Effects of elevated CO2, temperature and N fertilization on nitrogen fluxes in a temperate grassland ecosystem

The soil nitrogen cycle was investigated in a pre-established Lolium perenne sward on a loamy soil and exposed to ambient and elevated atmospheric CO2 concentrations (350 and 700 muL L-1) and, at elevated [CO2], to a 3 degreesC temperature increase. At two levels of mineral nitrogen supply, N-(150 kgN ha(-1) y(-1)) and N+(533 kgN ha(-1) y(-1)), N-15- labelled ammonium nitrate was supplied in split applications over a 2.5-y period. The recovery of the labelled fertilizer N was measured in the harvests, in the stubble and roots, in the macro-organic matter fractions above 200 mum in size (MOM) and in the aggregated organic matter below 200 muM (AOM). Elevated [CO2] reduced the total amount of N harvested in the clipped parts of the sward. The harvested N derived from soil was reduced to a greater extent than that derived from fertilizer. At both N supplies, elevated [CO2] modified the allocation of the fertilizer N in the sward, in favour of the stubble and roots and significantly increased the recovery of fertilizer N in the soil macro-organic matter fractions. The increase of fertilizer N immobilization in the MOM was associated with a decline of fertilizer N uptake by the grass sward, which supported the hypothesis of a negative feedback of elevated [CO2] on the sward N yield and uptake. Similar and even more pronounced effects were observed for the native N mineralized in the soil. At N-, a greater part of the fertilizer N organized in the root phytomass resulted in an underestimation of N immobilized in dead roots and, in turn, an underestimation of N immobilization in the MOM. The 3 degreesC temperature increase alleviated the [CO2] effect throughout much of the N cycle, increasing; soil N mineralization, N derived from soil in the harvests, and the partitioning of the assimilated fertilizer N to shoots. In conclusion, at ambient temperature, the N cycle was slowed down under elevated [CO2], which restricted the increase in the aboveground production of the grass sward, and apparently contributed to the sequestration of carbon belowground. In contrast, a temperature increase under elevated [CO2] stimulated the soil nitrogen cycle, improved the N nutrition of the sward and restricted the magnitude of the soil C sequestration.