Soil phosphorus availability influences nitrogen-fixing seedlings performance in tropical dry forests

Laura Toro, Damaris Pereira-Arias and Daniel Perez-Aviles, Plant and Microbial Biology, University Of Minnesota, St. Paul, MN, Jessica Gutknecht, Department of Soil, Water, and Climate, University of Minnesota, Twin Cities, St Paul, MN, Jennifer S. Powers, Department of Plant and Microbial Biology, University Of Minnesota, St. Paul, MN, Jennifer S. Powers, Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN

Background/Question/Methods
Tropical dry forests (TDF) have been described to have high nitrogen (N) and low phosphorus (P) soil availability, suggesting that P might limit early plant performance. Little is known about how species from different functional groups respond to soil P availability and which strategies these groups employ to access P. We grew eleven common species of TDF trees, under three different P treatments (control, low-P or high-P) to determine: (1) How N-fixing and non-N-fixing seedlings respond to different availability of soil P; (2) If there is coordination among traits that mediate how species respond to P addition. Four N-fixing and 7 non-N-fixing seedlings grew for 4 months in a shade house in Costa Rica. We harvested ~7 individuals per species per treatment and calculated total dry biomass (TDB), and relative growth rate (RGR). We also measured water use efficiency as δ13C (WUE), foliar P, root phosphatase activity (RPA), specific root length (SRL), and arbuscular mycorrhizal colonization (AMC).
Results/Conclusions
N-fixers grew 40% (low-P) to 17% (high-P) larger and faster under P addition, while non-N-fixers did not respond to P addition. They had lower biomass and RGR compared to N-fixers. N-fixers increased their WUE under added P (p>0.05). Both functional groups had similar foliar P, it increased by 41% under low-P and 25% under high-P. RPA was 150% higher for N-fixers compared to non-N-Fixers (p<0.001) but did not vary under P addition. Both functional groups had similar SRL across treatments (p>0.05). AMC increased by 13% under low but not high P for both functional groups (p<0.01). Our findings suggest: (1) N-fixing seedlings respond to P addition by increasing biomass and growth rates; (2) plant performance was often higher at the lower P addition rate, suggesting that there is a threshold of P that plants can use. Above that threshold other resources may become limiting; (3) both N-fixers and non-N-fixers are unable to down-regulate P acquisition traits, which may be related to the scarcity of P in TDF and the low plasticity of these traits; (4) for these TDF species, P limits early growth of N-fixers, but does not limit non-N-fixers growth implying that other resources might be limiting their performance.