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SCUAF
A Model to Estimate Soil Changes Under Agriculture, Agroforestry and Forestry

CYCLES worksheet

This Input worksheet sets values for processes in the Soil-Plant cycles for Carbon, Nitrogen and Phosphorus.


(A) Carbon cycle

The soil carbon cycle is similar to that in many comparable models, based on principles set out in the classic study of Nye and Greenland (1960), modified to take account of the different fractions of soil organic matter. Three such fractions are commonly recognised (with some variations in nomenclature): active, labile and stable (Young, 1997 p.100).

In SCUAF, the active fraction, the carbon that is present in the bodies of soil organisms, is not modelled as a carbon store. Instead, it is treated as two processes: the loss by oxidation of carbon (and nitrogen) during conversion from litter to humus; and the annual loss of these elements from humus. Hence, the soil carbon present at the end of each year is taken to be made up of two fractions:

Changes in soil carbon are then estimated as follows (see figure below):


The soil carbon cycle (Young 1997).


Nutrient cycling

The cycles of nitrogen and phosphorus are based on the stores and flows conventionally employed in soil science, agriculture and forestry, adapted to include the distinctive feature of agroforestry systems, the presence of tree and crop components. There are four nutrient stores in the plant–soil system:

The nutrient flows into the plant–soil system, out of it, and within it are detailed in the figure below. In addition to the Gains and Losses listed below, there are:

Internal flows: (1) uptake from soil to plants and (2) uptake of soil-mineral nutrients by plants.

Returns (plant to soil): (1) mineralisation of litter, (2) humification of litter, (3) throughfall and stemflow, (4) ash from burning.

Mineralisation of humus: Conversion of nutrients from soil-organic fractions to mineral form.

The gains, losses and internal flows apply differently to the various stores of nutrients. For example, fertilizers are input directly into the soil mineral fraction, and mineralisation of humus is treated as a transfer (with partial losses) from the plants to the soil organic fraction.

The inputs to SCUAF give estimates of the rates of all of these processes, or allow them to be calculated. The model applies rates to calculate annual changes in the four plant and soil stores of nitrogen and phosphorus.

There is a link between the estimates of erosion, soil carbon, and nutrient cycling. On sloping land, removal of carbon, nitrogen and phosphorus in eroded soil is a major source of loss of these elements from the system. Where erosion of soil exceeds 10 t/ha, this cause of loss of elements frequently dominates the nutrient cycle.


The nutrient cycle (Young 1997):

 


(B) Gains

The following nutrient gains are considered in SCUAF (* indicates variables entered in other worksheets):

(C) Losses

The following nutrient losses are considered in SCUAF (* indicates variables entered in other worksheets):


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