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This study evaluated the combined effects of biochar and straw on N2O flux and the community compositions of nitrifiers and denitrifiers in the maize season in an intensively farmed area in northern China. The experiment consisted of four treatments: (1) CK (only chemical fertilizer application); (2) C (biochar application); (3) SR (straw application to the field); and (4) C+SR (the application of both biochar and straw). The results indicated that during the maize growing season, N2O flux decreased by 30.3% in the C treatment and increased by 13.2% and 37.0% in the SR and C+SR treatments compared with CK, respectively. NO3--N, NH4+-N, and microbial biomass carbon (MBC) were the main soil factors affecting N2O flux, and they were positively correlated with NO3--N and negatively correlated with MBC in the C treatment and positively correlated with NH4+-N in the SR and C+SR treatments. Both biochar addition and straw return shifted the community compositions of nitrifiers and denitrifiers. N2O production was mainly reduced by promoting the ammonia-oxidizing bacteria (AOB) gene abundance and inhibiting the nirK gene abundance in the C treatment but promoted by inhibiting the AOB and nosZ gene abundances in the SR and C+SR treatments. Nitrosospira (AOB) and Rhizobium (nirK) were the main contributors among the treatments. NO3--N, NH4+-N, and MBC were the main soil factors affecting the denitrifier communities. The predominant species associated with the nirK, nirS, and nosZ genes were positively correlated with NO3--N and MBC and negatively correlated with NH4+-N. These results provide valuable information on the mechanism of N2O production and reduction in biochar- and straw-amended soil under field conditions.

Nitrous oxide (N2O) is a strong greenhouse gas, and it is of great significance for N2O reduction to study the effects of biochar on its production pathway. In this research, the contributions and mechanisms of biochar on autotrophic nitrification (ANF), heterotrophic nitrification (HNF), and denitrification (DF) to N2O emissions were studied by using 15N stable isotopes and high-throughput sequencing after laboratory incubation. The results showed that biochar addition at 2% (B2) significantly reduced the N2O emissions from the ANF by an average of 20.6%, while adding 5% biochar (B5) had no significant effect on the ANF. Both B2 and B5 significantly reduced the N2O emissions from the HNF by 15.7% and 13.2%, respectively, and reduced the N2O emissions from the DF by 40.9% and 11.7%, respectively. B2 enhanced the relative contribution rate of the ANF to N2O emissions by 6.3%, while B5 had little effect on it. Biochar addition significantly changed the copy numbers of the AOA and AOB, as well as the nirK, nirS, and nosZ genes, but it had no significant effect on the community composition of the AOA and had minimal effect on the AOB community. B2 significantly increased the abundance of the genus Rhodococcus of nirK type denitrifiers and had a significant effect on the relative abundance of Cupriavidus and Pseudomonas of the nosZ type denitrifiers. These results revealed that the inhibitory effects of biochar on N2O emissions from nitrification might be attributed to the direct immobilization and adsorption of inorganic N by biochar and to its promotion of the genus Rhodococcus of nirK-type denitrifiers and the genera Cupriavidus and Pseudomonas of the nosZ-type denitrifiers. The soil exchangeable NH4+-N and NO3−-N concentrations were the primary factors affecting the N2O emission rates. These results help to elucidate the effects and mechanisms of biochar on N2O production pathways in agricultural soil.

