Belowground Plant Production Research Articles

Synergistic Effects of Grassland Plant Spcies on Soil Microbial Biomass and Activity: Implications for Ecosystem-Level Effects of Enriched Plant Diversity

1. The effects of the grass species Lolium perenne L. and nine dicotyledonous grassland species, grown in monocultures and two-species mixtures, on (i) the soil microbial biomass, (ii) the respiration: biomass ratio and (iii) plant litter decomposition was investigated in a glasshouse experiment. 2. Microbial biomass was sometimes greater and sometimes less in the two-species mixtures than could be explained in terms of the additive effects of the two component species grown singly; this variation was independent of differences in below-ground plant productivity between monoculture and mixture treatments. 3. The microbial respiration: biomass ratios and plant litter decomposition rates in the two-species mixture treatments were either greater or less than expected based on the monoculture treatments; these differences were dependent on the combinations of species present. Because the respiration: biomass ratio is a measure of ecosystem stability, it is here proposed that stability does not respond predictably to shifts in species diversity. 4. These results provide evidence that increasing plant species richness (from one to two species) has the potential to influence soil processes positively or negatively in a non-additive way. The possible ecological implications of this are discussed.

Fire and topographic effects on decomposition rates and N dynamics of buried wood in tallgrass prairie

Decay rates and N dynamics of wood in soils of annually burned and unburned tallgrass prairie were measured over a 3-y period. Wooden dowels were placed at upland, mid-slope and lowland sites in two annually burned and two unburned watersheds. After 3 y, an average of only 15% of initial wood mass remained in burned watersheds, while 34% remained in unburned watersheds. Topographic position also significantly affected decay rates, with dowels decaying faster in the shallow-soil, upland sites and slope sites than in the deep-soil, lowland sites. This pattern is opposite of that generally observed for plant productivity (i.e. greater at lowland sites compared to uplands), and suggests that the controls of belowground decomposition and plant productivity are dissimilar. Dowels in both burned and unburned watersheds showed significant increases in N concentration over 3 y. Topographic position did not affect N concentration in the residual dowel material. Burn treatment, however, did affect N concentration, with dowels decomposing in burned watersheds having a higher average N concentration (0.5% after 3 y exposure) than dowels in unburned watersheds (0.43%). Relatively rapid decay rates resulted in net release of N, despite increased N concentration in the residual material. Faster net N release on the annually burned watershed was due to faster mass loss, since there were no differences in the rate of increase in N concentration per unit mass lost. Surface soil temperatures on burned prairie following spring fire usually exceed those on unburned prairie. However, average monthly summer soil temperatures (May–August) at a 10 cm depth in burned and unburned plots during the study were not statistically different and could not explain decay rate differences. Additionally, one of our unburned watersheds was accidentally burned during the first year of the study. Surprisingly, there were no significant differences in rates of wood decay between that watershed and the other unburned watershed. This suggests that indirect effects of annual fire (i.e. changes in the composition of soil flora and fauna) may override the short-term effects of fire (i.e. changes in soil temperature and moisture) on belowground decomposition in tallgrass prairie.

Long- and short-term effects of fire on nitrogen cycling in tallgrass prairie

Fires in the tallgrass prairie are frequent and significantly alter nutrient cycling processes. We evaluated the short-term changes in plant production and microbial activity due to fire and the long-term consequences of annual burning on soil organic matter (SOM), plant production, and nutrient cycling using a combination of field, laboratory, and modeling studies. In the short-term, fire in the tallgrass prairie enhances microbial activity, increases both above-and belowground plant production, and increases nitrogen use efficiency (NUE). However, repeated annual burning results in greater inputs of lower quality plant residues causing a significant reduction in soil organic N, lower microbial biomass, lower N availability, and higher C:N ratios in SOM. Changes in amount and quality of below-ground inputs increased N immobilization and resulted in no net increases in N availability with burning. This response occurred rapidly (e.g., within two years) and persisted during 50 years of annual burning. Plant production at a long-term burned site was not adversely affected due to shifts in plant NUE and carbon allocation. Modeling results indicate that the tallgrass ecosystem responds to the combined changes in plant resource allocation and NUE. No single factor dominates the impact of fire on tallgrass plant production.

Patterns of sulfate reduction and the sulfur cycle in a South Carolina salt marsh1,2

Rates of sulfate reduction and the magnitude of various sediment parameters related to the sulfur cycle were measured at about quarterly intervals for a 3‐yr period at sites in a South Carolina salt marsh containing tall‐ or short‐form Spartina alterniflora. Sulfate reduction rates were greater at all times in the short‐form sites and decreased markedly with depth. Seasonal changes were highly correlated with ambient temperature except in May and August when interactions with plants may have affected rates directly. In the short‐form site, the annual integrated sulfate reduction rate accounted for about 47.5–67.0% of the estimated belowground plant production. Dissolved sulfides and pH varied seasonally in the short‐ but not tall‐form sites; seasonal minima in the short‐form site occurred during May and were probably due to oxidation of the sediment with the onset of plant production. Mechanisms resulting in oxidation were not evident however. Chromium‐reducible sulfur, which is primarily pyrite, was substantially greater in the tall‐ than short‐form site but apparently stable temporally at both sites. The differences in pool sizes of pyrite were probably due to differences in iron dynamics and the extent of sulfide oxidation. Microprofiles of dissolved oxygen were consistent with greater oxidation at the short‐form site since the penetration of oxygen was deeper. Differences in sediment texture and tidal inundation may have contributed to greater oxygen inputs at the short‐form site.

Impact of management practices on the tallgrass prairie

The ELM ecosystem-level grassland model simulates the flow of water, heat, nitrogen, and phosphorus through the ecosystem and the biomass dynamics of plants, consumers, and the decomposers. This model was adapted to a tallgrass prairie site in northeastern Oklahoma, USA, the Osage Site of the U.S. International Biological Program Grassland Biome. Several range management manipulations were simulated by the model and the results compared to field data and literature information: (1) altering the grazing intensity, grazing system, and grazing time period; (2) adding nitrogen and phosphorus to the grassland; (3) adding water during the growing season; and (4) spring burning of the prairie.The model showed that cattle weight gain per head, above-ground and belowground plant production, transpiration water loss, standing dead biomass, and the net nitrogen balance decrease with increasing grazing intensity, while soil water content and bare soil water loss increase. A moderately stocked year-round cow-calf grazing system is more beneficial to the grassland than a more highly stocked seasonal steer grazing system because the former increases the aboveground and belowground primary production and the plant nutrient uptake rates. Range manipulations, such as fire, which stimulate uniform grazing of a pasture, increase primary production, cattle weight gains, and nutrient uptake of plants and animals. Model results indicated that adding fertilizer was the best strategy for increasing cattle weight gains per head, while adding water would produce the greatest increase in primary production. Simulation of yearly and triennial spring burns suggests that these treatments increase primary production, plant nutrient uptake, and cattle weight gain per head. Burning increases the nitrogen losses from the systems; however, these losses are greater with annual burns. The model results also suggest the spatial grazing pattern of cattle must be considered to correctly represent the impact of grazing on the prairie.The model is used to describe the behavior of the tallgrass prairie ecosystem, evaluate alternative management strategies, and identify future scientific research and management studies.