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

Nutrient depletion is an important limiting factor for agricultural sustain ability in shifting cultivation systems. This paper presents a case study e xamining nutrient dynamics for a hillrice-fallow system located on the east ern escarpment of Madagascar. A nutrient assessment was carried out, measur ing total C, N, P, K, Ca and Mg concentrations in phytomass, ashes and harv ests and total C and N, exchangeable K, Ca and Mg and available P concentra tions in topsoils, soil loss material and river discharges. At representati ve slash-and-bum sites, the soil-pool of P and K increased from 100% beneat h 5-year-oId fallow vegetation to 166% and 126% at harvest, but Ca and Mg d ecreased. 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. Pad dy at harvest only contained 1-7% of the nutrients in the burnt phytomass o f the previous stand. Nutrients regenerated rapidly in the fallow vegetatio n, which after 1 year contained already 36-57% of the previous phytomass po ol, whereas topsoil nutrient concentrations started to increase only after 3-5 years of fallow. The long-term nutrient depletion was studied by compar ing nutrient stocks at sites and watersheds, which were characterised by in creasing levels of degradation. The topsoil cation content increased during the early stages of shifting cultivation, but under long-term shifting cul tivation, 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 nutr ient stocks in a forested and a degraded watershed were calculated and comp ared. The established nutrient balances showed, that the dynamics and the d epletion depend greatly on the spatiotemporal scale of observation, on the topography of the sites and on the type of nutrients. (C) 1998 Elsevier Sci ence B.V. All rights reserved.