Impacts of MgO- and sepiolite-biochar composites on N-partitioning and dynamics of N-cycling bacteria in a soil-maize system: A field-based 15N-urea tracer study

Improving fertilizer-N retention and use efficiency (NUE) is imperative to mitigate the loss of reactive N into the environment. To reach additional improvements in N retention and NUE, we formulated MgO-and sepiolite-biochar nanocomposites with optimized N-retention capacity. The field-scale application of the modified bio-chars along with 15N-labeled urea (applied at 150 kg ha-1) was used to evaluate growth-stage-related N -parti-tioning, NUE, and N-recovery in a soil-maize (Zea mays L.) system. The effect of these biochars on growth-stage shifts in soil N-cycling enzymes, bacteria, and gene copies was reported for the first time. Results showed that during the 12-leaf stage, the MgO-biochar (MgOBF), sepiolite-biochar (SBF), and raw biochar (BF) amendments increased 15N-urea retention by 83.7 %, 26.7 %, and 33.0 %, respectively, than the sole fertilizer treatment (F). The improved 15N retention was attributed to the potential formation of an Mg-N complex, increased surface area, and cation exchange capacity (CEC) of the modified biochars particularly, in MgOBF, across maize growth stages compared to the raw biochar and SBF. Moreover, there was a simultaneous increase in the uptake of soil N across the plant growth stages in MgOBF and SBF compared to F and BF. At the physiological maturity (PM) stage, the total15N uptake by maize was 17.1 % and 10.5 % higher in MgOBF and BF, respectively, compared to F. Moreover, the slow-release of the retained 15N and its increased uptake across maize growth stages increased NUE, biomass, and grain yield in MgOBF compared to other treatments. Hence, a total N recovery (soil + plant) of 90.8 % was recorded in MgOBF compared to 81.7 %, 67.9 %, and 67.8 % in the BF, SBF, and F treatments, respectively. A higher relative abundance of key N-cycling bacteria and their gene copies was associated with an increase in N-cycling enzymes in MgOBF and SBF at the PM stage, hence, promoting the slow-release of the retained N for plants' use. Our findings provide new insights into the applicability of modified biochars, espe-cially, the MgO-modified biochar for improving fertilizer-N retention, NUE, and its recovery in the soil-plant system, while improving soil chemical properties.

Automated summary

Improving fertilizer-N retention and use efficiency is crucial for reducing reactive nitrogen loss into the environment. This study evaluated the effects of MgO- and sepiolite-biochar nanocomposites on N-retention and NUE in a soil-maize system. The results showed that the modified biochars significantly increased the retention and uptake of 15N-urea, resulting in improved NUE, biomass, and grain yield compared to the sole fertilizer treatment. The MgO-modified biochar, in particular, demonstrated the highest N retention and promoted the slow-release of retained N for plant use.