Soil inorganic carbon (SIC), a potential carbon sink especially in arid and semi-arid environments, contributes to soil development, landscape stability, carbon (C) sequestration, and global C dynamics but due to the lack of SIC scientific reporting in most C sequestration research, its importance is unclear. A detailed overview of primary and secondary carbonate occurrence, formation, and importance is much needed to understand the role of pedogenic (PC)/secondary carbonate (a common biogeochemically derived soil mineral over time) in the SIC. The mechanisms involved in the formation of PC including carbon dioxide (CO₂) from microbial respiration and precipitation, silicate mineral weathering, dissolution, and reprecipitation are highlighted. The isotopic composition of carbonates related to biological C₃ or C₄ carbon fixation pathways and other paleoecologic and/or climactic factors responsible for new soil carbonate formation are discussed in detail. To address the lack of knowledge associated with SIC, this review attempts to highlight the currently known aspects of the literature, and briefly describe the formation and methodologies that can aid in addressing the research gaps surrounding SIC sequestration. The authors also suggest that greater focus needs to be provided on the actual measurement of SIC to develop a more comprehensive SIC inventory to provide sound data for future research direction, and modeling efforts and to predict C terrestrial storage and change efficiently.

The amount of carbon (C) sequestered in soil is related to soil texture, soil management, vegetation, and climatic variation. However, in the Northern Great Plains, little information is available to quantify the effects of soil texture on the C sequestration potential of soils. This work was conducted to develop relationships for C sequestration potential based on soil texture under a variety of agricultural practices. Soil samples were collected from central and southeast North Dakota from sites with differing soil management and cropping systems; this includes native prairie, differing Conservation Reserve Program year classes, no-till, and conventional tillage practices. Particle size analysis was determined on the 0- to 15-cm soil depth using a hydrometer method. Sand fractions were determined by sieving. Carbon analysis was done by a high temperature combustion method. For all sampled soils, total silt (%) was found to be positively correlated (P ≤ 0.01) to organic C content (percent organic C) and organic C mass (kg m⁻² depth⁻¹). Sand was found to be negatively correlated (P ≤ 0.10) with % organic C and organic C mass. Soil clay content was correlated with organic C mass (P ≤ 0.05) but not percent organic C. Bulk density was found to be negatively correlated with percent organic C (P ≤ 0.10). The strong correlation between silt content and soil organic C reflects the greater water holding capacity and plant available water of silt-dominated soils, which, in turn, affect plant productivity and influences C sequestration in soil.

The Conservation Reserve Program (CRP) is a U.S. federal land conservation program that incentivizes grassland reestablishment on marginal lands. Although this program has many environmental benefits, two critical questions remain: does reestablishing grasslands via CRP also result in soil health recovery, and what parts of restored fields (i.e., topographic positions) recover the fastest? We hypothesized that soil health will recover over time after converting cropland to CRP grassland and that recovery will be greatest at higher topographic positions. To test this, we sampled 241 midwestern U.S. soils along a grassland chronosequence (0–40 yr, including native grasslands) and at four topographic positions (i.e., a chronotoposequence). Soils were measured for bulk density, maximum water holding capacity (MWHC), soil organic C (SOC), extractable inorganic N, potentially mineralizable C (PMC), and N. Native grasslands had superior soil health compared with cropland and most CRP soils, and even 40 yr since grassland reestablishment was not adequate for full soil health recovery. Topographic position strongly influenced soil health indicators and often masked any CRP effect, especially with MWHC and SOC. However, PMC (a measure of active C) responded most rapidly to CRP and consistently across the landscape and was 26–34% greater 19–40 yr after grassland reestablishment. Reestablishing grasslands through CRP can improve soil health, although topographic position regulates the recovery, with greatest improvements at shoulder slope positions. Patience is needed to observe changes in soil health, even in response to a drastic management change like conversion of cropland to CRP grassland.

