Short-term carbon dynamics from ¹³C‑labelled maize residue and its derived biochar in long‑term NPK and NPK plus biochar amended soil

Enhancing soil organic carbon (SOC) in croplands is fundamental to climate‑smart agriculture (CSA), yet the mechanisms by which labile and recalcitrant carbon (C) inputs build and stabilize SOC remain unclear. Crop residues supply labile C that improve microbial activity and particulate organic matter (POM) formation, whereas biochar is considered a recalcitrant, negative‑emission amendment. This study used ¹³C‑labelled maize crop residues and its derived biochar, applied at 1.5% w/w (1.46 atom% ¹³C) to trace amendment-derived C into different soil C pools such as particulate organic C (POC), mineral-associated organic C (MAOC), microbial biomass C (MBC), and microbial respiration. Soil was sampled (Grabenegg, Austria) from two long-term management practices: one fertilized with mineral NPK since 1954 (SM1) and one receiving the same NPK regime plus wood‑derived biochar since 2022 (SM2). In a 44‑day laboratory incubation we quantified how the laboratory amendment of ¹³C-crop residue and ¹³C-biochar, alone and interacting with the long-term soil management practices, affect microbial utilization and C stabilization.

The results showed that field biochar treated soil (SM2) had higher SOC (by 51%) than non-biochar amended field soil (SM1), indicating that longer-term biochar application enhanced soil C stocks. In the short-term, at day 44 of the incubation, SOC under laboratory applied biochar was highest in both soils (SM1: 21.1g C kg^-1; SM2: 26.8 g C kg^-1), followed by crop residue (SM1: 15.2 g C kg^-1; SM2: 21.1 g C kg^-1), compared to soil with no laboratory C amendment (SM1: 11.8 g C kg^-1; SM2: 17.8 g C kg^-1). This highlights that in the short-term SOC gains were higher for biochar (SOC gains of 9.0-9.3 g C kg^-1) than for crop residue (SOC gains of 3.0-3.3 g C kg^-1). In addition, both soils showed a strong positive relationship between SOC and POC (R² = 0.783) but not with MAOC, indicating that SOC increases were largely driven by changes in the POM. The ¹³C tracing will allow to partition and follow the amendment-derived allocation of C into microbial biomass, respiratory use, and the transfer from the POC into the MAOC pool by microbial turnover (data evaluation ongoing). Overall, biochar proved to be the most effective CSA amendment for enhancing SOC in both long-term and in the short-term.