Western North West Province

Historically, soil organic matter (SOM) levels in the North West Province region decline with the cultivation of natural veld (Figure 1).

Improved cropland management can increase SOM levels by sequestering atmospheric carbon into soil (SOC sequestration). This provides a practical and cost-effective means to reduce atmospheric CO₂ levels that can be implemented immediately and on a wide scale to mitigate the impacts of climate change. These impacts can be mitigated while improving soil quality and health, crop yields and ecosystem services in an environmentally friendly way. This creates a win–win situation for both farmers and the environment.

This page contains information on the potential of conservation agriculture (CA) to sequester soil organic carbon (SOC) for the western grain production region of the North West Province. The information is based on predictions obtained from numerical modelling. The range in climate and soil, with tillage, agronomic and grazing practices that were used to represent this region are also summarised, followed by notes on model calibration for the numerical modelling conducted.

For this study, the western grain production region of the North West Province includes the magisterial districts of Bloemhof, Delareyville, Ottosdal, Schweizer-Reneke, Vryburg and Wolmanransstad.

Carbon sequestration modelling

A detailed numerical modelling approach was followed that can simulate the effects of crop rotations and tillage with planned agronomic, forage and grazing practices to a level of detail that can predict how the SOC contents (and stock) change over time for region-specific climate, soil properties and farming systems.

Simulation of SOC sequestration was conducted for two periods, namely:

• Historic: SOC sequestration modelling assumed a baseline of natural veld conditions followed by 50 years simulation of conventional tillage (which includes mouldboard ploughing and discing). A decline in SOC levels was predicted due to the high disturbance and mixing of soil that occurred with mould board ploughing and deep discing.
• Conservation Agriculture (CA) farming systems: A 50-year simulation of CA farming systems was modelled with a baseline of the conventional tillage (with degraded soil carbon stocks). WinEPIC simulation results for 50 years of CA farming, per farming system, are summarised below.

Modelling scenarios of the simulated farming systems are summarised in Table 1.

Conservation Agriculture (CA) practices involve:

• Limit soil disturbance and mixing with minimum tillage (current CA scenario) and no tillage (future CA scenarios).
• Reduce incorporation of crop residue into soil with minimum- and no tillage.
• Controlled grazing of maize residue (current- and future CA scenarios).
• Include cover crops into the rotation cycle for the future CA scenarios.
• Soil fertility management that combines practices to improve total (inorganic and organic) soil nitrogen and phosphorus levels, and lime- and fertiliser application to:
  • Limit plant nutrient deficiencies to sustain biomass and yield production, and
  • Reduce imbalances in soil organic carbon and nitrogen to optimise SOC sequestration.
• Improved grazing practices of forage for future CA scenarios, such as high-density grazing for a limited period (1 day) to provide sufficient time for forage to recover.

Predicted soil carbon sequestration

For the purposes of this study, carbon stocks are defined as the soil organic carbon (SOC) contained in the upper 30 cm of soil and are expressed as tonnes carbon per hectare (tC/ha). The IPCC soil carbon analysis method specifies that a 30 cm depth must be used to report soil carbon sequestration. The predicted change in SOC stock and SOC sequestration potential following implementation of CA farming systems are shown in Figure 2 and Figure 3 respectively.  

Historic conventional tillage. Predicted changes in SOC and research results from du Toit et al., (1994) indicate initial high rates of decline in SOC stock during the first 5 years after cultivation of natural veld, followed by lower rates of decline (Figure 1). The high rates of SOC decline are the result of high oxidation (decomposition) rates of soil organic matter (SOM) and crop residue due to the extensive soil disturbance and mixing by fully inverting soil layers and incorporation of all/almost all crop residues into the plough layer.

Conventional tillage. Continuation of CT caused further deceases in SOC stocks (Figure 2), but at a lower rate than the preceding CT with mould board ploughing. Reduction in SOC stock with CT systems is supported by results from numerous international studies.

Current CA. A change from CT- to the current CA farming system caused an increase in SOC stocks. The increase in SOC stock can be ascribed to the effect of less soil disturbance and mixing, less residue incorporation into the soil with tillage and that more residue is left on the soil surface as a mulch layer. These changes resulted in a decrease in crop residue decomposition. The SOC sequestration potential is relatively low, both during the initial stage (0-6 years) and thereafter.

Future CA with initially recommended fertiliser rates. CA that includes cover crops and fertiliser applications only during planting led to higher SOC sequestration rates being achieved during the first 6 years compared to the current CA scenario. SOC stabilised (reached equilibrium) soon after the initial 6 years. SOC sequestration is limited because of nitrogen and (to a lesser extent) phosphorus deficits experienced by forage sorghum. Similar trends were reported by Nicoloso and Rice, (2021) and Causarano et al., (2008). SOC levels with current CA practices exceeded those of the recommended future CA after 10 years of simulation, mainly because the current CA cropping system (without forage sorghum) did not experience nutrient deficiencies.

