Mycorrhizal fungi colonize many terrestrial plants and often result in an increased uptake of P and several micronutrients. Little information is available on the relationships among vesicular-arbuscular endomycorrhizae (YAM), aquatic plants, and plant P concentrations. The major objectives of this study were to measure the degree of YAM colonization of wetland plants, and to determine the relationship between colonization and plant P concentrations when sampled from two contrasting water regimes (hydric soil with no surface water and hydric soil with a minimum of 10 cm of surface water). Two semipermanent wetlands in Lake County, South Dakota, were chosen for the study. Water regime did not have a significant effect on total or organic soil P concentration. Vesicular-arbuscular endomycorrhizal spore samples collected from wet soils averaged 5000 spores kg"' compared with 2740 spores kg"' in dry soils. The eight plant species evaluated to determine VAM colonization were reed canarygrass (Phalaris arundinacea L.), slough sedge (Carex atherodes Spreng.), river bulrush [Scirpus fluviatilis (Torr.) Gray], slender rush (Juncus tenuis Willd.), dogbane (Apocynum cannabinum L.), water smartweed (Polygonum amphibium L.), cattail (Typha Xglauca Godr.), and wild licorice (Glycyrrhiza lepidota (Nutt.) Pursh). Vescular-arbuscular endomycorrhizal colonization ranged from 78% (wild licorice) to none (slough sedge and slender rush). Roots from dry areas averaged 27% colonization compared with less than 1% in wet areas. Colonization levels of dogbane and reed canarygrass in dry zones were highly correlated to plant P concentrations. The results of this study indicate that several wetland plant species develop substantial mycorrhizal associations, particularly in relatively dry environments.

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Nitrogen mineralization, nitrification potentials, pH, total N, C, extractable P and cations were measured in soils under 4-year-old, mono-specific stands of six fast-growing, native tree species, an abandoned pasture, and a 20-year-old secondary forest, as part of a study on the use of indigenous tree species for rehabilitation of soil fertility' on degraded pastures at the La Selva Biological Station in the Atlantic humid lowlands of Costa Rica. Soil net nitrification potential rates were higher under two N-fixing, leguminous species, Stryphnodendron microstachyum Poepp. et Endl. (1.1-1.9 mg kg -1 day -1) and Dalbergia tucurensis Donn. Smith (0.7-1.5 mg kg-lday-1), than under the non-N-fixing trees in the plantation, Vochysia guatemalesis Don. Sm., Vochysiaferruginea Mart, Dipteryx panamensis (Pittier) Record and Mell and Hyeronima alchorneoides Fr. Allemao (0.2-0.8 mg kg-1 day-1). Values under the N-fixing trees were comparable to those found in secondary forest. There were no statistically significant: differences in soil total N or in other nurtients between the species. Results of pH measurements done before and after incubation did not show any clear evidence of a pH drop attributable to nitrification.

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The immobilization of P is a significant fertility limitation of Andisols in Central America. It is believed that soil Al and Fe fractions have an important influence on P availability. This study was conducted to obtain information on the various forms of Al and Fe in ten pedons derived from volcanic ash in Panama and Costa Rica. Correlations between these Al and Fe fractions and P immobilization were measured by different methods.The Al and Fe fractions, extracted by acid ammonium oxalate, 4M KOH, sodium pyrophosphate and dithionite-citrate, were determined, and the correlations between these fractions, P immobilization, and other soil properties made.It was observed that oxalate extractable Al correlated significantly with P immobilization, which had values of over 85% by the New Zealand method and 96% by the isotherm method. The Al concentration corresponding to this immobilization was over 2%. The difference between oxalate extractable Al and pyrophosphate extractable Al (inorganic Al fraction) correlated with P immobilization also. The pyrophosphate extractable Al fraction had the lowest concentrations and did not correlate with P immobilization. The NaF-pH and dithionite-citrate extractable Al correlated significantly with P immobilization.Oxalate extractable Fe correlated with the P retained by the isotherm method, but dithionite-citrate extractable Fe was negatively correlation with P immobilized. No correlations were found between the pyrophosphate extractable Fe and other soil properties.

Aboveground-tree biomass and nutrient content (nitrogen, phosphorus, calcium, magnesium and potassium) were measured in Cvear-old stands of four indigenous tree s~ecies: Smuhnodendron micr&tachywn Pcepp. et .End]. (gx S. excels& Harms), ~ochysia gua-remaIemis Donn. Smith (ex I! hondurensis S m m e ) , Vochysia f e m i ---. nea Mart and ~~e r o n i h alchorneoides (0), growing on hferiile 6 1 s in an experimental plantation in the Atlantic humid lowlands of Costa Rica Biomass and nutrient content among Ihe species, and among aboveground w e parts, forest-floor litter and understory vegelation were compared, as key factors that can be manipulated with different effects on site nubient conservation. Biomass and stemwood annual increments of the four species were similar to those reported for ocher Florencia Montagnini is Associate Rofessor of Tropical Ecology, Yale University, School of Forestry and Environmental Studies, 370 Rospect Street, New Haven. CT 065 11. JOURNAL OF SUSTAINABLE FORESTRYtropical tree plantations in the humid tropics. S. microstach~wn had the highest accumulation of N in stem, branch and total aboveground tree biomass. L! guatemalensis had the highest accumulation of Ca and Mg in the biomass, while H. alchomeoides had the highest stem K and P. In spite of heir relatively lower N tissue collcentrations. V. ferruginea and H. alchorneoides showed a high potential for N recycling due to its more even distribution in stems, branches and leaves. Nument accumulation by the understory in S. microstachywn and H. alchorneoides plots was 0.8-7.746 of aboveground see biomass numents.?he forest-floor litter repsented a major compartment for nutrient accumulation and recycling under the four species, especially for N, Ca, Mg and P.