Reducing peat in growing media: impact on nitrogen content, microbial activity, and CO 2 and N 2 O emissions

Renewable materials including coir, biochar, and composts are investigated worldwide in the horticultural industry to partially substitute peat in growing media. In this study, we assessed the effects of biochar and vermicompost as partial substitution of peat and compared these peat-based growing media with coir in terms of NH 4 + -N and NO 3 − -N content, CO 2 -C and N 2 O-N emissions and their microbial biomass carbon and nitrogen. Six growing media mixtures (peat; peat + biochar 9:1 v/v; peat + vermicompost 9:1 v/v; coir; coir + biochar 9:1 v/v; coir + vermicompost 9:1 v/v) replicated three times were incubated in growth chambers during a 60 d period. At day 0 of incubation (DAI), peat amended with biochar retained around 12.81% of NH 4 + -N compared with peat alone. The concentrations of NO 3 − -N peaked at 275 mg·kg −1 at 33 DAI for peat and 552 mg·kg −1 at 46 DAI for coir amended with vermicompost. The substitution of peat with biochar resulted in large CO 2 -C [2070 μg CO 2 -C·g −1 dry weight (DW)] and N 2 O-N (62.78 μg N 2 O-N·g −1 DW) emissions, but not coir. The substitution of coir with vermicompost increased N 2 O-N emissions at a much lower level (47.53 μg N 2 O-N·g −1 DW) than peat (111.82 μg N 2 O-N·g −1 DW). Our results showed that supplements of vermicompost in peat and coir improved N supply which could benefit plant growth, while substituting part of peat with biochar increased CO 2 -C and N 2 O-N emissions. In contrast, no effect of biochar was observed with coir, which is beneficial for the environmental footprint of short-cycle growing crops.