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Herbivores alter plant biodiversity (species richness) in many of the world's ecosystems, but the magnitude and the direction of herbivore effects on biodiversity vary widely within and among ecosystems. One current theory predicts that herbivores enhance plant biodiversity at high productivity but have the opposite effect at low productivity. Yet, empirical support for the importance of site productivity as a mediator of these herbivore impacts is equivocal. Here, we synthesize data from 252 large-herbivore exclusion studies, spanning a 20-fold range in site productivity, to test an alternative hypothesis-that herbivore-induced changes in the competitive environment determine the response of plant biodiversity to herbivory irrespective of productivity. Under this hypothesis, when herbivores reduce the abundance (biomass, cover) of dominant species (for example, because the dominant plant is palatable), additional resources become available to support new species, thereby increasing biodiversity. By contrast, if herbivores promote high dominance by increasing the abundance of herbivory-resistant, unpalatable species, then resource availability for other species decreases reducing biodiversity. We show that herbivore-induced change in dominance, independent of site productivity or precipitation (a proxy for productivity), is the best predictor of herbivore effects on biodiversity in grassland and savannah sites. Given that most herbaceous ecosystems are dominated by one or a few species, altering the competitive environment via herbivores or by other means may be an effective strategy for conserving biodiversity in grasslands and savannahs globally.

Understanding the effects of agricultural land use on wetland N cycling is critical in areas such as south-central Florida, where widespread agricultural activities intersect with extensive wetland systems. We examined annual net N mineralization ([N.sub.min]) (buried-core method) and denitrification rates (acetylene-block method) in 24 small seasonal wetlands on a cattle ranch in this region, 12 each in intensively managed improved pastures (IP) and less intensively managed semi-native pastures (SNP). Wetlands in IP had less detritus, higher N concentrations, lower C/N ratios (0-15 cm), and higher microbial biomass N than did wetlands in SNP. Cumulative annual net [N.sub.min] was lower in IP wetlands (98 [+ or -] 17 kg N [ha.sup.-1]) than in SNP wetlands (133 [+ or -] 18 kg N [ha.sup.-1]). Nitrification was much lower in IP than in SNP wetlands and dominated net [N.sub.min] during the dry season (December-June), but was negligible during the flooded period (July-October). Cumulative annual denitrification was lower in IP wetlands (17.7 [+ or -] 3.4 kg N [ha.sup.-1]) than in SNP wetlands (34.7 [+ or -] 6.3 kg N [ha.sup.-1]). Soil N cycling rates correlated with N[O.sub.3.sup.-] and N[H.sub.4.sup.+] concentrations, which correlated with soil C content. Our results show that the more intensive management of improved pastures was associated with declines in wetland soil C content and lower rates of nitrification and denitrification. Abbreviations: IP, improved pasture; OM, organic matter; SNP, semi-native pasture.

Land use practices exert a major influence on plant productivity, soil and plant nutrient content, and within-stand nutrient cycling in wetlands in agricultural landscapes. We examined differences between improved and seminative pastures in plant and soil nutrient characteristics in seasonally flooded wetlands in subtropical grazing land of south central Florida. The wetlands were embedded within either grazed improved pastures with a long-term history of fertilizer application or seminative pastures with no history of previous fertilizer application. Soil nutrient concentrations decreased with soil depth for both land use types. Total C, N, and P were significantly greater (P < 0.05) in the 0- to 15-cm mineral layer compared with the deeper layers (15-30, 30-45 cm) for both improved and seminative pasture wetland soils. Improved pasture wetlands had greater amounts of total P (22.3 kg P [ha.sup.-1]) in the upper 0- to 15-cm soil layer than did the seminative pasture wetlands (15.7 kg P [ha.sup.-1]). Plant and soil (0-15 cm) N/P and C/P ratios were lower in improved pasture wetlands compared with seminative pasture wetlands, suggesting greater P enrichment in improved pasture wetlands. Microbial biomass C and N decreased with soil depth in both pasture types. Soil microbial biomass C/total C ratios decreased with soil depth and were similar for both improved and seminative pasture wetlands. Our results suggest that plant and soil nutrient enrichment and storage in temporary wetlands may be impacted by adjacent land use practices, which potentially leads to the alteration of the structure and functions of these wetland ecosystems.

Expanding global commerce may be increasing the likelihood of exotic earthworm invasions, which could have negative implications for soil processes, other animal and plant species, and importation of certain pathogens. [...]

Earthworms incorporate organic matter into soil but their influence on cycling of N from incorporated materials is not well understood. This study examined the role of earthworms in the turnover of N from different (super 15)N-labeled inputs. We incubated intact soil cores with or without earthworms (Lumbricus terrestris) and with (super 15)N-labeled KNO3, rye litter (Secale cereale L.), or cow manure, added to the soil surface at the rate of 150 kg N ha(super -1). The cores were destructively sampled after 2, 6, and 10 wk and assayed for total soil N, KCl-extractable N, microbial-biomass N (MBN), two size fractions (53-2000 mu m and >2000 mu m) of particulate organic matter (POM), and (super 15)N enrichment of all N pools. Earthworms increased the incorporation of (super 15)N into the soil in organically treated cores but had little effect on the distribution of (super 15)N in the inorganically fertilized cores. The percentage of initial (super 15)N that was incorporated into the soil after 10 wk was 71 and 45% in cores with earthworms and 34 and 25% in cores without earthworms, in the manure and rye treatments, respectively. Earthworms incorporated the manure more rapidly than the rye, as shown by temporal patterns of incorporation of total N and POM. Earthworms increased MBN in the manure treatment by about 20 to 30%, but slightly decreased MBN on one sample date (2 wk) in the rye treatment. Interactions between earthworms and organic materials of different quality influence both the rate at which the materials are incorporated into the soil and the subsequent mineralization of N from those materials.