Response of soil biota to elevated atmospheric CO 2 in poplar model systems

Abstract

We tested the hypotheses that increased belowground allocation of carbon by hybrid poplar saplings grown under elevated atmospheric CO would increase mass or turnover of soil biota in bulk but not in rhizosphere soil. Hybrid poplar saplings (Populus×euramericana cv. Eugenei) were grown for 5 months in open-bottom root boxes at the University of Michigan Biological Station in northern, lower Michigan. The experimental design was a randomized-block design with factorial combinations of high or low soil N and ambient (34 Pa) or elevated (69 Pa) CO in five blocks. Rhizosphere microbial biomass carbon was 1.7 times greater in high-than in low-N soil, and did not respond to elevated CO. The density of protozoa did not respond to soil N but increased marginally (P < 0.06) under elevated CO. Only in high-N soil did arbuscular mycorrhizal fungi and microarthropods respond to CO. In high-N soil, arbuscular mycorrhizal root mass was twice as great, and extramatrical hyphae were 11% longer in elevated than in ambient CO treatments. Microarthropod density and activity were determined in situ using minirhizotrons. Microarthropod density did not change in response to elevated CO, but in high-N soil, microarthropods were more strongly associated with fine roots under elevated than ambient treatments. Overall, in contrast to the hypotheses, the strongest response to elevated atmospheric CO was in the rhizosphere where (1) unchanged microbial biomass and greater numbers of protozoa (P < 0.06) suggested faster bacterial turnover, (2) arbuscular mycorrhizal root length increased, and (3) the number of microarthropods observed on fine roots rose.

Mycorrhizal fungi have a potential role in soil carbon storage under elevated CO2 and nitrogen deposition

“…This interaction is not universal, however. Lussenhop et al (1998) noted no significant CO # by N effects in AM biomass of a constructed ecosystem of Populusieuramericana seedlings. Tingey et al (1995) also found no CO # and N interaction on the presence of ECM root tips and hyphae on P. ponderosa seedlings.…”

“…In experiments that have measured ECM biomass after weeks or months of growth, decreases Wallander et al, 1994) ; lack of response ; or increases (Ekblad et al, 1995) under higher N availability have each been observed. Likewise, the responses of AM biomass to N concentration have been positive (Lussenhop et al, 1998), negative , or not significant Bethlenfalvay et al, 1999).…”

“…0n3 across CO # levels (Table 1) Lussenhop et al (1998) Populusieuramericana saplings (constructed ecosystem)…”

Climate change goes underground: effects of elevated atmospheric CO2 on microbial community structure and activities in the rhizosphere

“…In contrast, in soils under L. perenne and Trifolium repens, Richter et al (2003) did not detect any changes in microbial biomass after 7 years of fumigation with 600 ppm CO 2 compared to current conditions. In addition, no changes in microbial biomass were detected in the rhizosphere and soils of poplar (Lussenhop et al 1998). In tall grass prairie system exposed to elevated CO 2 for 8 years, Williams et al (2000) showed a slight increase (not significant) in microbial C and N biomass, whereas there was a significant increase in overall microbial activity.…”

“…Nevertheless, microbial biomass measurements based on cell component ATP, phospholipid fatty acid analyses (PLFA), chloroform fumigation assays, or total cell counts have yielded mixed results. Quantitative alterations in C supply have been shown to decrease (Diaz et al 1993;Ebersberger et al 2004), increase (Hungate et al 2000;van Ginkel and Gorrisen 1998;van Ginkel et al 2000;Williams et al 2000;Zak et al 1993), or not affect (Chung et al 2006;Hungate et al 2000;Kandeler et al 1998;Lussenhop et al 1998;Randlett et al 1996;Richter et al 2003) microbial biomass and activities (e.g., decomposition and nutrient cycling (Hu et al 1999;Jones et al 1998). Indeed, after 3 months of fumigation with 700 ppm 14 CO 2 , a 42% increase in microbial biomass was measured in Lolium perenne soil (van Ginkel and Gorrisen 1998;van Ginkel et al 2000).…”