The Many Mysteries of Soil Organic Matter

Organic matter is arguably the most important component of soil. For almost any soil-related problem, organic matter seems to be the answer. Degraded soil structure? Needs organic matter. Poor water infiltration or retention? More organic matter. Low soil nutrient supply? Add organic matter. Climate change? Put that carbon (i.e. organic matter) back into the soil where it belongs! So, how does it work?

Much remains to be discovered about soil carbon dynamics. It would take many pages to address all the relevant factors. Instead, this article and those that follow will address some commonly-held misconceptions, as well as the directions in which recent research would point a carbon-concerned soil manager.

Conventional wisdom used to be that stable soil organic carbon (SOC) was made up of humus, the result of secondary synthesis of decomposing plant material into chemical structures so complex they were difficult to break down, or what scientists call "recalcitrant". However, over the past 20 years or so, advances in analytical techniques have made it clear that humus, as it was traditionally defined in soil science, doesn't actually exist in the soil at all, but was the result of the chemical extraction process in the lab (Lehmann et al., 2008; Lehmann and Kleber, 2015; Schmidt et al., 2011). That discovery caused quite a stir in the soil science community, which began searching for a new framework to explain the mechanisms of soil carbon stabilization that can incorporate all the evidence.

At this point you might be thinking, "That's fascinating and all, but you better be getting to the point of how this relates to me and my farm".

One of the most widely-accepted mechanisms for organic carbon accumulation in the soil is its physical protection through what is called occlusion. That is the scientific way of saying that the organic matter is made inaccessible to hungry microbes that are otherwise fully capable of decomposing it. This happens through the process of soil aggregation (Figure 1). Here is where we get to soil management on the farm, as aggregation is highly dependent on management (Figure 2).Figure 1. Soil aggregates and associated SOC storage time scale. (Tivet et al., 2013)

Figure 1. Soil aggregates and associated SOC storage time scale. (Tivet et al., 2013)

Figure 2. Management impacts on soil aggregation and SOC stocks. (Tivet et al., 2013)

Figure 2. Management impacts on soil aggregation and SOC stocks. (Tivet et al., 2013)

Occlusion and long-term (decades to centuries) organic matter protection happens mostly at the micro-aggregate scale. Macroaggregates also contribute, but only hold SOC for years to decades at most (von Lutzow et al., 2006). It's fairly common knowledge that microaggregates are pulled together by the action and byproducts of fungal hyphae and plant roots to create macroaggregates, but what is less commonly known is that within stable macroaggregates, new microaggregates form and physically protect organic matter in their structure (Six et al., 2000). Soil disturbance (i.e. tillage) destroys macroaggregates, disrupting this critical process. Reducing soil disturbance to a minimum is the first step on your path to increasing organic matter levels in your soils.

However, as many long-term no-tillers will know, simply removing the disturbance is not a golden ticket to higher organic matter; we need to be adding it as well. That's because the formation of aggregates and the organic matter they contain is highly dependent on soil microbes and their byproducts, and they need food to work.

We cannot think of soil systems without considering plants. Plants are the primary pathway for carbon inputs into the soil, and much of that carbon flows through root exudates. These are mostly relatively simple compounds that are used by microbes around plant roots in what is called the rhizosphere. Recent evidence shows that plant roots rich in soluble compounds have a positive impact on water-stable macroaggregates, mostly likely through stimulation of soil microbes (Poirier et al, 2017). Microbes can transform these simple compounds into the many forms of carbon found in soil (Kallenbach et al., 2016). This is one reason why there's so much talk recently around maximizing living roots in the soil.

The question then becomes: "Which plants do I grow if I want to improve soil aggregation?" That question brings us to the fascinating and rapidly expanding research into plant functional root traits; the specific architectural, physiological, chemical, and symbiotic characteristics of plant roots and how they influence the plant-soil system. Just like carbon dynamics, there's still a lot to be learned, and many species' root traits remain uncharacterized. Add to that the variety-level differences that can exist and we're a long way off from a complete picture, but here are a few things to know.

Root-associated microbes are very significant players in soil aggregation. Mycorrhizae promote microaggregate formation by producing glues and binding agents, and their network of hyphae bring microaggregates and larger soil particles together into macroaggregates.

Legumes (specifically their rhizobial partners) promote aggregation through the production of binding agents. This effect has been shown to be stronger in soils low in organic matter (Tisdall and Oades, 1982), which explains why macroaggregation is improved more in subsoil than topsoil under legume forage (Sainju et al., 2003).

However, legumes typically have less fibrous root architecture than grasses, which are associated with greater macroaggregation in topsoil (Angers and Caron, 1998). For example, ryegrass has high root length density (length of roots per volume of soil), and was found by Materechera et al. (1992) to result in stronger, denser and more stable soil macroaggregates enriched with SOC compared to peas and wheat. Phacelia has very high root length density (RLD) near the surface, which could explain why it's been indicated as having great benefits for topsoil structure. Cereal rye also has high RLD, almost twice as high as vetch, with mustard somewhere in between (Bodner et al., 2009). Root length density is correlated with root diameter, and other experiments have found that the positive impact of many fine roots is higher in soils with low organic matter levels.

There are many mysteries to be unraveled in the quest for higher soil organic matter. The belowground characteristics of the plants we choose to include in our cropping systems is an important piece of the puzzle, and as research progresses we should be able to use this kind of information to make better decisions about which cash or cover crops to include to achieve our soil management goals. These are exciting times, stay tuned!

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