Supply of Swietenia macrophylla King and Carapa guianensis Aubl. with K, Ca, and Mg in three different plantation systems

In the present study, the K, Ca, and Mg supply of the economically important timber tree species Swietenia macrophylla Aubl. and Carapa guianensis King were investigated in three plantation systems (monoculture, mixed culture, enrichment of a secondary forest) near Manaus, Central Amazon. In order to get insight into the element input in the 5-year-old plantation systems, the K, Ca, and Mg content of precipitation, throughfall, stemflow, and litter was quantified in one-week-intervals. The element output of the plantations caused by leaching out of the soil was calculated by soil solution samples collected in one-week-intervals in soil depths of 10, 20 and 60 cm (Cl-method). The element uptake of the vegetation was calculated from biomass data and the element content of different tree fractions carried out in December 1996 and in December 1997. The K, Ca, and Mg stocks of the soil were quantified by means of soil samples (exchangeable K, Ca, Mg content; total K, Ca, Mg content) and the analyses of the soil solution. The mean K, Ca, and Mg content of the precipitation was fairly low and varied between 0.07 and 0.7 mg/ l. A strong increase of the element content of the throughfall and the stemflow compared to the precipitation (2 to 16 times higher) was found, which was caused by leaching of mineral elements out of the crown and by dry deposition. A higher specific K demand was calculated for Swietenia (6788 ag/ g) compared to Carapa (5735 ag/ g), whereas a higher mean Ca and Mg content of the biomass of Carapa was found. Young leaves are a strong sink for K, whereas the Ca content of older leaves increased compared to younger leaves. This indicates that both species favour an "internal recycling" for K andan "external recycling" mechanism for Ca. The high Mg content of young leaves of Carapa already indicated a high photosynthetic activity of young leaves, whereas the Mg content of younger Swietenia leaves was reduced compared to older leaves. Litter decomposition experiments indicated an improved K, Ca, and Mg cycling in the enrichment system compared to 5-year-old monoculture and mixed culture plantations. Quantified input and output data of the plantations showed that the enrichment system is morestabilized with regard to K, Ca, and Mg supply than the 5-year-old monoculture and mixed culture system. Nevertheless, the quantification of the element stocks of the plantations indicated high K (872/1057 g/10 m2), Ca (818/1502 g/10 m2), and Mg (623/1011 g/10 m2) stocks in the monoculture system and, therefore, at this state it could not be predicted in how far sustainable growth is also possible in the monoculture system, as well. Higher K, Ca, and Mg stocks were found in Carapa plantations compared to Swietenia plantations, which indicates that the stabilization of the K, Ca, and Mg stocks during the initial phase of plantations is favoured more by Carapa than by Swietenia.