Agriculture Reference
In-Depth Information
(straw incorporation; yield potential) accounts for a variation in N t of
~500 kg N ha −1 , about half the spread observed in the catchment values.
The fraction of N mic in the soil is quickly decreasing after ploughing
grassland by ~200 kg N ha −1 to a value fairly stable over the next 30 years
(Fig. 2.3.2b). Increasing the fraction of microbial carbon (C mic /C org )inthe
simulation to values measured in soils could not explain the absolute height
of microbial N observed for grassland, N mic (G). To fit the value N mic (G),
the C mic fraction needed to be increased to 0.06, which is 70% higher than
before. The difference between model predictions and observations may be
caused by the distribution assumed for SOM and biomass. In arable soils, it
is assumed that SOM and biomass are distributed 80% in the top 25 cm
and 20% in the horizon below. Lavahun et al . (1996) found only minor
differences for the distribution of C mic in profiles of grassland and arable
soils. However, grassland contains a higher proportion of C org in the top
25 cm. This could be attributed to grass sod incorporated initially, and
the microbial biomass associated with it. Joergensen (2000) found that
considerable C mic in rhizosphere material and on grass roots was ignored
in standard CFE. Finally, the protective power of these silt-loam soils is
underestimated, and the model reaches the equilibrium content, N mic (A),
too quickly. Net mineralization is predicted as a quasi-linear process during
the first 30 years of simulation; however, with 150 kg N ha −1 year −1 ,itis
twice as high as estimated for continuous arable (65 kg N ha −1 year −1 ).
Conclusions
The static model predicts different equilibria and half-lives for the total
and active N fraction. Dynamic simulation proved to be a valuable tool
in detecting missing links: fitting a single parameter, we could quantify
microbial activity. The concept of 'microbial biomass' in the soil needs to
account for N mic and C mic associated with litter and crop debris, possibly by
regarding two different biomass fractions (bacteria and fungi) as explaining
flush and protection in ploughed-up grassland. Further improvements to
SUNDIAL should allow the model to be used explicitly to simulate ley-
arable rotations.
References
Bradbury, N.J., Whitmore, A.P., Hart, P.B.S. and Jenkinson, D.S. (1993)
Modelling the fate of nitrogen in crop and soil in the years following application
of 15 N-labelled fertilizer to winter wheat. Journal of Agricultural Science,
Cambridge 121, 363-379.
Brookes, P.C., Landman, A., Pruden, G. and Jenkinson, D.S. (1985) Chloroform
fumigation and the release of soil nitrogen: a rapid direct extraction method to
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