Nitrous oxide, methane emissions and grain yield in rainfed wheat grown under nitrogen enriched biochar and straw in a semiarid environment

Study site

The study was conducted during the 2014, 2015 and 2016 growing seasons at the Dingxi Experimental Station (35°28′N, 104°44′E, elevation 1971-m above-sea-level) of the Gansu Agricultural University in Northwestern China. The research station is located in the semiarid Western Loess Plateau, which is characterized by step hills and deeply eroded gullies (Feng et al., 2013). This area has Aeolian soils, locally known as Huangmian (Chinese Soil Taxonomy Cooperative Research Group, 1995), which equate to Calcaric Cambisols based on the FAO (1990) description. The soil type in the study area is sandy-loam with low fertility. The soil has a pH of ≈8.3, soil organic carbon (SOC) ≤8.13 g kg⁻¹, and Olsen-P ≤13 mg kg⁻¹ as described in Yeboah et al. (2018). The type of soil in the study area is the principal soil for cultivation of crops in the agro-ecological zone. Long term average rainfall, evaporation and aridity in the study area is 391.9 mm per annum; 1531 mm per annum and 2.53 respectively. The aridity index (AI) is the degree of dryness of the climate at the study area. In July, the daily maximum temperature can increase to 38 °C. Similarly, in January daily minimum temperature can drop to −22 °C. Annual cumulative temperatures >10 °C are 2240 °C and annual radiation is 5930 MJ m⁻², with 2477 h of sunshine as described in Yeboah et al. (2018). The agro-climatic conditions are similar to semiarid environments. The research site is characterized by continuous cultivation of the same field using conventional tillage practices. The preceding crop cultivated at the research site was potatoes (Solanum tuberosum L.). Seasonal rainfall recorded in 2014, 2015 and 2016 during the research was 174.6, 252.5 and 239.4 mm respectively (Fig. 1).

Experimental design and description of treatment

The experiment involved addition of different carbon (C) sources; namely: biochar and straw, and N fertilizer in the form of urea (46% N) arranged in a randomized block design with nine treatments and three replications (Yeboah et al., 2018). The treatments were: CN₀–control (zero-amendment), CN₅₀ –50 kg ha⁻¹ N applied each year, CN₁₀₀ –100 kg ha⁻¹ N applied each year, BN₀ –15 t ha⁻¹ biochar applied in a single dressing in 2014, BN₅₀ –15 t ha⁻¹ biochar applied in a single dressing in 2014 + 50 kg ha⁻¹ N applied each year, BN₁₀₀ –15 t ha⁻¹ biochar applied in single dressing in 2014 + 100 kg ha⁻¹ N applied each year, SN₀ –4.5 t ha⁻¹ straw applied each year, SN₅₀ –4.5 t ha⁻¹ straw applied each year + 50 kg ha⁻¹ N applied each year and SN₁₀₀ –4.5 t ha⁻¹ straw applied each year + 100 kg ha⁻¹ N applied each year as described in Yeboah et al. (2018). The two C sources (biochar and straw) were applied at the same quantity based on the straw returned to the soil every year and straw C mineralization. Biochar was spread evenly on the soil surface in March 2014 and incorporated into the soil using a rotary tillage implement to a depth of ≈10 cm. The biochar was obtained from Golden Future Agriculture Technology Company Limited, Liaoning in China. Biochar was produced from maize straw using pyrolysis process at a temperature of 350–550 °C. This process converted about 35% of the maize straw to biochar. The biochar in the form of granules was milled to a size of <5 mm to allow for even mixing with the soil. The wheat crop of the previous season from the research station was used as a source of straw for the study. In the plots that received straw treatment, the straw from the previous wheat crop was weighed and returned to the original plots. This was done after threshing. Biochar analysis was conducted using the procedure as describe in Lu (2000). Total C and N and soil pH were determined using a CN Analyzer (analytikjena; multi N/C, 2100S, Germany) and Kjeldahl digestion and distillation (Bremner & Mulvaney, 1982) and pH meter (model: Sartorius PB–10, Germany). The soil pH was determined using soil to water ratio of 1: 2.5. Similar protocol was used to determined total C and N, ash content and pH of the straw. Table 1 shows the chemical characterization of biochar and straw used in the experiment. All the treatments received a blanket application of Phosphorus (P) fertilizer which was applied equally at a rate of 46 kg ha⁻¹ P in the form of ammonium dihydrogen phosphate (12% N, 52% P₂O₅). No–tillage seeder was used to incorporate the fertilizer to about 20 cm soil depth at planting. Based on the protocol described in Yeboah et al. (2016) Spring wheat (Triticum aestivum L. cv. Dingxi 35) was sown in Mid-March at a rate of 188 kg ha⁻¹ seeds at 20-cm row spacing. The crop was harvested either at the end of July or early August. The individual plot’s measured 3 m by 6 m and the plots were separated by 0.5 m width protection rows.

