Watershed Scale Physically Based Water Flow, Sediment and Nutrient Dynamic Modeling System - Landscape Ecology for Sustainable Environment and Culture - page 148

Table 8.2 Mathematical expressions for soil nitrogen transformations

N pool N transformation equation

Fresh organic N (orgN frs )

d ð Dz orgN frs Þ
dt

¼ ON min j imb ON dec ON frs ; e þ ON frs ; s

Active organic N (orgN act )

d ð Dz orgN act Þ
dt

¼ ON dec ON trn ON min ON act ; e þ ON act ; s

Stable organic N (orgN sta )

d ð Dz orgN sta Þ
dt

¼ ON dec þ ON trn ON sta ; e þ ON sta ; s

Ammonium N (NH 4 )

d ð Dz NH 4 Þ
dt

¼ NH min NH nit j vol NH up R NH4 ; e þ NH s

Nitrate N (NO 3 )

d ð Dz NO 3 Þ
dt

¼ NH nit NO dnit NO up R NO3 ; f R NO3 ; e þ NO s

Table 8.3 Mathematical expressions for overland flow nitrogen transformations

N species N transformation equation

Particulate organic N (PON)

@ PON ov
@ t

¼ L ð PON ov Þ k hn PON ov

Dissolved organic N (DON) @ DON ov
@ t

¼ L ð DON ov Þþ k hn PON ov h DON ov k mn DON ov

Ammonium N (NH 4 ) @ NH 4 ov
@ t

¼ L ð NH 4 ov Þþ k mn DON ov h NH 4 ov k nit NH 4 ov R NH4 ; up

Nitrate N (NO 3 )

@ NO 3 ov
@ t

¼ L ð NO 3 ov Þþ k nit NH 4 ov h NO 3 ov R NO3 ; up

8.3.1.2 Nitrogen Transformations in Surface Runoff

The dominant nitrogen species in waters are dissolved inorganic nitrogen

, dissolved organic nitrogen (DON), particulate organic
nitrogen (PON) and particulate inorganic nitrogen (PIN) (Burt and Haycock 1993 ).
Models may consider particulate nitrogen as a single variable, or, alternately,
represent from one to many particle types or fractions. In NSM, dominant nitrogen
transformation processes in surface runoff are simulated for PON, DON,
NH 4 ; and NO 3 . Transformation processes include mineralization of DON to
NH 4 , nitrification of NH 4 ; to ; NO 3 , plant uptake of NH 4 ; and ; NO 3 , soil mass
transfer of NH 4 ; NO 3 , and DON, sediment sorption of NH 4 , and hydrolysis of
PON to DON. The mass balance equations used to simulate the nitrogen cycle in
surface runoff are summarized in Table 8.3 .

NH 4 ; NO 2 ; NO 3

8.3.2 Phosphorus Cycle

The phosphorus cycle differs from the other major biogeochemical cycles in that it
does not include a gas phase. The largest reservoir of phosphorus is in sedimentary
rock. When it rains, phosphates are removed from the rocks via weathering and are
distributed throughout both soils and water. Plants take up the phosphate ions from
the soil. Phosphorus is not highly soluble, binding tightly to molecules in
soil. Therefore it mainly reaches waters by traveling with runoff soil particles.