Soil-mediated effects of a CO 2 gradient on grassland productivity: Interactions with resources and species change. Philip A. Fay USDA-ARS Grassland, Soil, and Water Lab Temple, Texas U.S.A. Collaborators: Wayne Polley (USDA-ARS) Virginia Jin (USDA-ARS) Robert Jackson (Duke University) Richard Gill (Brigham Young University) Jack Morgan (USDA_ARS) Beth Newingham (University of Idaho)

Controls on ecosystem responses to CO 2 Species change Ecosystem Function Resources CO 2

Which resources (nutrients/water)? Which species? Where are each of these more/less important? When do they become important ? Sources of spatial variability: Which? Where? When? Species change Ecosystem Function Resources CO 2 Soils Climate

Working model CO 2 Water ANPP Species Nutrients Physiology Mesic Xeric Fine Coarse Aridity Soils

Experiments Mesic Grassland MAP: 850 mm y -1 Mesic Grassland MAP: 850 mm y -1 Semi-arid Grassland MAP: 400 mm y -1 Semi-arid Grassland MAP: 400 mm y -1 Mojave Desert MAP: 135 mm y -1 Mojave Desert MAP: 135 mm y -1

CO 2 gradient since to 500 µL L -1 Ambient temperature, average precipitation C 4 vegetation 3 soil types, 20 reps Silty claySandy loam Clay MollisolAlfisol Vertisol Mesic grassland Fay et al Ecosystems

Mesic grassland Strongest CO 2 effect on soils where: – High plant availability of soil moisture – Increased soil water content with CO 2 (not shown) – A more productive grass became dominant. Fay et al. 2012, Nature-Climate Change Which resource? Water

Mesic grassland Greater ANPP-CO 2 response when direct + indirect effects present. More CO 2 effects present on coarser-textured soils. Silty Clay CO 2 Soil water ANPP Species Nitrogen Physiology Sandy Loam CO 2 Soil water ANPP Species Nitrogen Physiology Clay CO 2 Soil water ANPP Species Nitrogen Physiology Fine Coarse Soils Fay et al. 2012, Nature-Climate Change

Mesic grassland Which species? – Sorghastrum nutans (Indiangrass) – Explains 45 – 65% of variation in ANPP Polley et al. 2012, Global Change Biol. Silty Clay Sandy Loam Clay

Species Mesic grassland When? Soil water Silty Clay Sandy Loam Clay Polley et al. 2012, Global Change Biol.

Semi-arid grassland Mixed-grass prairie: 400 mm yr -1 MAP C 3 dominated, 600 ppm CO 2 treatment CO 2 enrichment: increased: photosynthesis, soil moisture and aboveground biomass. No change in plant N or community composition CO 2 Soil Water ANPP Species Nitrogen Physiology Morgan et al. 2011, J. Morgan pers. Comm.

More xeric systems Mojave Desert: 135 mm yr -1 MAP 550 ppm CO 2 enrichment (FACE): – Increased photosynthesis (wet years) – Increased LAI – No effect on soil moisture, total aboveground biomass, or species biomass or cover. Treatment = 0.81 Species < Treatment x Species = 0.84 Nowak et al Newingham et al CO 2 Soil Water ANPP Species Nitrogen Physiology

Summary Mesic Xeric Sandy Loam CO 2 Soil water ANPP Species Nitrogen Physiology Soils Interactions fewer with increasing aridity Silty Clay CO 2 Soil water ANPP Species Nitrogen Physiology Mesic Grassland CO 2 Soil water ANPP Species Nitrogen Physiology Clay CO 2 Soil water ANPP Species Nitrogen Physiology Fine Coarse Soils Aridity/Precipitation Interactions more numerous on coarser soils Semi-arid Grassland CO 2 Soil water ANPP Species Nitrogen Physiology Desert CO 2 Soil water ANPP Species Nitrogen Physiology

Conclusion Local soils and regional climate are two important sources of spatial variability in the drivers of ecosystem responses to CO 2 enrichment.

Ecosystem processes Ecosystem processes Regional/ Continental climate Regional/ Continental climate Soils water/nutrients Soils water/nutrients Plant Populations/ Communities Plant Populations/ Communities Atmospheric CO2 Disturbance Multiple Factors: Interactive effects among multiple drivers determine CO2 effects (after Norby and Luo 2004, Polley et al. 2010) Controls on ecosystem responses to CO 2

Tallgrass prairie CO 2 Soil water content ANPP Sonu:Bocu Soil inorganic N Silty Clay CO 2 Soil water content ANPP Sonu:Bocu Soil inorganic N Sandy Loam CO 2 Soil water content ANPP Sonu:Bocu Soil inorganic N Clay Strongest CO 2 effect on ANPP where direct CO 2 effect and an indirect effect. Soil moisture and species, but not Nitrogen Fay et al. 2012, Nature-Climate Change

‘Direct Effects’‘Indirect Effects’ CO 2 Soil moisture Ecosystem Function Community change Soil N Physiological effects

Mojave desert No overall CO 2 effect (0.2, 0.5, 1.75 m) CO 2 significant on a few dates – Elevated CO 2 drier than ambient Nowak et al Ecology

Summary Mesic Xeric Tallgrass Prairie CO 2 Soil moisture ANPP Community change Soil N Physiological effects Midgrass Prairie CO 2 Soil moisture ANPP + BNPP Community change Soil N Physiological effects Mojave Desert CO 2 Soil moisture Soil C Community change Soil N Physiological effects Soils ANPP response to CO 2 when both direct + indirect effects occur. Indirect effects may be due to spatial (soils, regional MAP) or temporal (rainfall) variation. Requires species able to capitalize on the altered growing conditions. In xeric environments, indirect effects may not be possible – there’s just not enough water around. Next Steps: Structural equation modeling.

Summary Mesic Xeric Tallgrass Prairie CO 2 Soil moisture ANPP Community change Soil N Physiological effects Midgrass Prairie CO 2 Soil moisture ANPP + BNPP Community change Soil N Physiological effects Mojave Desert CO 2 Soil moisture Soil C Community change Soil N Physiological effects Soils ANPP response to CO 2 when both direct + indirect effects occur. Indirect effects may be due to spatial (soils, regional MAP) or temporal (rainfall) variation. Requires species able to capitalize on the altered growing conditions. In xeric environments, indirect effects may not be possible – there’s just not enough water around. Next Steps: Structural equation modeling.