Numerous studies have assessed the impacts of long-term nutrient enrichment on species diversity and net primary production, but the effects of soil nutrients on litter decomposition in aridlands remains uncertain. In general, decomposition of surface litter is governed to a large extent by abiotic drivers (e.g., UV radiation) while belowground processes generally depend on soil moisture and nutrient availability. In this study, we used the Tea Bag Index (TBI) method to test for the effect of nutrient enrichment (+N, +P, +K, +N+P+K, +N+K, +N+P and +P+K) on litter decomposition at the Sevilleta LTER Nutrient Network site in central New Mexico, USA. In each replicate of each treatment (N=5, total of 40 plots), two tea bags of Lipton green and two of rooibos were placed on top of the soil, and three bags of each tea were buried 8-cm below patches of black grama grass to measure rates of above- and belowground decomposition under various nutrient regimes. After 90 days of incubation, tea bags were collected, weighed, and the TBI standard protocol was used to calculate decomposition rate. We used Analysis of Variance with mixed effects models to test for the effect of nutrient enrichment on above- and belowground litter decomposition.
Results/Conclusions
The decomposition rate for aboveground litterbags was significantly lower than belowground litterbags (F=11.042, p<0.01). Furthermore, we did not find a significant effect of nutrient enrichment on aboveground decomposition (F=0.6453, p=0.72), but we did find a significant effect of nutrient enrichment on belowground decomposition (F=7.7125, p<0.001). A single nitrogen limitation was observed in belowground decomposition with +N+P+K (M=0.0059, SD=0.0015), +N+P (M=0.0054, SD=0.0008), +N (M=0.0052, SD=0.0003), and +N+K (M=0.005, SD=0.0009) plots reporting slightly higher decomposition rates than ambient (M= 0.0044, SD=0.0007) plots. In contrast, low decomposition rates were observed in +K (M=0.003, SD=0.0005), +P+K (M=0.0038, SD=0.0003) and +P (M=0.0038, SD=0.0006) plots. Results suggest that nitrogen enrichment enhances belowground decomposition while potassium enrichment may negatively affect decomposition rate. This may be due to the response of microbial communities to nutrient enrichment for belowground decomposition, and to a similar exposure to abiotic conditions for aboveground decomposition. Further studies are needed to test how precipitation, soil moisture, temperature, soil nutrients, and solar radiation regulate the effect of nutrient enrichment on above- and belowground decomposition rates of Chihuahuan desert grasslands. Understanding the effect of nutrient enrichment is important to better predict changes in grassland carbon pools and biogeochemical cycling under anthropogenic nutrient enrichment.