Disponibilidade e eficiência no uso de nutrientes em florestas secundárias sob diferentes níveis de alteração na região de Santarém e Belterra, Amazônia Oriental

Abstract

We have evaluated the availability and nutrients use efficiency in logged forests (EXP), logged and burned (EXPQ), and recovering forests after deforestation (CR), comparing them with undisturbed (PRI) at Tapajós National Forest. Thus, the soils were sampled following the RAINFOR Project standard protocol. In order to understand the complex dynamics of P in these forests, phosphorus sequential fractionation was applied (Hedley, 1982). In addition, nutrients use efficiency was addressed through the methodology used by Vitousek (1982, 1984). Although soil sum of bases was not statistically different between studied forests (p > 0.05), it tends to be higher where fire occurred, which in turn yielded greater pH values in EXPQ and CR forests (p < 0.05), 4.36 ± 0.25 e 4.36 ± 0.21, respectively. Nitrate, exchangeable K and phosphorus fraction Po NaOH were positively correlated (p < 0.05) with litter production (ρ = 0.67; ρ = 0.52; ρ = 0.35, respectively), which suggests that such nutrients are strongly associated with these forests productivity. We did not find differences in contents neither in soils phosphorus fractions distributions, either individually or in combined fractions (p > 0.05). Phosphorus use efficiency (EUP) was not different (p > 0.05). There is a strong evidence that forests studied here are limited by phosphorus, once there is low contents of this element in the soil, as well low annual return through litterfall and high efficiency cycling rates, regardless the vegetal cover alterations. Although a nitrogen use efficiency gradient is still noticeable (p < 0.05), markedly where the impacts were more drastic (CR), the soil nitrogen supply seems to getting close to the natural conditions, some time since the disturbances.