CO₂ Fertilization Study

SGS LTER investigators (Mosier, Parton, Milchunas) have successfully received renewal funding for our CO₂ fertilization study, which is also supported by the SGS LTER project. The study utilizes open-to chambers to address three objectives: 1) to determine the impact of doubling CO₂ in shortgrass steppe mixed C₃/C₄ plant communities on net primary production, net ecosystem CO₂/H₂O exchange, C and N allocation both above and below ground, and water and N use efficiency; 2) to determine the impact of doubling CO₂ on soil water and n dynamics on soil water content, C/N distribution in soil organic matter, changes in mineralizable N, NO and N₂O emissions, and consumption of atmospheric CH₄; ₃) to incorporate the knowledge gained from these studies into simulation models that will allow for realistic extrapolation through time and space of soil moisture, nutrient cycling, and plant productivity. Preliminary results from this study indicate significant enhancement of aboveground plant biomass in the double CO₂ treatment. An estimated 40% of annual N input into the soil is re-emitted to the atmosphere as NO_x and approximately 4% as N₂O. The double CO₂ soils emitted ~35% less N₂O but about the same amount of NO_x as soils under ambient CO₂ (Mosier et al. 1998).

Anthropogenic Additions of Nitrogen

We have initiated several new studies on the long-term effects of additional N on ecosystem structure and function. We have recently manipulated some long-term SGS-LTER N-addition plots, under funding from a separate NSF grant, in an effort to study processes responsible for n retention and availability. In a large-scale, replicated design, we are adding humus precursors to soils to stimulate n stabilization, as well as adding microbial C sources to stimulate immobilization's. 15N labeled additions are being tracked through soil organic matter pools using NMR. We are following n mineralization and plant community composition, with respect to weedy vs. native species. A graduate students associated with this project is conducting greenhouse studies to assess the influence of N and water availability on weed recruitment for shortgrass steppe soils (see also weed work listed in populations section above). In a long-term N addition study completed this year, we found that nitrogen fertilization through animal deposition or synthetic N application of pastures within the Colorado shortgrass steppe results in increased nitric oxide (NO) and nitrous oxide (N₂0) emissions for at least the following 15 years (Mosier and Parton 1998, Parton et al. 1998).

Species Effects on Biogeochemistry

We continue to focus a number of studies on the impacts of plant species composition on biogeochemical cycling. Much of our earlier work focused on the spatial patterns in biogeochemistry created by plant distributions. Recent studies (Epstein et al in press, Epstein et al in press b, Epstein et al. submitted) have demonstrated that plant function type (C₃ vs. C₄) can affect both the spatial and temporal patterns of nitrogen cycling in shortgrass steppe. Phenological and nutrient use efficiency patterns result in seasonal patterns of interactions between plants and soil microbial communities with respect to N cycling and uptake, with significant impacts on nitrogen and carbon trace gas fluxes. These differences may result in long-term and large-scale impacts on n cycling, retention, and storage.

Carbon and Nitrogen Distributions with Depth

A great deal of our past research has focused on the controls over organic matter distributions horizontally across the landscape and across microsites, while much less research has explored what controls the depth-distribution of SOM and nutrient availability. One of the interesting inconsistencies in the literature regarding soil organic matter in grasslands derives from the observation that although a very high proportion of plant roots are located in the soil surface, soil organic matter is distributed more evenly through the profile, not the expected pattern if most grassland SOM derives from root inputs. We have recently estimated decomposition and root production through the soil profile to attempt to explain this pattern. We estimated decomposition using buried litter bags distributed among several depths, and estimated root production using minirhizotrons. The data suggest that production and decomposition alone are insufficient to explain the patterns of SOM distribution. This may be the result of (1) movement of dissolved organic matter from the soil surface to depth, (2) geomorphic processes resulting in the accumulation or loss of mass from the soil surface, or (3) complex kinetics associated with the formation of SOM from root detritus (Gill and Burke 1998 ESA abstract, and manuscript in prep).

In a separate study, we evaluated the patterns of nitrogen availability with depth (Dodd 1997). Measurements of nitrogen availability through a loamy sand soil profile showed that the largest nitrogen resource occurred in the top 10cm, but that favorable soil moisture conditions could substantially increase available nitrogen at depths up to 60cm. This implies an important link between water and nitrogen availability.