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Trade-offs between residue incorporation and K fertilizer on seed cotton yield and yield-scaled nitrous oxide emissions
Field Crops Research 244 (2019) 107630
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Trade-offs between residue incorporation and K fertilizer on seed cotton yield and yield-scaled nitrous oxide emissions Nan Jianga, Li Zhaa, Wei Hua, Changqin Yangb, Yali Menga, Binglin Chena, Zhiguo Zhoua, a b
T
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Key Laboratory of Crop Physiology & Ecology, Ministry of Agriculture, Nanjing Agricultural University, Nanjing, China Jiangsu Academy of Agricultural Sciences, Nanjing, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Crop residue incorporation Seed cotton yield Nitrous oxide Yield-Scaled nitrous oxide
Crop residues can effectively replace inorganic potassium (K) fertilizers to increase crop yield. However, the effect of incorporating consecutive crop residues instead of applying inorganic K fertilizers on seed cotton yield and yield-scaled N2O emissions (N2O emissions per unit yield of seed cotton) over a long period has rarely been reported. Therefore, a six-year (2011–2016) study incorporating crop residue instead of applying inorganic K fertilizer was conducted in a cotton-wheat system (transplanting cotton after wheat harvesting within the same crop season). Results showed that no statistical difference was observed between crop residue treatments and K fertilizers application on seed cotton yield during 2011–2014. However, in 2015, the yields of the inorganic K treatments were superior to the yields of the residue retained treatments. In 2016, the residue retained treatments and inorganic K treatments had similar seed cotton yields except that the K300 (300 kg K2O ha−1) treatment had a significantly higher yield than the 9000 kg ha−1 wheat straw + 7500 kg ha−1 cotton residue treatment (C7500 + W9000). The seasonal N2O emissions and yield-scaled N2O emissions increased by 1.01–1.65 times and 1.02–1.94 times in residue incorporation compared with inorganic K application treatments during 2015 and 2016 (the fifth and sixth year of residue incorporation), respectively. Furthermore, the soil N2O fluxes were significantly correlated with soil dissolved carbon and available mineral nitrogen contents. In conclusion, crop residue as a K source is a suitable alternate to enhance cotton yield and to replace inorganic K fertilizers but this could also boost the soil N2O fluxes, seasonal N2O emissions, and yield-scaled N2O emissions.
1. Introduction Considering the scarce potash reserves in China, recently the incorporation of crop residue to replace inorganic potassium (K) fertilizers is gaining a great attention (Sui et al., 2015). According to our previous 3-year study (started from 2011), both cotton residue and wheat straw partially or totally replaced the inorganic K fertilizer as well as improved the lint cotton yield at the Dafeng research station (Sui et al., 2015; Yu, 2016b). However, residue quality, residue returning duration, and edaphic factors are variable, making the effects of crop residue incorporation on crop yield complex and variable (Putte et al., 2010; Kumar and Goh, 2000). For example, wheat straw returning and cotton residue incorporation have been reported as an effective strategy to compliment K compensating the soil K depletion and improving cotton yield (Jiang et al., 2019; Sui et al., 2017; Tan et al., 2017). Nevertheless, a contrary conclusion in which crop residue incorporation reduced the crop yield by 4.5% on average under European conditions was reported by Putte et al. (2010). Despite of positive and
⁎
negative aspects of crop residues incorporation, some previous studies also reported no effect on crop production (Soon and Lupwayi, 2012). Therefore, the effects of consecutive crop residue incorporation against inorganic K fertilization on seed cotton yield may be uncertain over a longer period, and continuous concern for the seed cotton yield under crop residue incorporation may provide valuable information on crop residue management. Indeed, most crop of the residues are used with low efficiency or wasted in China (Liu et al., 2008). For instance, in Shandong province where cotton-wheat system occupies a dominate position in cotton production, 73% of the cotton residue are directly burnt, 20% for industry materials, 5% for residue incorporation and the rest 2% are discarded (Dai and Dong, 2014; Wang et al., 2014). Likewise, wheat straw is primarily adopted as green manure, forage, industry material and fuel, after the part returning to fields or collection lost (Liu et al., 2008). Besides of the complex effects of crop residue incorporation on yield, numerous studies also highlighted a strong effect of residue incorporation on nitrous oxide (N2O, a potent greenhouse gas) emissions
Corresponding author. E-mail address: [email protected] (Z. Zhou).
https://doi.org/10.1016/j.fcr.2019.107630 Received 4 March 2019; Received in revised form 8 September 2019; Accepted 18 September 2019 0378-4290/ © 2019 Elsevier B.V. All rights reserved.
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inter-row and intra-row spacing, respectively keeping a planting density of 3.3 × 104 plants ha−1. Following cotton harvesting at the end of October or in early November, wheat was sown in rotary tilled soil using a seed rate of 150 kg ha−1 with an inter-row spacing of 0.15 m. After the harvest of the preceding crops, wheat straw or cotton residues were removed from the plots and then smashed for nutrient and water content measurement before being incorporated into the top 00.1 m of the soil. Based on the average yield of wheat straw and cotton residue of 9.0 × 103 kg ha−1 and 7.5 × 103 kg ha−1 in the Yangtze River Valley, respectively, wheat straw was always applied before cotton transplantation at rates of 0 and 9.0 × 103 kg ha−1 (W0 and W9000), and cotton residue was always applied before wheat sowing at rates of 0 and 7.5 × 103 kg ha−1 (C0 and C7500). Therefore, within the same growth season (cotton growing season + wheat growing season), 4 treatments with crop residue incorporation replacing inorganic K fertilization were conducted: W0 + C0 (CK), W0 + C7500, W9000 + C0, and W9000 + C7500. In addition, before cotton transplantation, two independent inorganic K fertilizer treatments without crop residue incorporation were applied as the basal fertilizer at rates of 150 and 300 kg K2O ha−1 as potassium sulfate (50% K2O and 18% sulfur, K150 and K300). During the wheat growing season, no inorganic K fertilizer was used. Therefore, a total of six treatments with or without crop residue incorporation were investigated (CK, W0 + C7500, W9000 + C0, W9000 + C7500, K150, and K300). In addition, inorganic N and P fertilizers were used at the same rates of 300 kg N ha−1 and 150 kg P2O5 ha−1 in both the cotton and wheat growing seasons. In the cotton growing seasons of 2015 and 2016, the N and P fertilization rates and timing are shown in Table 1. In the wheat growing season, N fertilizer was applied as urea at rates of 150 and 150 kg N ha−1 before wheat sowing and at the wheat jointing stage, respectively, and P fertilizer was applied as ordinary superphosphate before wheat sowing. The experiment was arranged in plots of 55 m2 (5 m × 11 m) in a randomized design with 3 replicates. Notably, the two inorganic K fertilization treatments were conducted before the cotton transplantation in 2012 (one year later than the stationary field experiment conducted in 2011). Therefore, the seed cotton yield and yield components under W0 + C7500 (cotton residue was always applied before wheat sowing within the same growth season) and the K150 and K300 treatments in 2011 are not listed in Fig. 2.
