Jin and Evans (2010) write, “resource limitations, such as the availability of soil nitrogen (N), are expected to constrain continued increases in plant productivity under elevated atmospheric carbon dioxide.” This is a common belief. Providing a glimmer of hope, however, they state, “one potential but under-studied N source for supporting increased plant growth under elevated CO2 is soil organic N.” They report, “in arid ecosystems, there have been no studies examining plant organic N uptake to date.”
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 injected with isotopically labeled 15N obtained from one of three different organic and inorganic sources—(1) organic 15N glycine, (2) inorganic 15NH4+, or (3) inorganic 15NO3-. They then destructively harvested the plants following zero, two, ten, 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. The scientists found “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 “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.” They note, “to date, this study is the first to report organic N uptake by a plant species from a hot, arid ecosystem.”
In further discussing their findings, Jin and Evans state “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 “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, they found several encouraging indications that the ongoing rise in the air’s CO2 content will significantly increase the vitality of arid-land ecosystems, just as it does for other ecosystems.
In a related study, Brantly and Young (2010) note woody plant encroachment in herbaceous ecosystems “represents a key shift in community structure that has [the] potential to alter regional and global C and N cycling.” However, they write, “there is considerable uncertainty regarding the effects of woody [plant] encroachment on global terrestrial C storage,” due to the possibility that “increases in C sequestration in woody biomass may be offset by associated increases in soil CO2 efflux (i.e., combined heterotrophic respiration and root respiration) resulting from increased litterfall, increased soil moisture, and associated increases in microbial activity that often accompany woody [plant] encroachment.”
To further investigate this situation by determining “if young, sandy soils on a barrier island became a sink for C after encroachment of the nitrogen-fixing shrub Morella cerifera, or if associated stimulation of soil CO2 efflux mitigated increased litterfall,” Brantley and Young “monitored variations in litterfall in shrub thickets across a chronosequence of shrub expansion and compared those data to previous measurements of ANPP [annual net primary production] in adjacent grasslands,” after which they “quantified standing litter C and N pools in shrub thickets and soil organic matter (SOM) , soil organic carbon (SOC), soil total nitrogen (TN) and soil CO2 efflux in shrub thickets and adjacent grasslands,” This field work was conducted on the north end of Hog Island (37°27’N, 75°40’W), a barrier island just east of the Virginia portion of the DelMarVa peninsula, USA.
The two researchers discovered that although soil CO2 efflux was indeed stimulated by shrub encroachment in the younger soils, “soil CO2 efflux did not vary between shrub thickets and grasslands in the oldest soils, and increases in CO2 efflux in shrub thickets did not offset contributions of increased litterfall to SOC.” In fact, they found “SOC was 3.6–9.8 times higher beneath shrub thickets than in grassland soils, and soil TN was 2.5–7.7 times higher under shrub thickets.” These facts led them to conclude the expansion of shrubs on barrier islands—which often have low levels of soil carbon but a high potential for ANPP—can “significantly increase ecosystem C sequestration.” In addition, “stimulation of N storage beneath shrub thickets will also favor future growth of species with lower nutrient use efficiencies than native grasses, including climax maritime forest species that could sequester additional C in biomass,” citing Ehrenfeld (1990) and Vitousek et al. (2002). The phenomena presaged by their work bode well for barrier islands and the planet’s less-productive grasslands.
Noting “human activities have greatly accelerated emissions of both carbon dioxide and biologically reactive nitrogen to the atmosphere,” Thomas et al. (2010) report, “as nitrogen availability often limits forest productivity, it has long been expected that anthropogenic nitrogen deposition could stimulate carbon sequestration in forests.” However, they note geographically extensive evidence for this phenomenon “has been lacking,” and, therefore, they proceeded to provide some, using “spatially extensive forest inventory data to discern the effect of nitrogen deposition on the growth and survival of the 24 most common tree species of the northeastern and north-central US, as well as the effect of nitrogen deposition on carbon sequestration in trees across the breadth of the northeastern US.”
They determined that “nitrogen deposition (which ranged from 3 to 11 kg ha-1 yr-1) enhanced the growth of eleven species and decreased the growth of three species,” while it “enhanced growth of all tree species with arbuscular mycorrhizal fungi associations,” leading to “a 40% enhancement over pre-industrial conditions,” This response “includes the direct effects of nitrogen deposition on tree growth through soil fertilization, foliar nitrogen uptake and other potential interactions between nitrogen deposition and other environmental changes, including CO2 fertilization.” To provide a sense of the significance of this response, they note it “exceeds the 23% enhancement of net primary production anticipated for the year 2050 from a doubling of atmospheric CO2 over preindustrial levels, as estimated using free-air CO2 enrichment studies,” citing Norby et al. (2005).
Thomas et al. thus conclude “nitrogen deposition is an important mechanism contributing to carbon sequestration within these temperate forests,” though this phenomenon is still “unlikely to explain all of the observed terrestrial carbon sink.”
In a study described previously in Section 7.6, Reich (2009) states, “levels of nitrogen deposition and CO2 have risen in recent decades and are expected to increase further.” He notes that in the case of natural ecosystems, the subsequent suppression of diversity by increasing N availability “is almost ubiquitous,” while “evidence of CO2 effects on species richness is scarce and shows mixed results, with positive, neutral, and negative responses seen in the few published reports.” So Reich explored what is most likely to happen when the two phenomena occur together using the ten-year-long BioCON study conducted at the Cedar Creek Long-Term Ecological Research site.
According to Reich, at the ambient soil nitrogen concentration, elevated CO2 had reduced observed species richness by 2 percent while at the ambient atmospheric CO2 concentration elevated nitrogen decreased species richness by 15 percent over the last seven years of the ten-year-long study. When the elevated soil N concentration was combined with the elevated atmospheric CO2 concentration, however, species richness declined by only 5 percent, leading Reich to conclude, “elevated CO2 reduces losses of plant diversity caused by nitrogen deposition.” With nitrogen deposition “expected to increase further,” as Reich notes, Earth’s many natural ecosystems will be protected by the rise in the air’s CO2 content that is expected to accompany it.
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