Assessing soil organic matter quality as an attribute of soil health in long-term tillage and crop rotation experiments

Principal Investigator: Amanda Diochon

Project Title: Assessing soil organic matter quality as an attribute of soil health in long-term tillage and crop rotation experiments

Scientific Summary:

There is a large knowledge gap on the quantifiable effects of agricultural management practices on soil health. One key attribute of soil health is soil organic matter and its measurement is included in many of the commercially available soil health tests, such as the Cornell Soil Health Test. Though measurement of soil organic matter is straightforward, it can be difficult to assess its status and response to management practices because the change in soil organic matter that occurs in response to management practices is small relative to the large background pool in the soil. Since soil organic matter is positively related to resilience, management practices that promote the accumulation of organic matter should also be resilient to external environmental pressures like extreme moisture conditions. Detecting change in soil organic matter in response to management may be enhanced by measuring attributes such as microbial biomass carbon, mineralizable carbon, and light fraction carbon that are highly responsive to changes in the inputs or outputs of organic matter.

The goal of the project was to examine the response of seven attributes of soil organic matter to tillage and crop rotation (corn, soybean and wheat) practices at four long-term agricultural experiments in Ontario (Elora, Delhi, Ottawa and Ridgetown) that have rotations with corn, soybean and wheat in fields that are no-till and those that are conventionally tilled using moldboard ploughing.

At the Elora site, significant effects of rotation were observed in five of the seven indicators but only one attribute showed a significant effect of tillage. Soils that were continuously cropped with alfalfa had significantly higher total organic carbon concentrations than soils cropped continuously with corn or rotations of corn and soybean. Rotations of corn with alfalfa had significantly higher total organic carbon concentrations than rotations of corn and soybean. Comparisons of the rotation treatments showed that carbohydrate carbon concentrations were significantly higher in soil under corn-soybean rotations than continuous corn. Soils under continuous alfalfa and corn in rotation with alfalfa had significantly higher concentrations of sand fraction carbon than the other rotations but rotations of corn and alfalfa had higher concentrations of sand fraction carbon than soils under continuous alfalfa. Comparisons of rotation showed that concentrations of microbial biomass were significantly greater in the continuous corn rotation than all other rotations. There were statistically significant differences for rotation on the ratio of microbial biomass carbon to total organic carbon. The ratio was significantly greater in the continuous corn treatment than all other rotation treatments. No-till had significantly higher concentrations of carbohydrate carbon than conventionally tilled soils.

At the Delhi site, two of the seven attributes were responsive to rotation and only one attribute was responsive to tillage. Comparisons of the rotation treatments showed that soil under continuous corn had higher concentrations of carbohydrate carbon and high rates of mineralizable carbon than soil under winter wheat in rotation with soybean. Soils under no till had a significantly greater concentration of sand fraction carbon concentrations than soils that under conventional tillage.

At the Ottawa site, sand fraction carbon concentrations and the ratio of mineralizable carbon to total organic carbon were responsive to tillage. The ratio of mineralizable carbon to total organic carbon was also responsive to rotation. In the Ottawa soils there was a statistically significant interaction between tillage and rotation on concentrations of carbohydrate carbon and mineralizable carbon, so statistical analysis was applied to determine differences in rotation for both conventional tillage and no till treatments independently. Statistically significant differences were observed for carbohydrate carbon concentrations among rotation treatments under no-tillage. Soil under continuous corn had significantly higher concentrations of carbohydrate carbon than continuous soybean, continuous wheat, and corn in rotation with soybean and wheat. Soils under continuous wheat had significantly higher concentrations of carbohydrate carbon than soils under continuous soybean, but were significantly lower than soils with a rotation of soybean and wheat. In soils under conventional tillage, the concentration of carbohydrate carbon was significantly greater under continuous corn, wheat and soybean than the rotation of all three crops. Among the continuous crop treatments, continuous corn had significantly higher concentrations of carbohydrate carbon than continuous soybean and continuous soybean was greater than continuous wheat. There was a significant effect of tillage for only the continuous wheat and the rotation of corn, soybean and wheat. For both rotations, soils that were conventionally tilled had significantly higher concentrations of carbohydrate carbon.

For mineralizable carbon in the Ottawa soils, there was no effect of rotation on the quantity of carbon mineralized under no till and conventional tillage systems; however, the soils under continuous wheat that were conventionally tilled had significantly great concentrations of carbon mineralized in the bioassay than the soils that were not tilled.

Ottawa soils under conventional tillage had higher ratios of mineralizable carbon:microbial biomass carbon and higher sand fraction carbon concentrations compared to no till systems.  Soils that were under rotations of continuous corn had higher ratios of mineralizable carbon: microbial biomass carbon than soils under continuous soybean or corn in rotation with wheat and soybean.

In the Ridgetown soils, total organic carbon, carbohydrate carbon and mineralizable carbon were responsive to rotation, while sand fraction carbon and carbohydrate carbon were responsive to tillage. Soils that were under soybean-wheat rotations had significantly higher total organic carbon concentrations than soils under soybean-corn rotations. Soils under a continuous corn rotation had significantly higher concentrations of carbohydrate carbon than soils under rotation continuous soybean, and rotation of corn and soybean. The continuous corn treatment also had significantly more carbon mineralized from its soils than the soils under continuous soybean. Soils under no-till had significantly higher carbohydrate carbon concentrations than soils that were conventionally tilled. Soils that were conventionally tilled had higher concentrations of sand fraction carbon that those soils that were not tilled.

There was no universal best management practice that enhanced concentrations in the measured soil organic carbon attributes, which speaks to the influence of climate and soil texture in influencing the effects of management on soil organic carbon concentrations. Responsiveness of the indicators to tillage and rotation treatments varied among the sites, which is likely a function of soil texture and climate. Though not all indicators showed significant differences among the treatments, and in some cases the indicators were themselves opposing with respect to the effect of management practice(s), there are management practices at each site that enhanced carbon indicator concentrations. At the Delhi site, no till systems under continuous corn had the highest carbon indicator concentrations. In Ridgetown, there was no clear effect of tillage but rotations of continuous corn, as well as soybean in rotation with wheat enhanced carbon indicator concentrations. In Ottawa, soil carbon concentrations were greatest under conventional tillage with continuous corn. In Elora soils, no till systems under continuous corn or corn in rotation with alfalfa enhanced carbon indicator concentrations. Though rotations of continuous corn were frequently shown to enhance soil organic carbon concentrations, the systems received optimal applications of mineral fertilizer that could have detrimental effects on other aspects for the environment, such as emissions of nitrous oxide and runoff to waterways.

We were able to identify tillage and rotation practices at each of the sites that did enhance soil organic carbon concentrations in the attributes but often times there were no differences among the treatments. The lack of effect of tillage and rotation on soil organic carbon concentrations in the attributes may be because the cycling of carbon in the soil is efficient and the soils are carbon limited. This means that adding inputs to the soil, such as manure rather than mineral fertilizer, may be more effective at increasing soil organic carbon stores.

Soils from these four long-term rotation sites are being used to develop an improved soil health scoring framework for Ontario by evaluating commercially available soil health tests. The findings from this project will contribute to a larger project that is developing an improved monitoring system for soil health in Ontario and to the establishment of sustainable management practices (no till or conventional; crop rotations) that enhance grower revenues and improve environmental quality. With respect to indicators that were responsive to management, concentration of microbial biomass carbon, the metabolic ratio of microbial biomass carbon to total organic carbon, and mineralizable carbon should be considered for inclusion in the development of a soil health test for Ontario.

Project Related Publications

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