An incubation study investigating the mechanisms that impact N2O flux from soil following biochar application

Abstract

An incubation study with four contrasting soils (Vertosol, Ferrosol, Calcarosol and Tenosol) and three biochars (oil mallee [OM-], wheat chaff [W-]) and poultry litter [PL-] all produced at 550 °C) applied at 1% w/w to each of the soils was conducted (n = 4). The soils were packed in cylindrical chambers and were subjected to five cycles of four weeks of wetting and four weeks of drying. The soils received 10 atom % 15N-KNO3- in the 1st and 5th wetting cycles. Two of the four soils (Ferrosol and Tenosol) were applied with labile carbon (C) between the 2nd and 5th wetting cycle, while the other two soils (Vertosol and Calcarosol) were amended with labile C in the 5th wetting cycle only. Peak nitrous oxide (N2O) emissions in the Tenosol, Ferrosol and Calcarosol occurred within the 1st wetting cycle, whereas the Vertosol without labile C did not emit N2O. However, the co-application of labile C with 15N-NO3- to the Vertosol in the 5th wetting supported N2O emissions. The greatest reduction in N2O emissions following biochar amendment occurred due to the use of OM-biochar in the Tenosol; which decreased the emissions from 1.95 kg N2O-N ha-1 to 0.58 kg N2O-N ha-1 across the first 4 wetting/drying cycles. The majority of N2O emissions occurred during the first wetting cycle (85% water filled porosity). In contrast, application of PL-biochar did not result in a statistically significant reduction in emission of N2O. Assessing the source of emissions, the initial N2O from the Ferrosol and the Calcarosol was principally from native N-sources, while in the Tenosol between 31-57% of N2O originated from the added NO3-. While biochars reduced the overall emissions of N2O in the Tenosol, they also reduced the proportion of the N2O originating from the supplied NO3- during the first wetting cycle, possibly by limiting NO3− availability to denitrifers. Towards the end of the incubation period bacterial nitrifier gene abundance (amoA) in the unamended Tenosol was lower (p < 0.05) compared to the earlier wetting cycles, a trend which was not evident in the biochar amended Tenosol treatments. The nitric oxide reductase (norB) component of the denitrifier community was not significantly affected by biochar amendment but increased (p < 0.05) with labile C addition in all of the Tenosol treatments (± biochar) in the later stages of the incubation. The Tenosol had the lowest abundance of nitrous oxide reductase (nosZ) suggesting that its capacity to further reduce N2O to dinitrogen (N2) was lower than the other soils tested. We hypothesise that biochar amendment lowered emissions of N2O in the Tenosol by providing conditions suitable for nosZ such as increased soil pH and microbial respiration. This was evidenced by higher nosZ gene abundance in the Tenosol amended with biochar, relative to the nil-biochar control.