Phosphorus availability and leaf type affect litter stoichiometry and decomposition

Background/Question/Methods

Resource-consumer stoichiometry in forested streams can influence nutrient dynamics. While studies have shown mixed results of resource P enrichment on consumer stoichiometry, little research has been done on quantifying elemental changes in detritus (namely the C:P ratio). We examined the stoichiometric changes of two leaf species (recalcitrant Quercus stellata and labile Acer saccharum) subjected to varying levels of P enrichment during 139 days of conditioning in laboratory microcosms. Oak and maple leaf disks were put in respective bags and placed in beakers filled with stream water and variable levels of P enrichment.  Disks were sampled periodically for C and P. Regression analysis was run on the response ratios (RR; relative to controls) for leaf %P and C:P to test if the slope was equal to 0 (no P enrichment effect). We also examined the relationship between stream TP and leaf litter C:P across a natural P gradient of streams in the Ozark Highlands Region, Arkansas. Mixed leaf species were collected across 14 streams in 2009 and 2010 along with water samples. Leaf elemental composition (C and P) and water TP were measured. Linear regression analysis was run to test for a relationship between leaf C:P and water TP.

Results/Conclusions

Leaf litter %P increased and the subsequent C:P ratio decreased with increasing P enrichment and with increasing lability. The RR for %P of oak and maple leaf litter both indicated a significant change through time and was dictated by level of P enrichment (oak low P: slope = 0.67, p = 0.0231; oak high P slope = 1.74, p = 0.0259; maple low P: slope = 2.73, p = 0.0011; maple high P slope = 4.83, p = 0.0009). Results were similar for the RR of C:P (oak low P: slope = -0.38, p = 0.0094; oak high P: slope = -0.59, p = 0.0019; maple low P: slope = -0.87, p = 0.0004; maple high P: slope = -0.81, p < 0.0001). We found a negative linear relationship in TP versus leaf litter C:P in the natural P gradient of streams in the Ozarks (R² = 0.59, p = 0.0013). These results demonstrated that P enrichment caused an increase in P content and a decrease in C:P ratio of oak and maple leaves and that the magnitude of the response was greater for maple leaves. Additionally, stream water TP can be a strong predictor of mixed leaf litter C:P.