Autochthonous nutrient recycling driven by soil microbiota could be sustaining high coconut productivity in...

The soils of Lakshadweep Islands are formed as a result of the fragmentation of coral limestone, that is carbonate-rich, with neutral pH, but poor in plant nutrients. Coconut palm (Cocos nucifera L.) is the main crop cultivated, supporting the life and livelihood of the islanders. No external fertilizer application or major plant protection measures are adopted for their cultivation as the Islands were declared to go organic decades back. Yet, Lakshadweep has one of the highest productivity of coconut compared with other coconut growing areas in India. Therefore, a question arises: how is such a high coconut productivity sustained? We try to answer by estimating in three main islands (i) the nutrients added to the soil via the litter generated by coconut palms and (ii) the role of soil microbiota, including arbuscular mycorrhizae, for the high productivity. Our results indicated that, besides adding a substantial quantum of organic carbon, twice the needed amount of nitrogen, extra 20% phosphorus to the already P-rich soils, 43-45% of potassium required by palms could be easily met by the total coconut biomass residues returned to the soil. Principal Component Analysis showed that soil organic carbon %, potassium, and organic carbon added via the palm litter and AM spore load scored >± 0.95 in PC1, whereas, available K in the soil, bacteria, actinomycetes, phosphate solubilizers and fluorescent pseudomonads scored above >± 0.95 in PC2. Based on our analysis, we suggest that the autochthonous nutrients added via the coconut biomass residues, recycled by the soil microbial communities, could be one of the main reasons for sustaining a high productivity of the coconut palms in Lakshadweep Islands, in the absence of any external fertilizer application, mimicking a semi-closed-loop forest ecosystem.