Effects of field-aged biochar on denitrification of vegetable soils under different temperature conditions in south China

Objectives In order to improve the understanding of how biochar reduces N₂O emissions, we compared the effects of fresh and aged biochar at different temperatures on N₂O emissions from intensive vegetable soils in southern China.

Methods An incubation experiment was conducted, and the treatments were soil without biochar addition control (CK), addition of fresh biochar (FB), and addition of field-aged biochar (FAB) at 10°C, 20°C, and 30°C. A parallel treatment was set up for each treatment with and without the addition of 10% volume content of acetylene to measure the N₂O emission, and the difference was made to obtain N₂ emissions. For the treatments without acetylene addition, soil pH, electrical conductivity (EC), dissolved organic carbon (DOC), \rmNO_3^- , \rmNH_4^+ , and \rmNO_2^- , as well as functional genes for denitrification (nirS, nirK, nosZ, and nosZII) were measured.

Results The elevation of incubation temperature significantly increased N₂O and N₂ emissions, increased soil pH and \rmNH_4^+ -N content, decreased soil DOC and \rmNO_3^- -N contents (P<0.01), but did not affect the abundance of nirK, nirS, nosZ and nosZII. FB and FAB treatments significantly reduced N₂O emissions by 18.8% and 22.3% at 30℃ respectively and the total denitrifying nitrogen amount (N₂O + N₂) (P<0.01). FAB significantly increased the nirK and nosZII gene abundance at 30℃ (P<0.05). The redundancy analysis showed that soil pH, NO₂^－ and DOC were the main factors affecting denitrification function genes, among which pH was the most important factor. Stepwise regression analysis showed that N₂O emission was mainly affected by DOC content in CK treatment, N₂O emissions in the FB treatment were mainly influenced by soil \rmNO_3^- content, and N₂O emissions in the FAB treatment were influenced by soil \rmNO_3^- and \rmNO_2^- content.

Conclusions N₂O emissions increased with temperature nonlinearly, and the greatest increment occurred from 10°C to 20°C. Fresh biochar enhanced soil pH and retain \rmNO_3^- from participating in denitrification process, thus reduced N₂O emission. FAB decreased soil \rmNO_3^- and \rmNO_2^- contents, nirK and nosZII genes abundance, and soil N loss as NO or N₂, not in form of N₂O emission. Low temperature inhibited microbial activity, therefore, both fresh and aged biochar perform better in reducing N₂O emissions at 30°C than at lower temperatures. Aged biochar can achieve better N₂O mitigation effects than fresh biochar.