Revisiting a short rotation coppice plot trial 10 years after biochar application: can we expect long-term effects on soil organic carbon content?

Biochar and related commercial products have been intensively studied in the past years with respect to their potential benefit for soil fertility and carbon sequestration. Yet, little is known about long-term effects of modern biochar application to soil organic carbon (SOC) content at plot scale and in temperate climate.

To evaluate potential effects of biochar application on soil organic carbon (SOC) and total nitrogen (N_tot) content, the soils of a field trial at Campus Klein-Altendorf, an experimental site of the University of Bonn (Germany), were re-sampled in 2022. The short rotation coppice plot experiment with four tree species had been established in 2012 on a Luvic Stagnosol formed from loess. Four treatments in threefold replication were realised: untreated control, compost, and two biochar–compost substrates with 15% (BCS15) and 30 % biochar (BCS30), respectively (in total 48 plots). All amendments were applied at 30 Mg dry matter ha^–1 and incorporated to 0.15 m depth before the trees were planted.

Prior to application of the amendments in 2012, the mean SOC and N_tot content were typical for arable loess soils in the region. In the first three years, no significant effects on soil fertility parameters and plant growth were observed, but C stocks were enhanced in the biochar treatments (von Glisczynski et al., 2016).

During the re-sampling campaign after ten years, soil samples were taken from 0-0.1, and 0.1-0.2 m depth, corresponding to the sampling design at the beginning of the trial. The samples were conventionally analysed for SOC and N_tot (elemental analysis). Priority was given to the control and the 30% biochar treatment to estimate the maximum effects. The first and preliminary results point to a significant increase of the SOC content in the BCS30 treatment, but also in the untreated control when compared to the initial contents published by von Glisczynski et al. (2016). The mean SOC content in 0-0.2 m depth over all tree species raised from 12.5 g kg^-1 to 13.8 g kg^-1 (control) and 16.9 g kg^-1 (BCS30) SOC, respectively. Total N also increased to 1.3 (control) and 1.4 g kg^-1 (BCS30), respectively. Resolving more in detail the depth distribution clearly showed that the SOC increase occurred predominantly in the uppermost 0-0.1 m (BCS30: 21.5 g kg^-1, control: 16.5 g kg^-1); here, the BCS30 treatment showed a significantly larger C:N ratio than the control. In contrast, little difference was observed in 0.1-0.2 m depth (BCS30: 12.2 g kg^-1, control: 11.2 g kg^-1). The increase of SOC and N_tot also in the untreated control as well as their depth profiles point to an effect not only of biochar application, but also of abandonment of soil cultivation for 10 years in the short rotation coppice. However, further in-depth investigations are necessary to confirm these presumed long-term effects.