The primary source of mineral nutrients for plants is the decomposition of organic matter by soil microbes. Plants are traditionally viewed as largely passive participants in the decomposition process, incapable of directly affecting rates of decomposition significantly and primarily assimilating nutrients unused by the microbial pool. We performed a 13C pulse‐chase experiment on a common grazing tolerant grass, Poa pratensis L., of Yellowstone National Park, to follow carbon flow into the soil rhizosphere and microbial biomass and the associated effects on soil N availability and plant N dynamics. Grazing promoted root exudation of carbon, which was quickly assimilated into a burgeoning microbial population in the rhizosphere of clipped plants. Moreover, these facilitating effects of defoliation on rhizospheric processes positively fed back on soil inorganic N pools, plant N uptake, leaf N content, and photosynthesis. Such findings are the first evidence, to our knowledge, that suggest (1) plants are capable of promoting rhizospheric microbial populations to facilitate uptake of a limiting soil resource and (2) that there is a general positive feedback mechanism by which herbivory promotes plant regrowth as well as energy and nutrient flows in grazed landscapes.

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High soil sodium (Na) is a common stress in natural and agricultural systems. Roots are usually the first tissues exposed to Na stress and Na stress-related impairment of mitochondrial function is likely to be particularly important in roots. However, neither the effects of NaCl on mitochondrial function, nor its protection by several potential adaptive mechanisms, have been well studied. This study investigated the effects of NaCl stress on maize (Zea mays) mitochondrial electron transport and its relative protection by osmoprotectants (proline, betaine, and sucrose), antioxidants (ascorbate, glutathione, and ␣-tocopherol), antioxidant enzymes (catalase and Cu/Zn-superoxide dismutase), and mitochondrial small heat shock proteins (sHsps). We demonstrate that Complex I electron transport is protected by antioxidants and sHsps, but not osmoprotectants, whereas Complex II is protected only by low concentrations of proline and betaine. These results indicate that NaCl stress damaged Complex I via oxidative stress and suggests that sHsps may protect Complex I as antioxidants, but NaCl damaged Complex II directly. This is the first study to demonstrate that NaCl stress differentially affects Complex I and II in plants and that protection of Complex I and II during NaCl stress is achieved by different mechanisms.Diverse environmental stresses often induce similar kinds of cellular damage. For example, many, or even most, environmental stresses induce oxidative stress and protein denaturation (e.g. temperature stress, salinity, and drought; Vierling, 1991; Bowler et al., 1992; Parsell and Lindquist, 1994; Navari-Izzo et al., 1996; Waters et al., 1996; Noctor and Foyer, 1998). As a consequence, diverse stresses often illicit similar cellular adaptive responses, such as the production of stress proteins, up-regulation of oxidative stress protectors, and accumulation of protective solutes (e.g. Vierling, 1991; Bowler et al., 1992; Parsell and Lindquist, 1994; Navari-Izzo et al., 1996; Hare et al., 1998; Noctor and Foyer, 1998; McNeil et al., 1999; Hamilton et al., 2001). In many cases, mitochondria are key sites of damage during environmental stress, especially mitochondrial electron transport (e.g. Chauveau et al., 1978; Zhang et al., 1990; Hernandez et al., 1993; Polla et al., 1996; Pobezhimova et al., 1997; Yan et al., 1997; Downs and Heckathorn, 1998). Most of the general cellular protective adaptations mentioned above are known to be present in mitochondria (e.g. Vierling, 1991; Parsell and Lindquist, 1994; Prasad et al., 1995; Waters et al., 1996; Jimenez et al., 1997 Jimenez et al., , 1998 Downs and Heckathorn, 1998); however, the relative importance of these adaptations is unknown. We predict that the relative importance of these adaptations will vary among stresses, because the specific nature of "damage" varies with the type of stress. For example, there are "weak links" within mitochondria during stress and these weak links are different for different stresses (Zhang et al

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Large mammalian herbivores in grassland ecosystems influence plant growth dynamics in many ways, including the removal of plant biomass and the return of nutrients to the soil. A 10-week growth chamber experiment examined the responses of Sporobolus kentrophyllus from the heavily grazed short-grass plains of Serengeti National Park, Tanzania, to simulated grazing and varying nitrogen nutrition. Plants were subjected to two clipping treatments (clipped and unclipped) and five nitrogen levels (weekly applications at levels equivalent to 0, 1, 5, 10, and 40 g N m), the highest being equivalent to a urine hit. Tiller and stolon production were measured weekly. Total biomass at harvest was partitioned by plant organ and analyzed for nitrogen and mineral element composition. Tiller and stolon production reached a peak at 3-5 weeks in unclipped plants, then declined drastically, but tiller number increased continually in clipped plants; this differential effect was enhanced at higher N levels. Total plant production increased substantially with N supply, was dominated by aboveground production, and was similar in clipped and unclipped plants, except at high nitrogen levels where clipped plants produced more. Much of the standing biomass of unclipped plants was standing dead and stem; most of the standing biomass of clipped plants was live leaf with clipped plants having significantly more leaf than unclipped plants. However, leaf nitrogen was stimulated by clipping only in plants receiving levels of N application above 1 g N m which corresponded to a tissue concentration of 2.5% N. Leaf N concentration was lower in unclipped plants and increased with level of N. Aboveground N and mineral concentrations were consistently greater than belowground levels and while clipping commonly promoted aboveground concentrations, it generally diminished those belowground. In general, clipped plants exhibited increased leaf elemental concentrations of K, P, and Mg. Concentrations of B, Ca, K, Mg, and Zn increased with the level of N. No evidence was found that the much greater growth associated with higher N levels diminished the concentration of any other nutrient and that clipping coupled with N fertilization increased the total mineral content available in leaf tissue. The results suggest that plants can (1) compensate for leaf removal, but only when N is above a critical point (tissue [N] 2.8%) and (2) grazing coupled with N fertilization can increase the quality and quantity of tissue available for herbivore removal.