Redes determinísticas y redes estadísticas

section 6.3.2. The schematic representation of the model organic intensive apple system and the equations in Stella® are presented in Appendix V. Information on the fertiliser programme of the model intensive system formed an input into the Overseer® nutrient budget programme, in order to predict the nutrient balances and leaching of N.

6.4.2.1 Energy ratio

The energy ratio of the model intensive system is 1.84. An energy ratio of more than one suggests that the system is efficient in converting input energy to fruit energy output.

The total energy input for the model intensive apple system is 64.00 GJ/ha (Table 6.14). Spraying is the most energy intensive of all the operations/inputs (Figure 6.3), followed by irrigation. Energy in diesel use (12.86 GJ/ha) is the most energy intensive input in the spraying operation, followed by embodied energy in fungicides (10.73 GJ/ha).

The yield of the intensive apple system is 54 t/ha. In energy terms, this comes to 117.72 GJ/ha (as per equation 4.2). Thus, the model intensive system is able to convert more energy into fruit, than the amount of energy being consumed in its production process.

Energy in mulching, mowing, lime application, human labour and aerial spraying and shelter-trimming contributed to less than 5% of the total energy use, respectively (Figure 6.3).

Table ‎6.14 Energy use and CO2 emissions from individual operation/processes of the model intensive organic apple system

Operation/processes Energy use

MJ/ha/yr CO2 emissions kg/ha/yr Mowing 2,183 175 Mulching 1,389 111 Harvesting 5,106 408

Providing tree support 528 42

Lime use 300 215

Human labour use 1,710 NA

Lubricant use 1,171 47 Shelter-trimming 733 59 Irrigating 10,234 614 Pruning/thinning 4,919 345 Fertiliser use 3,972 287 Spraying 27,130 2,170 Aerial spraying 686 43

Utility vehicle use 805 64

Transporting 3,141 251

CO2 equiv. N2O from Overseer® NA 122

CO2 from the soil NA 16398

Total 64,007 21,351

Spraying 41.8% Pruning/thining 7.6% Training 0.8% Harvesting 7.9% Irrigating 15.8% Human labour use

2.6% Mulching 2.1% Mowing 3.4% Lime use 0.5% Transporting 6.2% Fertiliser use 6.1% Utility vehicle use

1.2% Aerial spraying 1.1% Shelter-trimming 1.1% Lubricant use 1.8%

Figure 6.3 Energy use in the model intensive organic apple system.

6.4.2.2 CO2 ratio

The CO2 ratio of the model intensive organic apple system is 1.23. The CO2 emissions of the intensive organic apple system is 21.35 t/ha. Soil is the major source of CO2-equivalent emissions to the atmosphere, contributing to 77% of total emissions (16.52 t/ha) (Table 6.14). The CO2-equivalent emissions from the use of direct and embodied energy is 4.83 t/ha (23% of the total emissions). The total amount of CO2 sequestered in the apple trees is 26.31 t/ha/yr. Since the CO2- equivalent emissions are offset, the system is a net sink of CO2-equivalent emissions.

6.4.2.3 Changes in the soil carbon level

The model organic intensive apple system sequesters 980 kg C/ha/yr. From each tree in the model intensive system, 15.99 kg CO2 is returned to the soil, in the form of prunings and roots. There are 1250 trees per ha. Thus, 5446 kg C/ha is returned to the soil every year from the trees. However, 82% of the carbon entering the soil is returned to the atmosphere as CO2 in the same year, as described in the methodology. Thus, 4,466 kg C/ha is lost in the form of CO2 whilst the remainder 980 kg carbon can be expected to be sequestered in the orchard soil every year.

6.4.2.4 Nutrient balances

Overseer® estimates that of the primary nutrients, K is not supplied in adequate amounts (Table 6.15). This indicates that the K requirement of the crop has been met through depleting the soil‟s nutrient reserves. As such, K is being mined from the soil and therefore the nutrient management of the model organic intensive system is a threat to future yield and sutainability. For all other nutrients, the inputs are higher than the outputs: this suggests that these nutrients are adequately supplied and meet the crop‟s requirements.

Table ‎6.15 Nutrient balances of the model intensive organic apple system kg/ha/yr (from Overseer®)

N P K S Ca Mg

Inputs 73 26 40 75 281 65 Outputs 44 7 132 70 69 15

6.4.2.5 Leaching of N

The nutrient balances suggest that N is supplied in surplus amounts, than that which is exported in the output. Overseer® predicts leaching losses of about 8 kg N/ha/yr. In terms of leaching, this value comes to 5 mg N/L. This level is considered as a potential threat to aquatic ecosystem from eutrophication, although it is considered safe for human health.

Similar to the kiwifruit and semi-intensive organic apple systems, the grass-legume understorey was a significant factor for causing N surplus and N-leaching losses, in the intensive organic apple system. Overseer® also predicted that the absence of grass-legume understorey caused no N-leaching, at levels that can be a potential threat of eutrophication. However, the absence of this understorey meant that N would not be supplied in adequate amounts.

6.5 Model responses to changes in key model inputs

As described in the methodology, management scenario analyses and sensitivity analyses are undertaken, in order to identify how well the model for sustainability

assessment responds to changes in model inputs, in the two organic apple systems, respectively. These are described below.