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We resampled one of the earliest replicated experimental sites used to investigate the impacts of native tropical tree species on soil properties, to examine longer term effects to 1-m depth. The mono-dominant stands, established in abandoned pasture in 1988 at La Selva Biological Station, Costa Pica, contained six species, including one exotic, Pinus patula ssp. tecunumanil (Eguiluz & J.P. Perry) Styles, and five native species: Pentaclethra macroloba (Willd.) Ktze ([N.sub.2]-fixing); Hyeronima alchorneoides Allemao; Virola koschnyi Warb.; Vochysia ferruginea Mart.; and Vochysia guatemalensis J.D. Smith. Soil organic carbon (SOC) differed significantly among species in the surface (0-15-cm) layer, ranging from 44.5 to 55.1 g [kg.sup.-1], compared with 46.6 and 50.3 g [kg.sup.-1] in abandoned pasture and mature forest, respectively. The change in surface SOC over 15 yr ranged from -0.03 to 0.66 Mg C [ha.sup.-1] [yr.sup.-1]. The species differed in the quantity and chemical composition of their detrital production. Soil organic C was significantly correlated with fine-root growth, but not with aboveground detrital inputs. Soil organic C increased with potential C mineralization on a grams of C basis, indicating that species influenced both the quality and quantity of SOC. Contrary to expectations, SOC declined with increasing fine-root lignin concentrations, indicating that lignin-derived C did not dominate refractory SOC pools. We hypothesize that differences among species in the capacity to increase SOC stocks involved fine-root traits that promoted soil microbial turnover and, thus, greater production of recalcitrant, microbial-derived C fractions. Abbreviations: SOC, soil organic carbon.

High grain production of corn (Zea mays L.) can be maintained by adding inorganic N fertilizer, and also by using crop rotations that include alfalfa (Medicago sativa L.), but the relative impact of these management practices on soil quality is uncertain. We examined the effects on soil of N fertilization rate (0, 90,180, 270 kg [ha.sup.-1], corn phase only) in four cropping systems: CC, continuous corn; CS, corn--soybean [Glycine max (L.) Merr.]; CCOA, corn--corn--oat (Avena sativa L.)--alfalfa; and corn--oat--alfalfa--alfalfa (COAA). The 23- and 48-yr-old experimental sites, situated in northeast (Nashua) and north central (Kanawha) Iowa, were in a replicated split-plot design and managed with conventional tillage. At Nashua, we measured available N, potential net N mineralization and microbial biomass C (MBC) throughout the growing season; all were significantly higher in the CCOA system. At both sites, post-harvest N stocks, and soil organic C (SOC) concentrations were significantly higher in systems containing alfalfa. Grain yield was most strongly correlated with soil N properties. At Nashua, N fertilizer additions resulted in significantly lower soil pH (0- to 15-cm depth) and lower exchangeable Ca, Mg, and K and cation exchange capacity (CEC) in the CC and CCOA systems. In an undisturbed prairie reference site for Nashua, low available N, low pH, and high CEC suggested a strong influence of the vegetation on nutrient cycling. In terms of management of soil fertility, inclusion of alfalfa in the rotation differed fundamentally from addition of N fertilizer because high yield was maintained with fewer adverse effects on soil quality.

Growing interest in the potential for agricultural soils to provide a sink for atmospheric C has prompted studies of effects of management on soil organic carbon (SOC) sequestration. We analyzed the impact on SOC of four N fertilization rates (0-270 kg N [ha.sup.-1]) and four cropping systems: continuous corn (CC) (Zea mays L.); corn-soybean [Glycine max (L.) Merr.] (CS): corn-corn-oat-alfalfa (oat, Avena sativa L.; alfalfa, Medicago sativa L.) (CCOA), and corn-oat-alfalfa-alfalfa (COAA). Soils were sampled in 2002, Years 23 and 48 of the experiments located in northeast and north-central Iowa, respectively. The experiments were conducted using a replicated split-plot design under conventional tillage. A native prairie was sampled to provide a reference (for one site only). Cropping systems that contained alfalfa had the highest SOC stocks, whereas the CS system generally had the lowest SOC stocks. Concentrations of SOC increased significantly between 1990 and 2002 in only two of the nine systems for which historical data were available, the fertilized CC and COAA systems at one site. Soil quality indices such as particulate organic carbon (POC) were influenced by cropping system, with CS < CC < CCOA. in the native prairie, SOC, POC, and resistant C concentrations were 2.8, 2.6, and 3.9 times, respectively, the highest values in cropped soil, indicating that cultivated soils had not recovered to precultivation conditions. Although corn yields increased with N additions, N fertilization increased SOC stocks only in the CC system at one site. Considering the C cost for N fertilizer production, N fertilization generally had a net negative effect on C sequestration.