Biochar Versus Crop Residues: Modulating Net Nitrogen Mineralization-Immobilization and Lowering Nitrification in Calcareous Soils

The impact of plant residues and their biochars on nitrogen (N) transformation in calcareous soils, particularly regarding the nitrification rate and the nitrogen immobilization-mineralization process, is not yet fully understood. This study conducted an incubation experiment to investigate the influence of wheat and corn straw residues, as well as their respective biochars, on the transformation of ammonium-N to nitrate in calcareous sandy loam soil. Key parameters assessed included the potential nitrification rate (PNR), net N mineralization-immobilization patterns, the capacity of biochars to absorb and release N, microbial respiration rates, and the populations of ammonium- and nitrite-oxidizing bacteria. The experiment was designed to evaluate a series of treatments comprising nine levels of amendment application. These treatments encompassed a control group with no residue application; wheat residue at 2% and 5% (w/w); corn residue at 2% and 5% (w/w); biochar derived from wheat residue at 1% and 3% (w/w); and biochar produced from corn residue at 1% and 3% (w/w). Additionally, two nitrogen fertilizer regimes were applied: one without N fertilizer and another with a N fertilizer application at a rate of 200 mg kg−1 sourced from (NH4)2SO4. Samples were collected at six different intervals: on days 0, 7, 14, 28, 42, and 56 after the start of the incubation period. The study’s findings indicated that incorporating biochar, both from wheat and corn residues, at a rate of 200 mg N kg−1, decelerated the conversion of ammonium into nitrate. In the initial 7 days of incubation, soils treated with biochar retained a notably higher ammonium concentration than those without biochar. Wheat residues contributed to a reduction in soil nitrate levels, whereas corn residues led to an increase. Biochar-amended soils exhibited the lowest PNR, with the highest PNR recorded in soils amended with 2% corn residues and supplemented with N fertilizer. During the initial stages of incubation, the biochars demonstrated inhibition of the nitrification process in the soil, with the nitrification inhibition being more pronounced in corn straw biochar compared to wheat straw biochar. Corn residues favored the net mineralization of organic N. Yet, the biochar made from corn encouraged net immobilization of the added N, signifying a drop in N mineralization rates. Contrastingly, wheat residues and their derived biochar resulted in immobilization of the added N, primarily due to their elevated C:N ratios. Regarding N absorption, corn residue biochar outperformed wheat residue biochar. However, wheat residue biochar revealed a greater N desorption rate than its corn counterpart. The application of biochar was associated with an increase in soil microbial respiration relative to the control; nevertheless, this rate of respiration was considerably lower than that observed in soils treated with plant residues. Lastly, while plant residues stimulated an increase in nitrifying bacteria populations, biochar did not significantly impact these bacterial populations when compared to the control soil. The research concludes that the choice of plant residue and biochar, along with their application rates, can have a significant impact on N transformation in calcareous soils, highlighting the need for careful consideration in agricultural practices for effective N management.