Calcium is cycled tightly in Cryptomeria japonica stands on soils with low acid buffering capacity

Abstract Calcium (Ca) is a key cation in the soil acidification process and in the fertility of forest soils. We investigated total concentrations of Ca, magnesium (Mg), and potassium (K) in both soil and fresh leaf litter in eight plantation stands of Cryptomeria japonica divided into two groups of highly contrasting soil acid buffering capacity (ABC). In addition to conventional methods, we used a double-crystal high resolution X-ray fluorescence (HRXRF) spectrometer to determine Ca fractions in soils with low and high ABC. Total Ca had accumulated in the 0–10 cm depth horizons of the high-ABC soils, but not in the low-ABC soils. The exchangeable Ca concentration was correlated with total Ca concentration in the 0–10 cm depth of the low-ABC soils. Calcium- Kα 1,2 emission spectra of the 0–10 cm soils obtained by HRXRF spectrometry separated the total Ca into two fractions, suggesting that the predominant Ca constituents had either passed through biological processes or were derived from weathering hydration in both the high- and low-ABC soils. Calcium concentration in the fresh leaf litter was significantly lower in the stands with low-ABC soil than in those with high-ABC soil and was positively correlated with soil exchangeable Ca concentration in the low-ABC stands; no such correlation was observed in the high-ABC stands. Exchangeable Ca pools in the 0–10 cm depth range were equal to or less than the annual input of Ca via leaf litterfall in the low-ABC stands, but were seven times the annual input in the high-ABC soils. These results suggested that the accumulation of Ca in the upper soil occurs via the process of Ca circulation within C. japonica ecosystems, and that sufficient supply of Ca for tree needs was accompanied by the large accumulation of available Ca by C. japonica in the high-ABC soils. Nevertheless, it is likely that only small amounts of Ca were circulated in the stands on low-ABC soil. The lack of Ca accumulation in the surface soil and the accompanying tight circulation of Ca in the plant–soil system may explain the observed positive feedback in which ABC is gradually reduced in low-ABC soils.