Potential enhancement of relative phosphorus limitation in a warming world: Rising temperatures may not promote bioavailability of organic matter-derived phosphorus in soils

Pathways of generating bioavailable nutrients can be strongly influenced by temperature. In a warming climate, a predictive understanding of these changes is important because nutrient availability influences ecosystems' carbon (C) exchange with the atmosphere via primary production and soil microbial C mineralization. A key knowledge gap is how changing environmental conditions will influence microbially-mediated release of nutrients from decaying organic matter (OM). This is an especially intriguing question for phosphorus (P), a macronutrient derived in part from OM. Phosphatase exo-enzymes produced by roots and microbes mediate OM-P release by breaking ester-phosphate bonds; though rising temperatures can increase multiple exo-enzymes' activities in many soils, we do not know how phosphatase activities change with varying temperature and soil pH. To provide baseline kinetics unadulterated by substrate limitation or microbial adaptations to changing conditions, we performed assays of purified, plant-derived acid phosphatase activities at pH ranging from 3.5 to 7.5 and temperatures from 5 to 35°C. Temperature was an influence on exo-enzyme catalytic rate at pH 3.5, 4.5, and 5.5 as it increased from 5 to 15°C, but further increases in temperature at those pH had no effect. At pH 3.5, temperature affected catalytic rate only between 25 and 35°C. At pH 6.5, 25 and 35°C had statistically similar catalytic rates as both 5 and 15°C, and no temperature effect was observed at pH 7.5. These results contrast with two exo-enzymes that release C- and N-rich compounds upon decay, both of which exhibit positive responses to increasing temperature across all tested temperatures, and across diverse pH. The current study lends credence to a controversial hypothesis that higher temperatures may reduce the thermal stability of some exo-enzymes and thus catalytic activity. We suggest that pH may be more of a determinant of acid phosphatase activities than temperature at temperatures commonly occurring in many of the world's soils. In conjunction with knowledge about C- and N-liberating exo-enzymes, our work suggests that, unless exo-enzyme exuding organisms enhance their production of acid phosphatase, OM-derived P may become relatively more limiting to ecosystem processes in a warming environment.