Organic carbon convergence in diverse soils toward steady state: A 21-year field bioassay


Yanni, S.F., Janzen, H.H., Gregorich, E.G., Ellert, B.H., Larney, F.J., Olson, B.M., Zvomuya, F. (2016). Organic carbon convergence in diverse soils toward steady state: A 21-year field bioassay. Soil Science Society of America Journal, [online] 80(6), 1653-1662.

Plain language summary

The health of a soil is closely linked to the amount of carbon-rich organic matter it holds. Our study, based on relocation of initially diverse soils to a common field site, demonstrated the persistence and resilience of soil organic matter in prairie soils of western Canada. The rate of organic carbon loss (or gain) was primarily a function of initial organic carbon content. Our projections suggest that the total soil organic carbon content of these soils may not approach convergence for at least one or several centuries. More important than changes in total soil organic matter, however, may be the changes in labile fractions, which accounts for many of the benefits of soil organic matter to soil health and productivity. Comparatively rapid changes in labile soil organic matter suggest that these fractions may converge within one or several decades. At that time, many of these soils will have similar concentrations of labile soil organic cmatter but with still widely different levels of more stable organic matter. This experiment, therefore, if continued, may offer further insights into the relative contributions of labile and stable organic matter to the health and productivity of soils in the semiarid region of southern Alberta.


The response of soil organic matter (SOM) to an abrupt change in environment and management was studied in a long-term field bioassay. Thirty-six soils, with a wide range of SOM reflecting large differences in management, climate, and ecological histories, were selected and transported to a common site at Lethbridge, Alberta, Canada. The experiment included control and N-fertilized treatment plots. The plots were seeded annually to spring wheat and managed under no-till rain-fed conditions, with annual removal of aboveground residues. The soils were sampled every 7 yr and analyzed for organic C (OC), total N (TN), and light fraction C (LF-C) and N (LF-N). Most of the soils lost OC and TN over 21 yr, and the amount of loss was highly related to the original contents (9-74 g C ka-1, 1.4-6.4 g N ka-1). Similarly, LF-C and LF-N, indices of labile OM, declined in most soils, but the rate of loss was faster than for OC. In a few soils, SOM slowly increased from initially low levels. The soils seem to be progressing toward a steady state, although complete convergence of OC and TN may not occur for a century or longer based on an exponential model. Labile OM may converge within a few decades, so that these soils, conceivably, will still have widely different OC and TN but similar labile SOM, with possible implications for soil health. Our findings demonstrate the value of long-term field bioassays for improved understanding of SOM dynamics in response to management and climate change.

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