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Quality and Quantity...

In the last few days two papers have been published that fit together very nicely and have given us deeper insights into the dynamics of soil carbon after organic matter inputs differing in qualtiy and quantity. Christopher Poeplau first author of one paper and involved in the other paper.

Root samples in laboratory vessels
© Thünen-Institut/AK

One study is a 2-year incubation of Miscanthus roots with very different C:N ratios, which also had a clear effect on the decomposition of native soil organic matter. The less N in the biomass, the more the addition of biomass stimulated the mineralization of old carbon. The energy from the biomass is used to access more N-rich compounds in the soil. Thus, at least in the C:N range considered (50-124), a narrow C:N ratio was most positive for soil carbon accrual. The wide C:N ratio in biomass could not be compensated even by additional N inputs, which might suggest that the observed dynamics are relatedto more than just chasing N. Indeed, the overall chemical composition of the root biomasses also varied greatly. In any case, it is clear that the quality of C inputs controls not only their own decomposition, but also the mineralization of old carbon.

Poeplau, C., Begill, N., Liang, Z. et al.: Root litter quality drives the dynamic of native mineral-associated organic carbon in a temperate agricultural soil. Plant Soil (2023). https://doi.org/10.1007/s11104-023-06127-y

Winter cover crops on a field
© Thünen-Institut/AK

In the second study, the question was why sometimes no or even negative effects of cover crops on soil arbon are found. Here, different amounts of 14C labeled roots of fodder radish were incubated with soil and it was found that quantity had a strong effect on priming, i.e., increased mineralization of native soil carbon. The more biomass was added, the less priming occured. Then, in the soil considered, there was a threshold above which it turned from positive priming (increased mineralization compared to the control) to negative priming (attenuated mineralization). This was surprising and may indicate that little input (compared to no input) may have no effects on humus, or even negative effects in some circumstances. Problems of detectability of small changes are of course added, especially in Nordic trials where cover crop biomass tends to be low.

Zhi Liang, Jim Rasmussen, Christopher Poeplau et al.: Priming effects decrease with the quantity of cover crop residues – Potential implications for soil carbon sequestration, Soil Biology and Biochemistry (2023). https://doi.org/10.1016/j.soilbio.2023.109110

Taken together, the practical recommendations would then be almost trivial: Biomass inputs should not be too scarce and not of low quality to efficiently build up soil carbon.

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