Magnitudes of disturbance in the evolution of agro-ecosytems: N-flows in rice-based systems.

The processes underlying ecological sustainability are studied through a theoretical sequence of rice-based agro-ecosystems reflecting increasing intensities of land use. Data and assumptions are derived from existing literature in order to understand changes in the magnitude of N-flows across systems, based on N inputs equal to N outputs at the aggregated level. The three systems comprise (A) shifting cultivation, (B) mixed livestock-food crop farming, and (C) irrigated rice monoculture, that are compared on a total area basis (20, 2 and 1 ha respectively) and on a basis of per hectare crop land. Total and per hectare N inputs and outputs as well as the efficiency (ratio of N in useful outputs to N in other outputs) are estimated. On a hectare basis flows in the shifting cultivation system are less than 50 % of those in mixed livestock-cropping and only 4 % of those in rice monocropping, while efficiency increases from 0.12 to 0.29 and 0.41. Land use intensification thus leads to greater N-flows in the system, coupled with greater losses, proportionally to the increases in flows, and higher efficiency. The theoretical approach demonstrates the importance of including both magnitudes and efficiencies of N-flows at the aggregated system level in analysing ecological sustainability of agro-ecosystems