Do soil fertilization and forest canopy foliage affect the growth and photosynthesis of Amazonian saplings?

Abstract

Most Amazonian soils are highly weathered and poor in nutrients. Therefore, photosynthesis and plant growth should positively respond to the addition of mineral nutrients. Surprisingly, no study has been carried out in situ in the central Amazon to address this issue for juvenile trees. The objective of this study was to determine how photosynthetic rates and growth of tree saplings respond to the addition of mineral nutrients, to the variation in leaf area index of the forest canopy, and to changes in soil water content associated with rainfall seasonality.We assessed the effect of adding a slow-release fertilizer. We determined plant growth from 2010 to 2012 and gas exchange in the wet and dry season of 2012. Rainfall seasonality led to variations in soil water content, but it did not affect sapling growth or leaf gas exchange parameters. Although soil amendment increased phosphorus content by 60 %, neither plant growth nor the photosynthetic parameters were infl uenced by the addition of mineral nutrients. However, photosynthetic rates and growth of saplings decreased as the forest canopy became denser.Even when Amazonian soils are poor in nutrients, photosynthesis and sapling growth are more responsive to slight variations in light availability in the forest understory than to the availability of nutrients. Therefore, the response of saplings to future increases in atmospheric [CO 2 ] will not be limited by the availability of mineral nutrients in the soil.

Juvenile tree growth correlates with photosynthesis and leaf phosphorus content in central Amazonia

“…Values of Q und reported in this study are similar to those found in previous studies in the same area (Marenco & Vieira, 2005;Magalhães et. al.…”

“…The high N/P ratio (Koerselman & Meuleman, 1996) and the low leaf P content found in sapling leaves supports the belief that in the old and highly weathered Amazonian soils, P can limit tree growth in this ecosystem (Reich & Oleskyn, 2004). Nevertheless, Magalhães et al (2014) failed to detect any significant plant response to the addition of P, which suggests that the amount of P they added was not sufficient to cause any detectable effect on plant functioning; it is well known, for example, that oxisols have a high capacity for P fixation. Wright et al (2011) reported that the addition of P only marginally enhanced seedling growth rates in a lowland tropical forest of Panama.…”

“…Soil water content was determined after oven-drying the soil sample at 105°C until constant mass. A second set of soil samples (n = 4) similarly collected was used for chemical and physical analyses, as previously described (Magalhães et al, 2014). All data were submitted to analysis of variance (ANOVA) to assess the effect of rainfall seasonality on the studied parameters.…”

“…Amazonian soils are highly weathered, and consequently very low in phosphorus. In central Amazonia for example, Magalhães et al (2014) reported about 1 mg kg -1 of Mehlich-extractable P. Although the soil is poor in nutrients, biodiversity of central Amazonica is high. In one hectare more than 600 trees (diameter at breast height above 10 cm) from 280 species can be found (Oliveira & Scott, 1999).…”

Fisiologia de espécies florestais da Amazônia: fotossíntese, respiração e relações hídricas

“…A taxa máxima de transporte de elétrons (J max ) e V cmax (valores padronizados a 25°C) variam consideravelmente entre espécies. V cmax varia na faixa de 10 a 75 µmol m -2 s -1 (Domingues et al, 2014;Magalhães et al, 2014) e J max varia entre e 40 e 112 µmol m -2 s -1 (Domingues et al, 2007;Magalhães et al, 2014), estando ambos parâmetros altamente correlacionados. A relação J max / V cmax varia de 1 a 1,67 (Medlyn et al, 2002).…”