Overview of Plant-Soil Transfers
The land-use input worksheet is shown below.
There are two plant components, called Trees and Crops. Each component is divided into four plant parts: leaf (herbaceous matter); fruit (reproductive parts); wood (above-ground woody parts); and root (below-ground parts).
The Land-Use System is specified as a sequence of Periods (maximum 5), each lasting one or more years (maximum 100). After completion of the last period specified, the first period returns automatically.
For each period, the proportions of trees and crops are given as fractions, e.g. 0.4 (meaning 40%). Thus forestry will have Trees = 1.0, Crops = 0.0, and agriculture will have the reverse. Agroforesty systems consist of some combination of Trees and Crops.
If a Period is 1 Year only, it may be specified as a Cutyear. A Cutyear is a Year in which the Trees are felled, pollarded, or otherwise treated differently. To specify a cutyear enter a 1 under the Cut Year column, otherwise enter a 0. If the cut biomass will be burnt enter a 1 in the Burn column, otherwise enter a 0. Burning can only occur in cutyears.
Soil Conservation Practices can be applied in any or all of the Periods. The user indicates whether conservation practices will be applied in a given period by entering a 1 in the Cons. Prac column (enter a 0 otherwise). The actual effect of these practices, in terms of the extent to which they reduce erosion, are entered in the Erosion section of the Soil worksheet.
Note: The “Number of periods” cell at the top left of the land-use input worksheet is important, it determines how many periods will be read by SCUAF. If all five periods are filled up in the tables, but there is a 2 in the “number of periods” cell, only the first two periods will be simulated. In this version, a maximum of five periods and 100 years are allowed. This is done to keep the spreadsheet interface within a manageable size.
Additions refer to material brought into the Plant-Soil system from outside. Additions may be Organic Additions (mulch, compost, manure) or Fertilizer.
Additions may differ between Periods in the Land Use system. They may be applied to the whole area, or (as will often be the case) to the area under Crops only.
Note that if no Fertilizer is applied, then under the default values there is usually a substantial Nitrogen deficiency, causing a reduction in plant growth.
Organic Additions are entered as kg Dry Matter/ha per year, where “per hectare” refers to a hectare of the total land use system. The composition of organic additions is expressed in terms of the fraction of organic matter that is carbon and the carbon to nitrogen (C:N) and carbon to phosphorus (C:P) ratios.
Inorganic Additions (fertiliser) are entered as kg/ha per year, where “per hectare” refers to a hectare of the total land use system. The composition of inorganic additions is expressed as the ratio of N and P per kg applied.
Net Primary Production (NPP) can be allocated to different pools representing:
Note that, although NPP is measured in kg/ha/yr, the values used for this allocation are entered as fractions of the total production in a given year. Hence, for any given period and plant part, Harvest + Losses + Prunings + Retained must not exceed 1.0. If the sum of these pools is < 1.0 the remaining dry matter is allocated to litter:
LITTER = 1.0 - HARVEST - LOSSES - PRUNINGS - RETAINED.
Removals consist of Harvest and Losses. In the simulation, Harvest and Losses are summed, and treated as removals from the system. The reason to enter them separately is so that there can be an Output of Harvest, showing what are the useful plant parts harvested. Harvest in this section represents Annual Harvest. Cutyear Harvest is explained below.
Harvest refers to biomass that is removed from the Land Use system for human or animal use (grain, fruit, fodder, fuelwood, etc.). Below-ground Parts of Plants which are harvested (e.g. tubers) should be treated as Fruit, not Root. Harvest is entered as a fraction of the total NPP for the year.
Losses refer to Biomass that is lost from the Land Use system but is not Harvest. Losses may refer to burning of the NPP produced during the year. Burning of Standing Biomass can only occur during cutyears and is entered as explained above.The fraction of Nutrients in Annual Losses retained (default values 0.1 for Nitrogen and 0.75 for Phosphorus) also apply to cutyear burning. Losses are entered as fractions of the NPP for the year.
The term 'Prunings' is used in a restricted sense in SCUAF, to refer only to biomass that is cut from the Plant and applied to the Soil. It does not refer to the action of pruning, which can often lead to harvest (e.g. cutting branches and removing them for animal fodder). Prunings are retained on the Soil as Litter. There may also be tree litter transferred (e.g. by wind, or human agency) to Soil Under Crops. This can be treated as prunings. Prunnings and transfers are enetred as fractions of the NPP for the year.