from agricultural soils (Hu et al., 2016). The crop residues being big source of organic matter, contain a considerable quantity of carbon (C) and nitrogen (N) (Harper and Lynch, 2010; Smil, 1999). This exogenously applied organic matter source may accelerate or retard soil C and N cycling, resulting in higher or lower N2O emissions (Huang et al., 2004; Rasmussen et al., 1980; Witt et al., 2000; Yao et al., 2009). Thus, the application of crop residue may affect the soil N2O emissions by affecting soil C and N status. Similarly, the indicators of soil C and N status, such as soil dissolved organic C (DOC), available mineral N (AMN, sum of the soil NH4+ and NO3−), microbial biomass C (MBC), and microbial biomass N (MBN) contents, as well as the ratios of MBN to MBC (MBC/MBN), and MBC to total organic C (MBC/TC) should be correlated with soil N2O emissions (Anderson and Domsch, 1989; Grace et al., 1993; Song et al., 2007). Although numerous previous literatures have been reported on the effect of residue incorporation either on crop production or environmental aspects but a limited information is available on the environmental costs associated with crop production (yield-scaled N2O emissions) for residue management practices (Van et al., 2010; Qin et al., 2012). Studies addressing the impact of crop residue incorporation replacing inorganic K fertilization in a cottonwheat system on seed cotton yield-scaled N2O emissions and its relationship to the soil C and N status are even limited. Keep in view the previous literature, the present study was hypothesized that crop residue incorporation would increase cotton yield and yield-scaled N2O emissions compared with inorganic K fertilization. Further N2O fluxes would be positively correlated with the soil C and N supply attributes. So a six-year field experiment on crop residue and inorganic K fertilizer was conducted to verify the former set hypothesis under the climatic conditions of Dafeng, China. 2. Materials and methods 2.1. Site description A stationary field experiment was initiated in early June 2011. The location of the experimental station at Dafeng Basic Seed Farm (Dafeng, 33°24′ N, 120°34′ E) is in the Yangtze River delta in China. The location of the Dafeng site has a subtropical monsoon climate, with a hot and rainy summer and a cold and humid winter. The daily mean temperature and precipitation during the cotton growing seasons from 2014 to 2016 are shown in Fig. 1. The soil of the experimental site was sandy loam (6.8% clay and 57.0% sand) with the following topsoil (0–20 cm) properties before cotton transplantation in 2011: 1.18 g kg−1 total nitrogen (TN, sum of the organic and inorganic fraction of soil nitrogen), 26.4 mg kg−1 AMN, 12.1 g kg−1 soil organic carbon, 22.2 mg kg−1 Olsen phosphorus, 60.6 mg kg−1 water-soluble K, 255.8 mg kg−1 available K (NH4Ac extract), 1.1 g kg−1 nonexchangeable K, 18.4 g kg−1 structural K, 1.44 g cm-3 bulk density and pH 7.9 (soil:water = 1:2.5).
2.3. Seed cotton yield and yield components At harvest, 20 plants in the central row of each plot were harvested to measure seed cotton yield and yield components (number of bolls and seed cotton weight per plant). The seed cotton yield per hectare was calculated as the seed cotton weight per plant × number of bolls per plant × plant density. The percentage increase in seed cotton yield over the CK treatment was calculated by the following formula: (1) (YK - Y0) / Y0 × 100%
2.2. Experimental design and management
where YK is the seed cotton yield under different sources of K fertilization and Y0 is the seed cotton yield under the K control treatment.
The experiment was conducted with widely planted cultivars Gossypium hirsutum L. Siza 3 and Triticum aestivum L. Yangmai 16. Cotton seeds were sown on April 15, 2011 in a nursery bed and then transplanted to fields (after wheat harvesting) with 1.0 m and 0.3 m
Table 1 Chemical fertilizer rates and timing during the cotton growing season in 2015 and 2016. Chemical fertilization rates (N:P2O5:K2O, kg ha−1)
120:150:0 120:0:0 60:0:0
Fig. 1. Daily mean air temperature and precipitation in the cotton growing season from 2014 to 2016. 2
Application time (dd-mm) 2015
2016
15-Jun. 01-Aug. 04-Sep.
17-Jun. 01-Aug. 01-Sep.
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Fig. 2. Effect of crop residue incorporation and inorganic K fertilization on seed cotton yield increment (%) over the control treatment (CK).from 2011 to 2016. Different letters under a same treatment in different years indicate significant difference according to the shortest significant ranges (SSR) test (P < 0.05). W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively; the seed cotton yield increments (%) over the control treatment (CK) from 2011 to 2013 were cited from our previous research (Sui et al. 2014; Yu et al. 2016b).
and SPSS (ver. 23.0, IBM software). The data were normally distributed and one-way analysis of variance (ANOVA) was used to test the significant difference among treatments for soil TC, TN, AMN, DOC, MBC, MBN, Olsen-P, and Av-K content as well as the ratios of MBC/MBN and MBC/TC, N2O fluxes, seasonal N2O emissions, and yield-scaled N2O emissions. Pearson correlation analysis was used to verify whether there are association between the N2O fluxes and all the above tested parameters of the soil. Post hoc mean separation analysis was performed using the multiple-range test of Duncan’s SSR at P < 0.01 and P < 0.05 levels of significance.
