Changes in abundance and composition of nitrifying communities in barley (Hordeum vulgare L.) rhizosphere and bulk soils over the growth period following combined biochar and urea amendment

Abstract

To understand the effects of biochar and urea on soil N availability and plant growth, we conducted a pot experiment growing barley (Hordeum vulgare L.) under six treatments: control (N0), soil with 30 g kg−1 biochar (N0B), soil with 0.23 g kg−1 urea (N1), soil with 0.23 g kg−1 urea and 30 g kg−1 biochar (N1B), soil with 0.46 g kg−1 urea (N2), and soil with 0.46 g kg−1 urea and 30 g kg−1 biochar (N2B). The nitrifying community abundance and compositions in rhizosphere and bulk soils were analyzed using quantitative polymerase chain reaction (qPCR) and amplicon-based Illumina Hiseq sequencing. Adding urea with biochar (N1B) produced the greatest increase in above- and belowground plant biomass, followed by doubling the amount of urea with biochar (N2B); both treatments raised pH (p < 0.001) and lowered extractable N in the rhizosphere (p < 0.05). N1B treatment produced the greatest increase in ammonia-oxidizing bacteria (AOB) amoA gene copies, presumably because the combined amendment raised soil pH, which favored AOB access to NH4+. Nitrifier sequences were selected after blasting with reported nitrifiers in NCBI (similarity ≥ 97%). Nitrosospira dominated AOB communities during the plant seedling stage; however, during the mature stage, Nitrosomonas dominated over Nitrosospira and the nitrite-oxidizing bacteria (NOB) community became diverse. Redundancy analysis indicated that nitrifying community composition was affected by multiple soil properties, including N availability (i.e., exchangeable NH4+ and NO3−) and soil chemistry (i.e., pH, dissolved organic C, and exchangeable base cations). Our research suggests a positive application of combining biochar with urea in improving N bioavailability and promoting plant growth in the acidic soil.