Phosphorus fractions and organic matter chemistry under different land use on Humic Cambisols in Southern Brazil

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The Araucaria moist forest was once widespread in southern Atlantic Brazil but today, due to agriculture and timber harvesting, only remnants remain, mainly in mountainous regions of southern Brazil. This study combines Hedley sequential extraction to determine phosphorus (P) fractionation with 31P NMR, fluorescence spectroscopy, and ESI-FT-ICR-MS to examine soil P and organic matter chemistry at five different Araucaria moist forest sites, including two natural sites, grassland and native forest, and three altered sites, two apple orchard sites (fertilized apple rows and unfertilized soil between apple rows) and a pine plantation site without fertilization. This study examined how land use change has altered soil chemistry compared to native vegetation, focusing specifically on P and C fractions. Although the native grassland site contained the highest amount of soil C, the sites were similar in total P. The largest extractable fraction at all sites was NaOH-extractable organic P (Po), and the quantity of Po (0.1M plus 0.5M extractions) did not differ significantly among the soils. The 31P NMR analysis revealed that 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 (66 and 67% of P detected, respectively) and lowest in native grassland (20% of P detected), while Po was highest in the native grassland soil (75% of P detected). In our samples ~1% or fewer 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 due to 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 has 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.