Soil sampling, measurements and analyses

Based on the protocol described in Yeboah et al. (2016), soil bulk density (BD) was determined by taking small cores and relating the oven–dried mass of soil to the volume of the core. Soil saturated hydraulic conductivity (Ksat) was determined at two points per plot using the disc permeameter method according to Carter (1993). Soil samples were collected from 0–10 and 10–30 cm depth and bulked for analysis. The samples were processed for analysis using the protocol described in Yeboah et al. (2018). Soil organic carbon (SOC) in the fine ground samples was determined by the modified Walkley & Black (1934) wet oxidation method (Nelson & Sommers, 1982).

Gas sampling and analysis

Collection of N₂O and CH₄ gases were performed using the static chamber technique based on the procedure described by Zou et al. (2005). For each sampling event, gas collection was consistently performed between 08:00–12:00 h, based on the guidelines of Yeboah et al. (2016). Collection of samples for N₂O and CH₄ analyses was conducted at 0, 10, and 20 min after chamber closure. Samples were collected between March and September and detailed sampling procedure could be found in Yeboah et al. (2016); Yeboah et al. (2018). Based on earlier studies conducted in low rainfall areas (e.g., Wang et al., 2010) emissions occurring during the dry season were expected to be low and therefore did not justify measurements over that period. Gas fluxes were measured over 14 sampling events per year. Whilst acknowledging that accurate estimates of total emissions cannot be determined from relatively few sampling events, the main purpose of this work was to quantify relative differences between-treatments, which therefore justifies the approach used in this study. A similar approach was also employed by Tullberg et al. (2018) to quantify soil emissions of GHG from tillage and traffic treatments in conservation agriculture areas with seasonal rainfall. The N₂O and CH₄ concentration in samples were analyzed within 2 to 3 days after collection using gas chromatograph (GC). The GC system (Agilent 7890A, USA) equipped with flame ionization detector (FID) was used for CH₄ analysis and an electron capture detector (ECD) was used for N₂O analysis. Rates of CH₄ and N₂O fluxes were calculated by linear increment of the gas concentration at 0, 10 and 20 min. The calculation was only accepted when the R² of the linear correlation was higher than 0.90 (p < 0.05). The average GHG fluxes were a mean of three replicates of each treatment over the sampling dates. Further procedure for the analysis and conditions of the column could be found in Yeboah et al. (2018) and Zou et al. (2005).

Estimations of nitrous oxide and methane emissions

The N₂O (mg m⁻² h⁻¹) and CH₄ (mg m⁻² h⁻¹) emissions were calculated using Eq. (1) based on the protocol described in Yeboah et al. (2016): (1)${\displaystyle F=\frac{C_2\times V\times M_0\times 273/T_2-C_1\times V\times M_0\times 273/T_1}{A\times \left(t_2-t_1\right)\times 22.4}}$

where: F are fluxes of N₂O or CH₄(mg m⁻² h⁻¹), V is volume (m³), M₀ is the molecular weight of the gas, C₁ and C₂ are the concentration of previous (0 mins) and current (20 mins) gas concentrations inside the chamber (mol mol⁻¹), T₁ and T₂ are temperature (Kelvin) recorded inside the chamber during current and previous samplings, and t₁ andt₂ are previous and current sampling times (h).

