Examining Long-Term Experimental Manipulation of Season of Burn and Nutrient Addition On Productivity and Soil Characteristics in Subtropical Grasslands.

We examined the basic controls on productivity and nutrient cycling, inter-annual variability in productivity, and the interactive effects of season of fire and nutrients on productivity and soil processes in subtropical grassland. The experiment was a randomized split-plot design with fire as the main plot treatment and nutrient addition as the split-plot treatment. Fire and nutrient addition treatments were maintained annually for10-years, and sampling of annual net primary productivity (ANPP) and tissue nutrients was completed from 2002 to 2006 and again in 2011. Soil Mehlich-1 P was sampled in 2004, 2006 and 2011, and N mineralization was sampled in 2006. We expected that addition of nutrients would increase ANPP especially in burned plots due to greater availability of light. Average ANPP over the six years was 967.2 g/m^2­. Summer burn plots had greater ANPP than unburned or winter burned plots (unburned: 1,131.6 g/m², summer burned: 1,517.1 g/m², winter burned: 1,093.3 g/m²). There was a significant interaction between year and nutrient addition on ANPP. Tissue nitrogen did not differ among treatments but tissue phosphorus was significantly greater in phosphorus addition plots. Burned plots had lower tissue N in 2006, but this effect disappeared in 2011. Ten day aerobic incubations of soil collected in 2006 showed that net N mineralization rates were significantly higher (p = 0.02) in plots with N(12.5 ± 6.9 lg N g_1 day_1) and N + P (7.6 ± 4.0 lg N g_1 day_1) than in plots with P alone (2.8 ± 1.2 lg N g_1 day_1) or no fertilizer. These results suggest that ANPP is driven by top-down controls while belowground processes are primarily driven by nutrient resources in this subtropical grassland ecosystem.