_{Article written by Craig Galloway}

Trace & Save collects thousands of soil samples every year, aiming to assist farmers with their practices and management decisions towards healthier soils. One of the most important goals for soil health is to increase soil carbon levels. Not only does higher soil carbon levels support soil biology, higher nutrient holding capacity, higher water holding capacity and better soil structure, but it also leads to the removal of carbon from the atmosphere, to be stored in the soil. This is hugely important to mitigating the negative impact of excess carbon in the atmosphere on climate change.

As much as building carbon is clearly a desirable outcome for farms, it is not a straightforward process. And due to the insight gained from the numerous soil samples taken by Trace & Save, we have found it to be highly complex to track, predict and understand. Since the soil is a habitat to a multitude of living organisms, ongoing complex soil processes and the interactions between these living and non-living aspects, the result is constant change.

When we take soil samples, we are assessing the soil at a moment in time, trying to understand the various dynamics, including the levels of plant-important nutrients and the abundance and diversity of living organisms. It is not a straightforward task, nor is it infallible. But it is the best insight that we currently have as to the health of a given soil. That is also why we take soil samples in the same month and same spot each year, to limit, as much as possible, the impact of climate (especially soil temperature and moisture levels) from one sample to the next in the same camp.

Being able to better understand the processes that lead to healthier soils, especially soil carbon, will help us to direct management practices and farm systems towards one that will build the carbon levels in soil long-term.

Carbon cycle in the soil

Carbon cycles through the environment on a global scale, as depicted in the image below.

When focussing in on the soil aspect, there are a few processes which lead to carbon moving into the soil, and a few factors that lead to the loss of carbon from the soil. These are laid out below. The net effect of these process either leads to an accumulation of carbon or a loss of carbon from the soil.

Factors which influence the accumulation of soil organic carbon

As discussed, there are numerous processes taking place in the soil all the time. But there are certain factors which lead to the accumulation of carbon as an outcome of these processes, i.e., more carbon into the soil than out. These factors will be briefly discussed.

Soil texture

The clay content of the soil significantly influences its ability to build up and store organic carbon. Clay particles have a high surface area and negative charge, which allow them to form stable complexes with organic matter (OM). The higher the clay content, the more carbon the soil can store. In addition to that, in finer-textured soils (i.e., higher clay), carbon is not as vulnerable to loss at high temperatures.

Soil depth

The depth of soil also plays a role. Deeper soils have the ability to accumulate more carbon. Deeper soils allow for deeper root systems, which contribute to soil carbon through root exudates and decaying roots. These inputs can enhance carbon storage at greater depths. Carbon stored in deeper soil layers tends to be more stable and less susceptible to microbial decomposition because of lower oxygen levels, reduced microbial activity, and cooler temperatures.

Climate

Soil temperature has the greatest impact on soil carbon levels, since higher temperatures lead to higher decomposition and respiration rates, which both lead to higher loss of carbon from the soil. The other climatic factor which influences soil carbon accumulation is precipitation. High rainfall events can lead to the leaching of dissolved carbon from the upper part of the soil (0-30cm), which is generally the depth monitored for soil carbon levels.

Grazing management

There is no single effect that grazing has on soil carbon accumulation, but a wide range of effects which depend on the grazing management approach. The positive effects that good grazing management can have on carbon accumulation is that it: increases root carbon allocation; and increases growth of fast and deep growing fibrous roots. The negative effect that grazing can have is that defoliated plants use root reserves to recover leaf canopy.

Therefore, grazing intervals and pasture cover before grazing are extremely important, since the plants need enough time to recover, replenish and increase root reserves between grazing events. Grazing low cover leads to a reduction in the root to shoot ratio of plants (i.e., less carbon is stored in the soil) and to potential carbon reduction in the soil, whereas waiting to graze high cover, although good for carbon accumulation, can lead to poor pasture/grass quality. This is a tension that needs to be managed for the shared goals of grazing management – good quality grass, optimal utilisation and healthy soils.