The cumulative emission of N₂O and CH₄ in kg ha⁻¹ was estimated using the equation as follows (Yeboah et al., 2016): (2)${\displaystyle M=\sum \left(F_{N+1}+F_N\right)\times 0.5\times \left(t_{N+1}-t_N\right)\times 24\times 10^{-2}}$

where M is the N₂O and CH₄ cumulative emissions during the period of measurement (kg ha⁻¹), F is N₂O and CH₄ emission (in mg m⁻² h⁻¹); and previous and current sampling emissions were N+1 and N respectively. The number of days from first sampling is represented by t.

Biomass and grain yield

Biomass and grain yield was determined by cutting the plants using hand sickles to five cm height aboveground. The outer edges of about 0.5 m was discarded from each plot. Both yields were determined on a dry–weight basis by oven–drying the plant material at 105 °C for 45 min and then to constant weight at 85 °C (Yeboah et al., 2016).

Statistical analyses

Statistical analyses were undertaken with the SPSS 22 (IBM Corporation, Chicago, IL, USA) with the treatment as the fixed effect and year as random effect. Tukey’s honestly significant was used to determine the differences between-treatments means. Significance differences were declared at probability level of 5%.

Soil bulk density, saturated hydraulic conductivity and soil organic carbon

Soil samples taken during the study period showed significant differences in the bulk density depending on the type of treatment and the depth of sampling (Table 2). Bulk density increased with soil depth in many cases irrespective of treatment over the experimental period. Significant differences between treatments were minor in the upper layer in 2014, but significant treatment effect was recorded in the 5–10 cm soil depth as the experimental period progressed from 2014 to 2016 (Table 3). On average, the lowest bulk density (1.14 g cm⁻³) was recorded under biochar–amended soils, and the highest was observed under soils with carbon (1.21 g cm⁻³). The results obtained with the straw–amended soils showed a similar trend, except that differences were not significant at p < 0.05 in most cases. Saturated hydraulic conductivity (Ksat) was significantly (p < 0.05) affected by carbon, N fertilizer and year but there was no significant interaction between treatment factors (Table 2). Application of BN₁₀₀ treatment enhanced mean saturated hydraulic conductivity by 23.7%, 24.3% and 20.4% relative to CN₀, CN₅₀ and SN₀, respectively (Table 4). Carbon and year had significant interaction (p < 0.05) on soil organic carbon, except at the 10–30 cm soil depth (Table 2). Similarly, carbon and fertilizer-N also interactively affected soil organic C in all the soil depth evaluated. Application of fertilizer-N at the 50 and 100 kg ha⁻¹ rate influenced SOC significantly (p < 0.05) under biochar treated soils, particularly in the depth of 0–5 cm (Table 5). However, N₁₀₀ had greater effect compared to N₅₀.

Nitrous oxide emissions

All the treatments were sources of nitrous oxide (N₂O) emission throughout the sampling period and the maximum observed N₂O emissions occurred in early July in each year of this study (Fig. 2). These responses were consistent with recorded soil moisture and temperature data. Significant differences (p < 0.05) were found among treatments at certain periods of measurement (Fig. 3). For example, in 2014, the maximal N₂O emission of BN₁₀₀ was 79.5 µg m⁻² h⁻¹ and the minimal was 36.5 µg m⁻² h⁻¹; they were significantly lower than those for CN₅₀ (100.7 µg m⁻² h⁻¹ for maximum and 55.8 µg m⁻² h⁻¹ for minimum) and CN₀ (98.1 µg m⁻² h⁻¹ for maximum and 50.2 µg m⁻² h⁻¹ for minimum). At a lesser magnitude, SN₀ and SN₅₀ also produced significantly lower N₂O emission compared to CN₀ and CN₅₀. During this period the lowest seasonal N₂O emission was mostly recorded in the biochar treated soils and at a lesser magnitude in the straw treated soils.

