Soil phosphorus cycling across a 100‐year deforestation chronosequence in the Amazon rainforest

Abstract

AbstractDeforestation of tropical rainforests is a major land use change that alters terrestrial biogeochemical cycling at local to global scales. Deforestation and subsequent reforestation are likely to impact soil phosphorus (P) cycling, which in P‐limited ecosystems such as the Amazon basin has implications for long‐term productivity. We used a 100‐year replicated observational chronosequence of primary forest conversion to pasture, as well as a 13‐year‐old secondary forest, to test land use change and duration effects on soil P dynamics in the Amazon basin. By combining sequential extraction and P K‐edge X‐ray absorption near edge structure (XANES) spectroscopy with soil phosphatase activity assays, we assessed pools and process rates of P cycling in surface soils (0–10 cm depth). Deforestation caused increases in total P (135–398 mg kg−1), total organic P (Po) (19–168 mg kg−1), and total inorganic P (Pi) (30–113 mg kg−1) fractions in surface soils with pasture age, with concomitant increases in Pi fractions corroborated by sequential fractionation and XANES spectroscopy. Soil non‐labile Po (10–148 mg kg−1) increased disproportionately compared to labile Po (from 4–5 to 7–13 mg kg−1). Soil phosphomonoesterase and phosphodiesterase binding affinity (Km) decreased while the specificity constant (Ka) increased by 83%–159% in 39–100y pastures. Soil P pools and process rates reverted to magnitudes similar to primary forests within 13 years of pasture abandonment. However, the relatively short but representative pre‐abandonment pasture duration of our secondary forest may not have entailed significant deforestation effects on soil P cycling, highlighting the need to consider both pasture duration and reforestation age in evaluations of Amazon land use legacies. Although the space‐for‐time substitution design can entail variation in the initial soil P pools due to atmospheric P deposition, soil properties, and/or primary forest growth, the trend of P pools and process rates with pasture age still provides valuable insights.