Experiments E001 and E002

Dynamic Effects of N on Populations, Communities and Ecosystems

Experiment E001 - Long-Term Nitrogen Deposition: Effects on Plant Diversity, Composition, Productivity and Stability

Experiment E002 - Long-Term Nitrogen Deposition: Effects on Succession Following Major Disturbances

Introduction

Description of Experimental Methods

N Addition to Undisturbed Vegetation:
This experiment (E001) is replicated in three successional fields and in a savanna prairie opening. Treatments are a control, addition of all nutrients except N, and addition of all nutrients plus N, with N added at 1 of 7 rates (Tilman 1987 , www.lter.umn.edu). Plots are annually sampled for aboveground biomass (sorted to species), litter mass, and extractable soil NH4 and NO3, and periodically for belowground biomass, insect abundances, mycorrhizal fungal species densities, light penetration, small mammal densities, and microbial biomass. This experiment has provided insights into causes of successional dynamics (e.g., Tilman 1987 , 1988, 1990 ), effects of N deposition on C and N storage (Wedin and Tilman 1996), causes of diversity differences along productivity gradients (Tilman 1990 , 1993 , 1996a), impacts of diversity on ecosystem stability (Tilman and Downing 1994, Tilman 1996a, 1999a, Tilman et al. 1998, Lehman and Tilman, in review), impacts of climatic variation on biodiversity (Tilman and El Haddi 1992, Tilman 1996a), and long-term dynamics after a major drought (Haddad and Tilman, in prep)

A second N addition experiment in oak savanna (E095; fire frequency, 2 of 3 yrs), begun in 1983, has 9 plots (20 x 50 m) receiving N at 0, 5, or 17 g m-2 yr-1. It complements E001 by separating out responses to N of woody vs. herbaceous species, including the long-term consequences of alteration in plant composition and diversity. It complements E133 (fire frequency in savanna, below) by providing a different savanna N cycling gradient (in this case N deposition rather than long-term fire effects on N cycling). This experiment received increased attention in the late 1990s when we began documenting effects of N deposition on N cycling, soil water, NPP, and species composition and diversity, both beneath oak trees and in grassy openings between trees (Wrage and Reich, unpublished data; Reich et al. unpublished data).

N Addition to Disturbed Vegetation:
This experiment, E002, is identical to E001 except that plots were initially disturbed via disking. After undergoing rapid successional changes, E001 and E002 converged in composition, plant abundances, and diversity (Inouye and Tilman 1988, 1995). As of 1993 we use E002 for two new experiments. In each, we randomly chose 3 of 6 replicates of each treatment for a new manipulation and continued treatments in the other 3 replicates. In one experiment the new treatment was cessation of nutrient addition, designed to determine dynamics of recovery of productivity, composition and diversity after long-term N addition. In the other, we began annual spring burning in 3 plots per treatment.

Gradient in N Availability across a Successional Chronosequence:
Both total soil N (Knops and Tilman 2000; Fig. 3C [pdf], 17 [pdf]) and N mineralization rates (Pastor et al. 1982, 1984) increase during succession at Cedar Creek, providing an additional opportunity to examine effects of N on plants, arthropod and small mammal herbivores, and arthropod predators, parasitoids, and decomposers (Inouye et al. 1994, 1997, Siemann et al. 1999a, Pitt 1999, Lawson et al. 2000). We are exploring these relations in long-term observations in a series of over 2000 permanent plots in a successional chronosequence discussed in Set 5, below.

Key Results

— N addition impacts ecosystem C and N stores in E001 via effects on species composition and thus on litter C:N (Wedin and Tilman 1996). At higher N addition, diversity is lower, C4 grasses are less abundant, and litter and root C:N ratios are lower (Fig 18A-D [pdf]). The N-dependent shift to low C:N species corresponds with decreased ecosystem retention of added N (Fig. 19A [pdf]) and to lower C storage (Fig. 19B [pdf]), likely because of immobilization and decomposition effects (Fig. 19C [pdf]).
—The resistance of total plant biomass to drought was significantly greater at higher diversity in E001 plots, even after controlling for numerous potentially confounding variables (Fig. 2A [pdf]; Tilman and Downing 1994). For all non-drought years, interannual variation in total plant biomass (measured as CV) was greater at lower diversity (Fig. 20 [pdf]; Tilman 1996a, 1999a). Both results support the diversity-stability hypothesis.
— Addition of 1 g m-2 yr-1 of N for 18 yr, comparable to regional wet+dry deposition, caused the loss of 27% of plant diversity (Fig. 21A [pdf]). Greater rates of N addition led to greater species losses (Fig. 21B [pdf]). However, the N-dependent loss of species was non-linear (Fig. 21C [pdf]) and has not yet reached an asymptote for low rates of N addition, suggesting that effects of N deposition on species richness cannot be extrapolated from short-term but high-dose experiments.
— Since the 1988 drought, total plant biomass in the unfertilizeed control plots of this experiment has oscillated with a 2-3 y period (Fig. 22A [pdf]). These oscillations cannot be explained by variation in climate or densities of herbivores (Haddad et al, in review), but might be caused by effects of litter on either immobilization of N or on dynamics of fungal diseases that overwinter in litter. Similar oscillations in an burned field have biomass lows in unburned years (Fig. 22B [pdf]), supporting the hypothesized role of litter as a cause of these oscillations.
— In oak savanna, rates of soil net N mineralization were higher beneath trees than in grassy openings at all levels of N addition, and rates increased similarly with N addition in both vegetation patch types (Wrage and Reich, unpublished data).
— In 1999, the seventh field season after cessation of N fertilization in E002, there was no significant recovery in plant diversity or composition, suggesting that N deposition can have long-lasting effects. This experiment should continue to determine the trajectory of recovery.
— By 1999, burn treatments in E002 led to marked differences in composition, diversity, and standing crop, with Andropogon gerardi (big bluestem) becoming dominant in high-N, burned plots, but Agropyron repens (quack grass) in high-N, unburned plots. Schizachyrium scoparium (little bluestem) remains dominant in burned and unburned low N plots.

Past, Ongoing, and Future Work on Theory and Mechanisms

Ongoing mechanistic studies include work testing the ability of the disperal mechanisms and nutrient ecophysiology of the major plant species (Tilman and Cowan 1989, Tilman and Wedin 1991, Craine et al. 1999a, Craine et al., in review), when incorporated into resource competition theory (Tilman 1988, 1994a, 1997c; Tilman et al. 1994b, 1997c), to predict the species abundances, diversity and successional dynamics observed in our long-term N addition experiments and the chronosequence. This is aided by experimental studies of N and light competition (Tilman 1990, Wedin and Tilman 1993, Tilman and Wedin 1991). We are also using the N addition experiments and the successional chronosequence (Knops and Tilman 2000) for ongoing work on the effects of different plant species or functional groups on N mineralization and dynamics, and on C storage, via litter quality feedbacks (Wedin and Tilman 1990, 1996, Wedin et al. 1995).