Starved organic soils erode soil organic matter
By Robin Booker
Organic grain farms rely on micro-organisms to cycle soil nutrients into plant-available forms, but nutrient imbalances in organic soils can hamper this process.
“In organic systems, you don’t have inorganic fertilizers bringing in primarily nitrogen and phosphorus, but each year, you have grain being harvested so you are exporting those nutrients off the system,” said Bobbi Helgason of Agriculture Canada.
Helgason provided a review of research, including her own, of farming techniques that foster a healthy soil microbial environment during the University of Saskatchewan’s soils and crops meeting in Saskatoon.
One way of supporting the soil microbial community is through a balanced nutrient management regime, which Helgason demonstrated with a long-term alternative cropping experiment at the Scott Research Farm in central Saskatchewan.
The study compared organic versus conventional cropping systems with four treatments: an annual organic rotation, a perennial organic rotation, and paired comparisons of annual and perennial conventional rotations.
The treatments were not exactly the same because the experiment was designed around the best management practices of both the conventional and organic systems, but all treatments had a diverse crop rotation.
We’ve got an organic system that relies on soil biology to supply nitrogen and phosphorus, but it’s exhausted. – Bobbi Helgason
The organic system used legume crops to fix nitrogen instead of synthetic inputs, tillage was used to control weeds, and there was no use of livestock or organic amendments.
The conventional management system used best management practices for the region, including fertilizer and pesticides based on soil tests and crop scouting, as well as a no-till strategy.
Helgason said she doesn’t want to use this study to pick on organic systems, but it makes a terrific model system for soil biology in what has turned out to be a degraded soil.
“After 20 years of the organic and conventional management, one of the things that stands out is there is a lot more carbon and nitrogen in the conventional annual system then in the organic system, and that the perennial system under organic management is certainly very low compared to others in organic nitrogen and available phosphorus,” Helgason said.
She did note, however, that researchers struggled to establish a strong alfalfa stand during the perennial section in some of the rotations.
The focus of Helgason’s work was to examine how the different soil conditions, brought on by different management strategies, affected the health of the soil microbial community.
Samples of soil from each rotational treatment were collected and studied in the lab.
“The total microbial biomass was far healthier in that more fertile conventional annual system then it was in any of the other systems. The perennial systems were approximately equal, and the lowest fertility system, the organic annual, had the lowest organic biomass,” Helgason said.
Not only was there reduced microbial biomass in the annual organic system, the functioning of its microbial community was also reduced.
“The highest levels of enzyme production happen in the conventional annual system, followed by the perennial system and in some cases with the lowest production the annual organic system,” Helgason said.
To help study the microbial community structure, crop residue with a carbon 13 tracer molecule was added to the soil samples.
The microbial community in the conventional annual highly fertile system did not change significantly over time.
However, as residues were used by micro-organisms in the three less fertile soil samples, their microbial community structures reverted back to something that was different than the highly fertile conventional annual treatment.
A similar pattern was observed in the field at the experimental plots.
“Those nutrient deficiencies were resulting in lower crop yields that eventually led to less residue return and what turns out to be a carbon limitation, which is important because crop residue is important to the bugs,” Helgason said.
“We’ve got an organic system that relies on soil biology to supply nitrogen and phosphorus, but it’s exhausted. There is not enough carbon there to fuel it. It can’t make the enzymes to go out and scavenge those nutrients.”
A second study was performed to see how carbon was flowing through the microbial community in the soils of the rotation treatments, including which specific micro-organisms used the carbon.
Synthetic glucose was added to soil samples and the CO2 respiration was measured to gauge microbial activity.
In this analysis, a tracer molecule was followed not only into the phospholipid fatty acid profiles but also into DNA profiles, to help get better resolution on which micro-organisms were active in the soils samples.
Enzyme activity was also examined.
“In the organic system, there was a greater degree of soil organic matter priming. So, what that means is we added carbon and the microbial community used it, but in doing so it went to the soil organic matter and increased its level of soil organic matter breakdown,” Helgason said.
“This is important because it has implications for carbon cycling in the whole system, and not only the amount of soil organic matter but its composition in the soil.”
The high-resolution mapping of the active soil microbial organisms in each of the soil samples allowed researchers to demonstrate that the organic microbial community was very different than the conventional microbial community.
In the organic soils, the micro-organisms were associated with organic matter priming (which draws recourses form soil organic matter), while the active micro-organisms in the conventional systems were negatively associated with the priming effect.
“In the conventional system, the active organisms were positively correlated with things like overall CO2 respiration, fertility, so available phosphorus and nitrogen, dissolved organic carbon, so readily available resources as well as enzyme activities,” Helgason said.
Organic systems had to use other soil resources indirectly to capitalize on carbon availability.
“In the organic system, we fed them carbon, but they were totally starved for nitrogen and phosphorus so what they needed to do to use that carbon was to turn to the soil organic matter and actually mine those nutrients out of it,” Helgason said.
Soil amendments advised against
After her presentation, Helgason was asked if adding amendments such as compost teas or sugar to soils can improve the health of microbial communities in agriculture land.
In terms of adding compost teas for an inoculum source, “my knee jerk reaction would be that soil microbial communities are already abundant and really diverse, and everybody’s competing for ecological niches. And so, generally speaking, based on the environment in the soil, all of those niches are full,” Helgason said.
She said cropping inoculants are different, because applied mycorrhiza and rhizobia are given an ecological niche in a nodule or root where they can compete and become useful.
But adding a microbial inoculum to a soil is like adding a drop to a bucket that is unlikely to enhance soil health.
“It is unlikely there will be a lasting effect of dropping in that drop in the bucket. If you have a minor dysfunction and you’re trying to rest a system, perhaps that type of inoculation with a microbial consortium might be useful. But under general circumstances those organisms that are best suited for that environment are already there,” Helgason said.
In terms of adding a soil amendment such as sugar, the microbial community will use it, but there is unlikely to be any fundamental change to the microbial ecology.
“Soil organic matter composition is their home, it’s their food. So, I’d be skeptical that you could have, again unless you are addressing some major dysbiosis or dysfunction, that you would really have any fundamental lasting effect by those kinds of treatments.”