Another factor of grazing management is stocking intensity. High stocking intensity leads to a high level of plant removal (i.e., high utilisation). Although this is positive from a pasture allocation and cow diet perspective, it does lead to lower organic matter inputs and lower carbon accumulation. On the flip side, low stocking density leads to decreased plant removal (i.e., poor utilisation) and an increase in organic material inputted in the soil leading to higher carbon accumulation. Therefore, the goals of carbon accumulation and pasture utilisation need to be managed in tension to each other.

The third tension that needs to be managed is the amount of residual left after grazing events. This influences carbon accumulation since leaving too low residuals leads to an increased use of soil carbon stores for regrowth, and the loss of desirable plant species. Whereas, leaving too much residual, although positive for carbon accumulation, results in poor pasture utilisation.

When grazing management is implemented optimally, managing all three tensions towards the shared goals of utilisation and soil health, it can lead to carbon accumulation in the soil. Although there are definite trade-offs both ways, each farmer has to learn how to manage the tensions in their context.

Tillage

Soil tillage (any form of disturbance, e.g., ploughing, ripping, discing, etc.) leads to a negative impact on carbon accumulation. In some cases, lesser disturbances, such as ripping, are worth the loss of carbon in the short term in return for better growths and carbon building potential in the long term. This disturbance should only be considered if compaction is the big limiting factor to the desired growth of pasture and ripping would solve the limitation.

Fertilisation

Fertilisation has a complex interaction with carbon accumulation. Optimal fertilisation leads to optimal pasture growth, which is positive for carbon accumulation. However, the application of fertilisers (especially synthetic ones) leads to a disturbance of the soil microbial community. This disturbance normally has a negative effect on the balance of the soil microbial community. The effect of this on soil carbon accumulation is discussed later.  

Organic amendments, such as compost and dung and urine from grazing, can contribute to higher microbial biomass in the soil.

Pasture composition

A greater diversity in pasture species composition is associated with a greater diversity in the microbial community in the soil. Diversity is a key principle of functional ecosystems, leading to balance, resilience and health.  More diverse pastures also make it possible to manage for higher covers without the negative impact of poorer quality, which is necessary for carbon accumulation. Further to this, diverse pastures are associated with higher stable soil aggregates, which assist with carbon accumulation.

It should be noted that grasses, with their dense roots, are imperative in pastures to the accumulation of carbon. Grasses have a much higher root to shoot ratio than other plant species, and therefore are required in large proportions in pastures for carbon accumulation in the soil.

Microbial community

The soil microbial community (SMC) is the complex network of soil organisms which live and interact in the soil. A healthy SMC is a diverse web of species which are in balance with each other. There is not one group of species which dominates the space, but rather a balance between trophic levels and functions.

There are a few very important roles which the SMC plays in carbon regulation and accumulation.

• Decomposition of organic material. This leads to the loss of carbon through respiration, and the transformation of the nature of the organic material that is left into more stable forms of carbon. The higher the number of organisms in the SMC, the more decomposition is taking place.
• The formation of microbial biomass (total mass of living soil microorganisms), necromass (dead microbial material) and stabilised residues (organic matter that has been chemically protected in the soil). These all lead to the increase of stable carbon in the soil. And the healthier and more abundant the SMC, the greater the output of these processes.
• Nutrient turnover and supply, which provides nutrients to plants leading to optimised growth and therefore higher carbon input into the soil.
• Conservation of ecosystem services. This is a broad category, which just means that the SMC plays a role in how soil functions to provide for and protect plants. The healthier the SMC, the better functioning the ecosystem services, which leads to optimised plant growth and defence.

The importance of a healthy SMC to long-term soil carbon accumulation cannot be understated. One of the most important reasons for this is that the forms of carbon formed by the SMC and associated processes (i.e., microbial biomass, necromass and stabilised residues) have been shown to be the most stable forms of carbon in soil. Although they are subject to further processes and loss, they are the least susceptible. Other forms of carbon, especially particulate organic material and organic matter free in soil solution, are subject to rapid and large changes due to decomposition, movement and leaching. The diagram below, taken from Dynarski et al. (2020), shows a conceptual illustration of soil carbon flows and the importance of the SMC.

