Studies on litter characterization using 13C NMR and assessment of microbial activity in natural forest and plantation crops’ (teak and rubber) soil ecosystems of Kerala, India

The leaf litter is the major source of soil organic matter in natural and many plantation crop ecosystems. Quantity and quality of organic matter in a soil ecosystem is of utmost importance in regulating the soil health. Hence assessment of quality of organic matter input, viz., litter is important and is attempted in this study. The leaf litter of rubber (Hevea brasiliensis), pueraria (Pueraria phaseoloides), mucuna (Mucuna bracteata), teak (Tectona grandis) and forest (mixed species) were analyzed using solid state 13C nuclear magnetic resonance (NMR) to study the relative abundance of different carbon compounds present. The spectra revealed that litter of all species studied contain relatively larger amounts of polysaccharides compared to other C containing compounds. Also it could be observed that the alkyl-C to O-alkyl-C ratio of rubber litter was much higher compared to that of others. Aromatics and carbonyl compounds were also present in all litter species. The resource quality based on alkyl-C to O-alkyl-C ratio of the litter samples studied can be arranged in the order pueraria > teak > mucuna > forest > rubber. The respiration rate, substrate induced respiration rate and biomass-C (Cmic) of the litter samples were estimated. It could be observed that litter associated microbial activity decreased as alkyl-C to O-alkyl-C ratio increased. Resource quality derived from the NMR spectra and the litter biological properties were complementary. Soil samples (0–15 cm) from the five soil ecosystems (rubber, pueraria, mucuna, teak and forest) were analyzed for respiration rate, substrate induced respiration rate, Cmic, total-C and total-N. The forest soil had higher respiration rate, total-C and total-N compared to cultivated soil systems. Pueraria, mucuna and teak soils were comparable for their biological properties while rubber soil recorded comparatively lower microbial activity.