface layer. Soil samples from the top 15 cm plough layer were collected from 21 sites along an elevation gradient ranging from 78 to 2002 m a.s.l. in the central region of the country, and in eight sites in the lowland area in the eastern region to investigate regional differences in SOM status. SOM was fractionated into four components: (1) light fraction (LF, < 1.8 g cm⁻³), (2) heavy fraction (HF) consisting of physically stable aggregates, (3) oxidizable clay + silt fraction (OxF), and (4) nonoxidizable clay + silt fraction (NOxF) forming organo-mineral complexes with fine-textured minerals. The amounts of C in all fractions were determined, and the ∆¹⁴C values of selected samples were evaluated as indices of C turnover rate. The amount of stored C increased with elevation from 78 m (13.3 g kg⁻¹) to ca. 1700 m a.s.l. (28.0 g kg⁻¹). However, the total C content and C contents in LF, OxF, and NOxF exhibited decreasing trends from 1700 to ca. 2000 m a.s.l. (20.4 g kg⁻¹), probably because of decreased biomass production and decreased amorphous soil minerals at ca. 2000 m. The Δ¹⁴C values indicated that the C turnover rates in HF, OxF, and NOxF were faster at higher elevations (1221 m) than at lower elevations (78 m). These results suggest that mineralogy can have greater influence on C turnover than the climate difference in these mineral-associated C fractions through SOM stabilisation. In lowland, the amounts and turnover rates of stored C in the soil fractions were larger and slower in the central region than in the eastern region, respectively, reflecting differences in soil texture and mineralogy. Multiple regression analysis showed that the amount of C was negatively influenced by the mean annual temperature in all fractions and positively influenced by amorphous Al minerals (Alo–Alp) in OxF and NOxF. The coefficients for temperature further suggest that the relative vulnerability of C to temperature increase is in the order of LF>HF>OxF>NOxF. These findings can serve as a basis for the maintenance and improvement of paddy soil fertility in Nepal for sustainable agricultural management.