Gross fluxes of nitrogen in grassland soil exposed to elevated atmospheric pCO 2 for seven years

Abstract

Plant response to increasing atmospheric CO 2 partial pressure ( pCO 2) depends on several factors, one of which is mineral nitrogen availability facilitated by the mineralisation of organic N. Gross rates of N mineralisation were examined in grassland soils exposed to ambient (36 Pa) and elevated (60 Pa) atmospheric pCO 2 for 7 years in the Swiss Free Air Carbon dioxide Enrichment experiment. It was hypothesized that increased below-ground translocation of photoassimilates at elevated pCO 2 would lead to an increase in immobilisation of N due to an excess supply of energy to the roots and rhizosphere. Intact soil cores were sampled from Lolium perenne and Trifolium repens swards in May and September, 2000. The rates of gross N mineralisation ( m) and NH 4 + consumption ( c) were determined using 15N isotopic dilution during a 51-h period of incubation. The rates of N immobilisation were estimated either as the difference between m and the net N mineralisation rate or as the amount of 15N released from the microbial biomass after chloroform fumigation. Soil samples from both swards showed that the rates of gross N mineralisation and NH 4 + consumption did not change significantly under elevated pCO 2. The lack of a significant effect of elevated pCO 2 on organic N turnover was consistent with the similar size of the microbial biomass and similar immobilisation of applied 15N in the microbial N pool under ambient and elevated pCO 2. Rates of m and c, and microbial 15N did not differ significantly between the two sward types although a weak ( p<0.1) pCO 2 by sward interaction occurred. A significantly larger amount of NO 3 − was recovered at the end of the incubation in soil taken from T. repens swards compared to that from L. perenne swards. Eleven percent of the added 15N were recovered in the roots in the cores sampled under L. perenne, while only 5% were recovered in roots of T. repens. These results demonstrate that roots remained a considerable sink despite the shoots being cut at ground level prior to incubation and suggest that the calculation of N immobilisation from gross and net rates of mineralisation in soils with a high root biomass does not reflect the actual immobilisation of N in the microbial biomass. The results of this study did not support the initial hypothesis and indicate that below-ground turnover of N, as well as N availability, measured in short-term experiments are not strongly affected by long-term exposure to elevated pCO 2. It is suggested that differences in plant N demand, rather than major changes in soil N mineralisation/immobilisation, are the long-term driving factors for N dynamics in these grassland systems.