The application of a large amount of inorganic nitrogen (N) fertilizer resulted in an increasing N loss. It is an effective practice that biochar and organic fertilizer replace part of inorganic nitrogen fertilizer. Thus, it is necessary to identify and compare the effects of biochar and organic fertilizer on ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) community structure in saline–alkali soil. Three treatments in triplicate were included in the field experiment: (1) CK (no biochar and organic fertilizer), (2) biochar at 10.0 t ha−1 year−1 (C), and (3) organic fertilizer at 7.5 t ha−1 year−1 (M). The community structures and diversities of AOB and AOA were investigated by Illumina sequencing analysis of gene encoding ammonia monooxygenase subunit A (amoA) and followed by principal component analysis, least discriminant analysis effect size, and redundancy analysis. Biochar and organic fertilizers did not change the diversity of AOA and reduced the relative abundance of Candidatus-Nitrosoarchaeum in maize season, while only biochar increased the relative abundance of Candidatus-Nitrosotenuis in wheat season. The diversity of AOB was significantly reduced in the M treatment, but was not changed in the C treatment in wheat season. Moreover, the relative abundances of Nitrosomonas (CK, 15.1%; C, 6.8%; M, 3.4%) were decreased in the maize season, and the relative abundances of Nitrosovibrio (CK, 9.8%; C, 16.8%; M, 14.5%) were increased in the wheat season in the C and M treatments, which probably related to the changes in soil pH, soil NH4+–N content, and soil salt content (SSC). The lower soil pH, higher NO3−–N content, and SSC resulted in Nitrosospira (maize season: CK, 42.2%; C, 40.7%; M, 62.0%; wheat season: CK, 46.6%; C, 50.7%; M, 65.4%) becoming the dominant genus in the M treatment. Our results indicated that AOB was more susceptible than AOA to biochar and organic fertilizer. Organic fertilizer has significant effects on the diversities and community structure of AOB than biochar associated with the changing in soil pH, salinity, and mineral N.

To evaluate long-term effects of biochar addition and straw return (SR) on N2O fluxes in wheat-maize rotation system, a 2-year field experiment following a 7-year biochar addition and SR was investigated in an intensively managed agricultural soil in the North China Plain (NCP). Four treatments were included: 1) no biochar addition (CK); 2) biochar treatment with 4.5 t ha−1 yr−1 (C1); 3) biochar treatment with 9.0 t ha−1 yr−1 (C2); and 4) all the wheat/maize straw return (SR). The results showed the annual cumulative N2O emissions from C1 treatment were increased by 15.9–16.5%, while C2 treatment suppressed the annual cumulative N2O emissions by 22.8–26.3%. In comparison, straw return suppressed N2O emissions by 13.4–43.6% in the wheat season but increased N2O emissions remarkably by 45.3–53.9% in the maize season. Biochar addition enhanced the copies of AOA, AOB, nirK, nirS, and nosZ genes, while straw return decreased the copies of AOB, nirK, nirS, and nosZ genes at the high N2O emission period in the maize season. The production of N2O in the maize season was mainly driven by the AOB and nosZ genes, and by AOB gene in the wheat season. These results suggest that application of 9.0 t ha−1 yr−1 biochar is a more optimum agricultural strategy for reducing N2O emission in the wheat-maize system.

This research investigated the N2O fluxes and community compositions of nitrifiers and denitrifiers in a winter wheat-summer maize rotation system on the North China Plain. The experiment included three treatments: 1) a control treatment (CK); 2) biochar at 10.0 t ha–1 yr−1 (C); and 3) organic fertilizer at 7.5 t ha−1 yr−1 (M). The application of biochar reduced the cumulative N2O emissions by 47.7% and 62.2% in the maize and wheat seasons, respectively, compared with those of the CK treatment. Organic fertilizer increased the cumulative N2O emissions by 311.1% in the maize season and had no significant effects on them in the wheat season. Organic fertilizer reduced the cumulative N2O emissions by 39.8% in the nonfertilizer period of the maize season. The cumulative N2O emissions in the maize season accounted for 75.2–90.0% of the annual emissions among all the treatments. Biochar and organic fertilizer affected soil N2O emissions mainly by changing soil denitrifiers. In the maize season, the lower abundances of Candidatus Nitrosoarchaeum (AOA-amoA), Nitrosomonas (AOB-amoA), Mesorhizobium (nirK), Magnetospirillum (nirS) and Halomonas (nirS) may result in lower N2O emissions in the biochar treatment, and organic fertilizer had a similar influencing mechanism on N2O emissions during the nonbasic fertilizer and nontopdressing periods of the maize season. In the wheat season, the dominant genus Alicycliphilus (nosZ) was the major contributor to decreasing N2O emissions in the biochar treatment, while there was no biomarker related to N2O emissions in the M treatment. The soil pH, NO3−-N content and water-filled pore space (WFPS) were the key factors shifting the community compositions of nitrifiers and denitrifiers in this study. The application of biochar could be a better practice to improve saline-alkali soil with lower N2O emissions. These findings will improve our understanding of the nitrifiers and denitrifiers response to biochar and organic fertilizer.

