Biochar-fertilizer interaction modifies N-sorption, enzyme activities and microbial functional abundance regulating nitrogen retention in rhizosphere soil.

The impact of the excessive use of N fertilizer remains an environmental problem of global concern. The effect of biochar on soil N retention is still unclear, and knowledge on how a mixture of biochar and fertilizer (B-F) influence N-sorption, N-cycling enzymes activities, diversity and functional abundance of organisms regulating N-retention in rhizosphere soil is poorly understood. Therefore, biochars derived from bamboo, rice straw, cow and pig manure were characterized, and their interactions with NPK fertilizer were evaluated. Results showed that while the effect of biochar on N retention varied among biochar types, such variations increased after B-F. Unlike NH4+ retention, NO3- retention by biochar in fertilized soil was poor (<8 weeks), but were however increased after longer periods (15 weeks) in B-F due to plant uptake, sorption and stimulation of N-cycling enzymes activities. This stimulation proved that N-fertilizer provided substrates for N-cycling organisms which was confirmed by the dominance of Proteobacteria, Chloroflexi, Actinobacteria, and Gemmatimonadetes which are important in soil N-cycling, despite the reductions in total diversity, class, phyla and genera abundance of bacterial 16SrRNA genes by B-F. This suggested that B-F induced specific organisms involved in N-cycling, which out-competed other organisms not involved in N-cycling. The provision of substrates by N-fertilizer in B-F for bacterial groups involved in N-cycling modified the rhizosphere microbial structure. The abundance of N-cycling organisms was regulated by the persistence among dominant groups, soil pH, total N, and microbial colonization induced by different biochars interacting with fertilizer which led to enhanced N-retention.

Short term effects of biochar with different particle sizes on phosphorous availability and microbial communities.

Despite the increasing interest for biochar as a soil amendment, a knowledge gap remains on different particle size of biochar on soil phosphorous (P) availability and its impacts on microbial community. We hypothesized that biochar particle size and incubation temperature can significantly influence soil P availability and microbial community in subtropical acidic soil. A laboratory incubation study was established to investigate the effects of soil pH, available P and soil microbial responses to biochar addition having varying particle sizes using paddy soil and red soil under different incubation temperatures (15 °C & 25 °C). Biochar produced via pyrolysis of spent mushroom substrate feedstock was sieved into three particle sizes ((≤0.5 mm (fine), 0.5-1.0 mm (medium) and 1.0-2.0 mm (large)). The results exhibited that the fine particle biochar resulted in significantly higher release of P, soil pH, available P and bacterial species richness while simultaneously reducing the activities of phosphatase enzyme in both soils. Apprehending the impact of biochar particle size and incubation temperature, principal coordinate analysis (PCoA) predicted that soil microbial communities with fine particle biochar and high incubation temperature (25 °C) clustered separately. Redundancy analysis depicted that fine particle biochar had a direct association with available P and soil pH while high incubation temperature depicted a strong affinity for microbial communities. Hence, it is suggested that fine particle biochar and high incubation temperature may provide better habitat for microorganisms compared to the other particle sizes which may be due to improved soil pH and available P. However, a long term study of different biochar particles application in subtropical acidic soil needs to be pursued further for a more comprehensive understanding on this issue.

Evaluating the mechanisms of the impacts of key factors on soil soluble organic nitrogen concentrations in subtropical mountain ecosystems.

Soil soluble organic nitrogen (SON) concentrations in terrestrial ecosystems were influenced differently and substantially by both biotic and abiotic factors. This study aimed to ascertain the mechanisms of the impact of the key factors on the SON concentrations of subtropical mountain ecosystems in southeastern China using an integrative approach, which combined a field plot survey, gray relational analysis and structure equation modeling. The results showed that the soil organic matter, clay content, protease activity and bacterial biomass were the key factors controlling the dynamics of the SON concentrations in subtropical mountain ecosystems. Protease activity, by catalyzing the degradation of complex organic nitrogen to SON, had the highest direct influence on the SON concentrations among all of the impact factors with direct impact effect of 0.44. Organic matter, which serves as a primary source of SON and can increase soil protease activity and bacterial biomass, contributed the most significantly to the SON concentrations in both direct and indirect pathways with total impact effects of 0.87. Clay, by adsorbing SON and affecting organic matter accumulation and protease activity, also had important direct or indirect influences on the SON concentrations with total impact effects of 0.48. The impact of the bacterial biomass on the SON concentrations was likely to be concealed by accompanying nitrogen-degrading enzyme activity with total impact effects of 0.22. Thus, the organic matter, clay content and protease activity exerted greater total impact effects on the SON concentrations compared with the bacterial biomass. Protease activity and organic matter had a greater positive direct impact on the SON concentrations compared with the bacterial biomass and clay content, while organic matter also had greater positive indirect impacts on the SON concentrations than did the clay content. This study's results could help to elucidate the differential mechanism of SON dynamics among various terrestrial ecosystems.