Biochar addition increases the soil active nitrogen fractions in abandoned grasslands in ecologically fragile areas

The soil nitrogen (N) pool is a critical indicator of land degradation in ecologically fragile systems. The addition of biochar (BC) is an effective strategy for enhancing soil functions across various ecosystems. However, its impact on soil N dynamics in ecologically fragile regions remains poorly understood. We examined the influence of BC addition (BC 0 : 0 t·ha −1 ; BC 20 : 20 t·ha −1 ; and BC 40 : 40 t·ha −1 ) on total nitrogen (TN), active organic nitrogen (AON: dissolved organic nitrogen (DON) and microbial biomass nitrogen (MBN)), and dissolved inorganic nitrogen (DIN: nitrate nitrogen (NO 3 − -N) and ammonium nitrogen (NH 4 + -N)) in the soil of an abandoned grassland within the ecologically fragile zone of the Loess Plateau. Adding BC significantly augmented the concentrations of TN (54.36 %), AON (16.27 %), and NH 4 + -N (41.73 %) and the ammonium-nitrate ratio (>1) in the 0–40 cm soil layer. Concurrently, BC reduced the NO 3 − -N/DTN values (total dissolved nitrogen) ( P < 0.05) and inhibited the conversion of soil NH 4 + -N to NO 3 − -N in comparison to the BC 0 treatment. The MBN content under the various treatments decreased with soil depth. There were 20.30 % and 41.92 % average reductions in the 10–20 cm and 20–40 cm soil layers, respectively, compared with the surface layer. Concurrently, the concentrations of NO 3 − -N (3.92–4.50 mg·kg −1 ) and NH 4 + -N (3.54–4.78 mg·kg −1 ) were predominantly concentrated in the 10–20 cm soil layer. The addition of BC altered the cumulative trend of the (AON + DIN)/TN ratio in the 10–40 cm soil layer, resulting in an average decrease of 16.23 %. However, compared with the BC 20 treatment, the BC 40 treatment reduced the NH 4 + -N (16.64 %) and MBN (9.41 %) contents and increased the DON content (13.79 %) in the 0–40 cm soil layer. Partial least squares path modeling and random forest analyses indicated that microbial mediation, through enzyme activity and biomass pools, governed the dynamics of AON. In contrast, soil physicochemical parameters exerted predominant control over the variation in DIN. Collectively, BC addition affected the transformation of elemental N by increasing the content of soil active N fractions. Our results suggest that BC application at 20 t·ha −1 would be effective in mitigating soil degradation and improving N retention within ecologically fragile zones. • The addition of biochar (BC) suppressed the transformation of NH₄ + -N to NO₃ − -N in the soil. • The addition of high amounts of BC reduced the accumulation of active nitrogen and aggregated DON content toward 20–40 cm. • Soil enzyme activities and microbial biomass played key direct and/or indirect roles in the changes in AON. • Soil physicochemical properties primarily control the changes in soil DIN.