Apr 2, 2015

Building healthy soil crucial to organic production

Soil organic matter (SOM) is the foundation of a healthy, fertile soil because it drives critical soil functions.

That was the position stressed in a presentation at the Great Lakes Fruit, Vegetable and Farm Market EXPO in Grand Rapids, Michigan, by Lisa Tiemann, assistant professor of soil biology at Michigan State University.

“SOM is a critical component of healthy and productive agricultural systems, particularly organic systems, and microbes provide the means for building SOM,” Tiemann said. “It is therefore critical that we manage our soils with the microbes in mind.”

In addition to storing the largest fraction of terrestrial organic carbon and acting as a nutrient reservoir, SOM influences soil structure, water holding capacity, pH, ion exchange capacity and soil biological activity, Tiemann said.

“All of these factors in turn determine soil fertility, or the ability of soils to provide water and nutrients in support of plant growth,” she said. “Organic farmers in particular must rely heavily on the services provided by SOM and therefore need to understand how SOM is accrued or depleted in order to build healthy and productive soils.”

What are the sources of SOM? Tiemann said traditional views of SOM formation relied heavily on the “humification” paradigm, in which abiotic reactions convert organic, primarily plant-derived compounds into large, amorphous, polyaromatic structures that are recalcitrant – difficult for microorganisms to decompose.

More recent SOM research has emphasized a different set of mechanisms controlling SOM formation; processes related to microbial activity and growth rates.

“In the past, SOM formation was attributed to the recalcitrance of plant residues (how much lignin it has), but we now know that soil biota are more important regulators of SOM formation and accrual,” she said.

Microbial biomass has been described as the “eye of the needle through which all the natural organic material that enters the soil must pass.” As soil microorganisms process residues, microbial necromass (dead microbial biomass) and other growth and decomposition byproducts, such as polysaccharides, can accumulate.

These microbial products are responsible for SOM accrual as they are stabilized within the soil matrix because of their recalcitrant nature and reactivity with soil mineral surfaces.

In a soil system, most microbes are dormant, waiting for conditions favorable for their growth. Outside of the rhizosphere (the area immediately adjacent to actively growing roots), soil is a nutrient and energy desert for microbes so that the accrual of microbially derived SOM is “likely the result of microbial ‘boom’ and ‘bust’ cycles,” Tiemann said.

Microbes in the soil experience a “boom” in microbial growth and activity, with faster growth rates and biomass turnover when they are provided with high-quality organic matter, whether this is from residues, dissolved organic matter, dying roots or root exudates.

This high-quality material, however, is finite, leading to a “bust” where microorganisms starve and cells lyse. While some of this lysed cellular material is recycled, much of the resulting microbial necromass can end up associated with mineral surfaces, where it is physically and chemically protected from further decomposition, resulting in the accrual of long-lived SOM. It is this pool of slow-turnover, or long-lived, SOM that is critical for maintaining good soil structure, water-holding capacity and cation exchange capacity.

How do you “wake up” the microbes so that they can help build SOM?

The addition of high-quality residues (C:N ratio less than 25) such as legume cover crops, young cereals and poultry or slurry manures can increase microbial biomass and can stimulate microbial activity and growth rates.

“Microbial biomass or microbial activity and SOM (also referred to as soil C) are significantly related,” Tiemann said. “However, increases in biomass and microbial activity can have highly variable effects on SOM formation.”

Increased activity could either increase organic matter loss from the system through mineralization to CO2 or increase microbial contributions to SOM, depending upon microbial growth efficiency and biomass.

“If microbes are more efficient in their use of SOM for energy and growth, then more SOM is used to build biomass than is lost as CO2,” she said. “Managing this trade-off between stimulating microbial activity, while simultaneously increasing their overall growth efficiency, will result in SOM accrual.”

A balance between increasing microbial biomass and activity while decreasing SOM losses through mineralization to CO2 can be achieved using a number of methods.

“High-quality animal manures and green manure cover crops will result in SOM accrual to a greater extent than similar systems without this source of high-quality energy and nutrients,” Tiemann said. “Increases in SOM accrual in such systems are most easily explained by a greater retention of extant SOM; with high-quality manure inputs, and therefore greater N availability, microbes do not need to mine extant SOM for N and other nutrients to the same extent as microbes in systems without these high-quality residues.”

Tiemann said the use of a legume-cereal mixture to create a mixed-quality residue can simultaneously stimulate microbial communities while increasing efficiency.

“In these mixtures, the high-quality residues can be used by microbes as an energy source to power their initial growth after dormancy, followed by the decomposition of lower-quality residue,” she said. “The subsequent decomposition of the lower-quality residues is inherently more efficient (less CO2 loss) and therefore likely to result in SOM accrual.

“An increase in rotational diversity can also increase microbial biomass and activity, with positive impacts on SOM accrual,” she said. “The diversity of residue mixtures in diverse rotational systems increases microbial diversity.”

As this diversity increases, the mix of microbes with high and low efficiency growth strategies becomes more even, thus there is a greater potential for high biomass and activity coupled with high growth efficiency, Tiemann said.

An analysis of the benefits of diversity and high-low quality residue mixtures across a wide range of cropping systems and climates revealed an average increase in microbial biomass of 21 percent, and an average increase in SOM stocks of 9 percent.

These increases in microbial biomass and SOM were accompanied by a 13 percent increase in soil nitrogen.

In addition to their direct contributions to SOM, soil microbes also contribute to soil structure through aggregate formation and stabilization. Microorganisms produce materials that bind soil particles together, such as polysaccharides and glomalin or fungal hyphae, and thus promote the formation of soil aggregates.

“Soil aggregation plays a critical role in protecting SOM, limiting SOM breakdown and loss, because if SOM is occluded within aggregates, it is protected from further decomposition or mineralization.”

Tiemann said the large SOM losses typically seen during soil disturbances, such as land conversion or continuous tillage, are likely a function of increased SOM accessibility for microbes due to the breakdown of soil aggregates and thus soil structure.

“Evidence for the link between microbes, aggregates and SOM can be observed even in the sandy loam soils, where increased rotational, and therefore crop residue diversity, have led to increased microbial activity, aggregation and SOM,” she said.

– Gary Pullano