Easily mineralizable carbon in manure‐based biochar added to a soil influences N₂O emissions and microbial‐N cycling genes

Abstract

Manure‐based biochar is an effective amendment to increase both carbon sequestration and the fertility of degraded soils, whereas the responses of N2O emissions and microbial‐N cycling genes to its application and the underlying mechanisms are poorly understood. Here, four biochars were produced under two pyrolysis temperatures (300 and 700°C) and with two organic C extraction procedures (water and acetone extraction). The resulting biochars were either with relative enrichment or depletion in easily mineralizable carbon (EMC) compared with recalcitrant C. We added biochars, with urea and sodium nitrate, to a degraded red soil and incubated the amended soils at moisture levels of 60% and 130% field capacity. All the biochars decreased nitrification gene abundance, that is, amoA. Biochars with EMC had greater stimulatory effects on the abundance of denitrification genes (nirK, nirS, and nosZ) and N2O emission, regardless of moisture level and N form, compared with biochar without EMC. Biochar‐induced microbial activity, biochar aliphatics, and alkyl groups correlated positively with N2O emission and denitrification gene abundance. The water dissolved organic C of biochar facilitated the conversion of N2O to N2, whereas the acetone extractable C fraction postponed the completion of denitrification. In conclusion, the N2O emission and denitrification gene abundance increased with decreasing biochar aromaticity and increasing EMC content. Our study emphasizes that the EMC supply in manure‐based biochars also importantly mediates soil N2O emission and microbial N cycling genes, adding to current understanding of influencing factors such as biochar pH, porosity, N availability, and redox property.

Association of biochar properties with changes in soil bacterial, fungal and fauna communities and nutrient cycling processes

“…Therefore, biochars at high pyrolysis temperatures can compete for electrons with NO 3 − during denitrification, whereas biochars produced at low pyrolysis temperatures act as electron donors and provide the electrons for NO 3 − reduction. This was also supported by the stimulated N 2 O emission under supply of low-temperature biochar (e.g., 300 °C) (Dai et al 2019) and the inhibited N 2 O emission associated with the application of high-temperature biochar (e.g., 700 °C) (Harter et al 2014(Harter et al , 2016. Although NO 3 − is the substrate for denitrifiers, due to the low content of NO 3 − in biochar and the negative charges in biochar surface, the NO 3 − in biochar does not significantly affect denitrification.…”

“…Both processes can increase soil denitrification and hence N 2 O emission. Dai et al (2019) showed that biochars with high labile C contents stimulated soil denitrification and had greater stimulatory effects on the abundances for denitrification genes, such as nirK, nirS, and nosZ and also N 2 O emission, compared with biochar with low labile C. The close positive relationships between N 2 O emission and biochar-increased dissolved organic C (Feng et al 2018) and biochar volatile matter (Subedi et al 2016) also indicated the C-N coupled cycling in soil denitrification and N 2 O emission. As the labile C in biochar increased with decreasing pyrolysis temperature, we propose that the biochars produced at low pyrolysis temperatures (e.g., 300 °C) contribute to denitrification and N 2 O emission to a greater extent than those produced at high temperatures.…”

“…Some other bio-oils and condensates, such as polycyclic aromatic hydrocarbons, xylenol in labile C, are toxic to most microorganisms (Freddo et al 2012), while they may selectively stimulate the growth of specific microorganisms (Table 2). Dai et al (2019) used acetone to extract the major fraction of labile C from manure biochar and found that acetoneextracted C largely affected microbial biomass, community structure and microbial-mediated N cycling processes in both farmland and forest soils (Dai et al 2018a(Dai et al , 2019. These effects were significantly greater than the aromatic C and ashes of biochar, supporting the importance of labile C in determining microbial communities.…”

“…Third, biochar changes soil basic properties such as increase in soil pH. Furthermore, it acts as an electron shuttle for microbial metabolisms (Dai et al 2019;Saquing et al 2016;Wu et al 2018).…”

Effects of Organic Fertilizers on the Soil Microorganisms Responsible for N2O Emissions: A Review

“…Nevertheless, between 1 and 20% of the organic N in manure biochar can be mineralized in the short term and subsequently increase the nitrification rate. Biochars with lower aromaticity also have higher amounts of available C that can increase denitrification rates ( nirK , nirS , and nosZ ) and N 2 O emissions [ 79 ] ( Table 2 ). Additionally, the observed effects are dependent on biochar interactions with soil texture, pH, moisture and soil N [ 100 , 101 ].…”