Agricultural Practices

The SOMDY model can simulate the effects of agricultural practices in three different ways:

1. Organic matter amendment can be taken into account by adding new chemically defined organic matter (EOM) to the system, and distribute it in the proper model layer by a simple partition according to the different organic components of the EOM.

2. Irrigation is handled by direct addition of water to the water-balance submodel.

3. The impact of agronomic treatments is represented by a linear enhancing effect on the physical degradation of soil aggregates, following the soil management operations. The following practices are considered:

(a) Mouldboard plowing affects macro-aggregates stability producing a large release of meso-aggregates

(b) Weeding deteriorates macro-aggregates producing some release of meso-aggregates

(c) Rotary hoeing (tillage) enhances the degradation of both macro- and meso-aggregates, thus releasing meso- and micro-aggregates, respectively

11.3 Calibration

Two different steps were performed to calibrate the model in terms of parameters and functional responses to different environmental conditions. First, a simplified model application without activation of the physical aggregation module was run to compare the simulated data of organic matter mineralization rates with those of some experimental observations of decaying organic matter under controlled conditions. This procedure was used to define the optimal mineralization parameters (i.e., the potential maximal rates) and to assess the chemical protection factors, by best fitting simulated results with those observed in NMR spectra.

O 250

O 250

Fig. 11.3 Example of fitting between real and simulated data of CO2 emissions by mineralization processes of soil organic matter. The simulation refers to one summer week without application of agricultural practices time

Fig. 11.3 Example of fitting between real and simulated data of CO2 emissions by mineralization processes of soil organic matter. The simulation refers to one summer week without application of agricultural practices

An example of model performance to simulate changes of organic matter during decomposition in such calibration exercise is reported in Fig. 11.2.

Second, in order to assess the effects of different limiting factors (water, temperature, physical protection), the model was run at hourly time step, comparing simulated CO2 emissions with field measurements (see Chap. 9 for details). The model parameters were assessed by an iterative procedure of linear programming to optimize the fitting between simulated and observed values. An example of results is presented in Fig. 11.3 showing CO2 emissions for a period of four summer weeks in an irrigated field at the experimental MESCOSAGR site of Torre Lama, near Napoli (see Chap. 3 for details).

time

11.4 Simulations

The SOMDY model can be used to simulate soil organic matter dynamics under both theoretical and real scenarios. The model can be run to demonstrate specific processes and effects related to different environmental conditions and agricultural management options.

Figure 11.4 shows an example of a theoretical sequence of applications of different mechanical practices and their average impacts on physical aggregation state and soil carbon loss. It is evident that the occurrence of a plowing or weeding intervention affects, at different intensities, the physical soil structure. This happens because the destruction of Macro aggregates liberates OM and stimulates degradation processes (see Fig. 11.1), thereby reducing the physical protection of organic matter. Such impacts are much stronger in the case of rotary hoeing, and repeated tillage operations end up by completely degrading soil physical structure. As a

Fig. 11.4 Theoretical demonstration of dynamic behavior for SOMDY model as related to the effects of different agricultural practices on soil organic matter aggregation and associated CO2 emissions (P plowing; W: weeding; R rotary hoeing)

consequence of a reduction in soil aggregation stability, organic matter is exposed to greater biotic mineralization and larger CO2 flushes are emitted from soil.

The use of real input data in a simulation exercise allows the reproduction of complex scenarios of agriculture management. The panel of Fig. 11.5 reports the environmental conditions in the year 2007 at the Napoli experimental station (Torre Lama) of the Mescosagr project, for a maize field with traditional management. The model used inputs such as climatic conditions, precipitation, irrigation, and temperature data to calculate the soil-water balance. Further inputs were the initial SOM chemical composition and its physical aggregation state. During a model run, inputs of agricultural practices were applied at the corresponding dates. The result shows the dynamics of physical aggregation states and CO2 emissions in comparison with real field measurements.

In the case of physical aggregation, the system seems relatively stable during the observation period, despite the oscillations due to temporary impacts of mechanical practices. On the other hand, the model well reproduced CO2 emissions during the warmer period, whereas the simulated data were slightly overestimated in the colder season.

— Rain Irrigation

soil water (SWAT simulations)

o Average Temperature

— Rain Irrigation

soil water (SWAT simulations)

o Average Temperature

T 100

Fig. 11.5 Yearly simulation of SOMDY model with climatic data input (top), soil organic matter aggregation and impacts of agricultural practices (middle - P: plowing; W weeding; R: rotary hoeing) and CO2 emissions (bottom). See text for details day

Fig. 11.5 Yearly simulation of SOMDY model with climatic data input (top), soil organic matter aggregation and impacts of agricultural practices (middle - P: plowing; W weeding; R: rotary hoeing) and CO2 emissions (bottom). See text for details

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