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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 C-3 species at three CO2 concentrations and three nutrient treatments.

2.Community biomass increased at elevated (440 and 600 muL L-1) compared to pre-industrial CO2 (280 muL 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 muL L-1 at low nutrient and high P supply. CO2 effects were further increased up to 600 muL 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.