(F) Biomass Retained and Litter
Biomass Retained refers to the fraction of NPP that remains at the end of the year as standing biomass. This biomass will be added up to the standing biomass accumulated in previous years.
As well as the annual harvest, there may be an additional harvest in a cutyear, a year in which the tree component is felled, coppiced, pollarded or otherwise treated differently. The annual harvest will normally include crop fruit, and sometimes tree fruit or tree leaf (as fodder); the additional cutyear harvest will often include tree wood.
A Period may be specified as a Cutyear only if it is of 1 year length.
For land-use systems which begin with clearance of natural vegetation, Year 1 can be specified as a Cutyear and the initial standing biomass can be entered in the appropriate cells (in the Plants worksheet), this is allowed for trees only. Thus, land use systems based on clearance of natural vegetation can be modelled by specifying Initial Standing Biomass. The Litter and Nutrients resulting from vegetation clearance will be added to the Soil in Year 2.
For a cutyear, the user must enter the fractions of standing biomass that are harvested, lost or retained. Any additional biomass will become litter and contribute nutrients to the soil as it decomposes. Here is an example:
In this example there are 3 periods. In Period 1 the existing biomass is cut, some wood is harvested, and the remaining biomass becomes litter. This litter will provide nutrients for Period 2. In Period 2 the entire area is planted to a crop for 10 years. In period 3 the entire area is planted to a tree for 10 years. Thus, this represents a shifting cultivation system starting with forest clearing. If the simulation is run for longer than 21 years, the cycle will start again from period 1, burning the biomass that has accumulated to the end of Period 3.
The cutyear rules for this system are described by four sets of values (indicated by bracketed letters in the figure):
(a) The desired number of periods is entered at the top right of the Land-use worksheet.
(b) The cutyear is indicated by entering a 1 in the “Cut Year” column (note the length of this period is one year, as required for cutyears). The fact that the initial biomass is not burnt is indicated by a zero in the “Burn” column.
(c) Eighty percent (0.8 of the total) of existing wood is harvested.
(d) The initial biomass is set (in the Plants worksheet) for the tree component in terms of kg of DM per ha. This last step is not necessary if the cutyear does not represent clearance of existing forest but occurs in the middle of a simulation when the tree area is harvested.
In this example actual cutyear harvest will amount to 32,000 kg of DM per ha (0.8 x 40,000). The remaining biomass (20,000 kg of leaf, 8,000 kg of wood and 24,000 kg of root) will become litter and decompose, thus releasing nutrients to the soil.
Burning
In a cutyear, biomass that is not harvested can be burnt, this will release mineral nutrients. The fraction of biomass burnt is specified in the cutyear "Burnt/Lost" row. Only a certain amount of nutrients will be retained after burning (defaults are 0.75 for P and 0.1 for N).
This is the same shifting-cultivation system described in the cut year example above, but now initial biomass is burnt rather than harvested. There are two modifications relative to the harvest example above:
(b) Burning is enabled by entering a 1 in the “Burn” column.
(c) The amount of biomass to be burned is entered in the Burnt/Lost row in the "Cut Year" box. In this case the wood harvest has been set to zero and the entire leaf and wood biomass is burnt (indicated by the 1.00 entries). Note that root losses are set to zero, because roots do not burn. As in the case above, roots will die and become underground litter that will contribute nutrients to the soil.
It is important to note that, to enable burning, it is not enough to enter a 1 in the “Burn” column, it is also necessary to enter the fraction of biomass to be burnt as a loss.
Overview of Plant-Soil Transfers
The model calculates which plant parts are added to the soil. This is obtained by taking plant growth, subtracting the amount retained as standing biomass, annual harvest, cutyear harvest, and losses, and adding additions (see figure below). The resulting value, the addition of plant residues to the soil, is transferred to the soil section of the model. Using the specified composition of plant parts, together with conversion losses (such as oxidation of carbon), the inputs of carbon, nitrogen and phosphorus to the soil are calculated.
The land use and plant section of SCUAF (Young and Muraya, 1991):