Environmental controls on nutrient dynamics in a longleaf pine forest: How do soil moisture and fire interval affect plant-available nutrients?

Background/Question/Methods

Due to highly site-specific controls on fire behavior, there is little consensus on the effects of fire on soil nutrient dynamics. Consequently, our ability to predict community and ecosystem responses to fire regimes is limited. So, too, is our understanding of the relative importance of fire and soil moisture on structuring plant-available nutrients. Environmental factors that likely influence the availability of nutrients, such as ammonium (NH₄⁺) and nitrate (NO₃^-), vary both spatially (moisture gradients) and temporally (varying time since fire; TSF) across the longleaf pine (Pinus palustris) forests at Ft. Bragg, NC. We examined whether soil moisture or fire regime had a greater influence on nutrient availability in this system by measuring cation (NH₄⁺, K⁺) and anion (NO₃^-, PO₄^3-) supply rates using PRS™-probes (Western Ag Innovations, Inc.). This method more accurately approximates root uptake of nutrient pools than traditional extraction methods. We expected nutrient availability to be positively correlated with soil moisture and TSF. Since fires in this system are low-intensity and thus rarely reach volatizing temperatures, we also expected soil moisture to be a stronger driver at the landscape-scale. Lastly, we expected the relative availability of NO₃^- versus NH₄⁺to be negatively correlated with soil moisture.

Results/Conclusions

The relative abundance of NO₃^- and NH₄⁺ reversed along a moisture gradient; NO₃^- ranged from 0.00-3.51 μg 10cm^-2 day^-1 and was the more abundant inorganic N form at drier sites. NH₄⁺ ranged from 0.00-4.62 μg 10cm^-2 day^-1 and was more abundant at wetter sites. Total N was higher at wet sites than at dry sites. A mixed effects model with TSF + soil moisture as the predictors and “Site” as the random effect was consistently superior in predicting a number of nutrient responses than models that included either predictor separately, and superior to models that included elevation from the local minimum as a predictor. A PCA of the correlation matrix of nutrient concentrations and environmental variables revealed that both TSF and soil moisture were negatively loaded on PC1 (eigenvalue, 1.62; variance explained, 38%), and loaded positively with N-forms. Overall, nutrient availability in this xeric, fire-prone system is influenced by both TSF and soil moisture, indicating that even low-intensity fires may affect nutrient dynamics. Further work should focus on the short-term effect of fire on relative N-form availability along the moisture gradient, and on how plants compete for temporally variable NO₃^- and NH₄⁺ availability in this low nutrient system.