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 ra perturbation. We assessed measurable pools of SOM that are thought to correspond 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 from approximate to 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, we found that locations with little or no plant inputs for approximate to 45 yr had 28% 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 follows 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 overestimation 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 disturbances associated with tillage such as surface erosion and disturbance of soil aggregate structure.