Agriculture Reference
In-Depth Information
Table 9.6. Agronomic N inputs, N outputs and surplus (difference) for corn-
soybean-wheat rotations in the U.S. Midwest
a
and the annual MCSE systems.
b
Agronomic N Inputs,
Outputs, and Balance
U.S. Midwest
MCSE System
Conventional
No-Till
Reduced
Input
Biologically
Based
Agronomic Inputs (kg N ha
−1
yr
−1
)
Fertilizer
93
73
73
25
0
BNF
62
31
36
43
47
Total
155
104
109
68
47
Agronomic Outputs (kg N ha
−1
yr
−1
)
Harvest
145
97
109
96
81
Total
145
97
109
96
81
Surplus
+10
+7
0
-
28
-
34
a
Values for the U.S. Midwest from Vitousek et al. (2009) for a tile-drained corn-soybean rotation in Illinois.
b
Values for MCSE systems from Tables 9.2, 9.3, and 9.4, annualized to crop rotation cycle.
Worth noting is that the overall balance for each system is the sum of balances
for individual crops within each 3-year rotation, and that year-to-year balances dif-
fer by crop. In the Conventional and No-till systems, for example, only wheat has
an agronomic N budget in approximate balance; corn has a significant N surplus,
and soybean a significant N deficit. On average for 1993-2010, wheat in these
systems had a 3 kg N excess (76 kg N fertilizer less 73 kg N harvest), corn had a
55 kg N ha
−1
yr
−1
excess (141 kg N from fertilizer inputs less 86 kg N harvested),
and soybean had a 51 kg N deficit (100 kg N from BNF less 151 kg N harvested). N
balance over the entire rotation, then, is the result of soybean's deficit being made
up by corn's excess.
That leaching and presumably denitrification losses are significant in all sys-
tems, but especially in the annual cropping systems (see the prior section above),
suggests that overall N budgets are substantially out of balance: in the annual crop-
ping systems, more N appears to be lost via harvest, leaching, and denitrification
than is being gained via N deposition, BNF, and N fertilizer. This suggests either
that BNF has been underestimated, and that (1) more N is being mineralized from
soil organic matter than is being immobilized annually; (2) N leaching losses have
been overestimated; or (3) there is an unrecognized N source such as atmospheric
NH
3
adsorption. This apparent imbalance is a major knowledge gap that deserves
future attention.
Mitigation of Excess Nitrogen
The main management challenge for mitigating reactive N in the environment is
to maximize the efficiency with which N is used in agricultural systems (CAST
2011)—for field crops, this means implementing practices that minimize fertilizer
