Elevated CO2 effects on carbon and nitrogen cycling in grass/clover turves of a Psammaquent soil

Abstract

Effects of elevated C 0 2 (525 and 700 #L L-l), and a control (350 #L L -1 CO2), on biochemical properties of a Mollic Psammaquent soil in a well-established pasture of C3 and C4 grasses and clover were investigated with continuously moist turves in growth chambers over four consecutive seasonal temperature regimes from spring to winter inclusive. After a further 'spring' period, half of the turves under 350 and 700 #L L -1 were subjected to 'summer' drying and were then re-wetted before a further 'autumn' period; the remaining turves were kept continuously moist throughout these additional three consecutive 'seasons'. The continuously moist turves were then pulse-labelled with 14C-CO2 to follow C pathways in the plant/soil system during 35 days.Growth rates of herbage during the first four 'seasons' averaged 4.6 g m -2 day -1 under 700 #L L -1 CO2 and were about 10% higher than under the other two treatments. Below-ground net productivity at the end of these 'seasons' averaged 465, 800 and 824 g m -2 in the control, 525 and 700 #L L -l treatments, respectively.In continuously moist soil, elevated CO2 had no overall effects on total, extractable or microbial C and N, or invertase activity, but resulted in increased CO2-C production from soil, and from added herbage during the initial stages of decomposition over 21 days; rates of root decomposition were unaffected. CO2 produced h -1 mg -l microbial C was about 10% higher in the 700 #L L -1 CO2 treatment than in the other two treatments. Elevated CO2 had no clearly defined effects on N availability, or on the net N mineralization of added herbage.In the labelling experiment, relatively more 14C in the plant/soil system occurred below ground under elevated CO2, with enhanced turnover of !4C also being suggested.Drying increased levels of extractable C and organic-N, but decreased mineral-N concentrations; it had no effect on microbial C, but resulted in lowered microbial N in the control only. In soil that had been previously 'summer'-dried, CO2 production was again higher, but net N mineralization was lower, under elevated CO2 than in the control after 'autumn' pasture growth.Over the trial period of 422 days, elevated CO2 generally appears to have had a greater effect on soil C turnover than on soil C pools in this pasture ecosystem.

Increased Quantity and Quality of Coarse Soil Organic Matter Fraction at Elevated CO2 in a Grazed Grassland are a Consequence of Enhanced Root Growth Rate and Turnover

“…Although we found increased root growth at elevated CO 2 during the period of low soil moisture there was no difference in moisture content between the CO 2 treatments ( Figure 3). This effect was observed previously in turves of the same soil type growing in growth rooms; in this case, C assimilation was sustained longer into the moisture deficit period at elevated CO 2 allowing root growth to be maintained (Newton et al, 1996). Other indirect CO 2 effects may be relevant to the increased root turnover we observed under elevated CO 2 .…”

“…There have been several reports of increased root standing mass in grasslands under elevated CO 2 (Newton et al, 1996;Casella andSoussana, 1997, Loiseau andSoussana, 1999;but see Navas et al, 1995 andSto¨cklin et al, 1998). However, greater root standing biomass does not necessarily tell us much about the flux of C belowground (Canadell et al, 1996) as it is root turnover that is the major mechanism of C transfer to the soil (van Veen et al, 1991).…”

“…Microbial N biomass was calculated as N mic = N fumigated )N unfumigated /k EN , where k EN is the extraction yield for microbial N; k EN = 0.45. These coefficients are not affected by elevated CO 2 (Ross et al, 1996). Extraction yield EOM was extracted from the soil samples by autoclaving the equivalent of 10 g dry soil using a modification of the method described in Lemaitre et al (1995) (P. Loiseau, personal communication).…”

“…Because of the large quantities of organic C in grassland soils, changes in the size of soil C compartments are difficult to detect through direct measurements (Ross et al, 1996). The coarse particulate POM compartment (>1 mm) represents the first stage in a continuum of decomposition in grassland ecosystems that culminates in stabilized soil OM (Mellilo et al, 1989); consequently, in short-term experiments, we might expect to see any response to elevated CO 2 expressed in the coarse POM pool.…”

Influence of an Elevated Atmospheric CO 2 Content on Soil and Rhizosphere Bacterial Communities Beneath Lolium perenne and Trifolium repens under Field Conditions

“…Indeed, because of the competitive inhibition by O 2 of the ribulose-1,5-bisphosphate carboxylase, which is responsible for CO 2 fixation during photosynthesis [7], the C 3 plant photosynthetic rate increases under a CO 2 -enriched atmosphere [28,47,51]. An increased root biomass under CO 2 enrichment has been shown by several authors [25,30,47,52,53].…”