SOC compounds containing nitrogen (N) are highest in check/control treatments 

When estimating levels of soil organic carbon with nitrogen (SOC-N) compounds, a common assumption would be that high levels of SOC-N compounds could be higher in inorganic fertilizer amendment (NPKS) practices. However, as the results indicated in (Figure 12) the highest levels of SOC-N compounds are present in 5Y-Check/Control and WF-Check/Control treatments, which did not have any addition of amendments. This provides insights that the Breton plot soils may contain higher background levels or other confounding effects of SOC-N compounds. 

Fallow indicates a loss of SOC 

The comparisons between wheat fallow fertilized (WF-NPKS) and continuous grain fertilized (CG-NPKS) management practices provided the opportunity to compare the effect of fallow on SOC classes. (Figure 4 and 5, Data Page) illustrate that CG-NPKS resulted in significantly elevated concentrations of soil SOC compounds, particularly aliphatics, aromatics, carbohydrates, and phenolics, across both shallow (0–7.5 cm) and deep (7.5–15 cm) soil layers, possibly due to the effect of fallow in WF-NPKS. However, in contrast, the WF-NPKS exhibited significantly higher concentrations of SOC-N compounds, suggesting a distinct compositional shift in organic carbon associated with this treatment regime. This could also be due to the confounding effects of SOC-N compounds as identified above.  This indicates that effective management during fallow periods is essential to maximize the benefits of SOC accumulation and maintain soil fertility.

Inorganic amendments influence SOC classes in 5Y vs WF 

Both the five year rotational (5Y) and WF systems, encompass three different amendment treatments: control treatment (check), inorganic amendment (NPKS/fertilizer) and organic amendment (manure). Therefore, these 5Y and WF management practices were compared together to determine the amendment effects (organic vs inorganic) on the soil organic carbon classes.

There are no distinct differences that are common across the SOC classes between the four management practices, (5Y-Manure, 5Y NPKS, WF-Manure and WF–NPKS).  However, as indicated in (Figure 13), in shallow soil layers inorganic amendments (NPKS) are significantly higher in all SOC classes except for SOC-N compounds in 5Y compared to WF. In deep layers, this is evident only in SOC classes aliphatics and carbohydrates (Figure 13). There are no significant differences in organic amendments (manure) in SOC classes between 5Y and WF, except for SOC-N compounds in shallow layers. However, in deep layers aliphatics and carbohydrates have significantly higher organic amendments in 5Y compared to WF. Within the 5Y and WF management practices, there are no significant differences between inorganic and organic influences on SOC classes. 

In SOC-N compounds WF has higher levels compared to 5Y in both inorganic and organic amendments.  This could also be due to the confounding effects of SOC-N compounds identified above.

Foraging influence only SOC class: aliphatic

Comparison between the continuous grain (CG), 5Y-NPKS, and continuous forage (CF) where all treatments received conventional fertilizer were expected to show an increase in the amount of perennial forages (CF>5Y>CG) in cropping systems influencing soil organic carbon classes. The treatments primarily differ in the degree of perennial forage inclusion where CG receives no forage vs 5Y - every 2/5 years vs CF - continuously for 41 years. However, this is only evident in the SOC class aliphatic in shallow layers with a significantly higher concentration in CF compared to 5Y and CG (Figure 14). 

Highest SOC in shallow layers, and 8Y manure management practice

Shallow layers have higher concentrations of SOC among all classes (Figure 15 and 16) with a clear separation between the SOC concentrations in the two depths, specially in management practices such as CF-NPKS. However, this is not true for SOC-N compounds where some management practices have equal concentrations between the two depth layers. Samples from shallow and deeper soil layers also showed separation in the PCA biplot (Figure 11, Data Page) as well, suggesting chemical composition differences in the samples from shallow soil layers compared to samples taken from deeper layers.

In addition, as evident in both (Figure 15, 17, and 18),  the highest concentration of SOC is present in the 8Y Manure practice (in shallow layers). 

This study highlights the complex interactions between long-term agricultural management practices and soil organic carbon (SOC) composition in the Breton Plots. The results demonstrate that fallow practices, as observed in the WF-NPKS system, lead to a reduction in total SOC content, particularly in key carbon classes such as aliphatics, aromatics, carbohydrates, and phenolics. Interestingly, SOC compounds containing nitrogen (SOC-N) were consistently higher in unamended / control (check) plots, suggesting the influence of background soil characteristics or other confounding factors.

The study also revealed that inorganic amendments (NPKS) in shallow layers were more effective in increasing most SOC classes in 5Y systems while organic amendments (manure) showed limited differences. Forage inclusion also had minimal effects. Depth stratification was evident, with higher SOC concentrations in shallow layers across all treatments—particularly under the 8Y Manure practice, which yielded the highest overall SOC levels.

For future research, using this same data set a full overall view of the complete chemical signature could be identified for further understanding of chemical dynamics in Breton Plots - “a focal point for research and discovery, CONTINUING to provide science and best practices on soil health, higher yields, greenhouse gas emissions, nitrogen fertilizers, nutrient balances, crop rotation, and carbon sequestration” (Dyck & Puurveen, 2023).