Tropical forests are the most productive terrestrial ecosystem when compared to other biomes on Earth, but to fully understand their contribution to global carbon balance, it is also necessary to understand the respiration process in these ecosystems. Autotrophic and heterotrophic respiration, a process by which CO2 returns from the ecosystem to the atmosphere, can be greatly affected by soil nutrient availability. However, such interactions remain a major uncertainty in tropical forest ecology. Since the majority of forests in the Amazon grow in soils with very low availability of rock-derived nutrients (e.g: phosphorus, cations), we hypothesise that both auto and heterotrophic respiration would be affected by changes in soil nutrient in a Central Amazon forest. We used a fully factorial, N, P, and cation addition experiment (8 treatments x 4 blocks in 50 x 50 m plots) in an old growth Amazon rainforest near Manaus, Brazil, to explore how respiration changed with soil fertilisation. Here we show that the addition of P and cations together was associated with a decrease in leaf respiration (P:CATIONS: F1,25 = 4.09, p = 0.05). Stem respiration decreased by 12% with phosphorus addition (- P: 1.26 ± 0.05 versus +P: 1.11 ± 0.05 µmol m-2 s-1; F1,29 = 4.44, p = 0.04), and this trend was mainly driven by trees within the intermediate class of 35-45 cm diameter at breast height (DBH). Soil respiration had a non significant trend to be higher with phosphorus addition, mainly driven by heterotrophic respiration (soil microbes) that increased by 13% when compared to plots where P was not added (-P: 4.51 ± 0.18 versus +P: 5.10 ± 0.23 µmol m-2 s-1; F1,24 = 5.21, p = 0.03). Meanwhile, when N (plots with N addition) was added without cation (plots without cation addition) there was a decrease in autotrophic respiration in the soil (N:CATIONS: F1,28 = 6.05, p = 0.02). Our data indicate that multiple nutrients could limit soil respiration and leaf respiration, while phosphorus appears to affect stem respiration. In general, both increases and decreases in respiration occurred with nutrient addition. These results together with productivity are important to have insights if nutrients are acting as C sequestration, and the extent by which nutrients affect respiration, adds more confidence when incorporated into models to know how tropical forests will deal with a world of continuous rise in CO2 in the atmosphere.
CO2 efflux, phosphorus, cations, nitrogen, tropical forest