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Although the effects of fertilization on the abundance and diversity of soil nematodes have been widely studied, the impact of fertilization on soil nematode microbiomes remains largely unknown. Here, we investigated how different fertilizers: no fertilizer, mineral fertilizer, clean slurry (pig manure with a reduced antibiotic burden) and dirty slurry (pig manure with antibiotics) affect the microbiome of a dominant soil nematode and its associated antibiotic resistance genes (ARGs). The results of 16S rRNA gene high throughput sequencing showed that the microbiome of the soil nematode Dorylaimus stagnalis is diverse (Shannon index: 9.95) and dominated by Proteobacteria (40.3%). Application of mineral fertilizers significantly reduced the diversity of the nematode microbiome (by 28.2%; P < 0.05) but increased the abundance of Proteobacteria (by 70.1%; P = 0.001). Microbial community analysis, using a null hypothesis model, indicated that microbiomes associated with the nematode are not neutrally assembled. Organic fertilizers also altered the diversity of the nematode microbiome, but had no impact on its composition as illustrated by principal coordinates analysis (PCoA). Interestingly, although no change of total ARGs was observed in the nematode microbiome and no significant relationship existed between nematode microbiome and resistome, the abundance of 48 out of a total of 75 ARGs was enriched in the organic fertilizer treatments. Thus, the data suggests that ARGs in the nematode microbiome still had a risk of horizontal gene transfer under fertilization and nematodes might be a potential refuge for ARGs. Unlabelled Image • The microbiome of nematode Dorylaimus stagnalis is diverse and complex. • Fertilization significantly shifted the nematode microbiota and ARGs. • There may be a risk of horizontal gene transfer for ARGs in nematode microbiota. [ABSTRACT FROM AUTHOR]

Long-term nitrogen (N) fertilization affects soil aggregation and localizations of soil organic carbon (SOC), N and microbial parameters within aggregates. The mechanisms of these N effects are poorly understood. We studied these processes in a loamy soil from a 23-year repeated N addition field experiment under a rice–barley rotation. Nitrogen fertilization increased plant productivity and the portion of large macroaggregates (> 2 mm). However, SOC contents in macro- and micro-aggregates remained constant despite an N-induced increase of 27% in root C input into soil. Therefore, N fertilization accelerated SOC turnover. Nitrogen addition increased total N (TN) content in bulk soil and two macroaggregates (> 2, and 1–2 mm), but not in microaggregates (< 0.25 mm). Also, N fertilization increased the phospholipid fatty acids (PLFAs) contents of fungi in the large macroaggregates, but not in the microaggregates. In contrast, the effect of N addition on contents of bacterial and total microbial PLFAs was not apparent. Nitrogen fertilization increased N -acetyl- β -D-glucosaminidase (NAG) activities in the two larger macroaggregate size classes (> 2, and 1–2 mm), but not in the aggregates (< 1 mm). In both control and N fertilization, the large macroaggregates localized more TN, microbial PLFAs, and NAG activities than the microaggregates. In conclusion, long-term N fertilization not only directly promotes soil N resource but also indirectly improves soil structure by forming large macroaggregates, accelerates SOC turnover, and shiftes localization of microorganisms to the macroaggregates. [ABSTRACT FROM AUTHOR]

Long-term phosphorus (P) and nitrogen (N) applications may seriously affect soil microbial activity. A long-term field fertilizer application trial was established on reddish paddy soils in the subtropical region of southern China in 1998. We assessed the effects of swine manure and seven different rates or ratios of NPK fertilizer treatments on (1) the absolute and specific enzyme activities per unit of soil organic carbon (SOC) or microbial biomass carbon (MBC) involved in C, N, and P transformations and (2) their relationships with soil environmental factors and soil microbial community structures. The results showed that manure applications led to increases in the absolute and specific activities of soil β-1,4-glucosidase(βG), β-1,4-N-acetylglucosaminidase (NAG), and leucine aminopeptidase (LAP). The absolute and specific acid phosphatase (AP) activities decreased as mineral P fertilizer application rates and ratios increased. Redundancy analysis (RDA) showed that there were negative correlations between absolute and specific AP activities, pH, and total P contents, while there were positive correlations between soil absolute and specific βG, NAG, and LAP enzyme activities, and SOC and total N contents. RDA showed that the contents of actinomycete and Gram-positive bacterium PLFA biomarkers are more closely related to the absolute and specific enzyme activities than the other PLFA biomarkers ( P < 0.01). Our results suggest that both the absolute and specific enzyme activities could be used as sensitive soil quality indicators that provide useful linkages with the microbial community structures and environmental factors. To maintain microbial activity and to minimize environmental impacts, P should be applied as a combination of inorganic and organic forms, and total P fertilizer application rates to subtropical paddy soils should not exceed 44 kg P ha − 1 year − 1 . [ABSTRACT FROM AUTHOR]

