Combining field incubation with nitrogen-15 labelling to examine nitrogen transformations in low to high intensity grassland management systems

The N-15 isotope dilution method was combined with a field incubation technique to provide simultaneous measurements of gross and net rates of N turnover in three long-term swards: unfertilized (Z) or receiving N either from N fixation as clover (C), or as 200 kg fertilizer N ha(-1) year(-1) (F). Uniform N enrich ment of soil microplots was achieved with a multi-point soil injector to measure mineralization/immobilization turnover and nitrification over a 4-day incubation. Net rates of mineralization ranged between 0.6 and 2.9 mu g N g(-1) day(-1) and in all three treatments were approximately half the gross rates. Nitrification rates (gross) were between 1.0 and 1.6 mu g N g(-1) day(-1). In the F treatment, the turnover of NH4+-N and NO3--N pools was on a 2- and 4-day cycle, respectively, whereas in the N-limited treatments (C and Z) turnover rates were faster, with the NO3--N pools turning over twice as fast as the NH4+-N pools. Therefore, available N was recycled more efficiently in the C and Z treatments, whereas in the F treatment a higher N pool size was maintained which would be more vulnerable to leakage. A large proportion of the added N-15 was recovered in the soil microbial biomass (SMB), which represented a 4-5 times larger sink for N than the plant biomass. Although the C treatment had a significantly lower SMB than the grass-only treatments, there were no differences in microbial activity. Gross rates of nitrification increased along the gradient of N input intensity (i.e. Z<C<F), and the addition of a nitrification inhibitor (C2H2) tended to increase microbial immobilization, but did not influence plant N uptake. In this study, the value of combining different techniques to verify net: rates was demonstrated and the improved methodology for N-15 labelling of soil enabled measurements to be obtained from relatively undisturbed soil under natural field conditions.