Agriculture Reference
In-Depth Information
The Agricultural Nitrogen Cycle
A comprehensive understanding of the agricultural N cycle requires knowledge of
the transfers (movement) of N into and out of the soil-plant system (e.g., precipita-
tion, gas exchange, leaching) as well as the exchange of N between compartments
within the system (e.g., biological assimilation and release). Transformations in the
chemical form of N, often mediated by microbes, determine the availability and
mobility of N in soils and water.
Nitrogen Inputs and Outputs
New N is added to cropping systems through biological N fixation, N deposition,
and the application of compost, manure, and synthetic fertilizers (Fig. 9.1). In any
cropping system, to remain sustainable, the amount of N added must replace the
N removed in crop yield and in environmental losses. The amount of N removed
with crop harvest varies widely, largely as a function of crop species and growing
conditions. In high-yielding annual grain systems, 100-270 kg N ha
−1
are typically
removed during harvest (Robertson 1997).
In annual cropping systems of the KBS LTER Main Cropping System Experiment
(MCSE; Table 9.1), average harvest N removals range from 34 kg N ha
−1
yr
−1
by winter
wheat (
Triticum aestivum
L.) in the Biologically Based system to 163 kg N ha
−1
yr
−1
by soybean in the No-till system (Table 9.2). Among perennial crops, Poplar (
Populus
spp.) harvest removes only 107 kg N ha
−1
after 10 years of growth, or an annual-
ized average of 10.7 kg N ha
−1
yr
−1
, whereas alfalfa harvest removes 215 kg N ha
−1
yr
−1
. The Early Successional system is not harvested but with each annual burn loses
[N
2
]
x
[N
2
], [N
2
O]
Plant-N
Harvest-N
s
ng
d
Animal-N
Soil organic-N
Microbial-N
li
Sediment-N
[N
2
O]
NO
3
-
n
NH
4
+
Fertilizer-N
Precipitation-N
ri
Groundwater-N
F
N
Figure 9.1
. Schematic representation of the major elements of the terrestrial nitrogen (N)
cycle. Processes mediated by soil microbes appear in bold, and gases appear in brackets.
Redrawn from Robertson and Groffman (2015) with permission from Elsevier Limited.
