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

We investigated the relationship between plant nitrogen limitation and wate r availability in dryland ecosystems. We tested the hypothesis that at lowe r levels of annual precipitation, aboveground net primary productivity (ANP P) is limited primarily by water whereas at higher levels of precipitation, it is limited primarily by nitrogen. Using a literature survey of fertiliz ation experiments in arid, semi-arid, and subhumid ecosystems, we investiga ted 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 productio n in response to fertilization (the slope of ANPP vs. amount of added N at different levels of annual precipitation); (2) Relative Response (the perce nt increase in fertilized over control ANPP at different levels of N additi on); (3) Fertilizer Use Efficiency (FUE, the absolute gain in productivity per amount of fertilizer N), and (4) Maximum Response (the greatest absolut e increase in ANPP at saturating levels of N addition). Relative Response t o fertilization did not significantly increase with increasing precipitatio n either across the geographic gradient or across year-to-year variation wi thin sites. Nor did the Maximum Response to fertilization increase with inc reasing precipitation across the geographic gradient. On the other hand, th ere was a significant increase in the Absolute Increase and FUE indices wit h both geographical and temporal variation in precipitation. Together, thes e results indicate that there is not necessarily a shift of primary limitat ion from water to N across the geographic water availability gradient. Inst ead, our results support the hypothesis of co-limitation. The apparently co ntradictory results from the four indices of N limitation can best be expla ined by an integration of plant ecophysiological, community, and ecosystem mechanisms whereby plants are co-limited by multiple resources, species shi fts occur in response to changing resource levels, and nitrogen and water a vailability are tightly linked through biogeochemical feedbacks.