Agriculture Reference
In-Depth Information
Table 9.2. Nitrogen (kg N ha
−1
yr
−1
) contained in crop components and removed
in harvest of MCSE systems.
a
Component
MCSE System
Annual Crops
b
Perennial Crops
Successional
Conventional
No-Till
Reduced
Input
Biologically
Based
Alfalfa
c
Poplar
d
Early
Successional
e
Corn
Grain
84
88
80
61
Stover
22
f
26
f
25
f
18
f
Soybean
Grain
138
163
151
148
Stover
21
f
21
f
19
f
18
f
Wheat
Grain
61
66
57
34
Straw
9
10
10
f
6
f
Harvested N
g
97
109
96
81
215
11
41
a
Mean,
n
= 6 replicated plots.
b
Crop N in different components calculated as biomass × N content. Values are the average annual N amount in each
crop component over 18 years (1993-2010), covering six 3-year rotations of the annual cropping systems.
c
Annual N in alfalfa calculated as the sum of N harvested annually and averaged over 1993-2010 (15 years with crop
present).
d
N removed in aboveground woody biomass at clear-cut harvest of 1999 (107 kg N ha-1) divided by the number of
growth years (10; 1989-1998).
e
N in aboveground biomass volatilized by burning and calculated from aggregated postfrost aboveground biomass
and surface litter.
f
fNot harvested (left on soil surface).
g
N removed at harvest of annual crops (corn-soybean-wheat) determined as the annual sums of N removed in grain
(corn, soybean, and wheat) plus harvested straw for wheat as noted, averaged over 18 years (1993-2010) covering
six 3-year rotations. N removed in wheat straw is based on the straw N content and the ratio of grain to aboveground
biomass at peak biomass.
Nitrogen Fertilizers
In 1900 agriculture used very little inorganic N fertilizer, with less than 0.5 Tg of
N applied to crops worldwide, mainly as nitrate from Chile and as ammonium sul-
fate derived from coke-oven gas. The Haber-Bosch process—the production of
ammonia from its constituent elements N
2
and H
2
—was industrialized by 1913 in
Germany (Leigh 2004). Global ammonia production was ~2.4 Tg yr
−1
N in 1946 and
today is ~133 Tg yr
−1
, most of which is used to make N fertilizers (Kramer 2004).
Synthetic or industrially fixed N differs from organic N sources in that most
synthetic N is immediately available for plant uptake, whereas most organic N
must first be mineralized to NH
4
+
and then nitrified to NO
3
−
(see Internal Nitrogen
Transformations below) before it is available to plants (Robertson and Vitousek
2009). In the United States, synthetic fertilizer accounts for ~60% of the total
N added to agricultural land; legume N fixation and manure make up most of
