Effect of Soil pH Increase by Biochar on NO, N2O and N2 Production during Denitrification in Acid Soils

Abstract

Biochar (BC) application to soil suppresses emission of nitrous- (N2O) and nitric oxide (NO), but the mechanisms are unclear. One of the most prominent features of BC is its alkalizing effect in soils, which may affect denitrification and its product stoichiometry directly or indirectly. We conducted laboratory experiments with anoxic slurries of acid Acrisols from Indonesia and Zambia and two contrasting BCs produced locally from rice husk and cacao shell. Dose-dependent responses of denitrification and gaseous products (NO, N2O and N2) were assessed by high-resolution gas kinetics and related to the alkalizing effect of the BCs. To delineate the pH effect from other BC effects, we removed part of the alkalinity by leaching the BCs with water and acid prior to incubation. Uncharred cacao shell and sodium hydroxide (NaOH) were also included in the study. The untreated BCs suppressed N2O and NO and increased N2 production during denitrification, irrespective of the effect on denitrification rate. The extent of N2O and NO suppression was dose-dependent and increased with the alkalizing effect of the two BC types, which was strongest for cacao shell BC. Acid leaching of BC, which decreased its alkalizing effect, reduced or eliminated the ability of BC to suppress N2O and NO net production. Just like untreated BCs, NaOH reduced net production of N2O and NO while increasing that of N2. This confirms the importance of altered soil pH for denitrification product stoichiometry. Addition of uncharred cacao shell stimulated denitrification strongly due to availability of labile carbon but only minor effects on the product stoichiometry of denitrification were found, in accordance with its modest effect on soil pH. Our study indicates that stimulation of denitrification was mainly due to increases in labile carbon whereas change in product stoichiometry was mainly due to a change in soil pH.

Phylogenetic and functional potential links pH and N2O emissions in pasture soils

“…This suggests pH is part of a universally conserved mechanism selecting for both emissions and microbial communities. The range of pH observed in our soils (5.57–7.03) was sufficient to capture the range at which the N 2 O reductase and N 2 O emissions fluctuate in response to pH26444546. Soil pH controls not only the assembly of the N 2 O reductase2627, but also alters general expression patterns24 and selects for shifts in microbial community composition31 indirectly influencing the abundance and type of functional genes in soils.…”

Biochar reduces yield-scaled emissions of reactive nitrogen gases from vegetable soils across China

“…These findings potentially resulted from the effects of the biochars on soil aeration, C / N ratio and pH, which affected the N dynamics and N cycling processes (Zhang et al, 2010;Ameloot et al, 2015). In line with Obia et al (2015), biochar decreased NO emissions in low-pH Acrisol (Table 3b), probably by stimulating denitrification enzyme activity, which then resulted in less NO accumula- ± SD, n = 3) of each treatment for the sum of the four N fertilization events. See Fig.…”

“…Others have found that the lower mitigation capacity of high-N biochars (e.g., manures or biosolids) is probably due to the increased N release in the soil from the biochar (Schouten et al, 2012). To our knowledge, very few studies have investigated biochar effects on NO emissions (Nelissen et al, 2014;Obia et al, 2015), and the mechanisms through which biochar influence NO emissions have not yet been elucidated. Therefore, more research is needed to clarify the underlying mechanisms of biochar on NO emission.…”

Gas entrapment and microbial N2O reduction reduce N2O emissions from a biochar-amended sandy clay loam soil

“…by a pH increase) leading to a decreased N 2 O/(N 2 O + N 2 ) emission ratio11151640, or (iii) the contribution of abiotic processes such as sorption of N 2 O onto biochar particles and abiotic reduction of N 2 O to N 2 by redox active compounds4142. However, systematic studies focusing on the underlying geochemical and microbial processes causing N 2 O emission mitigation are rare and the contributing mechanisms are not fully understood yet.…”