In drylands, ecosystem function is primarily constrained by water, but also by nitrogen (N), causing potential co-limitation by water and N. Anthropogenic fixation of N has increased the global rate of N input into the biosphere, while climate change is altering the seasonality and amount of precipitation. Changes in the availability of water and N are known to cause changes to net primary production and to soil respiration. We hypothesized that increases in available water and N would affect both the production of biomass and soil respiration, with water having a larger effect than N because drylands are primarily water-limited. We conducted an experiment using a randomized block design incorporating two levels of water (control and +20% precipitation) and three of N (control, +10 kg N ha-1 yr-1, and +100 kg N ha-1 yr-1), yielding six treatment combinations. We installed seven replicates of the water and nitrogen treatments at each of three sites located across Wyoming and Colorado. Each site represents a different ecosystem type (shortgrass steppe, mixed-grass prairie, and sagebrush steppe). We measured changes in aboveground net primary production and soil respiration over two summer growing seasons.
Results/Conclusions
Our application of water altered soil water availability across all three locations. Total inorganic nitrogen in soil was also significantly elevated in the high-N treatments over the summer growing season during both years, regardless of the addition of water (p<0.01). However, the total inorganic N available during the spring following the first year of treatments displayed no significant changes, indicating that there was no significant hold-over effect from the previous year’s treatment applications. Aboveground net primary production was unchanged among treatments after the first year of water and N additions, though significantly altered during the second year of the study in the water and high N treatments, largely due to the response of perennial grasses. Soil respiration was significantly elevated in the water and high N addition treatments during both years of the study (p<0.01). Our results suggest that combining changes in water and N availability has the largest effect on available soil inorganic N, production of biomass, and soil respiration. These results agree with previous findings. Our results also show a lack of response in all but largest combined treatments of both water and N, potentially suggesting that some ecosystem processes may be co-limited by both water and N.