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In Situ Decomposition of Grass Roots as Affected by Elevated Atmospheric Carbon Dioxide

DLO Research Inst. for Agrobiology and Soil Fertility (AB-DLO), Dep. of Soil Ecology, P.O. Box 14, 6700 AA Wageningen, the Netherlands

*Corresponding author (j.h.vanginkel{at}ab.dlo.nl).

ABSTRACT

The effects of elevated CO₂ on belowground C input, on decomposition of roots in situ vs. decomposition of disturbed roots, and on soil microbial biomass were investigated in a perennial grass species. Forty ryegrass (Lolium perenne L.) plants were homogeneously ¹⁴C-labeled in two controlled environments for 115 d in a continuous ¹⁴CO₂ atmosphere at 350 and 700 µL CO₂ L^-1 and two soil N levels (low, LN, and high, HN). Thereafter, some of the plants were destructively harvested. Undisturbed root systems of the remaining plants were incubated in situ (IRS) for comparison with a disturbed incubation of the dried and ground roots (DRS) in their original soils. At the start of the incubation, elevated CO₂ had increased total ¹⁴C-labeled soil C input by 44 and 27% at LN and HN, respectively, compared with input at ambient CO₂. After incubation for 230 d, 40% of ¹⁴C soil content was mineralized to ¹⁴CO₂ in the disturbed system and 52% in the intact system. Native soil organic matter (SOM) decomposition of the DRS was lower than the SOM decomposition of the IRS. The formation of ¹⁴C-labeled soil microbial biomass (¹⁴C-SMB) in the soil with DRS was 130% higher than in the soil with the IRS. Elevated CO₂ decreased the decomposition of roots and root-derived products by 10% and increased the size of the ¹⁴C-SMB by 28% for both IRS and DRS, whereas the decomposition of SOM was not affected by CO₂ at either LN or HN. After plant growth and in situ incubation, the ¹⁴C-labeled C in the soil solution showed a highly positive correlation with the amount of ¹⁴C-SMB. The ratio between ¹⁴C-labeled microorganisms and total ¹⁴CO₂ evolved was not affected by elevated CO₂. It seems that microorganisms adapt to changing soil C input under elevated CO₂ and there is no effect on their turnover and behavior.

Received for publication April 30, 1997.

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