Integrated approach to nutrient cycling monitoring
There are two issues of social importance in relation to nutrient cycling. The first is the depletion of nutrients in croplands and rangelands. The second is the overabundance of nutrients through the enhancement of inputs, particularly in freshwater bodies and coastal systems, which results in loss of biodiversity and ecosystem services through eutrophication. Nutrient cycling encompasses 15 or so elements, each with multiple chemical forms and phases, four media (air, soil, biomass and water) and many transformation and transport processes. It is not cost-effective to monitor them all, even in intensive research sites. The two key elements involved in both fertility loss and eutrophication are nitrogen and phosphorus; thus their changes in nutrient pools and fluxes need to be monitored. Key anthropogenic nitrogen contributions are through atmospheric deposition and liquid waste streams. A sensitive impact indicator is the nitrogen saturation index, which rises abruptly when the absorptive capacity of the landscape is exceeded. Key anthropogenic phosphorus inputs are agriculture and, in certain locations, mining and industry. Monitoring phosphorus fertilizer application rates and local-to-regional nutrient balances is useful because phosphorus is highly conserved in ecosystems. Measurement of nutrients associated with sediment fluxes in rivers is important for both nitrogen and phosphorus, as well as for carbon balance. To place current fluxes and perturbations in perspective, historical records have to be established. Additionally, tools such as isotopic tracers, which can be used unequivocally to differentiate between the natural and anthropogenic components of nutrient cycles, need to be explored.