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We evaluated the hypothesis that soil microorganisms inhabiting wetland soils prefer plant litter and soil organic matter to fuel methanogenesis rather than root exudates. Root ethanol production, microbial respiration, concentrations of dissolved organic C (DOC), and the biochemical composition of litter from four contrasting plant species were compared to C[H.sub.4] production. Rates of C[H.sub.4] production varied from as little as 67 [micro]mol [kg.sup.-1] [d.sup.-1] associated with Lythrum salicaria L., to 675 to [micro]mol [kg.sup.-1] [d.sup.-1] with Carex lacustris Willd., 1555 [micro]mol [kg.sup.-1] [d.sup.-1] with Juncus effuses L., and 1647 [micro]mol [kg.sup.-1] [d.sup.-1] with Typha latifolia L. Ethanol production was pronounced in Typha roots in the spring and autumn, and in Lythrum in midsummer, but showed little seasonal pattern in Juncus or Carex. Concentrations of DOC did not vary with plant species and were greater in subsurface soil (13.3 nag C [L.sup.-1] at 20 cm) than in surface soil (8.6 mg C [L.sup.-1] at 5 cm). Our analysis of lignin oxidation products revealed plant- related differences in the lignin chemistry of soil. The p-coumaric acid content was 24 to 38% of the lignin yield in the surface soil, whereas vanillyl phenolics were dominant in the deeper soil. The ratio of acid to aldehyde was greater at depth in soils for all species except Juncus. The acid/aldehyde ratio for syringyl phenolics increased with depth in soil associated with Carex. Our results show that different plant species can create significant variation in soil methanogenesis, but plant-induced differences were probably caused by variation in the biochemical composition of litter rather than root ethanol production. Abbreviations: DOC, dissolved organic carbon; SFOW, soluble fats, oils, and waxes. doi:10.2136/sssaj2008.0395

A more detailed understanding of seasonal dynamics of carbon allocation between roots and shoots of temperate zone trees is needed. We labeled sapling stands of sugar maple (Acer saccharum Marsh) with [.sup.13][CO.sub.2] in four 3 m diameter chambers during a 3 week interval in September 2006 and traced the movement of this [.sup.13]C pulse through trees and soil during autumn and spring. High enrichment of [.sup.13]C in foliage was achieved ([delta][.sup.13]C = 387[per thousand] [+ or -] 16[per thousand]). Late-season photosynthate was strongly mobilized during leaf senescence and stored in twigs, wood, and roots. Very high [.sup.13]C enrichment of soil [CO.sub.2] emissions at this time ([delta][.sup.13]C = 766[per thousand] [+ or -] 82[per thousand]) indicated the role of late-season photosynthesis in supplying root metabolism after the growing season. Rhizosphere soil was weakly enriched in [.sup.13]C during fall, and increased significantly over the winter. Earthworms were highly enriched in fall, indicating that they consumed roots. In spring, [.sup.13]C was strongly mobilized to growing shoots but not to growing fine roots; apparently, fine root growth in spring was not supplied by late-season stored photosynthate. These results provide insights into the seasonal dynamics of temperate tree carbon allocation with implications for disruption of these dynamics by global warming. Resume: Nous avons besoin d'une comprehension plus detaillee de la dynamique saisonniere de l'allocation du carbone aux racines et aux pousses des arbres en zone temperee. Nous avons marque de jeunes peuplements d'erable a sucre (Acer saccharum Marsh.) avec du [.sup.13][CO.sub.2] dans quatre chambres de trois metres de diametre durant un intervalle de trois semaines en septembre 2006 et nous avons suivi les mouvements de cette impulsion de [.sup.13]C dans les arbres et le sol a l'automne et au printemps. Un enrichissement eleve en [.sup.13]C a ete obtenu dans le feuillage ([delta][.sup.13]C = 387 [+ or -] 16 [per thousand]). A la fin de la saison, les produits de la photosynthese ont ete fortement mobilises pendant la periode de senescence des feuilles et emmagasines dans les rameaux, le bois et les racines. Au meme moment, un enrichissement eleve en [.sup.13]C des emissions de [CO.sub.2] du sol ([delta][.sup.13]C = 766 [+ or -] 82 [per thousand]) etait une indication du role de la photosynthese a la fin de la saison dans l'approvisionnement du metabolisme des racines apres la saison de croissance. La rhizophere du sol etait faiblement enrichie en [.sup.13]C durant l'automne et l'enrichissement augmentait de facon significative au cours de l'hiver. Les vers de terre etaient fortement enrichis a l'automne, indiquant qu'ils consommaient des racines. Au printemps, le [.sup.13]C etait fortement mobilise par la croissance des pousses mais pas par la croissance des racines. La croissance des racines fines au printemps n'etait apparemment pas supportee par les produits de la photosynthese emmagasines a la fin de la saison. Ces resultats donnent une idee de la dynamique saisonniere de l'allocation du carbone chez les arbres de la zone temperee et des repercussions du rechauffement global sur la perturbation de cette dynamique. [Traduit par la Redaction], Introduction Interest in the movement of carbon in forest ecosystems has been stimulated by changes in greenhouse gas concentrations in the atmosphere and consequent global climate change. The complex interactions [...]

Abstract: While the spread of the exotic Lythrum salicaria L. across wetlands in North America is thought to Negatively affect the performance and diversity of native species, the actual impacts [...]

Although the hot, moist tropics in the Republic of Panama receive more than 2000 mm of rain per year, soils dry considerably during the 4-month dry season. We examined the effect of seasonal drought by irrigating two 2.25-ha plots of lowland tropical moist forest on Barro Colorado Island (BCI) for 5 consecutive dry seasons. Irrigation decreased soil permeability and improved soil nutrient status, which prompted this study of soil charge characteristics in the irrigated and control plots. Soil was an Alfisol, and thus it was not clear a priori whether variable-charge or permanent-charge components dominated. Surface soil (0-15 cm) had a pH([H.sub.2]O) of 5.5 and pH(KCl) of 4.8. Subsurface soil (30-45 cm) had a pH([H.sub.2]O) of 4.8 and a pH(KCl) of 3.5. The point of zero salt effect (PZSE), measured by titration, varied from 3.7 to 5.0 in surface soil and from 3.5 to 4.2 in subsurface soil. Variable charge of surface soil was 2.6 [cmol.sub.c]/kg.pH unit after the dry season in April versus 3.2 [cmol.sub.c]/kg.pH unit after the wet season in December in both control and dry-season irrigated plots, reflecting seasonal differences in pH and PZSE. The point of zero net charge (PZNC), measured by ion retention, was at pH Additional keywords: permanent charge, soil pH, tropical forest soil, variable charge, water regime., Introduction Water regime is critical to soil properties and processes (Jenny 1941). In the long-term, water regime largely determines weathering intensity of soil minerals and the types and amounts of [...]