Nitrogen limitation in dryland ecosystems: Responses to geographical and temporal variation in precipitation

We investigated the relationship between plant nitrogen limitation and water availability in dryland ecosystems. We tested the hypothesis that at lower levels of annual precipitation, aboveground net primary productivity (ANPP) is limited primarily by water whereas at higher levels of precipitation, it is limited primarily by nitrogen. Using a literature survey of fertilization experiments in arid, semi-arid, and subhumid ecosystems, we investigated the response of ANPP to nitrogen addition as a function of variation in precipitation across geographic gradients, as well as across year-to-year variation in precipitation within sites. We used four different indices to assess the degree of N limitation: (1) Absolute Increase of plant production in response to fertilization (the slope of ANPP vs. amount of added N at different levels of annual precipitation); (2) Relative Response (the percent increase in fertilized over control ANPP at different levels of N addition); (3) Fertilizer Use Efficiency (FUE, the absolute gain in productivity per amount of fertilizer N), and (4) Maximum Response (the greatest absolute increase in ANPP at saturating levels of N addition). Relative Response to fertilization did not significantly increase with increasing precipitation either across the geographic gradient or across year-to-year variation within sites. Nor did the Maximum Response to fertilization increase with increasing precipitation across the geographic gradient. On the other hand, there was a significant increase in the Absolute Increase and FUE indices with both geographical and temporal variation in precipitation. Together, these results indicate that there is not necessarily a shift of primary limitation from water to N across the geographic water availability gradient. Instead, our results support the hypothesis of co-limitation. The apparently contradictory results from the four indices of N limitation can best be explained by an integration of plant ecophysiological, community, and ecosystem mechanisms whereby plants are co-limited by multiple resources, species shifts occur in response to changing resource levels, and nitrogen and water availability are tightly linked through biogeochemical feedbacks.