Modelling the long-term productivity and soil fertility of maize/millet cropping systems in the mid-hills of Nepal

Abstract

Abstract Finding ways of maintaining or increasing crop yields without degrading soil fertility is an important goal in sustaining livelihoods in many parts of the world. In the mid-hills of Nepal, the problem is of particular importance due to the fragility of mountain ecosystems. Previously reported field experiments have investigated the influence of different combinations of inorganic fertilisers and farmyard manure (FYM) on crop yields in maize/millet cropping systems at two sites in this region, Pakhribas and Dordor Gaun, but due to the short-time period involved (three years), long-term trends in crop yields could not be determined, neither could changes in soil carbon and nitrogen levels be measured. In this paper, we describe the use of the PALM simulation model to address these questions, and its parameterisation and validation based on data from the field experiments. Using weather data generated for 100 years at each site, the model was then used to investigate the effect of different nutrient management treatments on long-term trends in crop yields and soil carbon and nitrogen dynamics. Overall, crop yields were higher at Pakhribas than Dordor Gaun, due both to the lower temperatures causing slower crop development and hence a longer period of biomass accumulation, and to the higher inherent soil nutrient status causing faster crop growth rates. At both sites, the highest crop yields were obtained in the treatment receiving only inorganic fertiliser at a rate of 90 kg N ha−1. Declines in soil organic C and N were predicted for all fertiliser and FYM treatments, with the severest rates of loss in the treatment receiving no nutrient inputs at all and the slowest rates of loss in the treatment receiving 7 t ha−1 of organic manure. Economic analysis showed that, at current prices, the use of inorganic fertilisers was by far the most rational option for farmers in terms of net returns, regardless of the site, and despite the associated decline in soil C and N levels. It was estimated that if greater use of organic manure is to be promoted as a way of increasing carbon sequestration in the soil, then farmers in the mid-hills of Nepal will need to be compensated at a rate of at least US$9.90 (t C)−1 for the lower crop yields they will obtain.