Climatic effects on soil phosphorus pools and availability in sub-Saharan Africa

Abstract

Phosphorus (P) is a key limiting nutrient in tropical and subtropical soils but with unknown responses to climate change. We hypothesized that storage and distribution of P pools differ between reference soil groups, but P availability increases with an increase in precipitation in native sub-Saharan Africa, an underrepresented region in the global P database. To test these hypotheses, we sampled topsoils (0–10 cm; Arenosols, Lixisols, Acrisols) of uncultivated Savannah woodlands along climate gradients stretching from Mozambique to Zambia. Mean annual precipitation (MAP) ranged from 365 to 1227 mm. We extracted P fractions using Hedley's sequential fractionation, yielding resin-exchangeable P, P in 0.5 M NaHCO3, 0.1 M NaOH, 1 M HCl and aqua regia. Extracts were analysed for total P using inductively coupled plasma–optical emission spectroscopy and inorganic P using the colorimetric molybdenum blue method. We found that total P contents were highest in Acrisols (235 ± 76 mg kg−1), followed by Lixisols (214 ± 45 mg kg−1) and Arenosols (133 ± 74 mg kg−1). Beside P storage, also P distribution differed between soils. Mean annual temperature (MAT) did not reveal significant correlations with P fractions. However, with increasing MAP, concentrations of almost all P pools increased; available Pi (Resin-P) increased significantly in Arenosols while potentially bioavailable P (NaHCO3-Po) increased in all soils, with MAP explaining 43% of data variability in Arenosols, 74% in Lixisols and 85% in Acrisols. Hence, P availability increased with an increase in MAP to different degrees, that is, climatic effects on P dynamics were soil-group-specific. Therefore, different regions in the native sub-Saharan woodlands are likely prone to different soil fertility responses when climate changes.