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 gradien ts in disturbance age because a better understanding of decay characte ristics of SOM pools may improve our ability to predict ecosystem resp onses ra perturbation. We assessed measurable pools of SOM that are th ought to correspond to active, intermediate, and passive SOM based upo n 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 rem oval (ant mounds). In a second experiment we assessed SOM pools across a temporal gradient in ant mounds ranging from approximate to 5 to 60 yr old. Finally, we compared our results from the second experiment t o a single human-induced plant-removal experiment. We found that reduc ed root biomass accounted for up to 90% of the variation in SOM across our spatial gradients. In our examination of temporal dynamics of SOM , we found that locations with little or no plant inputs for approxima te to 45 yr had 28% less total soil C than in native areas with averag e plant cover. We measured the greatest proportional losses (48-78%) o f SOM in response to reduced plant inputs (over 30-60 yr) in the activ e 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 fr om the passive pool were proportionately lower (57%). Though this gene ral pattern follows the course predicted by theory, and our measuremen ts 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 compa red our field estimates of plant-removal-induced SOM losses to simulat ion modeling results and cultivation studies. Our comparison of field results to Century model simulations indicated a possible model overes timation of the impact of plant removal on SOM loss. By comparing our plant-removal 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 disturbance s associated with tillage such as surface erosion and disturbance of s oil aggregate structure.