Initial soil pH affects the pH at which nitrification ceases due to self-induced acidification of microbial microsites

Abstract

The existence of microsites of low pH around active colonies of nitrifying soil bacteria has previously been suggested but has been difficult to verify. A study was undertaken to examine whether observed decreases in bulk soil pH that occur during nitrification are in accordance with the theory of acidified nitrification microsites. A red earth soil (sieved <2 mm) was retained at a pH of 5·3 or amended with KHCO3 to achieve a pH of 6·3. Ammonium [(NH4)2SO4] was added to the soils and they were incubated for 35 days. In both soils the pH dropped rapidly and severely limited further nitrification. The soil with the higher initial pH experienced limitations to nitrification at a pH which was 0·2 units higher than that of the soil with the lower initial pH. The explanation for this result is in terms of acidified nitrification microsites. It is suggested that an active nitrifying colony may lower the pH within its immediate vicinity to a critical pH at which nitrification almost ceases. This critical pH achieved at the nitrification microsite is probably unrelated to the initial pH of the soil, but the pH of the soil matrix which is distant from the immediate influence of the nitrification microsite would remain at a pH closer to that of the soil initially. This less acidified region of the soil matrix would have an overriding influence on the measured pH of the bulk soil and account for the discrepancy between the measured pH of the two soils at the end of the incubation. These data provide further evidence that acidified nitrification microsites exist in soil, and that the measured soil pH is a poor estimate of the pH experienced by the microbial biomass.

Current insights into the autotrophic thaumarchaeal ammonia oxidation in acidic soils

“…There is also evidence that potential nitrification rates were only enhanced by manure treatment but suppressed by mineral fertilizers (Fan et al, 2011) in some soils. Actually, nitrification correlated with nitrogen mineralization rather than ammonium availability in acidic soils has also been indicated in several previous environmental investigations (Kreitinger et al, 1985;Strong et al, 1997;Frank et al, 2000) and gross nitrogen mineralization in acidic forest soils has been recognized as a good index of ammonia availability for autotrophic ammonia oxidizers (Hart et al, 1997). Similar observations were also made in Chinese acidic red soils where potential nitrification rates were substantially reduced by long-term additions of mineral fertilizers but increased by manure addition (He et al, 2007).…”

Mineralização e desnitrificação do nitrogênio no solo sob sistema plantio direto

“…roSolem et al (2003) verificaram que a aplicação de sulfato de amônio, em solos sob plantio direto, promoveu a nitrificação das formas amoniacais para N-nítrico, contudo os autores discutem a influência da matéria orgânica como condicionador do pH do solo, uma vez que a aplicação de sulfato de amônia promove rápida acidificação do solo que pode prejudicar a ocorrência da nitrificação (STrong et al, 1997).…”

Nematicidal activity of volatile fatty acids generated from wheat bran in reductive soil disinfestation

“…Soil is a complex mixture of mineral, organic, gaseous, and aqueous phases and its properties vary at microsites of soil aggregates (Norton, 1978). The pH of nitrification microsites was considerably lower than the pH measured in the bulk soil (Strong et al, 1997). Hence, VFA generation is surmised to cause localized decreases in pH around wheat bran particles mixed into soil and exert sufficient nematicidal effect on nematodes present at these localized sites with low pH.…”