Effect of non-native invasive bluestems on soil nutrient pools in a south Texas coastal prairie

Background/Question/Methods

Invasive, non-native organisms pose a serious threat to native biodiversity and the overall ecological balance of the ecosystem at risk.  Some non-native, invasive plants have been shown to alter ecosystem functioning through plant-soil feedbacks which alter soil nutrient availability.  We hypothesized that non-native bluestems invading south Texas coastal prairies are altering soil nutrient pools through such plant-soil feedbacks.  To test our hypothesis, we collected 0-20 cm soil samples from six native bluestem and six adjacent non-native bluestem coastal prairie patches on clay and fine sandy loam soils at various time points throughout 2007 and 2008 at the Rob and Bessie Welder Foundation Wildlife Refuge in San Patricio County, Texas.  We aimed to compare inorganic nitrogen (N_inorg) pools and fluxes and total percent soil carbon and nitrogen between the two types of vegetation and between soil textures.

Results/Conclusions Our data indicate that soil N_inorg pools vary dramatically across time, vegetation, and soil types.  In early summer 2007, soil N_inorg  pools (mg N kg^-1 soil^-1) tended to be higher in the native bluestem patches in the sandy loam soils (14.1 and 11.6 mg N kg^-1 soil^-1 in native versus non-native bluestem patches, respectively).  However, this trend reversed in sandy loam soils throughout the remainder of 2007 (i.e., N_inorg was higher in non-native than native patches, 14 versus 9.7 mg N kg^-1 soil^-1, respectively).   In the clay soils, soil N_inorg  pools fluctuated dramatically over time, but also tended to be slightly lower in the non-native bluestem patches (14 and 17.4  mg N kg^-1 soil^-1 in non-native versus native bluestem patches, respectively, in August 2007).  Nitrogen mineralization rates were not different between patches in sandy loam soils (0.15 mg N/kg soil/day for both vegetation types), but it was higher in the non-native bluestem patches (0.05 mg N/kg soil/day) than the adjacent native (0.01 mg N/kg soil/day) on clay soils.  Interestingly, percent soil moisture was higher in the non-native bluestem patches on sandy loam throughout the 2007 growing season (2% to 5% higher than native patches)   We conclude that alterations in N cycling may accompany non-native bluestem invasion into coastal prairie, but these effects are relatively small and strongly influenced by soil texture and seasonal variability.  The striking differences in soil moisture between the two vegetation types suggest that alterations to the hydrologic cycle and/or rain use efficiency of the vegetation may be important in driving this particular invasion process.