Abstract: Semi-arid soils cover a significant area of Earth's land surface and typically contain large amounts of inorganic C. Determining the effects of biochar additions on CO2 emissions from semi-arid soils is therefore essential for evaluating the potential of biochar as a climate change mitigation strategy. Here, we measured the CO2 that evolved from semi-arid calcareous soils amended with biochar at rates of 0 and 20tha−1 in a full factorial combination with three different fertilizers (mineral fertilizer, municipal solid waste compost, and sewage sludge) applied at four rates (equivalent to 0, 75, 150, and 225kg potentially available Nha−1) during 182days of aerobic incubation. A double exponential model, which describes cumulative CO2 emissions from two active soil C compartments with different turnover rates (one relatively stable and the other more labile), was found to fit very well all the experimental datasets. In general, the organic fertilizers increased the size and decomposition rate of the stable and labile soil C pools. In contrast, biochar addition had no effects on any of the double exponential model parameters and did not interact with the effects ascribed to the type and rate of fertilizer. After 182days of incubation, soil organic and microbial biomass C contents tended to increase with increasing the application rates of organic fertilizer, especially of compost, whereas increasing the rate of mineral fertilizer tended to suppress microbial biomass. Biochar was found to increase both organic and inorganic C contents in soil and not to interact with the effects of type and rate of fertilizer on C fractions. As a whole, our results suggest that the use of biochar as enhancer of semi-arid soils, either alone or combined with mineral and organic fertilizers, is unlikely to increase abiotic and biotic soil CO2 emissions. [Copyright &y& Elsevier]

Biochar (BC) and nitrogen (N) fertilizers are frequently applied to improve soil properties and increase crop productivity. Nonetheless, our mechanistic understanding of plant-soil interactions under single or combined application of BC and N remains incomplete. For the first time, we applied a split-root system to evaluate how BC or N contributes to the changes in soil enzyme activities, N and phosphorus (P) cycling as well as root plasticity. Left and right parts of rhizoboxes were filled with silty-clay loamy soil amended with BC (15 g kg−1 soil, from wheat straw, 300 °C), N (0.05 g KNO 3 -N kg−1 soil) or a control (no amendments), resulting in the following combinations: BC/Control, N/Control, BC/N. Soil enzyme activities, available N and P, root morphology and plant biomass were analyzed after plant harvest. Plant biomass (shoot + root) ranged from 0.56 g pot−1 (BC/Control) to 0.91 g pot−1(BC/N). The decreased soil bulk density and increased P availability in the BC compartment (BC/Control and BC/N) stimulated root length by 1.4–1.8 times – an effect that was independent of N availability in the same rhizobox. Biochar stimulated activities of β-glucosidase and leucine aminopeptidase (by 33–39%) compared to N due to the coupling of C, N and P cycles in BC/N treated soil. Nitrogen fertilization also increased β-glucosidase activity compared to the unfertilized control, whereas root elongation remained unaffected. Thus, the combined application of BC/N had more efficient benefits for plant growth than BC or N alone. This is linked with i) the stimulation of enzyme activities at the BC locations to reduce N limitation for both microorganisms and plants, and ii) an increase of fine root production to improve N uptake efficiency. Thus, combined BC/N application is potentially especially sustainable to overcome nutrient limitation as well as to maintain crop productivity because it accelerates root-microbial interactions. Three-pairs design (BC/Control, N/Control and BC/N) in the rhizoboxes and main parameters (fine root length, enzyme activities and nutrients) related to plant growth strategy. Unlabelled Image • Split-root experiment was done to study the effect of fertilizers on maize growth. • Biochar (BC) and nitrogen (N) combination stimulated the highest growth of maize. • BC increased activities of β-glucosidase and leucine aminopeptidase in soil. • BC stimulated the root elongation of maize, whereas N had no effect. [ABSTRACT FROM AUTHOR]