Differential phosphorus and nitrogen effects drive species and community responses to elevated CO₂ in semi-arid grassland

1. Productivity of dryland communities is often co-limited by water and nutrients. Since atmospheric CO₂ enrichment induces water savings by plants, elevated CO₂ and nutrients could interact to reduce growth limitation, irrespective of the direct influence of CO₂ on photosynthesis. We studied CO₂ effects in model communities from the semi-arid Negev of Israel with 17 mostly annual C₃ species at three CO₂ concentrations and three nutrient treatments. 2. Community biomass increased at elevated (440 and 600 μL L^-1) compared to preindustrial CO₂ (280 μL L^-1) by 34% on average in the low-nutrient control, by 45% in the high P and by 50% in the high NPK treatment. Less evapotranspiration at elevated CO₂ increased soil water content by 30-40% on average. Significant CO₂-fertilization interactions indicated that plant responses to CO₂ enrichment were constrained by nutrient availability. 3. Responses of biomass and water-use efficiency (dry-matter accumulation per cumulative evapotranspiration) to CO₂ enrichment were non-linear and were saturated at 440 μL L ^-1 at low nutrient and high P supply. CO₂ effects were further increased up to 600 μL L^-1 only under full NPK fertilization. 4. The overall CO₂ effect on biomass depended on the differential response of plant functional groups, with the P-dependent legume response dominating at low nutrient and high P supply, and the N-dependent grass response dominating at high NPK. With the exception of grasses, species responded differently to combinations of CO₂ enrichment and nutrient addition, even within functional groups. 5. Biomass production was co-limited by CO₂ and nutrients in this semi-arid seasonal community, with both effects possibly mediated by water availability. Nutrient losses associated with desertification will thus diminish potential gains in biomass due to elevated CO₂. Growth stimulation by CO₂ enrichment beyond close-to-current concentrations will only be seen under nutrient-rich conditions in semi-arid and possibly other drought-stressed grasslands.