Site-and watershed-level assessment of nutrient dynamics under shifting cultivation in eastern Madagascar

Abstract

Nutrient depletion is an important limiting factor for agricultural sustainability in shifting cultivation systems. This paper presents a case study examining nutrient dynamics for a hillrice-fallow system located on the eastern escarpment of Madagascar. A nutrient assessment was carried out, measuring total C, N, P, K, Ca and Mg concentrations in phytomass, ashes and harvests and total C and N, exchangeable K, Ca and Mg and available P concentrations in topsoils, soil loss material and river discharges. At representative slash-and-burn sites, the soil-pool of P and K increased from 100% beneath 5-year-old fallow vegetation to 166% and 126% at harvest, but Ca and Mg decreased. Comparisons between fallow and burnt fields showed that 95–98% of phytomass-fixed and 22–24% of soil-fixed C and N were lost by burning. Paddy at harvest only contained 1–7% of the nutrients in the burnt phytomass of the previous stand. Nutrients regenerated rapidly in the fallow vegetation, which after 1 year contained already 36–57% of the previous phytomass pool, whereas topsoil nutrient concentrations started to increase only after 3–5 years of fallow. The long-term nutrient depletion was studied by comparing nutrient stocks at sites and watersheds, which were characterised by increasing levels of degradation. The topsoil cation content increased during the early stages of shifting cultivation, but under long-term shifting cultivation, the soil nutrients fell to approximately 2/3 of the initial stock. The nutrient stocks of the most degraded vegetation unit (grassland) was merely 1.1–6.5% of the nutrient stocks in the rainforest. Finally, the nutrient stocks in a forested and a degraded watershed were calculated and compared. The established nutrient balances showed, that the dynamics and the depletion depend greatly on the spatio-temporal scale of observation, on the topography of the sites and on the type of nutrients.