Effect of elevated atmospheric CO2 and vegetation type on microbiota associated with decomposing straw

Straw from wheat plants grown at ambient and elevated atmospheric CO2 concentrations was placed in litterbags in a grass fallow field and a wheat field. The CO2 treatment induced an increase in straw concentration of ash-free dry mass from 84% to 93% and a decrease in nitrogen concentration from 0.43% to 0.34%, After five months of decomposition, less than 50% of the straw was decomposed. The content of ash-free dry mass remaining in straw from plants grown at elevated CO2 was significantly higher than that from plants grown at ambient CO2 (4.02 vs. 3.69 g AFDM per litterbag in the fallow field and 3.40 vs. 2.67% AFDM per litterbag when buried in the wheat field). The immobilization of nitrogen during decomposition was significantly higher in the ambient straw, and there was a significant negative correlation between the content of organic matter remaining per litterbag and the nitrogen concentration in the recovered straw samples. After five months of decomposition, hyphal biomass was significantly lower in straw from plants grown at elevated CO2 (-30% and -13% in the fallow and wheat field, respectively). Bacterial biomass was not significantly affected by the CO2 induced changes in the litter quality, but the lower decomposition rate and fewer bacterial grazers in the straw from plants grown at elevated CO2 together indicate reduced microbial activity and turnover. Notwithstanding this, these data show that growth at elevated atmospheric CO2 concentration results in slower decomposition of wheat straw, but the effect is probably of minor importance compared to the effect of varying crops, agricultural practise or changing land use.