Background
The authors write that "resource limitations, such as the availability of soil nitrogen (N), are expected to constrain continued increases in plant productivity under elevated atmospheric carbon dioxide," which is a common mantra of climate alarmists. Providing a glimmer of hope, however, they state that "one potential but under-studied N source for supporting increased plant growth under elevated CO2 is soil organic N," although they report that "in arid ecosystems, there have been no studies examining plant organic N uptake to date."

What was done
To help remedy this situation, Jin and Evans grew seedlings of the desert shrub Larrea tridentata in environmentally-controlled chambers in ambient or CO2-enriched air (380 or 600 ppm) in pots filled with Mojave Desert (Nevada, USA) soils that they injected with isotopically-labeled ¹⁵N obtained from one of three different organic and inorganic sources -- (1) organic ¹⁵N glycine, or (2) inorganic ¹⁵NH4⁺, or (3) inorganic ¹⁵NO3^- -- after which they destructively harvested the plants following 0, 2, 10, 24 and 49 additional days of growth and determined the amounts of soil N they had taken up from each of the three N sources.

What was learned
The two researchers report that "elevated CO2 positively affected root uptake of N derived from all three N forms by day 10, with NO3^--derived N taken up at the highest rates," and that "added glycine was taken up as intact amino acid within one hour of treatment application, indicating that L. tridentata can directly utilize soil organic sources," while noting that "to date, this study is the first to report organic N uptake by a plant species from a hot, arid ecosystem."

What it means
Jin and Evans say "there is increasing consensus that organic N uptake could be a major plant N acquisition pathway (Lipson and Nasholm, 2001; Schimel and Bennett, 2004), with 10-90% of the total annual plant N requirement potentially met by the uptake of external soil organic N (Chapin et al., 1993; Kielland, 1994; Jones and Darrah, 1994)." In addition, they note that "long-term exposure to elevated CO2 has altered the quality and quantity of plant-derived carbon inputs into Mojave Desert soils, leading to higher extracellular enzyme activities indicative of a greater or more active soil fungal component (Jin and Evans, 2007)," such that "increased soil fungi may lead to the greater release of monomeric organic N under elevated CO2, enhancing substrate availability for soil microbes as well as for plant uptake." Hence, there are several encouraging indications that the ongoing rise in the air's CO2 content will significantly enhance the vitality of arid-land ecosystems, and other ecosystems as well.

References
Chapin III, F.S., Moilanen, L. and Kielland, K. 1993. Preferential use of organic nitrogen for growth by a non-mycorrhizal arctic sedge. Nature 361: 150-153.

Jin, V.L. and Evans, R.D. 2007. Elevated CO2 increases microbial carbon substrate use and N cycling in Mojave Desert soils. Global Change Biology 13: 452-465.

Jones, D.L. and Darrah, P.R. 1994. Amino-acid influx at the soil-root interface of Zea mays L. and its implications in the rhizosphere. Plant and Soil 163: 1-12.

Kielland, K. 1994. Amino acid absorption by arctic plants: Implications for plant nutrition and nitrogen cycling. Ecology 75: 2373-2383.

Lipson, D.A. and Nasholm, T. 2001. The unexpected versatility of plants: Organic nitrogen use and availability in terrestrial ecosystems. Oecologia 128: 305-316.

Schimel, J.A. and Bennett, J. 2004. Nitrogen mineralization: Challenges of a changing paradigm. Ecology 85: 591-602.