Agriculture Reference
In-Depth Information
Biological
fixation
Atmospheric
inputs
Biomass
store
(plants, animals)
Gaseous
losses
(microbes)
Soil
Animal
excreta
Uptake
Death
Compost,
biosolids
Immobilization
Litter and manure
Decomposition
Inorganic
store
Mineralization
Organic
store
Soil
Humus
Losses
Fertilizers
Weathering Leaching
A generalized nutrient cycle.
Figure 4.1
Nutrient Cycling
4.2
Nutrients are continually cycling in the soil-plant system as plants and other or-
ganisms go through repeated cycles of growth, death, and decay. In this dynamic
process, nutrients flow among these three major stores: an inorganic store , chiefly
the soil, a biomass store , comprising organisms living above and below the ground,
and an organic store , on and in the soil, formed by the dead residues and excreta
of living organisms.
The general nutrient cycle is illustrated in figure 4.1. The relative size of the
nutrient stores differs for each element, as does the importance of the pathways
for inputs and outputs, and the partitioning of an element between “soil” and
“nonsoil.” Within each store, nutrients can undergo transformations, changing
from one form (e.g., NH 4 ) to another (NO 3 ).
Differences also exist in the way the elements are distributed in the soil. El-
ements released at depth by mineral weathering are absorbed by plant roots and
transported to the shoots, to be deposited finally on the soil surface in litter or
excreta. Accessions from the atmosphere add to the surface store. N always accu-
mulates in the organic-rich A horizon, and the total N content declines with depth,
as shown in figure 4.2. Exchangeable NH 4 ions are retained in the A horizon,
but NO 3 ions can be leached and accumulate at depth. Although P distribution
 
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