Effect of active aluminum on soil phosphorus forms in a forested watershed in Akita, Japan

Abstract

Volcanic ash has fallen over a broad area in Japan and contributed to the formation of unique soil physical and chemical properties through the abundant supply of active aluminum (Al). However, the effect of two types of active Al, specifically Al-humus complexes and amorphous minerals, on soil phosphorous (P) forms remains unclear. We investigated soil P fractions in forest soil influenced by volcanic materials in igneous and sedimentary rock areas across an altitude of 80–451 m above sea level. Samples from the top 0–10 and 10–20 cm of the soil at the foot of a slope were collected from 29 Japanese cedar forests in a watershed in Japan. Soil samples were sequentially extracted to obtain operationally defined P fractions: calcium (Ca)-P, Al-P, and iron (Fe)-P. The soil samples were also extracted to obtain oxalate extractable silicon (Sio) and Al (Alo), and pyrophosphate extractable Al (Alp) and Fe (Fep). Regardless of the sampling location, the dominant soil P form was Fe-P, followed by Al-P and Ca-P. Each soil P fraction was positively correlated with slope angle. The sum of the fractionated P in the 0–20 cm depth on the igneous rock area was significantly higher than that in the sedimentary rock area. Al-P was significantly positively correlated with Sio, which represents amorphous minerals, while Al-P was significantly negatively correlated with Alp, which represents Al-humus complexes. Thus, these results suggested that amorphous minerals increased Al-associated P, while Al-humus complexes decreased Al-associated P. In contrast, the correlation between the Fe-P and Alp or Fep was positive. Because Alp and Fep represent metal-humus complexes, these results suggested that Al-humus complexes increased humus related P. The different effects of the two active forms of Al may result in distinctive P dynamics in forest ecosystems.