Volume 12, Number 44: 4 November 2009
De Graaff et al. (2009) introduce their study of the Progressive Nitrogen Limitation or PNL Hypothesis by stating it "posits that in unfertilized ecosystems, nitrogen availability progressively decreases under elevated CO2, because N retention in soil and vegetation is stimulated," and that "this ultimately leads to a decline in plant growth and a concomitant decrease in soil carbon sequestration." In a synthesis of results on plant growth and soil nutrient cycling under elevated CO2 in long-term field experiments, however, they say they showed that "under low N availability elevated CO2 still stimulated plant production by ~10%, even though data suggested that PNL had developed in these ecosystems (de Graaff et al., 2006)." In addition, they note that "plant production and soil C contents continue to increase under elevated CO2 in the Duke [Forest] FACE experiment, despite there being no evidence of increased net N mineralization or nutrient-use efficiency (Finzi et al., 2001; Johnson, 2006; Finzi et al., 2006)," and they say that "this suggests that an unexplained internal source of N can alleviate PNL in unfertilized ecosystems exposed to long-term elevated CO2."
So how is it done?
Giving others their due, the three researchers write that "Hungate and Chapin (1995) postulated that if mineral nutrients are scarce in soils, microbes utilize rhizodeposits as a carbon-source, and decompose more soil organic matter in order to acquire nutrients," so that "more N is then moved into the active N pool in the soil where, eventually, [it] may be made available to plants." Noting that "this process is referred to as priming, which is defined as the stimulation of soil organic matter decomposition caused by the addition of labile substrates (Jenkinson et al., 1985; Dalenberg and Jager, 1989)," de Graaff et al. (2009) go on to say that "since elevated CO2 frequently stimulates rhizodeposition - an important contributor to labile soil C inputs - and increases decomposition of soil organic matter, priming of more recalcitrant soil organic matter may be the mechanism partially responsible for alleviating PNL under elevated CO2 in low N environments."
This concept served as the stimulus for the study of de Graaff et al. (2009), where, as they describe it, various genotypes of two subspecies of spring wheat "were grown for one month in microcosms, exposed to 13C labeling at ambient (392 ppm) and elevated (792 ppm) CO2 concentrations, in soil containing 15N predominantly incorporated into recalcitrant soil organic matter pools," at the conclusion of which period the plants were harvested and numerous plant and soil properties assessed. This work revealed, in their words, that "decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial-13C was enhanced by 50% under elevated compared to ambient CO2," and that, concurrently, "plant 15N uptake increased (+7%) under elevated CO2 while 15N contents in the microbial biomass and mineral N pool decreased."
As for the implications of these findings, the three researchers say they suggest that "increased rhizodeposition under elevated CO2 can stimulate mineralization of N from recalcitrant soil organic matter pools," thereby "preventing N limitation in ecosystems exposed to long-term elevated atmospheric CO2." And in light of the many items we have archived in our Subject Index under the general heading of Nitrogen (Progressive Limitation Hypothesis) -- which demonstrate the absence of the PNL phenomenon in a wide variety of settings -- de Graaff et al.'s conclusions appear to be pretty much on the mark.
Sherwood, Keith and Craig Idso
References
Dalenberg, J.W. and Jager, G. 1989. Priming effect of some organic additions to C-14 labeled soil. Soil Biology & Biochemistry 21: 443-448.
de Graaff, M.A., van Groeningen, K.J., Six, J., Hungate, B.A. and van Kessel, C. 2006. Interactions between plant growth and nutrient dynamics under elevated CO2: a meta analysis. Global Change Biology 12: 1-15.
de Graaff, M.-A., Van Kessel, C. and Six, J. 2009. Rhizodeposition-induced decomposition increases N availability to wild and cultivated wheat genotypes under elevated CO2. Soil Biology & Biochemistry 41: 1094-1103.
Finzi, A., Allen, A., DeLucia, E., Ellsworth, D. and Schlesinger, W. 2001. Forest litter production, chemistry, and decomposition following two years of Free-Air CO2 enrichment. Ecology 82: 470-484.
Finzi, A.C., Moore, D.J.P., DeLucia, E.H., Lichter, J., Hofmockel, K.S., Jackson, R.B., Kim, H.S., Matamala, R., McCarthy, H.R., Oren, R., Pippen, J.S. and Schlesinger, W.H. 2006. Progressive nitrogen limitation of ecosystem processes under elevated CO2 in a warm-temperate forest. Ecology 87: 15-25.
Hungate, B.A. and Chapin III, F.S. 1995. Terrestrial Ecosystem Response to Elevated CO2: Effects on Microbial N Transformations Across Gradients of Nutrient Availability. GCTE Focus I Workshop, Lake Tahoe, California, USA.
Jenkinson, D.S., Fox, R.H. and Rayner, J.H. 1985. Interactions between fertilizer nitrogen and soil nitrogen -- the so-called "priming" effect. Journal of Soil Science 36: 425-444.
Johnson, D.W. 2006. Progressive N limitation in forests: review and implications for long-term responses to elevated CO2. Ecology 87: 64-75.