Recently, many studies related to carbon (C) sequestration by soils have been reported. However, little information has been reported related to the need for and effects of grinding soils on C analysis by high‐temperature combustion. We studied the effects of grinding five glacially derived soils of varying textural composition and organic matter content to 2‐mm, 1‐mm, 0.5‐mm, 0.25‐mm, and 0.15‐mm particle sizes on the measurement of total, inorganic, and organic C content using a high‐temperature combustion technique. Medium‐textured soils showed significantly higher total and organic C values for the 2‐, 1‐, and 0.5‐mm particle sizes than for the 0.25‐ and 0.15‐mm particle sizes compared to soils that were high in sand or clay. Grinding did not appear to affect inorganic C values. Grinding the soils to 0.15 mm, in all cases, greatly reduced the variability of C values by as much as two‐ to six‐fold when compared to 2‐mm soils. We recommend that soils be ground to pass a 0.15‐mm (100‐mesh) screen prior to C analysis by high‐temperature combustion.

Conservation crop residue management increases soil organic carbon (SOC) storage, nutrient cycling and availability and improves soil quality. This study was conducted to evaluate the amount of residue biomass, residue carbon to nitrogen (C:N) ratio, residue carbon (C) and nitrogen (N), and residue N fertilizer deficit (supplemental N fertilizer requirement) from crop residue decomposition in long-term no-till production. Aboveground aged and fresh residues were collected in spring 2011 and fall 2012, respectively. Results showed slightly greater residue dry matter weight in aged residue than fresh residue. C:N ratios were wider in fresh residue than the aged residue. Both aged and fresh residue also showed wider C:N ratio in the corn (Zea mays L.)-soybean (Glycine max L.) rotation (66.6 and 64.4, respectively) and narrower C:N ratio in the spring wheat (Triticum aestivum L.)-winter wheat (Triticum aestivum L.)-alfalfa (Medicago sativa L.)-alfalfa-corn (Zea mays L.)-soybean (Glycine max L.) (45.6 and 35.7, respectively). Individual fresh crop residues showed narrower C:N ratios for legume and cover crops than non-legume crops. Analysis of potential supplemental N fertilizer requirements showed greater potential N requirement for the fresh residue than the aged residue.

A 5-year study evaluated the change in the quantity of soil total C (STC), soil organic C (SOC), and soil inorganic C (SIC) stored in the surface 60 cm of the soil profile on two adjacent blocks of land with a long-term history of cropping (CH) or undisturbed grassland (NH) on similar soil types between 1999 and 2004. The NH area was tilled and a grass-legume species mix was seeded into plots on both the NH and the CH areas. Selected plots of restored grass were established so they could be grazed (GG) by livestock while other plots were left ungrazed (UG). Original undisturbed (and ungrazed) grassland plots within the NH area were used as a control treatment. Initially, STC and SOC in CH were lower than NH when compared under the semi-arid environmental conditions found in southwestern North Dakota. Over the study period, the undisturbed grass control plots had increases in STC and SOC levels in the soil profile of 3.90 kg·m⁻² and 3.34 kg·m⁻², respectively. Restored grass on the NH area with grazing showed increases in STC and SOC values of 2.11 and 1.26 kg·m⁻², respectively, while without grazing, profile STC and SOC had values of 3.80 and 3.28 kg·m⁻², respectively. Restored grass on the CH area showed increases in profile STC and SOC values of 0.55 and 1.96 kg·m⁻², respectively, for the grazed plots and 0.78 and 2.11 kg·m⁻², respectively, when left ungrazed. Soil inorganic C, though present in the soils, did not significantly change during the study. The lower C accumulation in the CH plots may be due to a lag time in the establishment of mycorrhizal associations with the seeded species, the inoculums of which were already present in the NH soils. Changes in STC were likely due to changes in water relationships in the soil profile where management changes affected water infiltration and its movement causing leaching of SIC below the 60 cm depth evaluated. Soils under undisturbed grassland continue to accumulate carbon while soils of the disturbed grassland or cropped prior to re-establishing grass showed losses that occurred due to either accumulating C at a lower rate or perhaps to C loss during the initial establishment period (1–2 years).