Abstract:
Depletion of soil organic matter (SOM) in Village tank cascade system (VTCS) in Sri Lanka has
been identified as a major constraint. This study determines the changes in labile and non-labile
fractions in different land uses in Mahakanumulla VTCS and assess the sustainability of their soil
organic carbon management. Soil samples (n=166) were taken from 0-30 cm depth along a
longitudinal linear transect and three cross-sectional linear transects at 50 m intervals,
representing the distribution of ten tanks and five predominant land uses (paddy, forest,
scrub/shrub, upland crops and home gardens). Permanganate oxidizable carbon (POXC) and
total organic carbon (TOC) contents of the soil were determined and the lability, carbon pool
index (CPI), lability index (LI) and carbon management index (CMI) were calculated.
Considerable variability of TOC and POXC was observed between the land uses (CV>25%). Even
though the POXC fraction in the soils of the scrub/shrub was 9% lower than that of the forest
soils, there was no significant differences. Forests recorded the highest TOC (1.33 ± 0.38%)
compared to all the other land uses. The CPI has decreased by 18-33% in lands converted from
forests to agricultural lands and home gardens and the highest reduction was observed in paddy
lands. However, the highest lability was recorded in paddy soils (0.056) in which the LI also
significantly higher than that of home gardens, forest and scrub/shrub. Harvesting practices
using combine harvesters, ensure the return of paddy straw back to the same field, rather than
burning and submerged conditions might have resulted the higher lability in OC in paddy soils.
The CMI of scrub/shrub lands (82.9), previously cultivated with upland crops or as chena but
now abandoned for about 10-20 years, was significantly lower (p<0.05) than that of all the other
land use types (98.0 to 103.7) that were not significantly different among themselves. While CMI
values indicates that SOM management practices in paddy lands, upland soils and home gardens
of the VTCS are sustainable, the need for introducing management strategies such as
agroforestry systems combined with the addition of organic matter/amendments for
scrub/shrub lands in the VTCS is highlighted
