Agriculture Reference
In-Depth Information
Table 9.5. Mean total soil N content, relative nitrification rate, and net N
mineralization rate during the growing season in the A/Ap horizon of MCSE soils. a
System
Total N b
Relative
Nitriication c (%)
Net N Mineralization
Rate d (kg ha −1 season −1 )
(g kg soil −1 )
(Mg ha −1 )
Annual Cropping Systems
Conventional
1.12
3.58
79
99
No-till
1.21
3.63
76
113
Reduced Input
1.24
3.72
80
178
Biologically Based
1.17
3.51
79
163
Perennial Cropping Systems
Poplar
1.17
3.28
32
62
Alfalfa
1.35
4.05
75
192
Coniferous Forest
Na
na
60
na
Successional and Reference Communities
Early
1.30
3.90
42
90
Mown Grassland
2.48
5.95
28
27
Mid-successional
1.36
4.08
31
113
Deciduous Forest
2.05
5.33
72
137
a n  = 6 plots for all systems, except Mown Grassland ( n  = 4) and the Mid-successional, Coniferous, and Deciduous
Forest systems ( n  = 3). na = not available.
b Calculated from Syswerda et al. (2011).
c Percentage of net mineralizable N as nitrate at the end of in situ incubations averaged over 18 years (1993-2010),
covering six 3-year rotations of the annual cropping systems (Robertson et al. 1999).
d Determined by extrapolation of daily net N mineralization rates (modified from Robertson et al. 2000, Table 1) over
the growing season (April to October, 1989-1995).
harvest, which removes ~0.1 Mg ha −1 yr −1 from cropped systems (Table 9.2). The
proportion of total N in the A/Ap horizon that is mineralized each year ranges from
0.5% (Mown Grassland never tilled system) to 4.8% (Reduced Input system; calcu-
lated from Table 9.5), which is consistent with values reported for cropping systems
elsewhere in the U.S. North Central Region (Cassman et al. 2002).
Temporal and spatial patterns in rates of net N mineralization with respect
to crop N requirements largely determine the degree of N synchrony within the
system (Robertson 1997). Where N mineralization is asynchronous with plant
growth—as might happen, for example, when N mineralization occurs largely
in the spring prior to crop growth or in the fall after plant senescence—the min-
eralized N will be susceptible to loss. Such asynchrony is unfortunately a nor-
mal situation for annual row-crop production in temperate climates. Managing
the system to maximize synchrony is an important strategy for conserving N in
cropping systems (e.g., McSwiney et al. 2010). Synthetic fertilizers owe their
effectiveness to adding a large pulse of available N to soil just as crops enter
their prime growth phase. Reproducing this pulse with biological management
is an extraordinary challenge, although evidence from the Reduced Input and
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