Over the last century, researchers and practitioners with diverse backgrounds have articulated the importance of improving soil organic matter (SOM) contents in agricultural soils. More recently, climate change scientists interested in CO2
sinks, and agroecologists interested in ecological intensification have converged on the goal of building SOM stocks in croplands. The challenge is that agriculture itself is responsible for dramatic losses of SOM. When grassland or forest ecosystems are first converted to agriculture, multiple mechanisms result in SOM declines of between 20% and 70%. Two of the most important mechanisms are the reduction in organic matter inputs from roots following the replacement of perennial vegetation with annual crop species, and increases in microbial respiration when tillage breaks open soil aggregates exposing previously protected organic matter. Many agricultural practices such as conservation tillage and integration of cover crops have been shown to achieve some degree of SOM improvement, but in general adoption of these practices falls short of accumulating the SOM stocks maintained by grasslands, forests or other native ecosystems that agriculture replaced. Two of the overarching reasons why native terrestrial ecosystems have achieved greater soil organic matter levels than human agroecosystems are because they direct a greater percentage of productivity belowground in perennial roots, and they do not require frequent soil disturbance. A growing body of research including that presented in this review suggests that developing perennial grain agroecosystems may hold the greatest promise for agriculture to approach the SOM levels that accumulate in native ecosystems. We present calculations that estimate potential soil organic carbon accumulation rates in fields converted from annual to perennial grains of between 0.13 and 1.70 t ha−1
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