Apparent Stability and Subtle Change in Surface and Subsurface Soil Carbon and Nitrogen under a Long‐Term Fertilizer Gradient

Abstract

Uncertainty exists regarding the depth and extent to which agricultural practices affect soil properties, in particular soil organic C (SOC). In this study we examined the impact of 53 yr of continuous corn (Zea mays L.) receiving varying rates of inorganic N fertilizer with complete stover return on soil properties including SOC, total N, and bulk density (BD) to a depth of 1 m. In the treatment receiving virtually no applied N there was a significant reduction in soil N content at 0 to 30 cm over the study period, while the treatment receiving N in excess of recommended application levels was similar to the treatment receiving recommended rates of N fertilizer. Trends in SOC content were similar to those for total N, but a significant treatment effect was detectable throughout the entire 1-m soil profile sampled. Over the course of the study, all experimental fields appeared to have lost approximately 6 cm of topsoil through erosion, with the lowest N rate plots subsiding a further 2 cm due to compaction. Despite using disruptive management practices (moldboard plowing and application of anhydrous ammonia), declines in soil C and N content were not apparent for this soil type under conditions of low slope and the linear gains in productivity realized under the two higher N rate treatments. Thus N fertilizer was a benefit to this cropping system, rather than a detriment, and was sufficient to allow maintenancebut not building-of SOC.

Surface‐soil carbon stocks greater under well‐managed grazed pasture than row crops

“…The other grain and forage treatments in the study lost SOC over this 20‐yr period, similar to findings from the same site by Collier et al. (2017). Comparing approaches with cool‐season pasture or hay management in a 2‐yr study, Oates and Jackson (2014) found rotational grazing had greater potential for C accumulation than continuous grazing, harvested hay, and unharvested pasture, but that the net C balance of these pastures was near zero.…”

“…In a 20-yr study of grain and forage systems in southern Wisconsin, no significant change in SOC was found under rotationally grazed cool-season pastures despite an increase in the surface 15 cm (Sanford, 2014). The other grain and forage treatments in the study lost SOC over this 20-yr period, similar to findings from the same site by Collier et al (2017). Comparing approaches with cool-season pasture or hay management in a 2-yr study, Oates and Jackson (2014) found rotational grazing had greater potential for C accumulation than continuous grazing, harvested hay, and unharvested pasture, but that the net C balance of these pastures was near zero.…”

Quantifying carbon for agricultural soil management: from the current status toward a global soil information system

“…cropland conversion to grassland with deep-rooted species) can have impacts deeper in the soil profile [42]. Over decadal time scales, relatively minor changes to subsoil SOC stocks that manifest under many cropping systems can constitute non-trivial quantities of C at the farm scale [43]. Because variability in SOC stocks tends to increase as a function of depth, while the impacts of most management practices on stocks tends to decrease with depth, efficient analyses of SOC changes should evaluate SOC stocks sequentially, from the surface to increasing cumulative depth layers, to the full depth of sampling [44].…”

Mechanisms of soil organic carbon stability and its response to no‐till: A global synthesis and perspective

“…Fertilizer application changed the contributions of wheat and maize to SOC, and caused a spatial variability in wheat–maize cropping systems (Shi et al, 2017; Wang et al, 2015). Nitrogen fertilizer alone could be sufficient to allow maintenance but not build‐up of SOC compared with unfertilized soil because mineral fertilizer application increased SOC mineralization rate in aggregates (Collier et al, 2017; Yu et al, 2012). Thus, understanding the processes of SOC stability is crucial to enhancing SOC sequestration.…”