Carbon emission and water use efficiency response to tillage methods and planting patterns of winter wheat in the North China Plain

Study site and soil properties

The field experiment was conducted at the Experimental Station of Shandong Agricultural University (117°9′03″E, 36°10′9″N) in the NCP from 2015 to 2017. The study area is characterized as a temperate continental monsoon climate. During the two experimental years (2015–2016 and 2016–2017), total precipitation in the growing season was 156.7 and 157.3 mm, respectively (Fig. 1).

Winter wheat was planted in 3.0 m ×3.0 m study plots under field conditions. The soil was classified as loamy clay. The available potassium, available phosphorus, and the available nitrogen content were 92.6, 16.2, and 108.3 mg kg⁻¹, respectively, in the topsoil layer (0–20 cm).

Study designs and management

The two tillage methods (conventional tillage, T and no tillage, NT) and two planting patterns (conventional planting, C and wide-precision planting, W) were arranged in random block design with three replicates each, for a total of 12 plots. The experiment began in 2015. The T treatments were manually ploughed to a depth of 25 cm using a shovel on 7 Oct 2015 and 6 Oct 2016, respectively. Soil in the NT treatment are not ploughed. The previous crop was summer maize, and maize straw was removed before sowing in both tillage methods. The sowing and row spacing of W and C are shown in Fig. 2. The sowing space for the W treatments was dug with an 8 cm hoe, while the C treatments was dug with a 5 cm hoe.

   We used the Jimai 22 with high yield winter wheat cultivar. It was sown at a seeding rate of 222 grain m⁻² by hand on 8 Oct 2015 and 7 Oct 2016. Each plot was given the same amount and type of fertilizers [potassium phosphate (30.0 g m⁻²), potassium chloride (7.5 g m⁻²) and urea (15.0 g m⁻²)] with irrigation (60 mm) before sowing. At the jointing stage (17 Mar 2016 and 15 Mar 2017), additional urea (15.0 g m⁻²) combined with irrigation (60 mm) was applied. The amount of irrigation was controlled using a flow meter. Winter wheat was harvested on 1 Jun 2016 and 3 Jun 2017.

Cumulative CO₂-C emissions

A gas analyzer (GHX–305; ADC Bio–scientific Ltd., Hoddesdon, UK) was used to measure the CO₂ flux. It was composed of a PVC pipe (the pipe size was 25.0 cm diameter and 15.7 cm height), covered with a lid to form a dark chamber. The chamber was placed on the surface of the soil before measurement and pressed down to ensure a sealed environment. The seal was maintained to ensure air tightness during measurements. Measurements were taken from 9:00 to 10:00 AM on sunny days, at the seeding, wintering, jointing, heading, milking, maturity stage per experimental year. We allowed a 2 min wait for data stabilization before each reading.

Cumulative CO₂-C emissions were computed following the method (Liu et al., 2014), (1)${\displaystyle \mathrm{E}=\frac{V}{A}\times 100\times \rho \times \frac{\mathrm{dc}}{\mathrm{dt}}\times 60\times \frac{\mathrm{P}1}{\mathrm{P}0}\frac{273}{\left(273+\mathrm{T}\right)}}$ (2)${\displaystyle \mathrm{CCE}=\frac{\sum \left({\mathrm{E}}_{\mathrm{i}}+{\mathrm{E}}_{\mathrm{i}+1}\right)}{2}\times \left({\mathrm{t}}_{\mathrm{i}+1}-{\mathrm{t}}_{\mathrm{i}}\right)\times 24}$

where E is the CO₂ flux of the soil surface (µg m⁻² h⁻¹); A is the area of the static chamber (76.0 cm²); V is the volume of static chamber (77.0 cm³); ρ is the standard atmospheric CO₂ density (1.963 mg m⁻³); dc/dt is the CO₂ concentration variation (10⁻⁶ min⁻¹); P₀ is the atmospheric pressure which is equal to the atmospheric pressure within the station under standard atmospheric conditions (1.013 × 10⁵Pa); P₁ is static chamber’s the atmospheric pressure; T is the air temperature (°C); CCE is the cumulative CO₂-C emissions (kg CO₂ ha⁻¹); t is the number of days after planting, and i is the number of the sample.

Evapotranspiration

Evapotranspiration was computed following the method Ren et al., 2018a; Ren et al., 2018b), (3)${\displaystyle \mathrm{ET}=P+I-\Delta S-S-SR}$

where ET is the evapotranspiration volume (mm); P is the effective precipitation in the experimental year (mm), provided by a meteorological station 10.0 m away from the study site; I is the irrigation volume (mm); ΔS is the change in soil water storage, calculated by the difference between the initial content of soil moisture and the latest value, which was measured every 7 days from planting date to harvest date; S is the seepage under the crop root zone (mm), which was negligible because the measured soil water value indicated that drainage was low; SR is the surface runoff which was negligible, because of a lack of heavy precipitation during the study period, and the presence of a 20 cm barrier above the soil surface which was built around each experimental plot to prevent surface runoff.

