3.1. Environmental variables and soil condition
The daily air temperature and precipitation, as well as soil temperature and WFPS, are shown in Figure 1. Strong precipitation occurred during the peanut growing season, especially in August. The daily average air temperature during the wheat growing season ranged from -6.9 to 28.8 °C in 2017-2018 and from -4.8 to 29.3 °C in 2018-2019, and those during the peanut growing season ranged from 21.2 to 31.8 °C and from 21.1 to 31.5 °C, respectively (Fig. 1A,B). The seasonal variation trend of soil temperature was comparable to that of air temperature, ranging from 4.8 to 26.7 °C in the wheat growing season and from 21.1 to 31.5 °C in the peanut growing season (Fig. 1C). The mean seasonal soil temperature in the peanut growing season was 13.8 °C greater than that in the wheat growing season. The soil WFPS significantly increased after irrigation and heavy rainfall. There was no significant difference in soil WFPS between the various fertilisation rates. However, the soil WFPS varied from 15.1 to 76.9% during the wheat growing season, which was generally lower than that in the peanut growing season due to soil evaporation and less precipitation.
The application of N fertiliser was found to significantly increase the soil concentration of NH+ 4-N and NO- 3-N compared with the control; these concentrations sharply increased after fertilisation in all treatments, especially in the 0–20 and 20–40 cm soil layers (Fig. 3). During the wheat growing season, peak soil NH+ 4-N and NO- 3-N concentrations were observed after the jointing fertilisation, the peak values in JCF100 was higher than that in JCF70 and JCRF70. Thereafter, the soil NH+ 4-N content rapidly decreased and remained low until the harvest period of wheat, while the soil NO- 3-N content of JCF100 reached another relatively large peak in the 0–20 cm soil layers in the filling stage of wheat. During the peanut growing season, soil concentrations of NH+ 4-N and NO- 3-N were significantly higher in treatments JCF70 and JCRF70 compared to treatment JCF100 due to the fact that 30% of the total fertiliser was applied in the initial peanut flowering stage. Under the same fertiliser rate, the peaks of the soil NH+ 4-N and NO- 3-N concentrations were larger in treatment JCF70 than in treatment JCRF70. However, in treatment JCF70, the soil NH+ 4-N and NO- 3-N concentrations were mostly lower compared to those in treatment JCRF70, especially in the late peanut growth stage. The annual average soil NH+ 4-N contents were 15.0, 21.5, 22.9, and 24.8 mg N kg-1 for CK, JCF100, JCF70, and JCRF70, respectively, and the annual average soil NO- 3-N contents were 5.4, 23.2, 25.3, and 27.4 mg N kg-1 for CK, JCF100, JCF70, and JCRF70, respectively. Overall, the soil concentrations of NH+ 4-N were lower than the NO- 3-N concentrations in all fertiliser treatments, except in the early wheat growing season in the 40–60 cm soil layer. Additionally, the concentration of soil NO- 3-N peaked not only after fertilisation but also after the rainfall events, and small peaks were observed on 07 June 2019.
3.2. CO2 emission
In the wheat growing season, except for a small peak which occurred after basal fertilisation during 2017-2018, the CO2 emission fluxes remained at a low level in the overwintering period in all treatments (Fig. 4A,B). However, the CO2 emission fluxes resumed and reached their maximum two and three days after the jointing fertilisation and irrigation events. For the CK, the CO2 emission fluxes was maintained at a low level until the harvest period of wheat, the application N fertiliser can significantly increase the flux of CO2 emission, and this change increased with increasing N application rate, the CO2 emission fluxes were higher by16.3–71.9% compared with CK over the two years (Fig. 4A-1,B-1). The largest CO2 emission flux occurred in treatment JCF100 (average 1495 mg m-2 h-1), followed by JCF70 (1285 mg m-2 h-1), JCRF70 (1014 mg m-2 h-1), and CK (868 mg m-2 h-1).
In the peanut growing season, the CO2 emission fluxes reached their maximum two days after the the second application of N fertiliser at the anthesis stage in 2018, while it reached their maximum approximately 4 to 5 days in 2019, with the largest flux being observed in treatment JCF70 (average 1509 mg m-2 h-1), followed by JCRF70 (1253 mg m-2 h-1), JCF100 (852 mg m-2 h-1), and CK (527 mg m-2 h-1). The peak CO2 emission fluxes of treatments JCF70 and JCRF70 were higher by 83.6% and 52.9%, respectively, compared with that of JCF100 (Fig. 4A-2,B-2). Due to a series of precipitation events (totalling approximately 86 mm) which occurred between 27 July and 02 August 2019 (Fig. 1B), an obvious and large peak in CO2 emissions was observed on 02 August. In addition, the CO2 emission fluxes during the peanut growing season were higher than during the wheat growing season, especially in August and September from 2018-2019. At the same fertiliser rate, the CO2 fluxes were lower in treatment JCRF70 than in treatment JCF70 for both the wheat and peanut growing seasons over the two years.
