Phosphorus Fractions and Organic Matter Chemistry Under Different Land Use on Humic Cambisols in Southern Brazil.

The Araucaria moist forest was once widespread in southern Atlantic Brazil but today, due to agriculture and timber harvesting, only remnants remain, mainly in southern Brazil. This study combines Hedley sequential phosphorus (P) fractionation with ³¹P NMR, fluorescence spectroscopy, and ESI-FT-ICR-MS to examine soil P and C chemistry at five different Araucaria moist forest sites: two natural, grassland and native forest, and three altered, two apple orchard sites (fertilized apple rows and unfertilized soil between apple rows) and a pine plantation site without fertilization. The sites were similar in total P, with the native grassland site highest in soil C. For all, the largest extractable fraction was NaOH-extractable organic P (Po), and the quantity of Po did not differ significantly among the soils. The ³¹P NMR analysis revealed the majority of NaOH-extractable P was present as either inorganic P (Pi) orthophosphate (49%) or Po monoesters (34%). Pi orthophosphate was highest in pine plantation and apple orchard in the row sites and lowest in native grassland, while Po was highest in the native grassland soil. Very few (~ 1%) of labile C compounds contained P, while 5-6% of adsorbed C compounds contained both N and P. Using ³¹P NMR and ESI-FT-ICR-MS approaches, we found evidence of chemical changes in native soil P and organic matter to due land use change, specifically loss of organic forms of P and increased organic matter aromaticity in apple soils compared to native soils. This research shows that the conversion of native grasslands to apple orchards led to a decrease in organic matter quality and Po depletion while conversion to pine plantations had a low impact on these parameters. These analytical techniques are promising approaches to improving our understanding of both P and C chemistry and the effects of land use changes on coupled biogeochemical cycles.