Effects of applications of fungicide, phosphorus and nitrogen on the structure and productivity of an annual serpentine plant community

Benomyl (a fungicide) and phosphate (as NaH2PO4-H20) were applied to patches of two annual serpentine communities dominated by forbs. After one year neither Benomyl nor phosphate had affected plant productivity (except for legume productivity which was increased by phosphorus application), the abundance of any species or mycorrhizal infection. However, at both research sites Benomyl decreased phosphorus concentration in the shoots whereas added phosphorus increased its concentration. At one site, added phosphorus also increased nitrogen concentration in the shoot. In the following year, nitrogen (as NH4NO3) was added to the experimental design. In that second year, both nitrogen and phosphorus applications decreased the abundance and total above-ground dry weight per unit area of Plantago erecta Morris, a dominant annual forb, and decreased the density of all plants taken together. Additionally, Benomyl increased total plant density. Phosphorus and nitrogen increased productivity of the whole community. The data presented herein support the hypothesis that communities occupying inherently infertile sites are not very responsive to nutrient amendment and that mycorrhizal infection may serve as a controller of community structure. Key-words: Annual grassland, Benomyl, nitrogen, phosphorus, serpentine soils, nutrient limitation, forbs, mycorrhiza, community structure * Address for correspondence: Dr R.T. Koide, Department of Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. t Present address: Dr L.F. Huenneke, Department of Biology, New Mexico State University, Box 3AF, Las Cruces, New Mexico 88003, USA t Present address: Dr S.P. Hamburg, Department of Systematics and Ecology, University of Kansas, Lawrence, Kansas 66045, USA. Introduction Plant productivity in serpentine soils can be limited by a number of factors. These include low levels of macronutrients, high concentrations of nickel or chromium, calcium deficiency or magnesium toxicity (Proctor & Woodell, 1975). It has become clear that no single factor is sufficient to explain growth limitations in all serpentine communities. Indeed, serpentine soils are quite variable chemically and physically (Walker, 1954) and the plant species occupying them appear to reflect this variability (Kruckeberg, 1954). Differential tolerance or preference of species to the particular soil chemical regime may thus influence the structure of the community (Tilman, 1986). Indeed, nutrient availability has been shown to affect species composition of both serpentine (Ferriera & Wormell, 1971) and nonserpentine substrates (le Roux & Mentis, 1986; Tilman, 1986). Turitzin (1982) identified both nitrogen and phosphorus as limiting nutrients in an annual serpentine grassland community in coastal central California (the Jasper Ridge Biological Preserve of Stanford University). He showed that total community productivity increased when either nitrogen or phosphorus were added, that nitrogen had a greater effect than phosphorus, and that annual grasses were more responsive than annual forbs. However, it was not reported how fertilizer application quantitatively altered species composition or other community properties. This same serpentine grassland supports welldeveloped vesicular-arbuscular mycorrhizal infections. As is well documented, infection may enhance plant phosphorus uptake (Smith, 1980), particularly under conditions of soil drought (Fitter, 1985). Because the benefit of infection may differ for each species in the community and because a mycorrhizal fungal mycelial network may mediate the transfer of materials from plant to plant within the community (Grime et al., 1987), mycorrhizal infection may serve as another controller of community structure. This content downloaded from 157.55.39.104 on Mon, 20 Jun 2016 07:32:35 UTC All use subject to http://about.jstor.org/terms 336 In order to document the effects of nutrient R. T. Koide supply and mycorrhizal infection on annual et al. serpentine community characteristics, a study was conducted in two serpentine sites in coastal central California. In the Jasper Ridge Biological Preserve, applications of Benomyl, a fungicide reported to suppress mycorrhizal infection [Menge, 1982]) and of phosphate were made during a single growing season. At another site (referred to as the Kirby Canyon site), applications of Benomyl and of phosphate were made over two growing seasons and nitrogen application was made during a single growing season. Effects on the abundance and shoot productivity of various species, mycorrhizal infection and nitrogen and phosphorus concentrations in the shoots of Plantago erecta Morris, a dominant annual forb, are reported. Materials and methods