There were no significant treatment interactions (p < 0.05) effect on cumulative N₂O emission (Table 6), but treatment factors independently influenced cumulative N₂O emission. The highest cumulative N₂O emissions were consistently observed in the fertilized soils compared to the unfertilized soils, but differences were not always significant (Table 6). Application of BN₀, BN₅₀ and BN₁₀₀ significantly decreased cumulative N₂O emission by 48.42%, 37.12% and 35.80% on average compared to CN₁₀₀, respectively (Table 4). The mean cumulative N₂O emission of biochar was averaged at 1.83 kg ha⁻¹ representing significant decrease of 10.93% and 38.61% compared to straw treated soils (2.03 kg ha⁻¹) and soils without carbon treatment (2.42 kg ha⁻¹). Straw treated soils had non–significant cumulative N₂O decrease of 0.39 kg ha⁻¹, or 19.40% less compared to no carbon soils.

Methane emissions

All the treatments had similar trends of seasonal CH₄ dynamics and were net carbon sinks over the three study years (Fig. 3). The minimum CH₄ consumption was recorded in April 2014 and 2015, and in September 2016. In the present study, a single peak was observed in June 2014, whiles double peaks were observed in May and July 2015 and 2016. During this period, the greatest seasonal CH₄ consumption of –79.94, –81.07 and –111.59 µg m⁻² h⁻¹ in 2014, 2015 and 2016 respectively were observed in BN₁₀₀ soils; it was 38.14%, 47.37%, 43.05% more compared to CN₀ (–57.87, –55.01 and –78.01 µg m⁻² h⁻¹). At a lesser extent, the maximum seasonal CH₄ consumption in SN₅₀ and SN₁₀₀ soils were significantly higher (p < 0.05) compared to the CN₀ and CN₅₀ soils. The results were clear that, the greater seasonal CH₄ consumption occurred with the higher N fertilizer soils and the greatest CH₄ uptake generally occurred in the biochar treated soils, followed by the straw treated soils and the least were observed in the no carbon soils

Year individually had a significant effect (p < 0.05) on cumulative CH₄ emission (Table 7), and interaction between carbon and year significantly affected cumulative CH₄ emission. The results of cumulative CH₄ emission showed that increasing N fertilizer rates generally enhanced CH₄ consumption in all treatments. The use of BN₁₀₀ boosted cumulative CH₄ uptake in 2014 (by 21.9% and 18.2%), 2015 (by 83.6% and 59.1%) and 2016 (by 30.5% and 18.4%) compared to CN₀ and CN₅₀, respectively. Increasing the fertilizer rate from N₅₀ to N₁₀₀ resulted in significantly higher cumulative CH₄ consumption (p < 0.05) on straw treated soils in 2014 relative to N₀ on soils without carbon; the increase was 16.8%. In 2015, application of SN₁₀₀ increased cumulative CH₄ sink by 41.0%, 73.0%, 22.8% and 26.8% compared with CN₀, CN₅₀ and CN₁₀₀, respectively. The mean cumulative CH₄ consumption was greatest in biochar treated plots (−2.8 kg ha⁻¹), followed by straw treated soils (−2.6 kg ha⁻¹) and the least in no carbon soils (−2.3 kg ha⁻¹).

Biomass and grain yield

There was significant interaction effects between carbon and nitrogen, and nitrogen and year on biomass yield at p < 0.05 (Table 2). In addition, carbon, nitrogen and year individually had significant effect on biomass yield. Application of N₁₀₀ treatments on biochar treated soils (BN₁₀₀) increased biomass yield by 39.05% in 2014, 37.31% in 2015 and 30.02% in 2016 on average compared to soils without carbon (Table 8). Similarly, BN₁₀₀ significantly increased biomass yield in 2014 (by 35.06% and 26.43%), 2015 (by 40.04% and 23.11%) and 2016 (by 21.86% and 13.45%) compared to SN₀ and SN₅₀ sites, respectively. Application of SN₁₀₀ also caused significant increases in biomass yield compared to no carbon soils, an average increase of 32.09%, 29.32% and 32.56% were recorded in 2014, 2015 and 2016 respectively. The grain yield under N₁₀₀ fertilization was significantly increased (p < 0.05) by 35.87%, 29.45% and 13.34% under no carbon soils; 33.64%, 37.02% and 39.16% under biochar soils, and 31.89%, 32.35% and 24.08% under biomass treated soils in 2014, 2015 and 2016, respectively, compared to their corresponding N₀ soils (Table 9).