The stable oxygen isotope compositions of soil phosphate (δ(18)O(p)) were suggested recently to be a tracer of phosphorus cycling in soils and plants. Here we present a survey of bioavailable (resin-extractable or resin-P) inorganic phosphate δ(18)O(p) across natural and experimental rainfall gradients, and across soil formed on sedimentary and igneous bedrock. In addition, we analyzed the soil HCl-extractable inorganic δ(18)O(p), which mainly represents calcium-bound inorganic phosphate. The resin-P values were in the range 14.5-21.2‰. A similar range, 15.6-21.3‰, was found for the HCl-extractable inorganic δ(18)O(p), with the exception of samples from a soil of igneous origin that show lower values, 8.2-10.9‰, which indicate that a large fraction of the inorganic phosphate in this soil is still in the form of a primary mineral. The available-P δ(18)O(p) values are considerably higher than the values we calculated for extracellular hydrolysis of organic phosphate, based on the known fractionation from lab experiments. However, these values are close to the values expected for enzymatic-mediated phosphate equilibration with soil-water. The possible processes that can explain this observation are (1) extracellular equilibration of the inorganic phosphate in the soil; (2) fractionations in the soil are different than the ones measured at the lab; (3) effect of fractionation during uptake; and (4) a flux of intercellular-equilibrated inorganic phosphate from the soil microbiota, which is considerably larger than the flux of hydrolyzed organic-P.

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Abstract. Respiration in tree stems is an important component of forest carbon balance. The rate of CO 2 efflux from the stem has often been assumed to be a measure of stem respiration. However, recent work in temperate forests has demonstrated that stem CO 2 efflux can either overestimate or underestimate respiration rate because of emission or removal of CO 2 by transport in xylem water. Here, we studied gas exchange from stems of tropical forest trees using a new approach to better understand respiration in an ecosystem that plays a key role in the global carbon cycle. Our main questions were (1) is internal CO 2 transport important in tropical trees, and, if so, (2) does this transport result in net release of CO 2 respired in the roots at the stem, or does it cause the opposite effect of net removal of stem-respired CO 2 ? To answer these questions, we measured the ratio of stem CO 2 efflux to O 2 influx. This ratio, defined here as apparent respiratory quotient (ARQ), is expected to equal 1.0 if carbohydrates are the substrate for respiration, and the net transport of CO 2 in the xylem water is negligible. Using a stem chamber approach to quantifying ARQ, we found values of 0.66 ± 0.18. These low ARQ values indicate that a large portion of respired CO 2 (∼ 35 %) is not emitted locally, and is probably transported upward in the stem. ARQ values of 0.21 ± 0.10 were found for the steady-state gas concentration within the stem, sampled by in-stem equilibration probes. These lower values may result from the proximity to the xylem water stream. In contrast, we found ARQ values of 1.00 ± 0.13 for soil respiration. Our results indicate the existence of a considerable internal flux of CO 2 in the stems of tropical trees. If the transported CO 2 is used in the canopy as a substrate for photosynthesis, it could account for up to 10 % of the C fixed by the tree, and perhaps serve as a mechanism that buffers the response of the tree to changing CO 2 levels. Our results also indicate, in agreement with previous work, that the widely used CO 2 efflux approach for determining stem respiration is unreliable. We demonstrate here a field applicable approach for measuring the O 2 uptake rate, which we suggest to be a more appropriate method to estimate stem respiration rates.

Respiration in tree stems is an important component of forest carbon balance. The rate of CO₂ efflux from the stem has often been assumed to be a measure of stem respiration. However, recent work in temperate forests has demonstrated that stem CO₂ efflux can either overestimate or underestimate respiration rate, because of emission or removal of CO₂ by transport in xylem water. Here we used the ratio between CO₂ efflux and O₂ influx in stems of tropical forest trees to better understand respiration in an ecosystem that plays a key role in the global carbon cycle. This ratio, which we defined here as apparent respiratory quotient (ARQ), is expected to equal 1.0 if carbohydrates are the substrate for respiration, and the net transport of CO₂ in the xylem water is negligible. However, using a stem chamber approach to quantifying ARQ we found values of 0.66 ± 0.18. These low ARQ values indicate that a large portion of respired CO₂ (~35%) is not emitted locally, and is probably transported upward in the stem. ARQ values of 0.21 ± 0.10 were found for the steady-state gas concentration within the tree, sampled by in-stem equilibration probes. These lower values may result from the proximity to the xylem water stream. In contrast, we found ARQ values of 1.00 ± 0.13 for soil respiration. Our results indicate, for the first time, the existence of a~considerable internal flux of CO₂in the stem of tropical trees. If the transported CO₂ is used in the canopy as a substrate for photosynthesis, it could account for several percent of the C fixed by the tree, and perhaps serve as a mechanism that buffers the response of the tree to changing CO₂ levels. Our results also indicate, in agreement with previous work, that the widely used CO₂ efflux approach for determining stem respiration is unreliable. We demonstrate here a field applicable approach for measuring the O₂ uptake rate, which we suggest to be a more appropriate method to estimate stem respiration rates

Improved tools for tracing phosphate transformations in soils are much needed, and can lead to a better understanding of the terrestrial phosphorus cycle. The oxygen stable isotopes in soil phosphate are still not exploited in this regard. Here we present a method for measuring the oxygen stable isotopes in a fraction of the soil phosphate which is rapidly available to plants, the resin‐extractable P. This method is based on extracting available phosphate from the soil with anion‐exchange membranes, soil organic matter removal by a resin, purification by precipitation as cerium phosphate, and finally precipitation as silver phosphate. The purified silver phosphate samples are then measured by a high‐temperature elemental analyzer (HT‐EA) coupled in continuous flow mode to an isotope ratio mass spectrometer. Testing the method with Mediterranean and semi‐arid soils showed no artifacts, as well as good reproducibility in the same order as that of the HT‐EA analytical uncertainty (0.3‰). Copyright © 2011 John Wiley & Sons, Ltd.