A neutron moisture meter (CNC 503D, Super Energy. Nuclear Technology Ltd., Beijing, China) was used to measure the volumetric moisture content. The measurement was taken every 10 cm from the soil surface to a depth of 120 cm. To reduce error, we calibrated the water content used an oven dry method of the topsoil (20 cm). The soil was dried at 105 °C in an oven until a constant weight was reached and compared to determine the soil moisture. The groundwater depth was greater than 5.0 m, so the influence of groundwater on water consumption was not considered in this study.

Grain yield and yield compositions

In each plot, two 1.5 m sections of rows were randomly selected with three independent replicates to measure grain yield, spike numbers, and 1000-kernel weight at maturity. The kernel numbers per spike were measured using an additional 20 spikes. The winter wheat was harvested by hand.

Yield carbon utilization efficiency

The YCUE was computed following the method (Liu et al., 2014), (4)${\displaystyle \mathrm{Y\; CUE}=\frac{\mathrm{Y}}{\mathrm{CCE}}}$

where Y is the grain yield (g m⁻²) and CCE is the cumulative CO₂-C emissions (g m⁻²).

Water use efficiency

The WUE (kg/m³) was computed following the method (Li et al., 2015), (5)${\displaystyle \mathrm{WUE}=\frac{\mathrm{Y}}{\mathrm{ET}}}$

where Y is grain yield of winter wheat (g/m²), and ET is the evapotranspiration during the experimental year (mm).

Carbon emissions per unit water consumption

The WUE_CE (g m⁻³) was computed following the method (Hu et al., 2017), (6)${\displaystyle \mathrm{WU}{\mathrm{E}}_{\mathrm{CE}}=\frac{\mathrm{CCE}}{\mathrm{ET}}\times 1000}$

where CCE is cumulative CO₂-C emissions (g m⁻²), and ET is evapotranspiration (mm).

Statistical analysis

Differences among treatments were assessed using analysis of variance (ANOVA) with a significance level of α = 0.05. When significance was observed, the least significant difference (LSD) post-hoc test was used to conduct multiple comparisons. The normality of variances was tested before performing the ANOVA. Microsoft Excel 2010 and SPSS were used to organize and analyze data, respectively.

Cumulative CO₂-C emissions

The patterns of CCE were similar over the two growing seasons (Fig. 3). The TC treatment had the highest CCE, followed by the TW treatment and the NTC treatment. The NTW treatment had the lowest CCE. The CCE were lower in NT than in T (30.8 and 21.3%, respectively) and were lower in W than in C (15.0 and 10.5%, respectively) in the first and second growing seasons. Moreover, CCE were slightly lower in NTW than in NTC (4.6 and 2.6%, respectively) and were lower in TW than in TC (21.6 and 16.3%, respectively). In 2015–2016 and 2016–2017, CCE were lower in NTW than in TW (23.2 and 14.8%, respectively) and were lower in NTC than in TC (36.8 and 26.8%, respectively). There was a significant interaction between tillage methods and planting patterns. The NT treatment combined with the W treatment appeared to inhibit CO₂-C emissions.

Evapotranspiration

Evapotranspiration was similar in the two growing seasons (Fig. 4). The NTW treatment had the lowest evapotranspiration, followed by the TW treatment, and finally the TC treatment. Evapotranspiration in winter wheat ranged from 275.7 to 316.7 mm and 310.4 to 340.7 mm in 2015–2016 and 2016–2017, respectively. Evapotranspiration was slightly lower in NT than in T (3.4 and 2.5%, respectively) and it was lower in W than in C (10.0 and 6.5%, respectively). Furthermore, evapotranspiration was lower in NTW than in TW (4.4 and 3.5%, respectively), and it was lower in NTC than in TC (2.6 and 1.6%, respectively). NT and W appeared to decrease evapotranspiration in the two experimental years.

Grain yield and yield components

Tillage pattern had a significant effect on winter wheat yield compositions (Table 1) in the two experimental years. Both the 1000-kernel weight and kernel numbers per spike were higher in the NT treatments than the T treatments (4.8 and 1.4%, and 5.7 and 7.1%, respectively, in 2015–2016 and 2016–2017). However, compared with T, NT had significantly lower spike numbers (12.0 and 21.7%, respectively). This meant that, overall, NT had a significantly lower winter wheat grain yield than T (9.0 and 9.5%, respectively).