2.4. N2O sampling and analysis N2O samples were collected using static chambers and stored in cold storage before the gas chromatography analysis (Zou et al., 2005). A chamber with a fixed PVC frame and size of 0.8 m × 0.8 m × 0.8 m or 0.8 m × 0.8 m × 1.6 m, depending on the cotton plant height, was installed in each replication of all six treatments. At the top edge of the frames, 5 cm deep grooves filled with water were installed to seal the chamber. The chambers in our experiment were equipped with fans to ensure that the gas in the chambers was evenly mixed. Additionally, to minimize the changes in air temperature the chambers were covered with insulation and aluminum foil layers. During the two cotton growing seasons in 2015 and 2016, N2O samples were collected from 9:00-11:00 am every 10 days. However, in case of precipitation, fertilization, and midseason drainage events, N2O samples were collected more frequently. After 5, 10, 15, and 20 min of the chambers being covered, N2O samples were collected from the chamber with a 60 ml syringe and then analyzed in a gas chromatograph (Agilent 7890A, Shanghai, China) equipped with an electron capture detector (ECD). The temperature for the column was set at 40℃, and the ECD detector was maintained at 300℃. N2O fluxes and standard errors on seasonal N2O emissions during the cotton growing season in 2015 and 2016 were the average of the triplicate plots. Unless a linear regression value of R2 > 0.90 is obtained, the sample sets would be rejected. The seasonal N2O emissions were analyzed by the methods of Zou et al. (2005). The yield-scaled N2O emissions in 2015 and 2016 were obtained by dividing seasonal N2O emissions by seed cotton yield (Venterea et al., 2011). The soil temperature at approximately 0.1 m depth was recorded on every N2O sampling day using soil thermometers throughout the entire cotton growing season in 2015 and 2016.
3. Results 3.1. Seed cotton yield and yield components The seed cotton yield and yield components from each treatment from 2011 to 2013 were previously reported by Sui et al. (2014) and Yu et al. (2016b). All the yield and yield components data are given in Table 3. The residue incorporation and inorganic K application treatments yielded statistically similar seed cotton from 2011 to 2014. While in 2015, the yields of the inorganic K treatments were superior to that of residue retained treatments (Table 3). However, in 2016, only K300 produced relatively higher seed cotton yield compared either to residue retained treatments or to K150 treatment. Compared with the CK treatment, different sources of K fertilization significantly (P < 0.05) increased boll numbers and seed cotton yields from 2011 to 2016 (Table 3 and Table 6). Over the six years, the highest improvement in seed cotton yield by crop residue incorporation (20.7%–38.4%) over the control treatment (CK) was recorded in 2013 (the third year of residue incorporation) and then decreased gradually in the subsequent three years (Fig. 2). Meanwhile, among the residue treatments, the W9000 + C7500 treatment highest improved the seed cotton yield over CK by 1.9%–38.4% during the first five years (2011–2015). However, in 2016, more increase in seed cotton yield (13.9%) was observed in W9000 + C0 compared with rest of the residue application treatments.
2.5. Soil sampling Five randomized soil cores (3 cm diameter) from the uppermost soil layer (0-0.2 m) of each plot were sampled during the cotton flowering and boll setting stage on August 19 in 2015 and August 14 in 2016. Then, the soil samples were sieved through a 2-mm mesh and divided into two parts for 4 ℃ refrigerator storage and air-drying. The soil AMN content is the sum of the soil ammonium- N (NH4+-N) and nitrate- N (NO3−-N) content and was determined by the method of Lu (1999). The soil TC and DOC content was measured by the method of Lu (1999) and Jiang et al. (2006), respectively. Soil TN, Olsen-P, and Av-K content was measured by the method of Lu (1999). The MBC and MBN were determined using the fumigation-extraction method of Vance et al. (1987) and Brookes et al. (1985), respectively.
3.2. N2O fluxes and seasonal N2O emissions N2O fluxes were significantly (P < 0.05) affected by crop residue incorporation across the two cotton growing seasons in 2015 and 2016 (the fifth and sixth years of residue incorporation, Table 6). The data in Fig. 3 and Table 5 show the variation of N2O fluxes of different treatments during the two cotton growing seasons. According to the presented data, the W9000 + C7500 showed the highest N2O fluxes with values of 0.16 mg N2O m−2 h-1 and 0.14 mg N2O m−2 h-1 in month of August in 2015 and 2016, respectively. Seasonal N2O emissions ranged from a high of 2.06 kg N2O ha-1 in the W9000 + C7500 treatment to a low of 1.25 kg N2O ha-1 in the K300 treatment in 2015. But in 2016,
2.6. Statistical analysis All statistical analyses were performed with Microsoft Excel-2016 3
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Fig. 3. Seasonal variations of N2O fluxes in cotton growing seasons in 2015 and 2016; the vertical bars represent the standard deviations of the means. W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively.
Fig. 4. Ratios of soil microbial biomass carbon (MBC) to soil microbial biomass nitrogen (MBN) and MBC to total carbon (TC) (%), the vertical bars represent the standard deviations of the means. Different letters within the same plot in a same year indicate significant difference according to the shortest significant ranges (SSR) test (P < 0.05). W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively.
