Nutrient dynamics and their control in land use systems of forest margins in Central Sulawesi, Indonesia

Abstract

Changes of soil fertility after rainforest conversion and nutrient sustainability in major land use systems were studied in rainforest margins in uplands of Central Sulwesi, Indonesia. The research area was characterised by continuing deforestation activity, mostly by smallholders. The study was devided in three parts: A) Influence of land use system and duration of maize- and agroforestry cultivation on soil parameters was studied, B) gross N transformation rates in natural forest, agroforest and maize were measured, and C) a case study of input-output balances of nutrients was conducted.A survey of 74 sites of 5 major land-use systems (maize fields, agroforestry, forest fallow, grass fallow and natural forest) showed that soils in the research region were generally fertile, with high base cation saturation, high cation exchange capacity, medium to high pH-values and sufficient stocks of nitrogen. Conversion of natural forest to both maize and agroforestry reduced topsoil C and N stocks by about 20 % compared to natural forest, but during continuous maize cultivation C and N stocks declined further, whereas in agroforestry levels stayed stable. These results indicate that agroforestry was the better land use option in terms of nutrient sustainability compared to maize. The study shows that legume shade trees are an important compartment of the agroforestry system, replacing N losses and keep soil organic matter stocks stable. This was supported by measurements of gross N transformation processes, which showed higher gross mineralisation, NH4+ uptake, and faster turnover of N in agroforestry systems than in maize. Results showed fast immobilisation of NO3-, which may lead to retention of N in the soil.Bulk density and base cation saturation was higher on converted sites than in natural forest, during cultivation of maize bulk density increased, whereas in agroforestry it stayed stable. Grass fallows had lowest pH and ECEC. The nutrient balances case study (nutrient input by precipitation and output by leaching and harvest) showed negative balances for all nutrients in forest, agroforest and maize, except for K and P in natural forest. N, K and P losses occurred mainly via harvest, Ca, Na and Mg losses mainly by leaching. Precipitation replaced only small fractions of N but larger parts of Na and K losses in agricultural sites. Although the partial nutrient balances were more negative for agroforestry than maize, nutrients were replaced through pathways which were not measured (biological N-fixation, weathering). This shows that partial nutrient balances alone should not be used to evaluate sustainability of land use systems.