Soil mineral nitrogen availability was unaffected by elevated atmospheric pCO(2) in a four year old field experiment (Swiss FACE)

The effect of elevated (60 Pa) atmospheric carbon dioxide partial pressure (p(CO2)) and N fertilisation on the availability of mineral N and on N tran sformation in the soil of a Lolium perenne L. monoculture was investigated in the Swiss FACE (Free Air Carbon dioxide Enrichment) experiment. The appa rent availability of nitrate and ammonium for plants was estimated during a representative, vegetative re-growth period at weekly intervals from the s orption of the minerals to mixed-bed ion-exchange resin bags at a soil dept h of 5 cm. N mineralisation was measured using sequential coring and in sit u exposure of soil cores in the top 10 cm of the soil before and after the first cut in spring 1997. High amounts of mineral N were bound to the ion e xchange resin during the first week of re-growth. This was probably the com bined result of the fertiliser application, the weak demand for N by the ne wly cut sward and presumably high rates of root decay and exudation after c utting the sward. During the first 2 weeks after the application of fertili ser N at the first cut, there was a dramatic reduction in available N; N re mained low during the subsequent weeks of re-growth in all treatments. Over all, nitrate was the predominant form of mineral N that bound to the resin for the duration of the experiment. Apparently, there was always more nitra te than ammonium available to the plants in the high N fertilisation treatm ent for the whole re-growth period. Apparent N availability was affected si gnificantly by elevated p(CO2) only in the first week after the cut; under high N fertilisation, elevated p(CO2) increased the amount of mineral N tha t was apparently available to the plants. Elevated p(CO2) did not affect ap parent net transformation of N, loss of N or uptake of N by plants. The pre sent data are consistent with earlier results and suggest that the amount o f N available to plants from soil resources does not generally increase und er elevated atmospheric p(CO2). Thus, a possible limiting effect of N on pr imary production could become more stringent under elevated atmospheric p(C O2) as the demand of the plant for N increases.