seasonal N2O emissions ranged from a high of 1.73 kg N2O ha-1 in the W9000 + C7500 treatment to a low of 1.01 kg N2O ha-1 in the K300 treatment. Compared with the CK, K150, and K300 treatments, treatments with crop residue incorporation had higher N2O fluxes and higher seasonal N2O emissions in both years. Moreover, compared with the W0 + C7500 treatment, the W9000 + C0 treatment released 13.7% and 19.4% more N2O during the cotton growing seasons in 2015 and 2016, respectively. (Fig. 4)
those under the inorganic K treatments in 2015 and 2016, respectively. 3.4. Soil fertility The soil TC, TN, DOC, MBC, AMN, MBN, Olsen-P and Av-K contents in the cotton flowering and boll setting stage in 2015 and 2016 are presented in Table 4. Compared with the CK, K150, and K300 treatments, the soil TC, TN and DOC contents were significantly (P < 0.05) improved by crop residue application (Table 6). The soil TC contents ranged from a high of 17.0 g kg−1 in W9000 + C7500 treatment to a low of 13.1 g kg−1 in the CK treatment in 2015 and ranged from a high of 17.4 g kg−1 in W9000 + C7500 treatment to a low of 13.8 g kg−1 in the CK treatment in 2016. The soil DOC contents ranged from a high of 101 mg kg−1 in the W9000 + C7500 treatment to a low of 74.2 mg kg−1 in the CK treatment in 2015 and ranged from a high of 106 mg kg−1 in the W9000 + C7500 treatment to a low of 75.5 mg kg−1 in CK treatment in 2016. The soil TN contents ranged from a high of 1.33 g kg−1 in the W9000 + C7500 treatment to a low of 1.22 g kg−1 in the CK treatment in 2015 and ranged from a high of 1.47 g kg−1 in the W9000 + C7500 treatment to a low of 1.35 g kg−1 in the CK treatment in 2016. Across the two cotton growing seasons, the wheat straw and cotton residue alone or in combination returning to the soil significantly (P < 0.05) decreased the MBC/TC ratio (Fig. 3). The ratios of MBC/TC
3.3. Yield-scaled N2O emissions Different K fertilization sources significantly affected the seasonal yield-scaled N2O emissions across two cotton growing seasons (Table 6). The yield-scaled N2O emissions from each treatment in the cotton growing seasons in 2015 and 2016 are presented in Table 5. K fertilization from residues significantly (P < 0.05) increased the yieldscaled N2O compared with the CK, K150, and K300 treatments during both seasons. Yield-scaled N2O emissions ranged from a high of 0.60 g N2O kg−1 seed cotton in the W9000 + C7500 treatment to a low of 0.31 g N2O kg−1 seed cotton in the K300 treatment in 2015 and ranged from a high of 0.34 g N2O kg−1 seed cotton in the W9000 + C7500 treatment to a low of 0.18 g N2O kg−1 seed cotton in the W0 + C7500 treatment in 2016. The highest yield-scaled N2O emissions under the residue treatments were 31.7%–93.5% and 1.7%–88.9% higher than 4
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alone (W9000 + C0 and W0 + C7500) were significantly lower in N2O fluxes and seasonal N2O emissions in the two cotton growing seasons of 2015 and 2016 (Fig. 3). This is because various crop residues consist of different C and N content with varying C/N ratios, which can affect soil N2O emissions in different ways (Shan and Yan, 2013). Higher C/N ratios or more exogenous C provision into the soil by residue application can enhance soil N immobilization and reduce soil available N for the microbial denitrification process and finally result in a higher soil N2O emissions (Baggs et al., 2010; Rezaei et al., 2017). Furthermore, residue returning duration, quantity of the residue returning and their interactions can affect seasonal N2O emissions (Table 6). The general trend of N2O fluxing was the same among the six treatments in both years, but significantly (P < 0.05) higher N2O emissions were observed during the cotton growing season in 2015 compared to 2016. This can be attributed to the higher mean air temperature in the cotton growing seasons of 2015 (Fig. 1). Because the higher temperatures can stimulate soil N2O emissions (Hu et al., 2016; Koponen and Martikainen, 2004; Zou et al., 2009). Zou et al. (2004) suggested an exponential relationship (y = 0.042e0.137x, R2 = 0.834, P < 0.001) between the normalized N2O emission and air temperature according to the datasets built from the non-waterlogged period of the rice-growing season.
were decreased under the W9000 + C7500 treatment compared with the inorganic K fertilizer treatments, lower than the W9000 + C0 treatment and the W0 + C7500 treatment in the 2015 and 2016 cotton growing seasons. The ratios of MBC/TC were 14.4%–15.9% lower under the W9000 + C7500 treatment compared to the inorganic K fertilizer treatments. We also found that wheat straw and cotton residue alone or in combination returning to the soil had significant effects on the ratios of MBC/MBN across the two cotton growing seasons (Fig. 3). The MBC/MBN ratios were decreased by 7.2%–16.4% under the W9000 + C7000 treatment compared to the inorganic K fertilizer treatments in the cotton growing seasons in 2015 and 2016. 3.5. Correlation analysis Soil TC, TN, MBC, MBN, Olsen-P, and Av-K contents did not show a significant correlation with soil N2O fluxes at flowering and boll setting stage of cotton in 2015 as well as in 2016. However, soil DOC contents showed a significant (P < 0.01) and positive correlation with N2O fluxes. Conversely, the soil AMN contents were significantly (P < 0.05) and negatively correlated with the soil N2O fluxes (Table 7). 4. Discussion 4.1. Seed cotton yield and yield components
4.3. Yield-scaled N2O emissions
In the current study, it was hypothesized that the seed cotton yield would increase with mid-long term incorporation of crop residues replacing inorganic K fertilizer, and the data in Fig. 2 and Table 3 confirms this. On substituting for inorganic K fertilizer, crop residues enhanced the soil K supplying ability in a similar manner resulting in a higher crop K content and better photosynthetic performance, which ultimately increased crop productivity in comparison to the K control treatments (Hu et al., 2015; Rebafka et al., 1994; Yu et al., 2016a). In the present study, incorporation of crop residue (as a replacement for inorganic K fertilization) significantly (P < 0.05) increased the soil Av-K content and the seed cotton yield during the six-year compared to the CK treatment (Fig. 2). Moreover, crop residue incorporation, similar to inorganic K fertilization, mainly improved boll numbers and therefore, improved cotton yield, which is consistent with the results of previous reported studies (Sui et al., 2014; Yu et al., 2016a). Furthermore, the effects of residue incorporation on crop production depend greatly on the environmental factors which are constantly changing (Hiel et al., 2016). Therefore, the effects of residue incorporation on the seed cotton production would change at different time span. In the present study, the highest improvement in seed cotton yield over the CK treatment was found in the third year of residue incorporation (2013), but decreased gradually in the subsequent years (Fig. 2). Meanwhile, crop residue incorporation always increased the seed cotton yield during the six-year period. The highest improvement in seed cotton yield by residue incorporation over the CK treatment was under the W9000 + C7500 treatment during the first five years (2011–2015), but in the sixth year (2016) of residue incorporation, seed cotton yield was the highest under the W9000 + C0 treatment (Fig. 2).
