Agriculture Reference
In-Depth Information
Internal Nitrogen Transformations
Microbial activity mediates the major N transformations within the ecosystem,
including mineralization, immobilization, nitrification, and denitrification (Fig. 9.1).
Mineralization and Immobilization
Nitrogen mineralization is the conversion of organic N to soluble inorganic forms
that can be taken up by plants and other microbes (Robertson and Groffman 2015).
If plant residues are rich in N, microbes release inorganic N in excess of their needs
to the soil solution. Nitrogen immobilization is the uptake of N by soil microbes—
the reverse of mineralization. If plant residues are low in N, microbes scavenge
additional soluble N from their surroundings, immobilizing it in microbial biomass.
In soil, both processes occur simultaneously, and the balance is known as net min-
eralization. When net mineralization is positive, inorganic N is added to the soil
solution; when net mineralization is negative, inorganic N is removed from the soil
solution.
At the MCSE, net N mineralization over the growing season (April to October)
is greater in the Reduced Input and Biologically Based systems (178 and 163 kg N
ha
−1
) than in the Conventional and No-till systems (99 and 113 kg N ha
−1
), respec-
tively (Table 9.5; Robertson et al. 2000). Higher rates reflect the build-up of min-
eralizable N and soil organic matter (Syswerda et al. 2011) in these cover-cropped
systems and, in particular, the importance of leguminous cover crops in providing
inorganic N to the subsequent primary crop (Sánchez et al. 2001). The cover crop
effect is also seen in the N content of soils—inorganic N concentrations are about
as high in the Reduced Input and Biologically Based systems as they are in the fully
fertilized Conventional and No-till systems (Fig. 9.4). Moreover, N appears to be
more available, especially as nitrate, from June through August (Fig. 9.5), the por-
tion of the growing season when plant uptake is greatest.
Among the perennial crops, net N mineralization between April and October
(Table 9.5) is slightly higher in Alfalfa (192 kg N ha
−1
) than in the annual cropping
systems with legume cover crops, but considerably lower in Poplar (62 kg N ha
−1
).
Low N mineralization in Poplar may reflect this system's lower quality (higher
C:N ratio) leaf litter, and is associated with low soil inorganic N pools (Figs. 9.4
and 9.5). Among the successional systems, net N mineralization generally fol-
lowed the pattern Deciduous Forest > Mid-successional > Early Successional
> Mown Grassland (never tilled) (Table 9.5). Net N immobilization appears to
only occur in the Mown Grassland (never tilled) system and only during July and
August (Fig. 9.6).
Total soil N contents of the A/Ap horizon of the Mown Grassland (never tilled)
system (5.95 Mg N ha
−1
) and the late successional Deciduous Forest (5.33 Mg N
ha
−1
) represent indigenous (precultivation) soil N, and are about 50-65% higher
than total soil N in the annual cropping systems (~3.6 Mg N ha
−1
; Table 9.5). This
difference reflects the loss of soil organic N due to a century or more of cultiva-
tion, which accelerated the mineralization of soil organic matter (Paul et al. 2015,
Chapter 5 in this volume) and subsequent N loss by various pathways—including
