Centennial deforestation impacts on soil phosphorus cycling in the Amazon rainforest

Deforestation 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 in the
Amazon basin has implications for long-term land use change and
productivity. We used a 100-year 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
assays, we assessed pools and process rates of P cycling in surface soils.
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, though the
relatively short but representative pre-abandonment pasture duration of
our secondary forest may not enable 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.