Robin Kelly

Plant-Soil interactions in temperate grasslands

We present a conceptual model in which plant-soil interactions in grasslands are characterized by the extent to which water is limiting. Plant-soil interactions in dry grasslands, those dominated by water limitation (‘belowground-dominance’), are fundamentally different from plant-soil interactions in subhumid grasslands, where resource limitations vary in time and space among water, nitrogen, and light (‘indeterminate dominance’). In the belowground-dominance grasslands, the strong limitation of soil water leads to complete (though uneven) occupation of the soil by roots, but insufficient resources to support continuous aboveground plant cover. Discontinuous aboveground plant cover leads to strong biological and physical forces that result in the accumulation of soil materials beneath individual plants in resource islands. The degree of accumulation in these resource islands is strongly influenced by plant functional type (lifespan, growth form, root:shoot ratio, photosynthetic pathway), with the largest resource islands accumulating under perennial bunchgrasses. Resource islands develop over decadal time scales, but may be reduced to the level of bare ground following death of an individual plant in as little as 3 years. These resource islands may have a great deal of significance as an index of recovery from disturbance, an indicator of ecosystem stability or harbinger of desertification, or may be significant because of possible feedbacks to plant establishment. In the grasslands in which the dominant resource limiting plant community dynamics is indeterminate, plant cover is relatively continuous, and thus the major force in plant-soil interactions is related to the feedbacks among plant biomass production, litter quality and nutrient availability. With increasing precipitation, the over-riding importance of water as a limiting factor diminishes, and four other factors become important in determining plant community and ecosystem dynamics: soil nitrogen, herbivory, fire, and light. Thus, several different strategies for competing for resources are present in this portion of the gradient. These strategies are represented by different plant traits, for example root:shoot allocation, height and photosynthetic pathway type (C3 vs. C4) and nitrogen fixation, each of which has a different influence on litter quality and thus nutrient availability. Recent work has indicated that there are strong feedbacks between plant community structure, diversity, and soil attributes including nitrogen availability and carbon storage. Across both types of grasslands, there is strong evidence that human forces that alter plant community structure, such as invasions by nonnative annual plants or changes in grazing or fire regime, alters the pattern, quantity, and quality of soil organic matter in grassland ecosystems. The reverse influence of soils on plant communities is also strong; in turn, alterations of soil nutrient supply in grasslands can have major influences on plant species composition, plant diversity, and primary productivity.

Soil Organic Matter and Nutrient Availability Responses to Reduced Plant Inputs in Shortgrass Steppe

We examined soil organic matter (SOM) dynamics in a shortgrass steppe ecosystem along spatial gradients in plant inputs and temporal gradients in disturbance age because a better understanding of decay characteristics of SOM pools may improve our ability to predict ecosystem responses to perturbation. We assessed measurable pools of SOM that are thought to correspolnd to active, intermediate, and passive SOM based upon turnover characteristics in three separate experiments. In a first, we evaluated SOM pools along a spatial gradient in plant inputs, from locations under individual bunch grasses to natural areas of plant removal (ant mounds). In a second experiment we assessed SOM pools across a temporal gradient in ant mounds ranging frm

Intra-annual and interannual variability of ecosystem processes in shortgrass steppe.

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HETEROGENEITY OF SOIL ORGANIC MATTER FOLLOWING DEATH OF INDIVIDUAL PLANTS IN SHORTGRASS STEPPE

5 to 60 yr old. Finally, we compared our results from the second experiment to a single human—induced plant removal experiment. We found that reduced root biomass accounted for up to 90% of the variation in SOM across our spatial gradients. In our examination of temporal dynamics of SOM, wer found that locations with little or no plant inputs for

Contribution of increasing CO2 and climate to carbon storage by ecosystems in the United States

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Using simple environmental variables to estimate below-ground productivity in grasslands

45 yr had 2% less total soil C than in native areas with average plant cover. We measured the greatest proportional losses (48—78%) of SOM in response to reduced plant inputs (over 30—60 yr) in the active SOM as indexed by C and N mineralization and microbial biomass C and N. The range of losses from intermediate organic matter as indexed by particulate organic matter (POM) ranged from 38 to 73% , and losses from the passive pool were proportionately lower (57%). Though this general pattern flows the course predicted by theory, and our measurements of turnover of intermediate SOM agree closely with models, results indicate a considerably slower turnover rate for the active pool and a considerably faster rate for the passive pool than expected. We compared our field estimates of plant—removal—induced SOM losses to simulation modeling results and cultivation studies. Our comparison of field results to Century model simulations indicated a possible model overesitmation of the impact of plant removal on SOM loss. By comparing our plant—revmoval results to studies of cultivation induced losses of SOM on the shortgrass steppe, we found that plant—removal is not as severe a disturbance as cropping, likely as a result of physical disturbances associated with tillage such as surface erosion and disturbance of soil aggregate structure.

ForCent model development and testing using the Enriched Background Isotope Study experiment

We used a daily time step ecosystem model (DAYCENT) to simulate ecosystem processes at a daily, biweekly, monthly, and annual time step. The model effectively represented variability of ecosystem processes at each of these timescales. Evolution of CO2 and N2O, NPP, and net N mineralization were more responsive to variation in precipitation than temperature, while a combined temperature-moisture decomposition factor (DEFAC) was a better predictor than either component alone. Having established the efficacy of CENTURY at representing ecosystem processes at multiple timescales, we used the model to explore interannual variability over the period 1949–1996 using actual daily climate data. Precipitation was more variable than temperature over this period, and our most variable responses were in CO2 flux and NEP. Net ecosystem production averaged 6 g C m−2 yr and varied by 100% over the simulation period. We found no reliable predictors of NEP when compared directly, but when we considered NEP to be lagged by 1 year, predictive power improved. It is clear from our study that NEP is highly variable and difficult to predict. The emerging availability of system-level C balance data from a network of flux towers will not only be an invaluable source of information for assessments of global carbon balance but also a rigorous test for ecosystem models.

Impact of precipitation dynamics on net ecosystem productivity

The shortgrass steppe of northern Colorado is characterized by patchy plant cover and associated spatial heterogeneity of soil resources. Zones of relatively high soil organic matter (SOM) under plants are maintained by direct litter inputs and physical stabilization of aeolian material. We studied the duration of plant-associated soil enrichment following plant death. We sampled soils between plants, under live plants, and under plants dead for 1, 9, and 36 mo. Live- and dead-plant soils were more enriched in total C and N than bare soils. We found a general pattern following plant death of initial active and total SOM increase due to greater litterfall than decomposition. Next, SOM decreased as substrate supplies declined, and decomposition continued. The temporal pattern depended upon SOM turnover. Though decomposition initially provided resources to maintain enriched plant-associated zones, our results suggest that enriched nutrient-supply zones under dead plants do not persist beyond several months.