Future CA based on Grain SA (2021) fertiliser guide for forage sorghum. SOC sequestration rates are about twice those predicted with the initially recommended fertiliser rates. Considerably higher rates of SOC sequestration were achieved than with current CA practices during the initial 6 years. For the scenario where a single top dressing of nitrogen (35 kgN/ha) was applied to forage sorghum, SOC reached an equilibrium after about 9 years. This can be ascribed to the levels of available nitrogen in the crop system. Modelling results indicate that a second top dressing of nitrogen (35 kgN/ha) is required for forage sorghum to sustain SOC sequestration in the long-term and for the farmer to fully realise the advantage thereof.

SOC sequestration that reached equilibrium after 6 and 10 years respectively is considerably earlier than the 20-30 years reported by the World Bank (2012), and the 40-60 years reported by West and Post (2002) from meta-analysis of 67 long-term experiments (as referenced by Lal, 2004). SOC sequestration reached quasi-equilibrium at 36 years for the future CA scenario with two nitrogen top dressings for forage sorghum, and equilibrium after 50 years.

After the initial 6 years of future CA cropping systems, SOC sequestration with two top dressings of nitrogen is lower than the mean rate of 0.20 t/ha/yr determined from a meta-analysis conducted by the World Bank (2012) for areas in Africa that receives <500 mm/yr precipitation. The sequestration potential for the current CA after 6 years compares with the lower range of 0.05 tC/ha/yr determined from the meta-analysis of the World Bank (2012).

Lessons learned from modelling results and practical implications:

The future CA scenario (no-till planting, retention of crop residue as mulch and inclusion of cover crops in the rotation cycle) can lead to relatively high rates of SOC sequestration, if soil fertility is managed to limit nutrient deficiencies. Modelling results demonstrate the importance of cover crops with high root and vegetation mass to effectively sequestrate soil carbon.

The combination of cover crops and soil fertility management clearly emerged as the most important element in facilitating SOC sequestration. The potential benefits obtained from cover crops are determined by soil fertility and nutrient management, where healthy root development and high forage yield are a function of nutrients that are readily available in soil.

Modelling clearly demonstrated that SOC sequestration (both initial rates and time to equilibrium) can be severely constrained by deficits in nitrogen (N) and phosphorus (P). Nutrient deficits inhibit the development of a healthy root system and high above-ground biomass production, which provides the building blocks of SOM. Deficiencies in N and P availability in the soil and crops will inhibit SOC sequestration when SOC and soil organic nitrogen levels are not adequately balanced (Lal, 2004). It is vitally important that soil fertility is assessed regularly and managed to enable SOC sequestration. The elemental requirements for SOC sequestration need to be resolved for soil type, crop rotations and tillage method. Soil specific and demand specific (yield of grains and biomass, and desired rates of SOC sequestration) fertiliser application rates are required.

Overgrazing of maize residue can reduce the SOC potential significantly, especially for farming systems that do not include a cover crop. Modelling results indicate that not more than about 50% (live weight) of the residue should be removed since as much as 20% of the residue can be trampled into the soil (e.g. model default value for SOC modelling) and a further 10% (can be as high as 20%) of the residue is incorporated into the soil during planting.

The advantage of high density grazing of the cover crop on SOC sequestration could be clearly noticed from the modelling simulations. High density grazing with limited duration provide the cover crop sufficient time to recover without losing root mass. Continuous (over)grazing can result in significant losses in root mass and in forage production if the cover crop is not allowed sufficient time to recover between grazing activities. Overgrazing will result in a reduced SOC sequestration potential.

The background value (veld) was not achieved. This indicates that the inclusion of a single cover crop in the crop rotation is not sufficient to achieve background values. Single-cropping systems lack the necessary amount carbon inputs to provide the SOC needed to achieve background values.

Model input data

Climate

The region is characterised by hot summers and cold to warm winters with frost in the winter (Figure 5). Most of the rain (about 90% of annual rainfall) occurs between October and April as thunderstorms late afternoon. Rainfall usually exceeds 50 mm/month during the months of November to March. A mean annual precipitation of 489 mm/yr was used for the region.