The application of N fertiliser was found to significantly increase the cumulative CO2 emissions compared with CK; with N fertilisation, the cumulative CO2 emissions were between 8648 and 16609 kg CO2 ha-1 in 2018 and 10918 and 16870 kg CO2 ha-1 in 2019 during the wheat growing season, while those during the peanut growing season were between 9206 and 16940 kg CO2 ha-1 in 2018 and 16494 and 27700 kg CO2 ha-1 in 2019 (Table 2). Furthermore, split applications of N were found to significantly increase the cumulative CO2 emissions. The annual cumulative CO2 emission in treatment JCF70 was 9.0% higher than that in treatment JCF100 over the two years. However, under the same fertiliser rate, the annual cumulative CO2 emission in treatment JCRF70 was lower by 14.1% compared with that of treatment JCF70. The annual accumulated CO2 emissions were significantly influenced by the fertilizer treatment and year, but not significantly affected by the interaction between the fertilizer treatment and the year (Table 3). Moreover, the results of the correlation analysis (Table 4) showed that the daily CO2 emission was significantly positively correlated with the soil temperature, the WFPS, and the soil NH+ 4-N concentration, with correlation coefficients of 0.43, 0.55, and 0.64, respectively, at the P<0.01 level.
Table 2 Global warming potential (GWP) and global warming potential intensity (GHGI) under different nitrogen treatments.
Year
|
Treatment
|
Wheat
|
Peanut
|
|
Annual
|
|
|
CO2
|
N2O
|
CO2
|
N2O
|
CO2
|
N2O
|
Yield
|
GWP
|
GHGI
|
(kg ha-1)
|
(kg ha-1)
|
(kg ha-1)
|
(kg CO2-eq ha-1)
|
(kg -1)
|
2017-2018
|
CK
|
8648 c
|
1.12 c
|
9206 c
|
1.46 d
|
17854 d
|
2.38 d
|
10200 d
|
18624 d
|
1.83 c
|
|
JCF100
|
12035 b
|
1.71 b
|
16940 a
|
2.55 b
|
28976 b
|
4.26 c
|
13257 c
|
30243 b
|
2.81 a
|
|
JCRF70
|
13176 b
|
1.82 a
|
12197 b
|
2.60 c
|
25373 c
|
4.42 b
|
14627 b
|
26689 c
|
1.82 c
|
|
JCF70
|
16609 a
|
2.77 a
|
16094 a
|
2.80 a
|
32704 a
|
5.56 a
|
15490 a
|
34361 a
|
2.22 b
|
2018-2019
|
CK
|
10918 d
|
1.31 d
|
16491 c
|
1.28 d
|
27409 c
|
2.59 d
|
12464 d
|
28180 c
|
2.26 c
|
|
JCF100
|
16870 a
|
2.33 b
|
24573 b
|
1.88 c
|
41443 b
|
4.21 c
|
15879 c
|
42698 b
|
2.69 a
|
|
JCRF70
|
14872 c
|
1.66 c
|
26140 a
|
2.89 b
|
41013 b
|
4.55 b
|
19134 a
|
42368 b
|
2.19 c
|
|
JCF70
|
15880 b
|
2.52 a
|
27700 a
|
3.32 a
|
43581 a
|
5.84 a
|
18618 b
|
45321 a
|
2.47 b
|
Different letters within a column represent significantly different mean values at P<0.05, according to the LSD test.
Table 3 ANOVA for the effects of N treatment and year (Y) on annual CO2 and N2O emissions, and total grain yields.
