Biodiversity and sustainability of ancient agroecosystems of southwest India

Maintenance of soil fertility is a critical aspect of sustainability of agroecosystems, especially in the humid tropics. In India’s state of Kerala, farmers manage soil organic carbon (SOC), and thus soil fertility, by manipulating the composition, number, and rotation of crop species. Given that this agricultural system has been utilized for >4000 years, it provides a setting for investigating relationships between biodiversity and sustainability. Using a paired-plot design to control for extraneous factors, I measured SOC (0-15 cm) in 140 plots, pairing a monoculture with a polyculture, within each of 70 farms for which long-term agricultural history was known. I sampled in 1997, and again in 2005. Based on 1997 results in which the SOC quantity depended on both species composition and richness, I hypothesized that over time SOC would decline in monocultures of annuals and palms, and remain the same or increase in monocultures of trees and polycultures. Over the 8-yr period, 13% of the plots had been converted to non-agricultural uses and 49% to another crop. Only 8% of the annual monocultures remained in 2005, as this system is not sustainable unless part of a rotation. In palm monocultures, 60% of the plots remained, and SOC declined in 31% of them. In comparison, 88% of the tree monocultures remained, and SOC declined in only 14%. In the 51% of the polycultures that remained, SOC declined in 33%. Although biological processes play a key role, a complex interplay of social and economic factors also influenced sustainability, especially of the ancient polycultures.