The yield-scaled N2O emissions provide useful information for evaluating the crop production and influence of greenhouse gases (Van et al., 2010). We hypothesized that incorporation of crop residues replacing inorganic K fertilizers would significantly increase yield-scaled N2O emissions, and the data in Table 5 confirms this. Across the two cotton growing seasons (2015 and 2016), wheat straw and cotton residue applied to the soil alone or in combination significantly (P < 0.05) increased the yield-scaled N2O emissions in both years. Treatments with residue incorporation showed higher yield-scaled N2O emissions than the CK, K150, and K300 treatments. The yield-scaled N2O emissions were obtained by dividing seasonal N2O emissions by seed cotton yield (Van et al., 2010; Qin et al., 2012). Compared to the CK, K150, and K300 treatments, with crop residue incorporation, the increase was greater by 0.83–1.19 times for seed cotton yield and by 1.01–1.71 times for seasonal N2O emissions (Table 5). The increase in seed cotton yield by crop residue incorporation was not enough to counterbalance the increase in seasonal N2O emissions, which finally resulted in higher yield-scaled N2O emissions in the present study, consistent with the results of previous studies (Plaza-Bonilla et al., 2014). In addition, compared to inorganic sources of fertilizers, exogenous N provided to the soil by crop residue was reported to promote N2O fluxes and seasonal N2O emissions simultaneously, which resulted in higher yield-scaled N2O emissions (Table 2, Pittelkow et al., 2013;Plaza-Bonilla et al., 2014). Table 2 Accumulated carbon (C), nitrogen (N), phosphors (P), potassium (K) and ligin (kg ha−1) brought in by crop residues incorporation to the soil before cotton transplanting in 2015 and 2016.
4.2. Seasonal N2O emissions We further hypothesized that residue incorporation taking the place of inorganic K fertilization would significantly increase N2O fluxes and seasonal N2O emissions during the fifth (2015) and sixth (2016) cotton growing seasons. The differential impacts of crop residue of different types and quantities on N2O fluxes and seasonal N2O emissions were also hypothesized. Compared with the CK, K150, and K300 treatments, the residue incorporation treatments showed higher N2O fluxes and higher seasonal N2O emissions. Moreover, compared to the treatment with both wheat straw and cotton residue incorporation (W9000 + C7500), treatments with wheat straw or cotton residue
Year
Treatment
C Ligin (× 103 kg ha−1)
N P (kg ha−1)
K
2015
W0 + C7500 W9000 + C0 W9000 + C7500
14.9 21.0 36.0
5.6 7.2 12.8
351 416 767
73 86 159
883 785 1669
2016
W0 + C7500 W9000 + C0 W9000 + C7500
18.6 25.0 43.6
6.7 8.7 15.4
434 463 897
87 100 187
995 914 1910
W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively. 5
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Table 3 Effect of crop residue incorporation and inorganic K fertilization on seed cotton yield (kg ha−1), boll number (m-2), boll weight (g) and seed cotton yield nitrogen (kg ha−1) from 2011 to 2016. Year
Treatment
Seed cotton yield (× 103 kg ha−1)
Boll number (m−2)
Boll weight (g)
2011
CK W9000 + C0
4.90 a 5.18 a
90.4 a 95.0 a
5.4 a 5.5 a
2012
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
5.04 4.91 5.01 5.15 5.19 5.44
a a a a a a
88.4 88.4 93.1 93.7 92.4 99.3
a a a a a a
5.7 5.6 5.4 5.5 5.6 5.5
a a a a a a
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
4.87 5.89 6.73 6.76 6.38 6.64
b ab a a a a
104 126 140 143 133 138
b ab a a a a
4.7 4.7 4.8 4.7 4.8 4.8
a a a a a a
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
3.29 4.00 4.35 4.45 4.29 4.48
b ab a a a a
64.0 78.4 85.5 86.5 83.0 86.2
b ab a a a a
5.1 5.1 5.1 5.1 5.2 5.2
a a a a a a
2015
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
2.87 2.94 3.39 3.41 3.77 4.12
d d c c b a
62.4 66.4 75.4 74.3 84.3 90.8
d d c c b a
4.6 4.4 4.5 4.6 4.5 4.5
a a a a a a
2016
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
4.80 5.37 5.47 5.10 5.42 5.57
c ab ab bc ab a
87.1 c 100 ab 101 ab 94.3 bc 99.8 ab 101 a
5.5 5.4 5.4 5.4 5.4 5.5
a a a a a a
2013
2014
Table 5 Effect of crop residues incorporation and inorganic K fertilization on soil N2O flux in the cotton flowering and boll setting stage, seasonal N2O emissions and yield-scaled N2O emissions after five and six years crop residues incorporation in 2015 and 2016. Treatment
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
Soil N2O fluxes (mg N2O m−2 h-1)
Seasonal N2O emissions (kg N2O ha−1)
Yield-scaled N2O emissions (g N2O kg−1 seed cotton)
2015
2015
2015
0.08 0.12 0.14 0.16 0.12 0.07
2016 c b ab a b c
0.07 0.11 0.13 0.14 0.10 0.05
c b a a b d
1.37 1.61 1.83 2.06 1.55 1.25
2016 e c b a d f
1.06 1.29 1.54 1.73 1.29 1.01
d c b a c d
0.48 0.55 0.54 0.60 0.41 0.31
2016 bc ab ab a c d
0.22 0.24 0.28 0.34 0.24 0.18
bc bc ab a bc c
Different letters within the same column in a same year indicate significant difference according to the shortest significant ranges (SSR) test (P < 0.05). W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively.
4.4. Soil fertility After the fifth and sixth years of continuous crop residue incorporation, in 2015 and 2016 respectively, most of the soil fertility parameters were significantly (P < 0.05) improved by crop residue incorporation compared to the CK treatment (Table 4), which is consistent with the results of previous studies (Banger et al., 2009; Hao et al., 2001; Tan et al., 2017). However, the soil MBN contents rather than the soil MBC contents were significantly increased by crop residue incorporation compared with the CK, K150, and K300 treatments in 2016. This finding is contradictory with the observations of Powlson et al. (1987) and Kaur et al. (2010), who reported that the soil MBC and MBN contents were both higher in soils with crop residue incorporation. Moreover, the ratios of MBC/TC and MBC/MBN indicate soil organic matter quality and provide an extra information on soil C and N dynamics (Anderson and Domsch, 1989). In the present study, the lowest ratios of MBC/MBN were found in the control and W9000 + C0 soils in 2015 and 2016, respectively. Similarly, Ebhin et al. (2006) reported that the MBC/MBN ratio was 3.87 times higher with farmyard manure + inorganic fertilizers. The MBC/TC ratio was lowest in the W9000 + C7500 soil in both years, which is in contrast to results from a study conducted by Kaur et al. (2010). The reduced MBC/MBN and MBC/TC ratios might be due to the change in microbial species and
Different letters within the same column in a same year indicate significant difference according to the shortest significant ranges (SSR) test (P < 0.05). W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively; data from 2011 to 2013 were cited from our previous research of Sui et al. (2014) and Yu et al. (2016b).
