Decomposition of 14C-labeled roots in a pasture soil exposed to 10 years of elevated CO 2

Abstract

The net flux of soil C is determined by the balance between soil C input and microbial decomposition, both of which might be altered under prolonged elevated atmospheric CO 2. In this study, we determined the effect of elevated CO 2 on decomposition of grass root material ( Lolium perenne L.). 14C-labeled root material, produced under ambient (35 Pa pCO 2) or elevated CO 2 (70 Pa pCO 2) was incubated in soil for 64 days. The soils were taken from a pasture ecosystem which had been exposed to ambient (35 Pa pCO 2) or elevated CO 2 (60 Pa pCO 2) under FACE-conditions for 10 years and two fertilizer N rates: 140 and 560 kg N ha −1 year −1. In soil exposed to elevated CO 2, decomposition rates of root material grown at either ambient or elevated CO 2 were always lower than in the control soil exposed to ambient CO 2, demonstrating a change in microbial activity. In the soil that received the high rate of N fertilizer, decomposition of root material grown at elevated CO 2 decreased by approximately 17% after incubation for 64 days compared to root material grown at ambient CO 2. The amount of 14CO 2 respired per amount of 14C incorporated in the microbial biomass ( q 14CO 2) was significantly lower when roots were grown under high CO 2 compared to roots grown under low CO 2. We hypothesize that this decrease is the result of a shift in the microbial community, causing an increase in metabolic efficiency. Soils exposed to elevated CO 2 tended to respire more native SOC, both with and without the addition of the root material, probably resulting from a higher C supply to the soil during the 10 years of treatment with elevated CO 2. The results show the importance of using soils adapted to elevated CO 2 in studies of decomposition of roots grown under elevated CO 2. Our results further suggest that negative priming effects may obscure CO 2 data in incubation experiments with unlabeled substrates. From the results obtained, we conclude that a slower turnover of root material grown in an ‘elevated-CO 2 world’ may result in a limited net increase in C storage in ryegrass swards.