Urine as an important sodium source increases decomposition in a Na-poor but not Na-rich tropical forest

Background/Question/Methods In nutrient-poor environments, the addition of essential nutrients can have profound impacts on ecosystem processes. Urine provides nutrients that can increase decomposition, alter nutrient cycling, and change community structure. Urine is a point source fertilizer largely containing nitrogen (N) and potassium (K), but also significant amounts of sodium (Na), which is critical for proper neural function, osmoregulation, and reproduction and limits detritivores in Na-poor environments. Here we tested how Na in urine affects decomposition and arthropod recruitment in tropical forests. We conducted experiments in Ecuador and Panama and predicted increased decomposition and detritivore recruitment, specifically termites, with the addition of Na particularly in Ecuador due to its lower environmental Na levels. To separate the effects of Na from other nutrients in urine, we used four treatments (+Na, H₂O, Urine+Na, and Urine-Na) in a randomized block design (20 blocks). We added 250ml of solution to respective 0.25m² plots in each block every other day. All plots contained a decomposition bag with pre-weighed filter paper and wooden sticks. After two weeks we harvested decomposition bags and collected invertebrate communities via Berlese funnels.

Results/Conclusions In Na-poor Ecuador, decomposition was higher on Urine+Na and +Na plots. Specifically, wooden sticks but not filter paper had higher percent mass loss on Urine+Na and +Na plots compared to H₂O and Urine-Na plots. Neither filter paper nor wooden stick decomposition increased on Urine+Na, +Na, or Urine-Na plots in Na-rich Panama. The increased decomposition on +Na and Urine+Na plots in Ecuador is likely due to increased termite recruitment. Decomposition did not increase on Urine-Na plots despite the presence of other nutrients such as N and K in the treatment. No response to Na in Panama may be due to naturally occurring high Na levels via oceanic aerosols (ca. 9x higher than Ecuador). These results suggest that the Na component of urine is an important factor shaping the heterogeneity of the tropical forest floor, particularly in Na-poor environments which account for ca. 75% of Tropical Ecoregions.