Factors
|
Cumulative CO2 (kg ha-1)
|
Cumulative N2O (kg ha-1)
|
Yield (kg ha-1)
|
|
F value
|
df
|
p
|
F value
|
df
|
p
|
F value
|
df
|
p
|
Year
|
558.008
|
1
|
<0.001
|
0.6410
|
1
|
0.4351
|
37.186
|
1
|
<0.001
|
N treatment
|
172.898
|
3
|
<0.001
|
129.5880
|
3
|
<0.001
|
27.058
|
3
|
<0.001
|
N×Y
|
6.5160
|
3
|
0.4542
|
0.4310
|
3
|
0.7335
|
0.9180
|
3
|
0.0004
|
Table 4 Correlations between greenhouse-gas emissions and environmental factors
Correlations
|
CO2
|
N2O
|
Soil temperature
|
0.43**
|
-0.40**
|
WFPS
|
0.55**
|
0.15
|
NH+ 4-N
|
0.64**
|
-0.13
|
NO- 3-N
|
0.45*
|
-0.05
|
∗ significant at P < 0.05; ∗∗ significant at P < 0.01
3.3. N2O emission
The N2O emission fluxes exhibited the same trends as the CO2 emission fluxes for both the wheat growing season and the peanut growing season in the two years(Fig. 5A,B). The application of N fertiliser was found to significantly increase the N2O emission fluxes relative to CK, especially after irrigation and precipitation. In the N fertilisation treatments, the average N2O emission fluxes ranged from 13.8 to 243.3 µg m-2 h-1 in 2018 and from 13.6 to 260.5 µg m-2 h-1 in 2019 during the wheat growing season, while those during the peanut growing season ranged from 26.5 to 143.3 µg m-2 h-1 in 2018 and from 24.3 to 130.9 µg m-2 h-1 in 2019. The N2O emission fluxes reached a first peak on the second day after jointing fertilisation and irrigation for all treatments in the two years. The average value of this peak was 260.5 µg m-2 h-1 under treatment JCF100, which was significantly higher than the values obtained for treatments JCF70 and JCRF70. After which N2O emission sharply decreased except a relatively large peak was seen on 19 May in 2019 due to the irrigation event. During the peanut growing season, the peak in N2O emissions after anthesis fertilisation was lower than that after jointing fertilisation due to the lack of irrigation in this fertilisation event, but emission valus were maintained at a relative high level until the harvest period of peanut. The N2O emission fluxes reached the peak on the second day for treatments JCF70 and JCRF70 after anthesis fertilisation, and the N2O emission flux for JCF70 was higher than for JCRF70 all the time from day two to day six (Fig. 5A-2,B-2). In all the N fertilisation treatments, the mean N2O emission flux was higher in the growing season of peanut than in the growing season of wheat due to the higher temperature and greater amount of rainfall in the former in both years(Fig. 1A,B). The second peak of N2O emissions was 70.6% and 49.5% higher in JCR70 and JCRF70, respectively, compared to that in the JCF100 treatment (Fig. 5 a-2,B-2). Due to an irrigation event on 18 May 2019, peaks of N2O emission fluxes occurred on 19 May 2019 for all treatments (Fig. 5A).
In both years, splitting N application into three applications was shown to significantly increase the cumulative N2O emissions compared with two N applications, while no significant difference was found during the wheat growing season. Across the two seasons, the average total cumulative N2O emissions in CK, JCF100, JCF70, and JCRF70 were 2.49, 4.24, 5.70, and 4.49 kg N2O ha-1, respectively (Table 2). The emission fluxes of N2O in treatment JCRF70 were generally lower than those in treatment JCF70 (Fig. 5A,B) and the annual cumulative N2O emissions in treatment JCRF70 were lower by 34.3% in 2018 and by 51.8% in 2019 during the wheat growing season, and those lower by 7.9% and 13.0% during the peanut growing season, respectively, compared with treatment JCF70. The annual accumulated N2O emissions were significantly influenced by the fertilizer treatment, but not significantly affected by the year and the interaction between the fertilizer treatment and the year (Table 3). Additionally, in all treatments, soil variables were found to have no significant correlation with daily N2O emission fluxes, while there was found to be a negative correlation between N2O flux and soil temperature (Table 4).
3.4. Grain yields and GHGI
Compared to CK, wheat grain yields were found to be significantly higher by 36.7–54.3% in 2018 and by 31.8–57.6% in 2019 under all N fertiliser treatments (Fig. 6 A, B). Meanwhile, the wheat grain yield of treatment JCF70 was higher than that of treatment JCF100 despite the fertiliser use being a 30% lower. Compared with treatment JCF100, the grain yields in treatments JCF70 and JCRF70 were higher by 10.9% and 12.8%, respectively, in 2018, and by 16.7% and 19.6%, respectively, in 2019. However, no significant difference in grain yield was found between treatments JCF70 and JCRF70. Split N fertiliser application was found to significantly increase the pod yield and kernel yield of peanut in both growing seasons compared with CK (Fig. 6 C, D). Compared with treatment JCF100, the pod yields in treatments JCF70 and JCRF70 were higher by 9.7–21.0% in 2018 and by 14.6–24.8% in 2019, respectively. Additionally, under the same N fertiliser rate, the pod yields for treatment JCRF70 in 2018 and 2019 were 5.2% and 8.9% higher than for treatment JCF70, respectively.
As shown in Table 2, N application was found to significantly increase GWP and GHGI in both the wheat and peanut growing seasons. During the wheat growing season, the GWP and GHGI in JCF100 were higher than that in treatments JCF70 and JCRF70 due to all the fertiliser were applied to wheat. However, there were no statistically significant difference of GWP between treatments JCF100 and JCRF70 for the peanut growing seasons and the annual. In addition, the annual lowest GHGI was obtained in treatment JCRF70 due to a higher total grain yield in both years. In comparison with the respective urea treatments, CRF treatments were found to significantly decrease GHGI in both the wheat and peanut growing seasons.