This study intended to evaluate the combined effects of both biochar and organic fertilizer on nitrous oxide (N2O) fluxes and composition of nitrifier and denitrifier of saline-alkali soil. Therefore, four different treatments such as CK (only chemical fertilizer), B (only biochar), M (only organic fertilizer), BM (B:M = 1:1) were used in this experiment. The results showed that N2O emissions were decreased in B and BM treatments compare to the control. In contrast, N2O emissions were highest before day 12 but lowest after day 19 in M treatment compare to the control. Application of biochar, organic fertilizer and biochar plus organic fertilizer decreased the nirS and nirK genes copies and enhanced the nosZ gene copies which resulting in the lower N2O fluxes. The ammonia-oxidizing bacteria (AOB) amoA and nirK genes copies were significantly increased by organic fertilizer before day 12, leading to high N2O emissions. The genera Nitrosospira (AOB) and Nitrososphaera (ammonia-oxidizing archaea, AOA) assumed absolute superiority. Additionally, Nitrosospira (AOB) was also appeared in nirK-type denitrifiers, illustrating denitrification was carried out by nitrifiers. The genera Azospirillum (nirS), Burkholderia (nosZ) and Polymorphum (nosZ) were dominant in CK. There was only one dominant genus, Mesorhizobium (nosZ) in the B treatment. The genera Mesorhizobium (nirK), Azoarcus (nirS), Kocuria (nirS) and Pseudomonas (nosZ) occupied the main status in the M treatment. The relative abundance of Rhodanobacter (nirS) and Azospirillum (nosZ) were higher in the BM treatment compared with other treatments. Soil water content (SWC), pH, NH4+-N and NO3−-N were the main factors affecting AOB and denitrifiers, which influencing N2O emissions. Overall, combined application of biochar and organic fertilizer can reduce the N2O emission where AOB and nirK-type denitrifier were the main contributors to the N2O emission.

Soil enzymes catalyze key biochemical processes in organic matter decomposition and nutrients cycles and are regarded as indicators of soil quality. In this study, we investigated the impact of 4 consecutive years of biochar (corncob; 360°C) amendment on dynamics of enzyme activities (i.e., invertase, urease, catalase, and alkaline phosphatase) during the winter wheat (Triticum aestivum L.) growing period in Northern China. The experiment consisted of four treatments: control (CK), biochar rate at 4.5 t · ha · y (B4.5), biochar rate at 9.0 t · ha· y (B9.0), and crop straw return (SR). Four times throughout the wheat growth period (i.e., 5 Nov. 2010; 26 Mar. 2011; 25 Apr. 2011; 5 Jun. 2011), we determined the enzyme activities down to the 30-cm depth of the soil profile (i.e., 0–5, 5–10, 10–20, and 20–30 cm). Results showed that there were considerable fluctuations in enzyme activities across the observed period and depths. All enzyme activities decreased significantly in deeper soil horizons. High biochar amendment rates (B9.0) lead to the peak activities of invertase, urease, and phosphatase occurring in the 0- to 5-cm layer on 25 Apr. 2011 samples. The greatest catalase activity under B9.0 was found on 26 Mar. 2011 samples. In addition, the effect by B4.5 and SR on enzyme activities was variable and limited. These data support the conclusion that consecutive application of biochar for 4 years increased enzyme activities, which potentially influence soil nutrients dynamics under field condition, although the effects were dependent on sampling depth and time.

