Soil Organic Carbon Stock of Some Upland Use System Under Tropical Monsoon Climate and Their Interrelationship with Soil Water Retention

Soil organic matter is the most important component of soil, contributing to soil biological, chemical and physical properties. A study was undertaken to assess the organic carbon stock of dominant land use/systems in the upland of the eastern region (Kadalipal watershed, Dhenkanal, Odisha, India). The dominant land use systems selected were forest grazing land (C0), maize (C1), rice (C2), groundnut (C3), cucumber (C4), okra (C5), cowpea (C6), cashew plantation (C7) and barren land (C8). A correlation matrix was developed among SOC, water retention at field capacity, permanent wilting point, bulk density, particle size distribution (sand, silt and clay) and pH. The R2 and slope of different relationships (single/multiple/regression) were computed. The organic carbon and water retention at saturation, field capacity (−33 kPa) and permanent wilting point (−1500 kPa) were grouped depth-wise, and the R2 and slope of the relationship between water retention at field capacity and permanent wilting points of the particular depth and SOC of respective layers were derived. Soil water retention at field capacity (−33 kPa) was found to be correlated with SOC at 0–0.15 m depths only rather than the SOC of whole profile. Soil pH, bulk density and porosity had significant relationship (P ≤ 0.05) with SOC content. From this fact, it can be concluded that organic carbon content appeared to be an important soil property to improve the estimation of soil water retention at lower suction values. No significant relation was observed between organic carbon at different depths and soil water retention at higher suction (−1500 kPa, PWP). This may be related to the fact that the structure-forming ability of organic matter affects soil water retention at water content close to field capacity to a larger extent than water retention close to wilting point.