Agricultural practices have been shown to have a negative impact on the SMC. It is not always possible to isolate the impact of each practice, but agricultural soils generally have much lower species abundance and diversity than natural (uncultivated) soils. The practices which appear to have the largest negative impact on the SMC are:

• Soil tillage
• The application of fertiliser and plant protection products (i.e., pesticides)
• Use of heavy machinery

Practices that have a positive effect on the SMC are all of those listed above which contribute to carbon accumulation, since the two are inextricably linked.

Indicators of healthy soil microbial community

Since most of the factors that influence soil carbon accumulation from a management and context perspective are relatively straightforward to understand and measure, the one which needs to be further explored is the SMC. The health and abundance of the SMC is imperative to the long-term build-up of soil carbon.

Trace & Save measures various indicators of soil biology, which we can explore to better understand the dynamic influencing the SMC. These include:

• Total carbon (TC)
  • Indicator of total available organic matter.
  • Source of food for soil organisms.
  • More carbon equals more potential for life.
• Active carbon (AC)
  • Fraction of total carbon.
  • Readily broken down by soil organisms.
  • More direct indicator of soil biological activity.
• Total nitrogen (TN)
  • Measure of total organic (±98%) and inorganic (±2%) nitrogen in the soil.
  • Indicator of the amount of organic nitrogen that can be converted to inorganic through mineralisation.
  • Indicator of the excessive build-up of nitrogen in soil.
• Active mould (AM)
  • Mould count grown on agar plate which were reproductive within five days.
  • Indicator of the amount of mould actively growing in the soil.
  • Mould is a fundamental role player in the effective breakdown of organic matter, enhancing soil structure and cycling of nutrients in the soil.

Context

Trace & Save noticed a trend in the soil carbon results between 2021 and 2023 on some farms, where the carbon levels decreased, but some of the other soil biology indicators showed a positive change. This was a bit strange, since the goal is to build carbon, and this should be associated with the other soil biology indicators following suit.

The graphs of the trends in the various soil biology indicators show that on average across all the results, and for most regions (Alexandria; Cookhouse; Cradock; Humansdorp; Midlands and Oyster Bay) there was an increase in TC and TN from 2021 to 2022, but then a decrease from 2022 to 2023. This was associated with an average decrease in AC from 2021 to 2022, and an increase in AC from 2022 to 2023. And an increase in AM from 2022 to 2023. So, the positive trend of an increase in TC was associated with the negative trend of a decrease in AC, whereas the negative trend of a decrease in TC was associated with the positive trend of an increase in AC and AM.

These are obviously averages, this was not the case on every farm and every camp. But it was enough of a trend for us to take note of, and desire to examine further. There were also regions that do not follow the trend, such as Tsitsikamma and Southern Cape.

The research about the principles of carbon accumulation discussed above were a result of wanting to understand this trend better. We also examined at the relationships between the soil biology indicators to try and understand these dynamics better.

Data analysis

We carried out Spearman’s correlations (since the data is non-normally distributed) on the change in TC, TN, AC and AM to understand how they relate to each other. The results are below.

There are a few insights that can be gained from this data.

• There is a strong positive correlation between TC and TN change, as would be expected – they are two of the main elements of organic matter, therefore would be expected to follow a similar trend.
• There is a strong positive correlation between AC and AM change. This is good to see, since they are both indicators for soil biological activity.
• There is a weak positive correlation between TC and AM change, which is a bit surprising, since carbon forms the main source of food for mould.
• There is no correlation (although a negative relationship) between TN and AM change. This helps us to understand the dynamic, since TC and TN are correlated, but one is positively associated with AM and the other negatively. It is not statistically significant, but there is an indication that an increase in TN is not positive for mould activity.
• The main conclusion we can draw from this data is that AM is the main driver of AC increase. Since AC is a by-product of decomposition, which is the main role of mould in the soil.

