The biological basis for nitrogen management in agroecosystems - Microbial Ecology in Sustainable Agroecosystems - page 118

6.7 Temporal synchrony
In annual cropping systems, soil nitrogen uptake primarily occurs during a narrow win-
dow of peak crop growth that lasts only weeks ( Figure 6.2 ). The period of maximum N
demand is thus short and intense, but seasonal N availability in temperate cropping sys-
tems can fluctuate dramatically depending on management and environmental condi-
tions. Herein lies the problem: The rates of internal inorganic N production, as well as
inputs of fertilizer N, are often highest when plant demand is low. Both exogenous inputs
and N mineralization-immobilization dynamics play key roles in soil N dynamics, but
both may be strongly influenced by factors unrelated to those driving plant N demand.
The problem of temporal synchrony is thus a complex one that involves the supply of inor-
ganic and organic nutrient sources and their relationships to environmental factors and
plant and soil management (Robertson and Groffman, 2007).
Depending on the quality of available organic matter, N mineralization rates are often
highest in the spring before, or in the summer after, crop N demand crests. Legume residues
and other organic matter with low C:N ratios and low lignin concentrations decompose
quickly in warm soils. The release of N from these materials typically occurs before plants
are able to use it. Organic materials with high C:N ratios and high lignin concentrations
break down more slowly and often immobilize inorganic N before there is a net mineraliza-
tion of N. The decomposition of these residues can deplete soil N when it is most needed
by crops only to release it once crop N demand wanes. Asynchrony can also be a problem
in the fall, particularly in temperate cropping systems. The incorporation of legume cover
crops into warm soils followed by a winter fallow can increase soil N concentrations when
there is no crop N demand (Crews and Peoples, 2005). A second wave of N mineralization
is common in the fall when crop residues have accumulated in the soil and temperatures
still promote microbial activity. Further, residual N fertilizer that has not been utilized by
plants or microbes may be lost in the fall. These losses may be exacerbated by fall tillage,
which remains a common practice in many regions where weather or soil type make tillage
in the spring difficult.

Improve timing of N
availability

Increase crop demand
for N

Asynchrony Excess N

Cover crop

N availability

Crop

Growing season Growing season

Figure 6.2 Temporal asynchrony between N availability and plant demand in temperate cropping
systems. Nitrogen availability often exceeds crop demand early in the growing season and fol-
lowing crop senescence or harvest. Management practices that affect the timing of N release from
fertilizer applications or N uptake by the crop, and that maximize N uptake during periods when
cash crop demand is low, will improve the synchrony of N and reduce N losses to the environment.