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Soil organic carbon (SOC) is broadly recognised as an important parameter affecting soil quality, and can therefore contribute to improving a number of soil properties that influence crop yield. Previous research generally indicates that soil organic carbon has positive effects on crop yields, but in many studies it is difficult to separate the effect of nutrients from the effect of SOC in itself. The aim of this study was to analyze whether the SOC content, in itself, has a significant effect on potential yields of commonly grown cereals across a wider range of soil types in Denmark. The study draws on historical data sets from the Danish national field trials consisting of 560 winter wheat ( Triticum aestivum L.) trials and 309 spring barley ( Hordeum vulgare L.) trials conducted over the past 20 and 17 years, respectively. We hypothesised that for these two crops, the potential grain yield, the yield with no fertiliser N application and the N use efficiency would be positively affected by SOC level. A statistical model was developed to explore relationships between SOC and potential yield, yields at zero N application and N use efficiency (NUE). The model included a variety of variables and aimed to elucidate the sole effect of SOC by controlling for potential confounding variables. No significant effect of SOC on potential winter wheat was found, whilst for spring barley, only for the course sandy loam soil type was a borderline significantly positive effect of SOC on potential yields found. The relationship between unfertilized plot yields and SOC was positive for winter wheat, although not significant, whilst for spring barley a significant positive effect of SOC was found only for the coarse sandy soil type, and a borderline significant positive effect of SOC was found for the coarse sandy loam soil type. A significant negative relationship was found between SOC and NUE for both winter wheat and spring barley. Based on the large dataset analyzed, we cautiously challenge the importance of SOC in contributing to crop productivity in contexts with similar soils and climate, and we speculate that in situations where nutrient limitation does not occur, SOC levels above 1% may be sufficient to sustain yields. In light of the findings presented in this study, further work should be conducted which can further elucidate the effect of SOC on yields. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) is believed to play a crucial role for many soil functions and ecosystem services. Despite much research, a lower threshold of SOC for sustainable crop production has not been identified across soil types. We addressed a comprehensive dataset with yields of winter wheat for nearly a 1000 field nitrogen (N) response experiments performed over 3 decades in Denmark. The soils ranged from coarse sand to clay loam (0.016-0.406 kg clay kg-1 minerals) with SOC ranging from 0.005 to 0.039 kg kg-1 minerals. All experiments were located on arable soil that was fully fertilized with all other plant nutrients. Based on the yield response at 4-6 levels of applied mineral N, we estimated the potential yield not restricted by any nutrient, Ypot, the yield at no applied N, YN0, the marginal agronomic efficiency (AEN0) in terms of the increase in yield per unit of applied N at zero N, and the level of mineral N, Nopt, needed to obtain Ypot. YN0 increased with Fines20 (proportion of soil minerals < 20 μm). AEN0 decreased with SOC independent of soil type. Nopt decreased with increase in SOC. Ypot increased significantly with a squared expression of Fines20 and decreased significantly with SOC. Ypot was better explained by RH = Fines20/SOC than SOC per se. Our results indicate a positive effect of SOC in terms of a reduction of mineral N needed to obtain the potential crop yield, and a negative effect on Ypot. We suggest a multigate regulation concept for future studies addressing the non-N effect of SOC. We further draw attention to the importance of the quality of the SOC for better understanding effects on crop yields. [ABSTRACT FROM AUTHOR]