Understanding the effects of crop rotation on soil organic carbon stabilization
Term: 1 year ending Feb 2023
Status: Complete
Researcher(s): Bobbi Helgason, U of S; Reynald Lemke, Jennifer Town, Mervin St. Luce, AAFC; Jeff Schoenau, U of S, Meagen Reed, U of S
SaskCanola Investment: $5,443
Total Project Cost: $50,893
Funding Partners: ADF, WGRF, SWDC
Grower Benefits
Functionally important mineral-associated organic matter (MAOM) and particulate organic matter (POM) were not consistently affected by long term continuous canola or wheat vs. diverse crop rotations in canola and wheat systems.
Over the long-term, the quantity of MAOM and POM pools in the annual cropping systems studied may be determined more strongly by intrinsic soil properties, with quantity and quality of organic matter inputs playing a secondary role.
Project Summary
Soil organic matter (SOM) plays an important role in soil health and sustained soil productivity. Increasing soil carbon (C) storage is beneficial for soil health, mitigating rising atmospheric CO2 concentration and off-setting greenhouse gas emissions from crop production systems. POM consists of coarse, light organic material that is vulnerable to loss following disturbance and is considered the fast cycling pool of soil organic carbon (SOC). MAOM consists of more stable, persistent compounds that are formed by microbial metabolism of plant carbon and are sorbed through chemical bonding on clay surfaces. Both of these components (along with water extractable organic matter, or WEOM) make up SOM. Some types of SOM can be destabilized through microbial decay more easily than others, which leads to differences in SOC turnover. This turnover serves as fuel for other biological processes and can affect susceptibility of the remaining SOC to future loss. Prediction of how management decisions affect the stability of SOC is needed. Reduced tillage and continuous cropping have contributed to large quantities of C being stored in the soil, so researchers wanted to understand how different crop species and their use in rotations affect the cycling, protection, and stabilization of C. Researchers also wanted to quantify C storage in different soil functional pools and learn more about the relationship of microbial abundance, community structure and activity with soil organic matter storage and utilization.
This study used samples that were previously collected from two long-term crop rotation studies in locations in Saskatchewan and Alberta. These samples were originally collected with the intention to provide functional information about nutrient cycling and plant root-microbe interactions of key rotation crops. For the canola frequency study which compared continuous canola to canola-wheat and canola-pea-barley rotations, the canola phase was sampled in both 2018 and 2019 which represented the 11th & 12th year of the long-term study and the researchers focused on glufosinate-resistant canola. The samples were taken at Swift Current, Scott and Lacombe to represent the three major soil zones. Multiple assessments were conducted on the soil collected including soil organic matter fractionation, phospholipid fatty acid profiling, and extracellular enzyme activity.
The differences in POM and MAOM fractions resulting from long-term crop rotation or canola monocropping varied between locations. Significantly more carbon was stored as MAOM than POM at all sites. There were no differences in the mass fraction of POM at any of the sites, but the C concentration of the POM responded more dynamically to differences in canola frequency compared to MAOM resulting in some difference between rotations. These year-to-year differences between rotation treatments did not follow consistent trends within or across sites. At Lacombe and Scott, there were no differences in the mass fraction of POM carbon content in the canola-wheat soil in 2018 and the continuous canola soil in 2019, and was the lowest in canola-pea-barley in 2019. There were no differences in the mass fraction as POM at Swift Current, but the POM carbon content was highest in the continuous canola 2018 soils compared to all other soils sampled at that location.
At Lacombe, there was no effect of crop rotation treatment on the total microbial biomass or in the abundance of any of the functional microbial groups in 2018 or 2019. The fungal-bacterial biomass ratio was higher in continuous canola versus canola in 2- and 3-year rotations. At Scott, continuous canola had the highest total and bacterial biomass as well. At Swift Current, there were no significant effects of crop rotation on any of the biomarkers, and total biomass and general bacterial biomass was higher in 2019 versus 2018. The microbial community structure at each of the three sites showed clear effects of sampling year. Crop rotation had a significant effect on community structure only at Scott.
Phosphatase (enzymes released from canola roots that mineralize phosphate from organic phosphorus pools) and arylsulfatase (enzymes that control the buildup of organic sulphur and soil sulphur cycling) activity were both significantly impacted by canola frequency in 2018. Soils with continuous canola had greater phosphatase activity compared to canola-pea-barley in 2018, but arylsulfatase was lowest in this rotation. There were no effects on other enzymes including Beta-D-glucosidase (responsible for the catalysis of the last stage for glucose release from cellulose), N-acetyl-glucosaminidase (plays a role in soil nitrogen remineralization rates by breaking down chitin), and phenol oxidase (involved in degradation of many recalcitrant aromatic compounds).
The lack of consistent differences in POM and MOAM carbon pools observed under different canola cropping frequencies indicates that the interplay between quality and quantity of crop root and residue inputs did not lead to differences in C stabilization as reflected in these important pools. Soils were sampled in the canola phase (at peak flowering), the long-term impact of differences in residue input quality may have been masked partially by the actively growing canola crop, particularly for the POM pool.
Full Report PDF: Understanding the effects of crop rotation on soil organic carbon stabilization