LAUSR

Earth scientists find themselves within an unsettling timeframe in history, where they are discovering the human impacts on the climate system while, at the same time, being under considerable societal pressure to develop ways to remediate it. Until sometime in the 20th century, the combined effects of land use—both biomass removal and soil carbon oxidation through farming—were the largest source of emissions to the atmosphere (1) (Fig. 1), and, even today, they represent roughly 10% of net emissions. Given the 133-Gt soil carbon deficit that has accrued over time (2), it is to be expected that, beginning about 20 y ago (e.g., ref. 3), proposals for repaying this carbon debt, through enhanced farming practices, began to emerge as a climate mitigation strategy. In the intervening time, more and longer-term studies have emerged, somewhat tempering the optimism for this strategy (e.g., refs. 4–6). In a significant, multidecadal research study established on the former prairie landscape of southern Wisconsin, Rui et al. (7) find that tillage and cultivar practices commonly proposed to regain soil C had minimal impacts, and that the only mechanism that began to repay the soil C debt, in the upper 30 cm, was the conversion of the plots back to permanent grassland. The amount of carbon in soils is the balance between plant inputs and the annual loss, by microbial respiration, of CO2. In most climatically stable native systems, this balance is close to a steady state. But soils are complex biochemical systems, and the cultivation of these systems unleashes an array of unintended consequences, including the rapid decline, in less than a century, of around 50% of a soil’s C. Like the mythical Pandora’s box, it is, unfortunately, easier to let the C out of soil than to put it back in. The Wisconsin Integrated Cropping Systems Trial (WICST) was started in 1989 to examine how alternative cropping systems perform on the loess landscapes of this region. Included among the treatments are some of those commonly suggested for gradually replacing soil C stocks in US row-crop agriculture: minimum tillage of corn and soybean as well as organic crop rotations. Included in the initial experiment was a managed permanent grassland treatment. In 1999, three native grassland treatments were also installed (not part of the study here). The paper by Rui et al. (7) focuses on the differences between treatments after 29 y, revealing that only the permanent pasture had somewhat higher soil C (both in concentration [Fig. 2A] and mass) than the other treatments. The addition of data collected at the start of the experiment, and again in 2009 by Sanford et al. (8), reveals the sluggish nature of C change over time (Fig. 2A). A serious problem with C sequestration efforts is knowing the local natural “limit” to soil C imposed by the local environmental boundary conditions (9). There are no datasets for uncultivated Plano soils that underlie the research site, so we do not know the original C baseline. The C contents for the experiments fall within the range of soil C found in three nearby soils sampled in 1962 by the Natural Resource Conservation Service (NRCS) (Fig. 2A), although, of course, these sites likely had approximately a century of use before sampling, and thus may represent the regional “new normal.” The slowness of the C recovery might be attributed to the challenges of capturing C while simultaneously working the land for crop or forage production. However, in a unique long-term prairie restoration study just 15 km south (near Madison, WI), soil C in a 69-y-old restored prairie remained at about 1/2 that of an adjacent prairie remnant (10). It seems that, at least in this region of Wisconsin, restoring significant amounts of preagricultural soil C is a long-term proposition. One of the important recommendations of recent research is the need for deep soil monitoring (to a meter or more) to accurately capture the total net soil C changes under differing managements. This is needed because agricultural practices change rooting depths of plants, and also change the depth of surficial inputs (due to tillage). Rui et al. (7), unfortunately, only report total, mineralassociated, and particulate C to a depth of 30 cm. However, Sanford et al. (8) measured soil C at the WICST site to a depth of 90 cm, and found that all treatments in the WICST lost C below the upper 15 cm over the initial 20 y of the trial (Fig. 2B). Annual crops, and some of the cultivars chosen for the pasture mix, do not have the same deep rooting systems as the native grassland species, which Fig. 1. Fifty-year windows of integrated C emissions from land use versus fossil fuel emissions. Data are from ref. 18.