Can soil phosphorus availability in tropical forest systems be increased by nitrogen-fixing leguminous trees?

Understanding the role of N-fixing leguminous trees for phosphorus (P) cycling in highly weathered tropical soils is relevant for the conservation of natural forests as well as the sustainable management of agroforests and forest plantations with low P input in the Brazilian Atlantic Forest region. We hypothesized that N-fixing leguminous trees can increase the availability of soil P by exploiting different P sources without causing a depletion of soil organic P due to efficient biogeochemical cycling, but empirical evidence remains scarce. For this purpose, ³¹P nuclear magnetic resonance spectroscopy (³¹P NMR) was used for quantifying soil P forms and the Hedley sequential extraction to determine soil P fractions. The studied sites were forestry systems with leguminous trees: mixed forest plantations with different proportions of fast-growing N-fixing leguminous trees; pure plantations, and agroforestry systems with leguminous trees. The results show that all N-fixing leguminous trees and N mineral fertilization positively affected the concentrations of available soil P in relation to the control treatments. There were increases of all P fractions through cycling in all forest sites. ³¹P NMR spectra clearly identified and quantified that a large amount of phosphomonoesters followed by phosphodiesters in the form of DNA, as well as high reserves of P_i species (ortho-P and pyrophosphate) in the first eleven years of growth at pure plantations, mixed plantations or agroforests. The relations between both ortho-P and DNA with the resin-P_i, NaHCO₃-P_i and NaOH-P_i fractions suggest that both analysis methods provide complementary information about the soil P transformations. Thus, the paper highlights the importance of the use of different N-fixing leguminous tree species under different environmental conditions, production systems and management practices for recovering heavily degraded areas, which may be a suitable strategy through efficient management of P in highly weathered tropical soils in the Brazilian Atlantic Forest biome.