Soil

Soils of the Avalon, Bainsvlei, Bloemdal, Clovelly, Glencoe, Hutton, Kimberley, Molopo and Plooysburg soil forms were identified as suitable soils for crop production for the region. Soil profiles of these soil forms that are included in the Digital National Soil Profile Database
(Soil Survey Staff, 1972-2010) were used to determine representative soils for the region, based on soil texture and clay content (discussed in Section 4.3 of Numerical modelling webpage). The median (50^th percentile) clay content determined from 93 soil profiles is 9, 20 and 17% for the A-, B₁ and B₂ soil horizons respectively. The soil texture of the horizons is a loamy sand A-horison, and a sandy loam B₁– and B₂ soil horizons. Soil phosphorus- and exchangeable potassium levels of 25 and 120 mg/kg was used for the A-soil horizon respectively for the initial conditions (after 50 years of historical conventional tillage) of the implementation of CA farming systems scenarios. The levels are based on several decades of historic fertiliser application to increase soil fertility to these optimum levels for crop production.

Tillage-, agronomic- and grazing practices

The simulation of cropping systems required information related to the following activities:

• Planting.
• Tillage before, during and after planting.
• Fertiliser and lime application.
• Harvesting.
• Grazing of forage sorghum and maize residue after harvesting.

Activities summarised in Table 2 and the scheduling thereof, are based on data provided by Grain-SA and ASSET Research

Data on suggested nitrogen, phosphorus, potassium and lime application rates were provided by Grain SA and ASSET Research for each crop system scenario (Table 4). The rates were increased if the modelling indicated that a crop experience a nutrient deficit that limits the potential to sequester SOC. Proposed nitrogen application rates of forage sorghum were adjusted in the model to both sustain high (forage) biomass production rates and to limit the effect of nitrogen deficiency stress on the crops that sequester SOC.

Data on suggested nitrogen, phosphorus, potassium and lime application rates were provided by Grain SA and ASSET Research for each crop system scenario (Table 3). The rates were increased if the modelling indicated that a crop experience a nutrient deficit that limits the potential to sequester SOC. Proposed nitrogen application rates of forage sorghum were adjusted in the model to both sustain high (forage) biomass production rates and to limit the effect of nitrogen deficiency stress on the crops that sequester SOC.

Model calibration

Model calibration is discussed in Section 5 of the numerical modelling webpage for this study. This section focusses on the results from the calibration.

Crop growth model component

Selected parameter values in the WinEPIC plant growth and development module were refined until the predicted above ground dry matter and grain yield corresponded within 5% of the data provided by ASSET Research (Table 4). The dry matter and grain yields predicted corresponded with the data provided by ASSET Research for maize and sunflower. Predicted dry matter for forage sorghum was 2.8, 7.6 and 12.2 t/ha, which indicates the effect of nitrogen deficiencies on the above-ground biomass production. A maximum of 12.2 t/ha was predicted for the climate of the region, which is marginally lower than 13 t/ha forage yield potential indicated by Grain SA (2021) guide for forage sorghum. 

Soil organic matter model component

WinEPIC could predict the non-linear change in SOC contents that were determined by
du Toit et al., (1994) following calibration of the crop growth component and simulating mouldboard ploughing (which typically occurred before the 1990s) (Figure 6). This includes realistic predictions on the change in SOC contents for the initial period of about 7 years with high rates of changes, followed by a second period with lower rates in changes in SOC contents for the conventional tillage scenario.

References

Causarano, HJ, Doraiswamy, PC, McCarty, GW, Hatfield, JL, Milak, S and Stern, AJ, 2008. EPIC Modeling of Soil Organic Carbon Sequestration in Croplands of Iowa. Publications from USDA-ARS / UNL Faculty. Paper 1363.

Department of Agriculture, 2003. Maize production. Directorate Agricultural Information Services, Department of Agriculture publication.

Department of Agriculture, Forestry and Fisheries, 2010. Sunflower production guideline. Directorate Agricultural Information Services, Department of Agriculture publication.

du Toit, ME, du Preez, CC, Hensley, M and Bennie, ATP, 1994. Effek van bewerking op die organiese materiaalinhoud van geselekteerde droëlandgronde in Suid-Afrika. SA J. Plant Soil 11(2): 71-79.

Grain SA, 2021. Integrated crop and pasture-based livestock production systems – Part 11: Forage sorghum (Sorghum spp.). https://www.grainsa.co.za/integrated-crop-and-pasture-based-livestock-production-systems—part-11.

Lal, R, 2004. Soil carbon sequestration. SOLAW Background Thematic Report – TR04B.

Nicoloso, RS and Rice, CW, 2021. Intensification of no-till agricultural systems: An opportunity for carbon sequestration. Soil Sci. Soc. Am. J. 1: 1–15.

Soil Survey Staff, 1972-2010. Soil profile descriptions and soil analyses data. In: ARC-ISCW Soil Profile Information System. ARC-Institute for Soil, Climate and Water, Pretoria.

West, TO and Post, WM, 2015. Soil Organic Carbon Sequestration Rates by Tillage and Crop Rotation: A Global Data Analysis. Soil Sci. Soc. Am. J. 66: 1930–1946.

World Bank (2012). Carbon sequestration in agricultural soils. Report No. 67395-GLB.