The effect of planting pattern on yield compositions was significant (Table 1). Compared with C, the W treatments had fewer kernel numbers per spike (3.8 and 4.0% less, respectively) and lower 1000-kernel weights (6.4 and 1.0%, respectively), but it had more spikes (6.3 and 3.8%, respectively) and higher grain yields (by 3.8 and 7.8%, respectively).

The TW treatment had the highest grain yields (727.5 and 864.1 g m⁻², respectively) and spike numbers (685.3 and 985.7 spike m⁻², respectively), and had the lowest kernel numbers per spike (39.4 and 39.3 kernels spike⁻¹, respectively) and 1000-kernel weights (35.3 and 36.7 g, respectively). Compared with that of TW, grain yields in TC, NTW, and NTC were significantly lower (2.2, 7.5, and 12.4%, respectively, in 2015–2016, and 11.1, 13.3, and 15.7%, respectively, in 2016–2017). It appears that the NT treatments had significant negative effects on grain yield and spike numbers and significant positive effects on kernel numbers per spike and 1000-kernel weight. The W treatments compensated for the NT treatments with regards to grain yield loss. The W treatments enhanced grain yields mainly through the increase of spike numbers.

Yield carbon utilization efficiency

The YCUE was influenced by tillage methods and planting patterns in winter wheat, and ranged from 0.33 to 0.51 in 2015–2016 and from 0.38 to 0.52 in 2016–2017 (Table 2). NT had a significantly higher YCUE than that of T (32.4 and 13.3% in 2015–2016 and 2016–2017, respectively). Compared with C, W had a higher YCUE by 20.5 and 16.0% in the two experimental years, respectively. The YCUE in NTW, NTC, and TW was significantly higher than in TC by 54.5, 39.4, and 27.2%, respectively, in 2015–2016 and by 36.8, 28.9, and 34.2%, respectively, in 2016–2017.

Water use efficiency

The NT treatments had lower WUE than the T treatments (5.4 and 7.3% in 2015–2016 and 2016–2017, respectively), and W had higher WUE than in C (15.3 and 15.4% in 2015–2016 and 2016–2017, respectively) (Table 2). In the two experimental years, NTW had higher WUE than in NTC (18.9 and 11.1%, respectively), and TW had higher WUE than in TC (12.9 and 19.0%, respectively). It appeared that the NT treatments decreased WUE, but the W treatments had a compensatory effect on the NT treatments with respect to WUE.

Carbon emissions per unit water consumption

In the two experimental years, WUE_CE was significantly lower in NT than in T (by 28.0 and 19.1%, respectively), and WUE_CE was significantly lower in W than in C (5.5 and 4.2% in 2015–2016 and 2016–2017, respectively) (Table 2). WUE_CE was significantly lower in NTW than in that of TW (19.6 and 11.7%, respectively) and significantly lower in NTC than in that of TC (35.2% and 25.6%, respectively). These results suggest that both the NT and W treatments reduced WUE_CE.

CO₂ emissions

In this study, NTW had lowest CCE because of the beneficial interaction of tillage methods and planting patterns, and TC had highest CCE. For tillage methods, tilling intensively disturbed the soil, broke down soil aggregates, and exposed organic matter that was protected by soil aggregates for microbial decomposition (Six, Elliott & Paustian, 2000); the root length density was significantly higher in T than NT treatments deeper than 10 cm in the soil profile (Qin, Stamp & Richner, 2006). These factors all increased carbon emissions. During our two-year experimental period, CCE was higher in T than NT by 1.3 to 1.5 times. This finding was consistent with a previous study, which indicated that total seasonal CO₂ emissions were 1.6 times higher in T soils than NT soils (Zhang et al., 2016). Compared with C, W increased the photosynthetically active radiation capture ratio at 40 and 60 cm above the ground, and improved the leaf area index (Zhang et al., 2013). This then decreased the exposure and temperature of the soil surface (Liang & Richards, 2012), and reduced soil respiration. The reduction in CCE from combining planting patterns with tillage methods will improve the methods used for mitigating CO₂ emissions from agricultural soil.

Differences were found in CCE under T between the two years, which may be related to precipitation. In 2015–2016, total precipitation from April to harvest was 106.3 mm (67.84% of the whole growing season), while in 2016–2017 it was 70.3 mm (44.69%) in 2016–2017. A loose soil structure and suitable water content enhanced microbial activity and root system activity, resulting in higher soil respiration (Zhang et al., 2011). Hence, leading to higher CCE under T in 2015–2016 than 2016–2017.

