A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Relationship between soil nutrient availability and carbon fixation in seedlings andtrees in central AmazoniaRomilda M. Q. Paiva; Claudio Y. Yano; Flávio J. Luizão. INPA-Ecology. E-mail:romilda@inpa.gov.brTropical rain forests generally grow on nutrient-poor soils. Despite of efficient mechanismsfor nutrient conservation and recycling, developed by the forest, nutrient limitations mayprevent higher carbon fixation, and consequent plant growth. The present study aim to testthe hipothesis that soil nutrient limitations can influence both, the initial growth ofseedlings as well as carbon sequestration and additional growth in adult trees. The studyhas been conducted at the ZF-2 INPA’s Reserve, ca. 80 km north of Manaus, in two phases:i) test of the effect of fertilizer additions (N + P and Ca + Mg) on the growth of seedlings ofpioneer and climax species; and, ii) study of the relationship between C and nutrientconcentrations, and the possible nutrient limitation to the atmospheric CO 2 uptake by adulttrees in the forest. In a toposequence (plateau, slope and valley), the ratios C/N, C/P,C/bases, were calculated in selected trees and surrounding soil. The addition of N+P to thesoil induced a higher seedling mortality; however, there was a positive effect on the growthof surviving seedlings, suggesting a possible N and P limitation to the establishment ofseedlings. Adult trees located in the valley plots showed the most favorable C/bases(K+Ca+Mg) ratio: 83, against 145 in both, plateau and slope plots. Mean N concentrationswere similar in tree leaves at plateau (1.9%) and slope (2%); however, both were higherthan in the valley (1.3%). That is also reflected in the C/N ratio, significantly lower in theplateau and slope than in the valley plots (26.1%, 26% and 36.6%, respectively). Carbonconcentrations in mature leaves were higher (49.2%) in the slope than in both, plateau andvalley (47.5% and 47.8% respectively). Suggestions are made that: (i) lower CO 2 fixationin the valley plots may be influenced by low soil nutrient availability for plants; and, (ii)slopes may have better supplies of other elements (e.g., water in soil) which favor Cfixation in trees.