It is important to note the correlations between TC levels in the soil and TN, AC and AM (table above). There are very strong relationships, which make sense, since TC is the food for AM. Therefore, the higher the TC levels, the more AM there will be in the soil. What is interesting, when examining the relationships between the change, is that you would expect that when TC increases, this results in more food for AM, and therefore it would increase too. But what we are finding is that there is no relationship.

Our hypothesis as to what is taking place in the soil is that in some situations, the increase in AM leads to the decrease in TC since the AM is decomposing particulate organic matter that has been built up over time in the soil. As this material is decomposed, the AC levels increase, but the TC levels decrease. But as was discussed earlier, the process of decomposition and transforming of particulate organic matter into more stable forms by the SMC is positive for the long-term accumulation of carbon in the soil. Therefore, in the short-term, it is more beneficial to see the increases in AM and AC, than it is to see an increase in TC. Obviously, in an ideal world, we would like to see all three increase together, but it is likely that TC will fluctuate year to year as this process of decomposition plays out. In the long-term, it is imperative, for soil health and for climate change mitigation, to see an increase in TC.

Insight for farmers

Having researched the factors which influence carbon accumulation, and examined the data from Trace & Save soil samples, the following are the main takeaway points which should be of interest to farmers. This is especially for farmers who have seen the trend of a decrease in carbon in their soils, and are concerned.

• There is an obvious climate effect on soil life indicators, therefore we should not over-react to one year of change.
• The SMC plays a fundamental role in creating long-term carbon in the soil.
• In the short term, it is more important to build the SMC (the indicators Trace & Save uses to asses this are AC and AM) than to see an increase in TC.
• In the long term, TC needs to build to reflect the impact of good management.
• There are trade-offs between the current conventional dairy farming practices and ideal soil carbon building practices – it is up to each farmer to navigate what will work best for them.
• The main outcome of dairy farming is not soil carbon sequestration, but it can be a positive outcome with a holistic, sustainable approach that focusses on soil life improvement. It may not happen as quickly as you want, but the soil will improve in the long run.  

Management trade-offs

There are some very clear trade-offs between the goal of improving the SMC and accumulating carbon in the soil, and common pasture-based dairy farming goals. The diagram below depicts some of these trade-offs.

All of these trade-offs have been explored earlier, so we will not discuss them in detail again. But what is very important to note, is that there are management practices that find the balance between the practices and can result in achieving both goals simultaneously. The key is finding the balance for each farm, in its context. And the mechanism is usually through the farmer, and their management team, paying close attention to all of what is happening on the farm.

Conclusion

It is well-established that building soil carbon is an important goal for agriculture for the future. When considering how to manage a farm in a manner that will lead to soil carbon accumulation, there are six very important principles to consider.

1. Adaptive grazing management

Optimal grazing management will result in good pasture utilisation, while not leading to overgrazing and/or leaving too low residuals.

2. Pasture cover management

Optimal pasture cover management will result in sufficient grazing intervals that allow pasture and soil to recover in a manner that will contribute to soil health, while not allowing the cover to become so long that it negatively impacts on milk production through poor quality.

3. Perennial pastures

Planting and maintaining perennial pastures on as much of the farm as possible will lead to soil cover and living roots throughout the year.

4. Diverse pastures including legumes and grass

Diverse pastures will contribute to diverse and healthy soil microbial communities, and dense roots throughout the soil profile. Diversity also contributes to pasture quality at higher covers.

5. Efficient/strategic fertiliser and pesticide inputs

Optimal fertiliser and pest management ensures healthy pasture growth without excessive inputs that will have a negative impact on the soil microbial community.

6. Reduced tillage

Reduced tillage protects soil aggregates and allows for long term accumulation of carbon in the soil.

Soil carbon accumulation is not a straightforward process. There are many complex relationships and processes in the soil that we cannot predict or understand. But there are broad principles that farmers can follow, which in the long term will lead to an increase in soil carbon levels. Farmers should not get despondent if they do not see the short-term results but should rather focus on improving the soil microbial community, and in the long term, the carbon will accumulate.