Soil pH
Soil pH varied from 4.78 to 6.97 in acidic vegetable soil, from 7.05 to 8.00 in neutral rice soil and from 8.18 to 8.76 in alkaline soil among all treatments (Fig. 1). For all soils, biochar amendment increased soil pH compared to no biochar treatments (Fig. 1). In the last day of the incubation, biochar amendment increased soil pH by 1.43 and 1.56 units, 0.70 and 0.57 units and 0.37 and 0.29 units in the N addition and no N addition treatments for acidic vegetable soil, neutral rice soil and alkaline soil, respectively. The increase was more pronounced in acidic vegetable soil. In contrast, application of N fertilizer reduced soil pH in comparison with the control across the entire incubation period. N plus biochar amendment showed an increase in soil pH when compared with the control among all soils. While soil pH for the NB treatments was lower than that for the B treatments. The two-way ANOVA showed that soil pH was significantly influenced by biochar and N fertilizer for each individual soil type (p<0.001, Table 2), whereas the interactions between them were not significant for all soils after the 40-day incubation.
Soil NH4+-N and NO3--N
In the beginning of incubation, soil NH4+-N concentrations were increased after N addition for all soils (Fig. 2a, c, e). In neutral rice soil, NH4+-N decreased sharply on day 5 for N addition treatments. In acidic vegetable soil, NH4+-N for N addition treatments decreased to the level of non-nitrogen treatments at the middle of the incubation on day 20. Whereas NH4+-N showed an increase on day 5 and then decreased gradually with incubation time in alkaline soil. Soil NH4+-N concentration for NB treatment was lower than that for N treatment. For no N addition treatments, soil NH4+-N concentrations were kept low and changed rarely in all soils. The two-way ANOVA showed that addition of biochar and N not significantly affected NH4+-N concentrations in acidic vegetable and neutral rice soil after 40-day incubation (Table 2). Nevertheless, the interactions of them significantly influenced NH4+-N concentration in acidic vegetable soil. Converse to acidic vegetable soil, NH4+-N concentration in alkaline soil was significantly influenced by biochar and N fertilizer (p<0.05), but not significantly affected by their interactions.
Relative to no N addition treatments, soil NO3--N concentrations remained high in acidic vegetable and neutral rice soil throughout the entire incubation period for the N addition treatments (Fig, 2b, d). While in alkaline soil, soil NO3--N concentration was relatively low for the N addition treatments at the beginning of the incubation and increased with incubation time (Fig. 2f). The concentration of NO3--N was higher in acidic vegetable soil (118.8-174.8 mg kg-1) than that in neutral rice (48.9-103.0 mg kg-1) and alkaline (16.4-83.8 mg kg-1) soils. The NO3--N concentrations for the control and biochar-only (B) treatments maintained low during the entire incubation period in all soils. Across all the treatments, soil NO3--N concentrations were highest in the nitrogen-only treatment and were lowest in the biochar-only (B) treatments. The two-way ANOVA showed soil NO3--N was significantly affected by biochar and N fertilizer (p<0.05), but not significantly affected by their interactions in all soils (Table 2).
Nitrous oxide emissions
The N2O fluxes showed different temporal variations in the three soils across the incubation (Fig. 3). The flux peak was highest in neutral rice soil (122.12 μg N kg -1 h -1) and was lowest in acidic vegetable soil (10.59 μg N kg -1 h -1). For acidic vegetable soil, N2O flux peaks occurred on day 4 and day 5, for NB and N treatment, respectively. The N2O fluxes for NB treatment declined more quickly than that for N treatment and reached steadily low level 6 days earlier than the N treatment (Fig. 3a). The pattern of N2O fluxes in neutral rice soil showed a typical pulse release followed by the N addition. The N2O fluxes climbed rapidly to the top on day 2 and day 3, for N and NB treatment, respectively. Afterward, N2O fluxes declined sharply during the next 2 or 3 days and remained relatively low till the end of incubation (Fig. 3b). For alkaline soil, after N addition, the fluxes of N2O increased and reached the peaks on day 12 and day 15 for N and NB treatment, respectively, while declined till day 25 of the incubation then kept relatively stable for the rest period (Fig. 3c). The flux peak was lowest in acidic vegetable soil (10.59 μg N kg -1 h -1) and was highest in neutral rice soil (122.12 μg N kg -1 h -1). Across the entire incubation period, N2O fluxes were relatively low and steady for the control and B treatments in all soils (Fig. 3).
The highest cumulative N2O emissions were measured in N treatment, averaged 2.72, 9.03 and 11.28 mg N kg-1, for acidic vegetable, neutral rice and alkaline soil, respectively (Fig. 4). In acidic vegetable soil, the cumulative N2O emissions were significantly depressed by biochar addition for B (47.6%) and NB (20.8%) treatments relative to the control and N treatments, respectively. Although biochar addition decreased cumulative N2O emissions by 10.6% and 15.4% for B and NB treatments in neutral rice soil, the differences were not significant. In alkaline soil, NB treatment significantly reduced cumulative N2O emissions by 22.3% relative to N treatment. Whereas, the difference between the control and B treatment was not significant. The two-way ANOVA showed cumulative N2O emission was significantly affected by biochar, N fertilizer and their interactions in acidic vegetable and alkaline soils (p<0.05). Nevertheless, cumulative N2O emission was only significantly influenced by N fertilizer in neutral rice soil (Table 2).