NT reduced carbon emissions, thereby increasing soil organic carbon storage (Huang et al., 2015; Liu et al., 2015). W also reduced CCE, but it was unclear whether W increased soil carbon. More research is needed into the relationship between soil carbon emissions, soil carbon storage under W.

Grain yield and yield carbon utilization efficiency

The responses of different ecosystems to agronomic management practices vary (Brouder & Gomez-Macpherson, 2014). In general, the reasons for crop yield losses under NT are plant diseases (Wang et al., 2020), lower flag leaf fluorescence parameters and leaf area index (Liu et al., 2019), and reduction of spike numbers (Ren et al., 2018b). Winter wheat spike numbers were lower in NT than in T in this study. However, crop yield also varied under different planting patterns. Spike numbers in W treatments were significantly higher than C treatments (Li et al., 2015), which was consistent with the finding of our study, and this was the crucial factor which affected grain yield (Zhao et al., 2013). Although, NT decreased spike numbers, W enhanced spike numbers. Compared with TC, NTC decreased spike numbers by 17.1% while NTW decreased spike numbers by only 6.4% in the 2015–2016 growing season. The difference in spike numbers between NTC and TC, as well as NTW and TC, provided support for the compensatory effect of W under NT on the reduction of grain yield.

Spike numbers were higher in the second experimental year than in the first. The total precipitation was same from Nov. to Feb. in the two experimental years, but precipitation frequency was different. There was no precipitation in Dec. and Jan. in 2015–2016; however, precipitation occurred in all months in 2016–2017. The months from Nov. to Feb. are vital for tiller formation on winter wheat. A certain amount of in-season soil moisture is necessary to meet the water demands of crop components and enable root development to access the deeper soil moisture, and the well-distributed growing season precipitation is important for achieving higher grain yield (Thapa et al., 2020). Grain yield was closely related with tiller numbers. Precipitation frequency may explain the difference in spike numbers in the two experimental years.

Grain yield and CCE had an effect on YCUE. Although NT decreased grain yield, it also decreased CCE. T improved the porosity of the soil surface and the activity of root; this combined with the wet conditions of the soil, increases carbon emissions, thereby leading to a decrease in YCUE. W increased grain yield and decreased CCE. Hence, the NTW treatment had the highest YCUE among the other treatments. The interaction between tillage methods and planting patterns was significant for YCUE.

Although the yields of winter wheat in the NT other treatments were lower than those of the T treatments in the NCP, W had a compensatory effect on grain yield and increased YCUE. To mitigate GHG emissions, NT and W together appeared to provide a viable approach for cleaner production.

Water use

NT decreased evapotranspiration in this study, which was similar to Huang et al. (2012), because NT increased interception of precipitation, and it reduced water absorption because of the lower root volume (Ali et al., 2018). WUE was lower in NT than T, mostly because of the reduction in grain yield under NT, which was similar to previous study (Ren et al., 2018a; Ren et al., 2018b). W had higher WUE than C, mostly because of higher improving grain yield, as well as, reduced evapotranspiration. The reduction of evapotranspiration under W may be related to the change of sowing width (6–8 cm under W; 3–5 cm under C), which could increase the leaf area index (Zhao et al., 2013), reduce the exposure of the soil surface, and decrease evaporation between crop rows. The W treatments appeared to have a compensatory effect on WUE under NT treatments, and the combination may provide a more suitable strategy for sustainable agriculture.

In Northwestern China, another study found that reduced tillage in wheat production had mean WUE_CE values of 2.3 kg C ha⁻¹mm⁻¹, which was 4.7% lower than that of T (Hu et al., 2015), much less than the 28.0 and 19.0% differences observed for the 2015–2016 and 2016–2017 seasons, respectively, in the present study. Differences in temperature and precipitation between the study areas probably caused this difference. In the previous study, spring wheat was planted in Mar. and harvested in Jul. in Northwestern China and in the present study, winter wheat was planted in Oct. in the first year and harvested in Jun. in the second year in the NCP. There were large differences in the temperature and precipitation conditions experienced by the crops.

Limitations and future research

Because of climate variability and soil heterogeneity, there are high temporal and spatial differences in GHG emissions. We only measured the CO₂-C of the soil surface in this study, so we could not better explain trends in total GHG emissions. Decreasing carbon emissions might also be related to the soil organic carbon pool and root growth in the W treatment. Further investigations into crop root development and the soil carbon sequestrations under different tillage and planting methods should be considered in the NCP area. However, we have shown that there is a clear difference in carbon emissions and WUE under different planting patterns and tillage methods. This provides options for investigating cleaner agricultural production. In the long run, the combination of NT and W may benefit food security and environment conditions. This study was an analogous tillage experiment, it was not a mechanical tillage field experiment. We recommend that a field experiment should be established to verify the results of our research.