Table 4 Effect of crop residue incorporation and inorganic K fertilization on soil total carbon (TC), total nitrogen (TN), dissolved organic carbon (DOC), microbial biomass carbon (MBC), available mineral nitrogen (AMN), microbial biomass nitrogen (MBN), Olsen P (Olsen-P) and available potassium (Av-K) contents of the top 20 cm soil layer of the experimental field in the cotton flowering and boll setting stage at 19th August in 2015 and 14th August in 2016. Year
Treatment
TC (g kg−1)
TN
DOC (mg kg−1)
MBC
AMN
2015
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
13.1 13.1 14.0 17.0 14.5 14.9
b b b a b b
1.22 1.26 1.32 1.33 1.29 1.26
2016
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
13.8 14.9 15.7 17.4 14.1 14.5
c bc b a c c
MBN
d cd ab a bc cd
74.2 c 79.0 bc 87.2 b 101 a 80.5 bc 80.6 bc
500 534 554 579 562 573
b ab a a a a
56.5 55.6 61.2 63.2 60.2 60.6
b b ab a ab ab
82.0 90.8 88.6 95.0 79.7 75.0
1.35 c 1.37 bc 1 .45 a 1.47 a 1.36 bc 1.39 b
75.5 c 82.6 bc 89.6 b 106 a 87.1 b 86.5 b
570 582 603 647 620 630
c c bc a ab ab
58.3 58.0 64.0 67.1 60.8 64.6
c c abc a bc ab
93.5 b 96.6 a 106 a 110 a 95.1 bc 94.1 b
a a a a a a
Olsen-P
Av-K
23.7 24.2 24.4 25.2 23.9 24.2
a a a a a a
180 199 190 215 216 237
c bc bc abc ab a
26.1 24.0 24.1 27.6 26.5 25.2
ab b b a ab ab
164 199 202 215 218 260
c b b b b a
Different letters within the same column in a same year indicate significant difference according to the shortest significant ranges (SSR) test (P < 0.05). W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively. 6
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Table 6 ANOVA showing the effects of year (Y), treatment (T), and their interactions (Y × T) on soil total carbon (TC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), total nitrogen (TN), water-soluble inorganic nitrogen (WIN), microbial biomass nitrogen (MBN), Olsen P (Olsen-P) and available potassium (Av-K) contents of the top 20 cm soil layer from the experimental fields, seasonal N2O emissions (Es), seed cotton yield (Yil) and yield-scaled N2O emissions (Yil-Es). Source of variance Y T Y×T
TC
TN ns
1.45 3.88 * 0.54 ns
276 ** 26.3 ** 1.70 ns
DOC
MBC ns
2.55 9.14 ** 0.11 ns
F values and significance levels (**P < 0.01, *P < 0.05 and
13.7 ** 1.80 ns 0.13 ns ns
WIN 1.48 2.64 0.10
MBN ns ns ns
21.1 ** 31.2 * 0.40 ns
Olsen-P 3.84 0.99 0.64
ns ns ns
Av-K ns
0.51 17.1 ** 1.11 ns
Es
Yil
Yil-Es
4.68 * 39.0 ** 31.1 **
230 ** 5.44 ** 1.36 ns
179 ** 12.7 ** 2.60 ns
P ≥ 0.05).
5. Conclusions
Table 7 Pearson correlation coefficients among N2O fluxes (mg m−2 h-1) and soil total carbon (TC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), total nitrogen (TN), water-soluble inorganic nitrogen (WIN), microbial biomass nitrogen (MBN), Olsen P (Olsen-P), available potassium (Av-K) contents, ratios of MBC/MBN and MBC/TC of the top 20 cm soil layer from the experimental fields. Soil C, N correlate parameter
N2O fluxes (mg N2O m−2 h-1)
TC DOC MBC TN WIN MBN MBC/MBN MBC/TC Olsen-P Av-K
0.479 ns 0.626 * −0.034 ns 0.230 ns −0.812 ** 0.394 ns −0.157 ns 0.394 ns 0.073 ns −0.123 ns
Crop residue as a K source can promote seed cotton yield as similar to inorganic K fertilizer, but the effects varied over the six years. A consecutive crop residue incorporation simultaneously increased the soil N2O flux and seasonal N2O emissions compared with the inorganic K fertilizer applicant. However, the increase in seed cotton yield by residue incorporation was not enough to counterbalance the increase in seasonal N2O emission, which finally resulted in a higher yield-scaled N2O emissions. Furthermore, the findings of this study suggest that the crop residue as a K source is beneficial for increasing cotton productivity but could lead to a rise in soil N2O fluxes, seasonal N2O emissions, and yield-scaled N2O emissions. Acknowledgement This work was funded by the Special Fund for Agro-scientific Research in the Public Interest (201503136), National Key Research and Development Program of China (2017YFD 0201900), China Agriculture Research System (CARS-18-14) and Jiangsu Collaborative Innovation Center for Modern Crop Production (JCIC-MCP).
Pearson correlation coefficient and significance levels (**P < 0.01, *P < 0.05 and nsP ≥ 0.05).
populations (Joergensen et al., 2010). On the one hand, treatments without organic sources fertilizer adversely affected the N storage of total biomass (Joergensen et al., 2010). The other possible reason of down-regulated of the MBC/TC ratio might be due to the inputs of lowquality litter (higher C: N ratio) immobilizing N (Deng et al. 2016).