Little information is available regarding the effects of biochar amendment on soil thermal properties and soil temperature, especially under field conditions. The possible changes in soil thermal conductivity, surface reflectance, and temperature resulting from biochar addition might affect other biophysical–chemical processes in the soil. We examined the effects of biochar amendment on the thermal conductivity, surface reflectance, and temperature of soil. Soil thermal conductivity, reflectance, and temperature at a 5-cm depth were measured and monitored in an experimental field using a portable soil thermal property analyzer, a portable full-range spectrometer, and temperature and water potential probes. The field was located in the North China Plain and had been cropped in a winter wheat (Triticum aestivum L.)–maize (Zea mays L.) system for 5 yr. With biochar amendment, soil thermal conductivity was decreased significantly by 3.48 and 7.49% with 4.5 (B4.5) and 9.0 t ha⁻¹ yr⁻¹ (B9.0) of biochar addition, respectively, which was consistent with a decrease in soil bulk density. Soil water potential slightly increased under B4.5 treatment but decreased under B9.0 treatment relative to the control. Reflectance increased in the near-ultraviolet and blue-light wavelengths (350–513 nm) and decreased in the infrared wavelength range (520–2350 nm) with biochar amendment. Biochar amendment reduced diurnal soil-temperature fluctuations on both daily and seasonal scales, although the annual average daily soil temperatures at a 5-cm depth showed no significant difference among treatments. Comparison with the control showed that biochar treatment moderated soil temperature extremes, lowering the temperature when soil temperature was high and raising it when soil temperature was low. This moderating capability was mostly within ±0.4 and ±0.8°C for mean daily temperature and mean diurnal temperature of soil, respectively. The effect of biochar amendment on soil temperatures can be explained by the combined action of changes in soil thermal conductivity and reflectance.

The long term effect of biochar application on soil microbial biomass is not well understood. We measured soil microbial biomass carbon (MBC) and nitrogen (MBN) in a field experiment during a winter wheat growing season after four consecutive years of no (CK), 4.5 (B4.5) and 9.0 t biochar ha(-1) yr(-1) (B9.0) applied. For comparison, a treatment with wheat straw residue incorporation (SR) was also included. Results showed that biochar application increased soil MBC significantly compared to the CK treatment, and that the effect size increased with biochar application rate. The B9.0 treatment showed the same effect on MBC as the SR treatment. Treatments effects on soil MBN were less strong than for MBC. The microbial biomass C∶N ratio was significantly increased by biochar. Biochar might decrease the fraction of biomass N mineralized (KN), which would make the soil MBN for biochar treatments underestimated, and microbial biomass C∶N ratios overestimated. Seasonal fluctuation in MBC was less for biochar amended soils than for CK and SR treatments, suggesting that biochar induced a less extreme environment for microorganisms throughout the season. There was a significant positive correlation between MBC and soil water content (SWC), but there was no significant correlation between MBC and soil temperature. Biochar amendments may therefore reduce temporal variability in environmental conditions for microbial growth in this system thereby reducing temporal fluctuations in C and N dynamics.

The effect of biochar on soil respiration (Rs) over one maize-growing season was studied after 5 years of consecutive application in an intensive cropland in the North China Plain. The experiment was carried out in randomly arranged plots with four treatments being evaluated. Three replications were conducted per treatment: a control plot without biochar addition (CK), biochar incorporated at 4.5 t ha–1 year–1 (BC4.5), biochar incorporated at 9.0 t ha–1 year–1 (BC9.0), and incorporated wheat straw (SR). The Rs was determined throughout the growing season of maize in 2012. Soil temperature and moisture were measured simultaneously at 5 cm depth. The results showed that the seasonal and diurnal variations of Rs in the four different treatments were approximately equal, and there was a positive correlation between Rs and soil temperature. The Rs values of treatments BC4.5 and BC9.0 were significantly lower than of SR but not CK. Significant correlations between Rs and soil temperature and soil moisture were observed. Soil temperature had a stronger effect on Rs than did soil moisture, and Rs was more sensitive to soil temperature in the biochar treatments than in the SR and CK treatments. The application of biochar and straw increased the soil active organic carbon content, but an obvious relationship between Rs and the soil active organic carbon content was not found.