Plant-Based Biogas Production for Improved Nutrient Management ...

Hansen et al. (2005) reported that the residual effect of a harvested
clover-grass ley was larger following first-year ley than following secondyear
ley, but the opposite was found by Granstedt & L-Baeckström (2000).
Hansen et al. (2005) attributed the reduced effect to organic N being easier
to mineralise if formed more recently. This was indeed also observed by
Granstedt & L-Baeckström (2000), as the third-year ley had a substantially
lower first-year residual N effect than the second-year ley although a larger
amount of N was incorporated with the third-year ley. The explanation was
not only to be found in a lower N concentration due to reduced clover
content, but also to the fact that the third-year ley had a humification
coefficient of 0.4, in comparison with 0.35 for harvested first- and secondyear
leys. Similarly, Granstedt (1995) was able to predict the first-year
apparent N mineralisation from different one-year green manure crops by
using a humification coefficient of 0.35.
In conclusion, factors of importance for residual N effects include:
o species
o N-concentration
o time of incorporation
o harvested ley or green manure ley
o age of ley
1.2.3 Calculation of N use efficiency, residual effect and fertiliser value
Nutrient efficiency can be expressed in several ways depending on the main
focus of the study (Weih et al., 2011). For practical farming or advisory
service organisations, mineralisation of crop residues needs to be translated
into some expression of fertiliser value. The literature contains a mix of
different ways to evaluate fertiliser value. For example, Schröder et al. (1997)
evaluated the first-year fertiliser value of cover crops by their effect on: (1)
economic optimum N rates; (2) marketable yield; and (3) N yield when no
mineral fertiliser N was supplied. In a split-plot experiment with cover crops
and a control without cover crop as main plots, five N fertiliser levels
(including a 0-level) were applied to the following crop in sub-plots. This is
an unusually ambitious way of studying residual N effect, but it showed that
the N fertiliser value was lower when evaluated in terms of economic
optimum N rates and highest in terms of effect on N yield when no mineral
fertiliser was supplied.
A more common way of studying residual N effects in field experiments
is to compare yield level and/or N uptake between the pre-crop or cover
crop in question and a control, which is often a cereal without a cover crop
(e.g. Wallgren & Linden, 1994). The comparison is typically made without
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