Development and use of a mass-balance model to calculate the likely effects of agrichemicals on trace element accumulation in soils supporting palm oil production

Trace elements (TEs) can reduce both the quantity and quality of agricultural produce when essential trace elements are deficient or when any trace element accumulates beyond threshold concentrations in soil. Therefore, TEs in agricultural systems should be managed to ensure that soil concentrations are kept within guideline levels and do not affect production. Oil palm agriculture in Indonesia occurs on weathered, acidic soils that require substantial fertilizer inputs to maintain production. To understand whether TE accumulation in soils resulting from fertilizer and pesticide use may affect production in the future, we have developed a mass-balance model to calculate likely concentrations of TEs in soils after given time periods under production conditions. Our model was developed to simulate processes which occur in the soil-plant system and involve the transport of TEs. It was developed to be rationally convenient, that is, parsimonious, working at environmentally relevant concentrations and able to be applied at the hectare scale to large agricultural systems. The model uses the mass-balance equation Sigma(d) M-Y,M-d = Sigma(d) M-0,M-d + Sigma(y) M-y,center dot(+) - Sigma(y) (M-y((P)) + M-y((L)) + M-y((R))) and uses the critical parameters of rainfall, evaporation, erosion, soil adsorption coefficient, initial plant and soil contaminant concentrations, as well as soil-and- crop specific parameters. We validated our model using a data set from a 64-year fertilizer trial, which had known inputs and application rates of fluorine (F), cadmium (Cd) and uranium (U). We applied the validated model to a smallholder oil palm plantation in Indonesia, modelling accumulation of copper (Cu), F, Cd and U under recommended production conditions, to determine which TEs might limit production and the likely timeframes for this. Our results indicate that topsoil concentrations of Cu and F were likely to reach phytotoxic concentrations in soils (289 and 719 mg kg(-1), respectively), within 50 years under these conditions. The future feasibility of intensive oil palm production on weathered, low fertility soils such as those in Indonesia should be assessed to avoid long-term negative impacts on soil quality.