Agriculture Reference
In-Depth Information
large, all the N is incorporated into microbial tissue and some mineral N may also
be taken from the surrounding soil—net immobilization then occurs. Figure 3.5
shows that the critical C-to-N ratio for the tipping point between net mineraliza-
tion and net immobilization is about 25.
As table 3.2 shows, materials such as cereal straw or vine prunings, which
have high C-to-N ratios, are likely to induce net N immobilization in the soil.
Low C-to-N ratio materials such as legume residues from a mid-row cover crop,
for example, should promote net N mineralization. Although net mineraliza-
tion can occur when well-humified organic matter decomposes, because its
decomposition is slow, the release of mineral N is small compared with that
released from fresh legume residues. However, some naturally fertile soils with
more than 5% organic C and a low C-to-N ratio can mineralize substantial
amounts of N that may predispose to too much N uptake and excess vigor in
grapevines.
CO
2
Metabolized C
compounds in the
substrate
Microbial cell
substance
Net immobilization if
C:N ratio >25
Figure 3.5
How the
carbon-to-nitrogen ratio affects the
balance between net mineralization
and immobilization of nitrogen in the
soil. (Redrawn from White, 2003.)
N in the substrate
Net mineralization if
C:N ratio <25
NH
+
Table 3.2
Expected Net Nitrogen Mineralization from Soil Organic Matter or
Returned Plant Residues
Net mineralization expected in
one year (kg N/ha)
Organic material
C-to-N ratio
Well-humified soil organic
matter
10-15
Small (10-15)
Legume residues (clover,
medics, peas, beans)
15-25
Significant (25-50)
Grape leaves
30
<5
Cereal straw
40-120
Nil
Winter prunings
100
Nil
