Temperature and pH mediate stoichiometric constraints of organically derived soil nutrients

It remains unclear how warming will affect resource flows during soil organic matter (SOM) decomposition, in part due to uncertainty in how exoenzymes produced by microbes and roots will function. Rising temperatures can enhance the activity of most exoenzymes, but soil pH can impose limitations on their catalytic efficiency. The effects of temperature and pH on enzyme activity are often examined in environmental samples, but purified enzyme kinetics reveal fundamental attributes of enzymes' intrinsic temperature responses and how relative release of decay-liberated resources (their flow ratios) can change with environmental conditions. In this paper, we illuminate the principle that fundamental, biochemical limitations on SOM release of C, N, and P during decay, and differential exoenzymes' responses to the environment, can exert biosphere-scale significance on the stoichiometry of bioavailable soil resources. To that end, we combined previously published intrinsic temperature sensitivities of two hydrolytic enzymes that release C and N during decay with a novel data set characterizing the kinetics of a P-releasing enzyme (acid phosphatase) across an ecologically relevant pH gradient. We use these data to estimate potential change in the flow ratios derived from these three enzymes' activities (C:N, C:P, and N:P) at the global scale by the end of the century, based on temperature projections and soil pH distribution. Our results highlight how the temperature sensitivity of these hydrolytic enzymes and the influence of pH on that sensitivity can govern the relative availability of bioavailable resources derived from these enzymes. The work illuminates the utility of weaving well-defined kinetic constraints of microbes' exoenzymes into models that incorporate changing SOM inputs and composition, nutrient availability, and microbial functioning into their efforts to project terrestrial ecosystem functioning in a changing climate.

Flow ratios to express results obtained with the human in vivo 'perfused forearm technique'

AIMS

To determine the accuracy of forearm blood flow (FBF) ratio (flow in infused arm/flow in control arm) to detect unilateral increases in forearm blood flow.

METHODS

In nine healthy male volunteers, we measured the effect of infusion of saline into the brachial artery at a rate of 2 ml/100 ml forearm min-1 on FBF ratio during control, mental arithmetic (MAR) and lower body negative pressure (LBNP) at -40 mmHg.

RESULTS

Saline infusion increased FBF ratio from baseline by 115.9+/-17.4, 82.0+/-19.0 and 159.6+/-53.3% for control, MAR and LBNP, respectively (P<0.05 for MAR vs control).

CONCLUSIONS

FBF ratio may underestimate unilateral increases in forearm blood flow during simultaneous mental arousal.