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The productivity of forests is often considered to be limited by the availability of phosphorus (P). Knowledge of the role of organic and inorganic P in humid subtropical forest soils is lacking. In this study, we used chemical fractionation and 31 P nuclear magnetic resonance (NMR) spectroscopy to characterize the form of P and its distribution in undisturbed perhumid Taiwan false cypress (Chamaecyparis formosensis Matsum.) forest soils. The toposequence of transects was investigated for the humic layer from summit to footslope and lakeshore. The clay layer combined with a placic-like horizon in the subsoil may affect the distribution of soil P because both total P and organic P (P o) contents in all studied soils decreased with soil depth. In addition, P o content was negatively correlated with soil crystalline Fe oxide content, whereas inorganic P (P i) content was positively correlated with soil crystalline Fe oxide content and slightly increased with soil depth. Thus, P i may be mostly adsorbed by soil crystalline Fe oxides in the soils. Among all extractable P fractions, the NaOH-P o fraction appeared to be the major component, followed by NaHCO 3-P o ; the resin-P and HCl-P i fractions were lowest. In addition, we found no typical trend for P i and P o contents in soils with topographical change among the three sites. From the 31 P-NMR spectra, the dominant P o form in soils from all study sites was monoesters with similar spectra. The 31 P-NMR findings were basically consistent with those from chemical extraction. Soil formation processes may be the critical factor affecting the distribution of soil P. High precipitation and year-round high humidity may be important in the differentiation of the P species in this landscape.

The productivity of forests is often considered to be limited by the availability of phosphorus (P). Knowledge of the role of organic and inorganic P in humid subtropical forest soils is lacking. In this study, we used chemical fractionation and 31P nuclear magnetic resonance (NMR) spectroscopy to characterize the form of P and its distribution in undisturbed perhumid Chamaecyparis forest soils. The toposequence of transects was investigated for the humic layer from summit to footslope and lakeshore. The clay layer combined with a placic-like horizon in the subsoil may affect the distribution of soil P because both total P and organic P (Po) contents in all studied soils decreased with soil depth. In addition, Po content was negatively correlated with soil crystalline Fe oxide content, whereas inorganic P (Pi) content was positively correlated with soil crystalline Fe oxide content and slightly increased with soil depth. Thus, Pi may be mostly adsorbed by soil crystalline Fe oxides in the soils. Among all extractable P fractions, the NaOH-Po fraction appeared to be the major component, followed by NaHCO3-Po; the resin-P and HCl-Pi fractions were lowest. In addition, we found no typical trend for Pi and Po contents in soils with topographical change among the three sites. From the 31P-NMR spectra, the dominant Po form in soils from all study sites was monoesters with similar spectra. The 31P-NMR findings were basically consistent with those from chemical extraction. Soil formation processes may be the critical factor affecting the distribution of soil P. High precipitation and year-round high humidity may be important in the differentiation of the P species in this landscape.

The chemical and physical properties, clay mineralogy, and 14C radiocarbon age of iron nodules found in a Ultisol located on a low elevation terrace of Taiwan were investigated. The results indicated that the amount and size of iron nodules increase with soil depth. Free iron contents in nodules were markedly higher than in the surrounding soil matrix. Feo/Fed ratio was low in all iron nodules, indicating that the predominant form of free iron was crystalline. Al-substituted goethite was found as the predominant oxide mineral and Al substitution ranged from 16 to 24% mole. Illite and hydroxy-interlayered vermiculite and kaolinite were the dominant clay minerals in the iron nodules. X-ray fluorescence (XRF) analysis of the iron nodules are composed primarily of Fe, Si, and Al, and minor amounts of Mn. Based on the similarity of clay mineralogical compositions and radiocarbon dating ages, it is suggested that the major driving force promoting the formation of iron nodules was in situ weathering of saprolite by the frequent wetting/drying cycles. We speculate that illuviated iron played only a minor role in their formation.