Land-Use Types Depth (cm) Soil pHTotal N (g N/kg soil) Soil Particle Size Density (%)Land-Use Types ClaySiltSand Forest Forest 20 – Coconut Stand Coconut Stand 20 – Rubber Plantation Rubber Plantation 20 – Large Homegarden Large Homegarden 20 – Small Homegarden Small Homegarden 20 – Rice-Paddy Field Rice-Paddy Field 20 – Carbon Sequestration under Tropical Homegardens and Related Land-use Systems in Kerala, India Subhrajit K. Saha 1 *, P.K. Ramachandran Nair 1, Vimala D. Nair 2, B. Mohan Kumar 3 1 School of Forest Resources and Conservation, 2 Soil and Water Science Department, University of Florida, Gainesville, FL 3 Department of Silviculture and Agroforestry, Kerala Agricultural University, Kerala, India Fig. 2: Mean Soil C of All Study Sites (Fractionated Soil) in Thrissur, Kerala, India. Soil is a major sink of terrestrial carbon (C). Tree-based land-use systems such as forests and agroforests are likely to retain substantial stable C in the soil as a consequence of their litter and root dynamics, the smaller soil aggregates retaining relatively more stable C than larger aggregates. The perceived carbon sequestration potential of agroforestry systems, however, remains largely unexplored, especially in systems like tropical homegardens (HG). Hypotheses: 1. Tree based land-use systems promote soil C sequestration. 2. Carbon content is higher in smaller soil aggregate size classes than in larger aggregates in tree-based systems. Objective: To compare soil C storage within a meter profile in whole- and size-fractionated soils of homegardens with four other common land-use systems (natural forests, rubber plantations, sole stand of coconuts, and rice-paddy fields) in Kerala, India. Materials and Methods Introduction Selected Land-use Systems (Treatments) Homegardens (HG): An agroforestry system where multipurpose trees are grown in intimate association with shrubs and herbaceous species, and livestock around the homesteads (Fernandes and Nair, 1986). In this study, homegardens are categorized into two classes: large (≥ 0.4 ha/ 1 acre) (HGL) and small (<0.4 ha/ 1 acre) (HGS). Primary Forest (PF): Undisturbed patches of moist deciduous forests located near the (other) study sites. Approximately 25% of the state is under forest. Coconut Stands (CN): Kerala Agricultural University’s experimental plantings of 30-year old coconut palms (Cocos nucifera) spaced 8m × 8m. Rubber Plantations (HB): Rubber trees (Hevea brasiliensis) planted at 6 m x 6 m spacing; the age and size of selected plantations varied at different sites. Rubber is the fastest growing cash crop in Kerala Rice-Paddy Fields (OS): Rice (Oryza sativa)-paddy fields in close proximity to the HG sites. Approximately 14% of the state is under the rice-paddy cultivation. Results and Discussion Land-use systems with higher tree density and less soil disturbance contributed to greater soil C sequestration. Homegardens retained more C in soil than agricultural systems such as rice-paddy. Across the systems, the stable soil fraction (<53 µm) contained the highest amounts of C. Carbon content in soil profiles decreased with soil depth; but lower depths up to 1 m contained substantial amount of C, indicating the importance of considering the soils below the surface horizon in soil C studies. Conclusions References 1. Elliot E.T Aggregate structure and carbon, nitrogen and phosphorus in native and cultivated soils. Soil Sci. Soc. Am. J. 50: Fernandes, E.C.M. and Nair, P.K.R An Evaluation of the Structure and Function of Tropical Homegardens. Agroforest. Syst. 21: Six, J., R.T. Conant, E.A. Paul, and K. Paustian Stabilization mechanism of soil organic matter. Implications for C-saturation of soils. Plant Soil 241: Acknowledgments We thank the Kerala Government Departments of Agriculture and Forest for data sources, the farmers and land-owners for their cooperation in the study, and the Kerala Agricultural University staff for their help with soil sampling and land-use survey. Soil Sampling Collected from four replications of each treatment (land-use systems). Each sample, a composite of three sub-samples from three random sampling points. Samples from four soil depths (0 – 20, 20 – 50, 50 – 80, 80 – 100 cm) at each sampling site. Particle Size Fractionation Wet sieving of air-dried, 2 mm sieve samples, through 250 and 53 µm sieves to obtain three fraction size classes (250 – 2000 µm, 53 – 250 µm, and <53 µm) (Elliot 1986). The whole and fractionated soil samples were finely crushed to homogenize them for analyses. Analysis: Total soil C determination by dry combustion on an automated FLASH EA 1112N C elemental analyzer. Statistical Analysis: Waller-Duncan K-ratio T test was performed following ANOVA. Fig. 1: Mean of Total C from All Study Villages (Whole Soil) in Thrissur, Kerala, India. Among the land-use types, total soil C was highest in the forest (p< ), followed by the rubber plantations. Lowest amount of soil C was found in the rice-paddy systems (p< ). Thus, soil C content was directly related to tree density and long-term undisturbed nature of the systems. Homegardens had less C in top soil compared to forests and rubber plantations, which could be a result of soil disturbance (periodic tillage) consequent to cultivation of herbaceous crops. Across systems, highest total C was found in the top soil (0 – 20 cm) ( p< ), which gradually decreased with depth resulting in lowest total C at 80 – 100 cm depth in all land-use systems. The overall trend of total soil C in the three aggregate size classes was similar to that of total soil C: forest and rubber plantation having higher amounts and rice-paddy the lowest (p< ). Also, across systems, the amount of C decreased with depth in all three aggregate size classes. No difference was observed among large and small homegardens and coconut stands. In all three aggregate size classes, the amount of C decreased with depth. Rice-paddy soils had a sharp decrease in the amount of C from top soil to sub-soil in small aggregate size class (<53 µm). Amount of C was higher in the small aggregate size class (<53 µm) in all selected land-use types and at all depth classes. Carbon is known to be present in most stable form in small soil fractions (Six et al. 2002). No difference was found between other two aggregate size classes (250 – 2000 µm and 53 – 250 µm). Fig. 3: Mean Soil C in Three Fraction Size Classes across Five Land-use Systems in Thrissur, Kerala, India. Table 1: Soil Characteristics of Pandiparambu (Site 1), Thrissur, Kerala, India. Table 2: Mean of Total Soil C from All Study Villages (Whole Soil) in Thrissur, Kerala, India. Soil Collection HG Soil Profile Land-use Types Soil C (Mg/ha) at different depths (cm) 0 – 5050 – 100 Forest126.8 a a Coconut56.39 c c HGL58.65 c b HGS59.05 c b Rubber77.85 b b Rice- Paddy d d Lower case letters next to the mean values indicate significant differences in soil C among land-use types (within a given depth) cm Study Sites Location: Three villages (Pandiparmbu, Chirakkakode, and Vellanikkara) and an adjacent forest area from the Madakkathara subdivision (Panchayat),Thrissur district (10°0' – 10°47' N; 75°55' – 76°54' E), Kerala State, India. Soils: Inceptisols and Ultisols. Climate: Mean annual rainfall: 2783 mm. Mean annual temperate: 27.7°C. Study Location in India N Kerala Thrissur The overall pattern in total soil C was, Primary Forest> Rubber Plantations> Homegardens> Coconut Stands> Rice-Paddy fields.