Soil microbial activity responses to long-term nitrogen addition and annual burning in tallgrass prairie

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Land management practices affect ecosystem states and processes.  In tallgrass prairie, annual burning reduces soil nitrogen (N) availability, while lack of fire allows woody vegetation to dominate over grasses and forbs.  Raising N availability via fertilization increases aboveground plant production but decreases plant diversity.  Belowground responses to these contrasting management practices are less well understood, despite the importance of soil microbial communities in mediating N availability via N uptake and mineralization of soil organic matter (SOM).  In particular, it is not clear whether the broadly observed differences between grassland and forested soils in soil microbial responses to N addition are reflected in the transition from annually burned grassland to unburned shrubland.  To begin to address these knowledge gaps, we collected 0-20 cm mineral soil cores from replicate field plots at the Konza LTER Belowground Plots Experiment (BGPE) at Konza Prairie Biological Station. The BGPE includes factorial N fertilization and annual burning treatments, and has been ongoing for 30 years as of 2015: woody vegetation is now well established in unburned treatment plots.  We have collected soils monthly since November 2014, and are measuring microbiological parameters including extracellular enzyme activities (EEA) on all samples; to date, EEA data from November 2014 – April 2015 have been analyzed.  We predicted that annual burning would maintain higher soil microbial N limitation, while microbial P limitation would be more apparent in N fertilized soils; and we also expected soil microbial communities in unburned soils, with more woody vegetation, to have a higher capacity for plant litter decomposition.  These differences would be indicated by relatively higher N- and P-acquiring EEA in N- and P-limited soils, respectively; and higher cellulose and lignin degrading EEA in unburned soils.  Also, a shift in microbial functional response to N addition in long-term unburned grassland soils might be indicated by a decrease in oxidative EEA, analogous to responses seen in forest soils.  Results show consistently greater soil microbial N limitation in burned soils.  However, in samples analyzed to date, the N fertilization treatment does not relieve this effect, and increased soil microbial P limitation is not apparent.  Cellulose degradation activity is higher in unburned soils, as predicted and consistent with higher plant litter inputs.  There are interactive N fertilization effects, apparent in only the unburned treatments, which indicate lower microbial C and N acquisition potential in fertilized soils. Also, despite some lower oxidative EEA values in unburned soils with N added, there is no statistical support for this particular predicted functional state change.  In summary, burning treatment affected soil microbial functional potential to a greater extent than N addition, with interactive effects of N fertilization in unburned soils only.  While differential responses to N addition in unburned soils are not as predicted, they do suggest a possible shift in soil microbial functional feedbacks associated with long-term lack of burning, specifically towards decreased soil C and N mineralization potential in response to fertilization.  As we produce the longer time series of data, we predict to see burning and fertilization impact the seasonality of microbial function, with N treatment effects becoming more pronounced during the summer growing season, as plant growth increases soil N demand.