Effect of Hydrothermally Carbonized Char Application on Trace Gas Emissions from Two Sandy Soil Horizons

The application of biochar to soil is a potential tool for the long‐term sequestration of C and a possible mitigation of greenhouse gas (GHG) emissions. Among the various processes available to produce biochar, hydrothermal carbonization is one technique that is suitable for moist feedstock like digestates from biogas production. The aim of this study was to investigate the stability of C and emissions of N2O after the addition of (i) digested wheat (Triticum aestivum L.) straw (digestate) and (ii) hydrothermally carbonized (HTC) char of wheat straw as well as (iii) HTC char of digested wheat straw to two soil horizons that differed in C content. The HTC chars were obtained from wheat straw and digested wheat straw that were hydrothermally carbonized at 230°C for 6 h. The digestate and HTC chars were mixed with soil and incubated in 125‐mL vessels. The GHG emissions of CO2 and N2O were measured at regular intervals. Additionally, after 108 d, N was applied in the form of NH4NO3 equivalent to 100 kg N ha−1. After 500 d of incubation, the digestate had lost 34% of C, while the soil mixture with the corresponding HTC char lost 12% of C in the form of CO2 from the topsoil. The estimated bi‐exponential half‐life of the recalcitrant C was more than 50% longer for the carbonized material than for the untreated digestate. The N2O emissions from both HTC chars were significantly reduced compared with untreated digestate. The reductions were up to 64% for the topsoil and 60% for the subsoil samples. These laboratory results show that HTC holds the potential to increase the C stability of fermented and carbonized biomasses and to reduce N2O emissions.