The progressive nitrogen limitation hypothesis suggests that low concentrations of soil nitrogen will gradually curtail the ability of the productivity-enhancing effect of rising atmospheric CO2 concentrations to maintain increased plant growth and ecosystem carbon sequestration rates as time progresses (Hungate et al., 2003; Luo et al., 2004). But is this really so?
In a study that comes to bear upon this important question, Butler et al. (2012) examined how warmer-than-current temperatures influenced various fluxes and pools of nitrogen (N) within an even-aged, mixed deciduous forest that is dominated by red oaks (Quercus rubra) in central Massachusetts (USA). There, during the summer and fall of 2001, they installed by hand (to minimize disturbance in a 30 x 30 meter area) about 5 km of heating cables that were buried at 10 cm depth and spaced about 20 cm apart, while a similar sized unaltered plot was maintained nearby as a control against which to compare the climatic and biological impacts of the cable-heated plot, where they say that “each minute, heating cables were turned on or off automatically to maintain a 5°C temperature differential between heated and control areas.” So what did they learn?
“Since the start of the experiment,” in the words of the thirteen researchers, they say that they “have documented a 45% average annual increase in net nitrogen mineralization [the process by which organic forms of nitrogen found in dead plant material are converted by microbes to inorganic forms that may be taken up by living plants] and a three-fold increase in nitrification [the biological process that converts nitrogen-containing organic compounds into nitrates and nitrites that can be used by living plants] such that in years 5 through 7, 25% of the nitrogen mineralized [was] then nitrified.” In addition, they further state that “the increase in N availability in the warmed area has led to increases in leaf N and plant C storage relative to the control (Melilloet al., 2011) and to an increase in relative growth rates, especially for red maples,” although they add that “leaf N is positively correlated to photosynthetic rate and carbon storage in many plants growing across the globe,” citing Field and Mooney (1986), Reich et al. (1994, 1995, 1997) and Ollinger et al. (2008).
In discussing their findings, Butler et al. write that “the increase in N mineralization in response to warming that we documented in this study has also been observed in other studies; some in forests (Peterjohn et al., 1994; Hartley et al., 1999; Rustad et al., 2001; Melillo et al., 2002), some in grasslands (Shaw and Harte, 2001) and some in tundra (Chapin et al., 1995).” And they add that “this sustained increase in net N mineralization with warming has been accompanied by increases in net nitrification, which has also been reported by others (Hartley et al., 1999; Barnard et al., 2004; Emmett et al., 2004).”
And thus it is that the US research team concludes that “as CO2 concentrations increase and warming stimulates increases in N availability, it is possible that we may see further increases in growth rates of these species,” as has, in fact, already been demonstrated to be the case in several free-air CO2 enrichment or FACE studies, including those of Finzi et al. (2006), Norby and Iversen (2006), Zak et al. (2007), McCarthy et al. (2010) and Hofmockel et al. (2011).