Soil Organic Carbon Fractions and Aggregation of a Tropical Alfisol as Affected by Plant Residues

Incorporation of plant residues into soil plays a vital role in enhancing the quality of fragile tropical soils. This study evaluated the short-term effect of plant residues of different quality on organic carbon (OC) fractions, carbon management indices and stability of a sandy loam soil with low (< 2%) initial OC content. The residues are Banana clippings (BC), Chromolaena odorata (CO), Leucaena leucocephala (LL), Maize Stover (MS), Neem clippings (NC), and Panicum maximum (PM) applied at 0 t ha-1 (control), 10 t ha-1 and 20 t ha-1. These treatments were arranged in a randomized complete block design and replicated three times. Undisturbed soil samples were collected at 0-20 cm depth for determination of percent water stable aggregates (%WSA), aggregate ratio (AR) and Mean Weight Diameter (MWD). Pre-sieved (2 mm) soil samples from same depth were collected to determine structural index (SI), micro aggregate stability, total OC (CT), labile OC (CL) and non-labile OC (CNL). Carbon indices, i.e. lability index (LI), carbon management index (CMI) and carbon pool index (CPI) were computed from the OC fractions to evaluate the best management practice. The OC fractions were significantly (p≤ 0.05) higher with LL applied at 20 t ha-1 (19.29 g kg-1 for CT, 1.20 g kg-1 for CL and 18.09 g kg-1 for CNL) than other treatments. This was followed by PM 10 t ha-1, with 17.16 g kg-1 for CT and 16.44 g kg-1 for CNL. The LI (1.29) was highest with MS 10 t ha-1, while CMI (166.6) and CPI (1.85) were highest with LL 10 t ha-1 and 20 t ha-1 respectively. Significant increase in %WSA of > 0.25 mm size over < 0.25 mm was observed with the addition of residues which indicates binding of smaller aggregates into large fractions, while AR (11.97) was highest with CO 10 t ha-1. Aggregate stability showed MWD (2.02 mm) was highest in soils treated with CO followed by PM (MWD = 1.86 mm) both at 20 t ha-1, but SI trend was higher with the addition of LL and PM. Computed micro-aggregate stability indices were generally similar for all treatments. High positive correlations were obtained between MWD and CL (r = 0.634), LI (r = 0.686) and CMI (r = 0.641) with addition of Leucaena leucocephala. In conclusion, L. leucocephala, followed by P. maximum and C. odorata provided short-term improvement in physical quality of this soil over other plant residues.