Nitrogen (N) inputs to terrestrial regions have more than doubled in less than a century as a result of increased fertilizer use and N deposition, with high variation in the rates of inputs to different locations. Many studies have sought to understand how N availability affects plant species biodiversity. Previous studies of N additions in terrestrial systems show that N supply that overcomes growth limitation often causes species losses. However, many of these studies have sought to understand the effect of overcoming N limitation in an environment in which no other nutrients are limiting; they accomplish this by adding high rates of N along with a mix of phosphorus (P), potassium (K), and micronutrients (µ). Yet evidence is growing that even low rates of N addition may reduce biodiversity, and many terrestrial ecosystems are limited by more than one nutrient. In this study we asked: how does nitrogen affect grassland biodiversity in a co-limited environment? By adding a gradient of N (0, 1, 5, 10 g m-2 yr-1) annually for a decade to a central Minnesota grassland, we quantified the rates of species turnover and richness decline. To compare to earlier work focused on overcoming nutrient limitation, in an additional treatment, we added P, K, and µ (hereafter the “NPKµ” treatment) to the highest (10gN m-2 yr-1) N addition rate. We hypothesized that increasing N addition would lead to higher rates of species turnover, with greater rates of N addition increasingly favoring fast-growing, poor N competitors. We also predicted faster turnover with NPKµ compared to N alone.
Results/Conclusions
Species richness did not significantly differ among the N-alone treatments; however, richness was significantly lower in the NPKµ treatment (p = 0.03). Through the ten-year study, rates of among-year species turnover did not significantly differ from control in any of the treatments. These results suggest that low rates of N addition, comparable to those currently experienced in many locations around the world due to N deposition, may not strongly alter species diversity on a decadal time span, but that a relief from co-limitation by P, K, or µ may cause a decline of species persisting from one year to the next. Studies that add nutrients to relieve co-limitations from other nutrients should thus be cautious in their inference about the impacts of N deposition.