Combining field incubation with nitrogen-15 labelling to examine nitrogen transformations in low to high intensity grassland management systems |
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| Authors: | D J Hatch S C Jarvis R J Parkinson R D Lovell |
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| Institution: | (1) Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon, EX20 2SB, UK e-mail: DAVID.HATCH@BBSRC.ac.uk Tel.: +44-1837-82558, GB;(2) Seale-Hayne Faculty of Agriculture, Food and Land Use, University of Plymouth, Newton Abbot, TQ12 6NQ, UK, GB |
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| Abstract: | The 15N 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 enrichment 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 μg 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 μg
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 15N 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 15N labelling of soil enabled measurements to be obtained from relatively undisturbed soil under natural field conditions.
Received: 25 May 1999 |
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| Keywords: | Nitrogen-15 mean pool dilution Mineralization Immobilization Nitrification Field incubation |
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