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4.5. Correlation analysis The soil N2O fluxes showed positive and significant correlations with soil C pools and these results are in accordance with the findings of Bhattacharyya et al. (2012). In the present study, however, a significant positive (P < 0.05) relationship was found only between the soil N2O fluxes and soil DOC contents. Meanwhile, a significant but negative (P < 0.05) relationship was observed between the soil N2O fluxes and soil AMN contents (Table 7). This is contrary to findings of Verchot et al. (2006), who reported that there were no notable correlations between the soil N availability and the soil N2O fluxes. During the cotton flowering and boll setting stages in 2015 and 2016, the soil TC, TN, Olsen-P, and Av-K content was not significantly (P < 0.05) correlated with the N2O fluxes. Crop residue incorporation and inorganic fertilizer application, however, significantly impacted the soil MBC and MBN contents. Previous studies have also shown that soil N2O fluxes are significantly and positively (P < 0.05) associated with the soil MBC and MBN content (Lou et al., 2007). However, the present study found no significant correlations between the N2O fluxes and the soil MBC and MBN contents. Higher MBC/MBN and MBC/TC ratios in the soil receiving crop residues showed a higher activity of soil C and N metabolism (Carter, 1991; Ebhin et al., 2006). The weak correlation between soil N2O fluxes and MBC/MBN and MBC/TC ratios indicate the nitrification and denitrification processes that produce N2O in the soil may not be directly affected even though the soil C metabolism is more active in residue incorporated soils.
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Crop. Res. 179, 120–131. Huang, Y., Zou, J., Zheng, X., Wang, Y., Xu, X., 2004. Nitrous oxide emissions as influenced by amendment of plant residues with different C:N ratios. Soil Biol. Biochem. 36, 973–981. Jiang, N., Hu, B., Wang, X., Meng, Y., Chen, B., Zhou, Z., 2019. Effects of crop residue incorporation and inorganic potassium fertilization on soil potassium supply power. Arch. Agron. Soil Sci. 65 (9), 1223–1236. Jiang, P., Xu, Q., Xu, Z., Cao, Z., 2006. Seasonal changes in soil labile organic carbon pools within a Phyllostachys praecox stand under high rate fertilization and winter mulch in subtropical China. Forest Ecol. Manag. 236, 30–36. Joergensen, R.G., Mäder, P., Fließbach, A., 2010. Long-term effects of organic farming on fungal and bacterial residues in relation to microbial energy metabolism. Biol. Fert. Soils. 46, 303–307. Kaur, K., Kapoor, K.K., Gupta, A.P., 2010. Impact of organic manures with and without mineral fertilizers on soil chemical and biological properties under tropical conditions. J. Plant Nutr. Soil Sc. 168, 117–122. Koponen, H.T., Martikainen, P.J., 2004. Soil water content and freezing temperature affect freeze-thaw related N2O production in organic soil. Nutr. Cycl. Agroecosys. 69, 213–219. Kumar, K., Goh, K.M., 2000. Crop residues and management practices: effects on soil quality, soil nitrogen dynamics, crop yield, and nitrogen recovery. Adv. Agron. 68, 197–319. Liu, H., Jiang, G., Zhuang, H., Wang, K., 2008. Distribution, utilization structure and potential of biomass resources in rural China: with special references of crop residues. Renew Sust Energ Rev. 12 (5), 1402–1418. Lou, Y., Ren, L., Li, Z., Zhang, T., Inubushi, K., 2007. Effect of rice residues on Carbon Dioxide and nitrous oxide emissions from a paddy soil of subtropical china. Water Air Soil Pollut. Focus. 178, 157–168. Lu, R.K., 1999. Method for Agro-Chemical Analyses of Soil. Agricultural, Science and Technology Press of China, Beijing (in Chinese). Pittelkow, C.M., Adviento-Borbe, M.A., Hill, J.E., Six, J., Kessel, C.V., Linquist, B.A., 2013. Yield-scaled global warming potential of annual nitrous oxide and methane emissions from continuously flooded rice in response to nitrogen input. Agric., Ecosyst. Environ., Appl. Soil Ecol. 177, 10–20. Plaza-Bonilla, D., Álvaro-Fuentes, J., Arrúe, J.L., Cantero-Martínez, C., 2014. Tillage and nitrogen fertilization effects on nitrous oxide yield-scaled emissions in a rainfed Mediterranean area. Agric., Ecosyst. Environ., Appl. Soil Ecol. 189, 43–52. Powlson, D.S., Prookes, P.C., Christensen, B.T., 1987. Measurement of soil microbial biomass provides an early indication of changes in total soil organic matter due to straw incorporation. Soil Biol. Biochem. 19, 159–164. Putte, A.V.D., Govers, G., Diels, J., Gillijns, K., Demuzere, M., 2010. Assessing the effect of soil tillage on crop growth: a meta-regression analysis on European crop yields under conservation agriculture. Eur. J. Agron. 33, 231–241. Qin, S., Wang, Y., Hu, C., Oenema, O., Li, X., Zhang, Y., Dong, W., 2012. Yield-scaled N2O emissions in a winter wheat-summer corn double-cropping system. Atmos. Environ. 55, 240–244. Rasmussen, P.E., Allmaras, R.R., Rohde, C.R., Roager Jr, N.C., 1980. Crop residue influences on soil carbon and nitrogen in a wheat-fallow system. Soil Sci. Soc. Am. J. 44, 596–600. Rebafka, F.P., Hebel, A., Bationo, A., Stahr, K., Marschner, H., 1994. Short- and long-term effects of crop residues and of phosphorus fertilization on pearl millet yield on an acid sandy soil in Niger, West Africa. Field Crop. Res. 36, 113–124. Rezaei, R.M., Wang, W.J., Chen, C.R., Reeves, S.H., Scheer, C., 2017. Assessment of N2O emissions from a fertilized vegetable cropping soil under different plant residue
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Trade-offs between residue incorporation and K fertilizer on seed cotton yield and yield-scaled nitrous oxide emissions Nan Jianga, Li Zhaa, Wei Hua, Changqin Yangb, Yali Menga, Binglin Chena, Zhiguo Zhoua, a b
T
⁎
Key Laboratory of Crop Physiology & Ecology, Ministry of Agriculture, Nanjing Agricultural University, Nanjing, China Jiangsu Academy of Agricultural Sciences, Nanjing, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Crop residue incorporation Seed cotton yield Nitrous oxide Yield-Scaled nitrous oxide
Crop residues can effectively replace inorganic potassium (K) fertilizers to increase crop yield. However, the effect of incorporating consecutive crop residues instead of applying inorganic K fertilizers on seed cotton yield and yield-scaled N2O emissions (N2O emissions per unit yield of seed cotton) over a long period has rarely been reported. Therefore, a six-year (2011–2016) study incorporating crop residue instead of applying inorganic K fertilizer was conducted in a cotton-wheat system (transplanting cotton after wheat harvesting within the same crop season). Results showed that no statistical difference was observed between crop residue treatments and K fertilizers application on seed cotton yield during 2011–2014. However, in 2015, the yields of the inorganic K treatments were superior to the yields of the residue retained treatments. In 2016, the residue retained treatments and inorganic K treatments had similar seed cotton yields except that the K300 (300 kg K2O ha−1) treatment had a significantly higher yield than the 9000 kg ha−1 wheat straw + 7500 kg ha−1 cotton residue treatment (C7500 + W9000). The seasonal N2O emissions and yield-scaled N2O emissions increased by 1.01–1.65 times and 1.02–1.94 times in residue incorporation compared with inorganic K application treatments during 2015 and 2016 (the fifth and sixth year of residue incorporation), respectively. Furthermore, the soil N2O fluxes were significantly correlated with soil dissolved carbon and available mineral nitrogen contents. In conclusion, crop residue as a K source is a suitable alternate to enhance cotton yield and to replace inorganic K fertilizers but this could also boost the soil N2O fluxes, seasonal N2O emissions, and yield-scaled N2O emissions.
