In the Jasper Ridge Global Change Experiment – an annual grassland with elevated carbon dioxide (CO2), nitrate deposition, temperature, and precipitation – we used six indices of phosphorus (P) limitation to test the hypothesis that global changes that increase net primary production (NPP) increase P demand or limitation. All indices indicated that nitrate deposition, the only factor that stimulated NPP, increased P demand or limitation: (1) soil phosphatase activity increased by 14%; (2) P concentration in green and (3) senescent leaves of the dominant grass genus, Avena, dropped by 40% and 44%, respectively; (4) N : P ratios in green and (5) senescent Avena widened by 99% and 161%, respectively; and (6) total aboveground plant P decreased by 17% with elevated nitrate deposition. The other three factors, which did not stimulate NPP, did not increase P demand: based on two indices, enhanced precipitation decreased P demand (11% decrease in phosphatase activity, 19% increase in total aboveground P), and there was no evidence that elevated CO2 or temperature altered P demand. In a meta-analysis to assess the generality of P constraints on growth increases from global change factors, we found that six of 11 N-limited ecosystems responded to N deposition with enhanced P limitation or demand, but did not detect significant effects of elevated CO2 or warming.
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