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Nitrogen Uptake by Soil Microbes: The Long (Term) and the Short (Term) of It
Barnard, R., Barthes, L. and Leadley, P.W. 2006. Short-term uptake of 15N by a grass and soil micro-organisms after long-term exposure to elevated CO2. Plant and Soil 280: 91-99.

It has periodically been claimed that when atmospheric CO2 enrichment increases plant growth leading to greater soil carbon input, decomposing micro-organisms require more nitrogen, and that this phenomenon can reduce nitrogen mineralization, which is one of the primary sources of nitrogen for plants. Hence, it is often further claimed that in a high-CO2 world of the future, the availability of nitrogen in forms usable by plants will be too low to support large increases in carbon storage, leading ultimately to more rapid and extreme global warming.

What was done
Barnard et al. injected 15N-labelled NH4 into the soil of mesocosms of Holcus lanatus (L.) that had been grown for over 15 months at either ambient or elevated atmospheric CO2 concentrations "to determine whether the uptake capacity of soil micro-organisms had remained higher at elevated CO2, and to shed further light on the short-term (48 hours) partitioning of N between plants and soil micro-organisms." In addition, they reviewed the findings of other pertinent studies in coming to their conclusions.

What was learned
The researchers report that their "results and literature data from plant-microbial 15N partitioning experiments at elevated CO2 suggest that the mechanisms controlling the effects of CO2 on short- vs. long-term N uptake and turnover differ [our italics]." Even more importantly, they say that "short-term immobilization of added N by soil micro-organisms at elevated CO2 does not appear to lead to long-term increases in N in soil microbial biomass." How does it happen? They say that "the increased soil microbial C:N ratios that [they] observed at elevated CO2 suggest that long-term exposure to CO2 alters either the functioning or structure of these microbial communities."

What it means
Barnard et al. conclude that "short-term immobilization of inorganic soil nitrogen or exploitation of nutrient pulses may be altered under conditions of elevated atmospheric CO2 concentration," and this alteration is such that it takes most of the wind out of the sails of the progressive nitrogen limitation hypothesis, likely allowing long-lived plants and ecosystems to maintain positive growth responses to atmospheric CO2 enrichment for as long as the enrichment continues.

Reviewed 19 July 2006