1. Introduction Considering the scarce potash reserves in China, recently the incorporation of crop residue to replace inorganic potassium (K) fertilizers is gaining a great attention (Sui et al., 2015). According to our previous 3-year study (started from 2011), both cotton residue and wheat straw partially or totally replaced the inorganic K fertilizer as well as improved the lint cotton yield at the Dafeng research station (Sui et al., 2015; Yu, 2016b). However, residue quality, residue returning duration, and edaphic factors are variable, making the effects of crop residue incorporation on crop yield complex and variable (Putte et al., 2010; Kumar and Goh, 2000). For example, wheat straw returning and cotton residue incorporation have been reported as an effective strategy to compliment K compensating the soil K depletion and improving cotton yield (Jiang et al., 2019; Sui et al., 2017; Tan et al., 2017). Nevertheless, a contrary conclusion in which crop residue incorporation reduced the crop yield by 4.5% on average under European conditions was reported by Putte et al. (2010). Despite of positive and
⁎
negative aspects of crop residues incorporation, some previous studies also reported no effect on crop production (Soon and Lupwayi, 2012). Therefore, the effects of consecutive crop residue incorporation against inorganic K fertilization on seed cotton yield may be uncertain over a longer period, and continuous concern for the seed cotton yield under crop residue incorporation may provide valuable information on crop residue management. Indeed, most crop of the residues are used with low efficiency or wasted in China (Liu et al., 2008). For instance, in Shandong province where cotton-wheat system occupies a dominate position in cotton production, 73% of the cotton residue are directly burnt, 20% for industry materials, 5% for residue incorporation and the rest 2% are discarded (Dai and Dong, 2014; Wang et al., 2014). Likewise, wheat straw is primarily adopted as green manure, forage, industry material and fuel, after the part returning to fields or collection lost (Liu et al., 2008). Besides of the complex effects of crop residue incorporation on yield, numerous studies also highlighted a strong effect of residue incorporation on nitrous oxide (N2O, a potent greenhouse gas) emissions
Corresponding author. E-mail address: [email protected] (Z. Zhou).
https://doi.org/10.1016/j.fcr.2019.107630 Received 4 March 2019; Received in revised form 8 September 2019; Accepted 18 September 2019 0378-4290/ © 2019 Elsevier B.V. All rights reserved.
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inter-row and intra-row spacing, respectively keeping a planting density of 3.3 × 104 plants ha−1. Following cotton harvesting at the end of October or in early November, wheat was sown in rotary tilled soil using a seed rate of 150 kg ha−1 with an inter-row spacing of 0.15 m. After the harvest of the preceding crops, wheat straw or cotton residues were removed from the plots and then smashed for nutrient and water content measurement before being incorporated into the top 00.1 m of the soil. Based on the average yield of wheat straw and cotton residue of 9.0 × 103 kg ha−1 and 7.5 × 103 kg ha−1 in the Yangtze River Valley, respectively, wheat straw was always applied before cotton transplantation at rates of 0 and 9.0 × 103 kg ha−1 (W0 and W9000), and cotton residue was always applied before wheat sowing at rates of 0 and 7.5 × 103 kg ha−1 (C0 and C7500). Therefore, within the same growth season (cotton growing season + wheat growing season), 4 treatments with crop residue incorporation replacing inorganic K fertilization were conducted: W0 + C0 (CK), W0 + C7500, W9000 + C0, and W9000 + C7500. In addition, before cotton transplantation, two independent inorganic K fertilizer treatments without crop residue incorporation were applied as the basal fertilizer at rates of 150 and 300 kg K2O ha−1 as potassium sulfate (50% K2O and 18% sulfur, K150 and K300). During the wheat growing season, no inorganic K fertilizer was used. Therefore, a total of six treatments with or without crop residue incorporation were investigated (CK, W0 + C7500, W9000 + C0, W9000 + C7500, K150, and K300). In addition, inorganic N and P fertilizers were used at the same rates of 300 kg N ha−1 and 150 kg P2O5 ha−1 in both the cotton and wheat growing seasons. In the cotton growing seasons of 2015 and 2016, the N and P fertilization rates and timing are shown in Table 1. In the wheat growing season, N fertilizer was applied as urea at rates of 150 and 150 kg N ha−1 before wheat sowing and at the wheat jointing stage, respectively, and P fertilizer was applied as ordinary superphosphate before wheat sowing. The experiment was arranged in plots of 55 m2 (5 m × 11 m) in a randomized design with 3 replicates. Notably, the two inorganic K fertilization treatments were conducted before the cotton transplantation in 2012 (one year later than the stationary field experiment conducted in 2011). Therefore, the seed cotton yield and yield components under W0 + C7500 (cotton residue was always applied before wheat sowing within the same growth season) and the K150 and K300 treatments in 2011 are not listed in Fig. 2.
from agricultural soils (Hu et al., 2016). The crop residues being big source of organic matter, contain a considerable quantity of carbon (C) and nitrogen (N) (Harper and Lynch, 2010; Smil, 1999). This exogenously applied organic matter source may accelerate or retard soil C and N cycling, resulting in higher or lower N2O emissions (Huang et al., 2004; Rasmussen et al., 1980; Witt et al., 2000; Yao et al., 2009). Thus, the application of crop residue may affect the soil N2O emissions by affecting soil C and N status. Similarly, the indicators of soil C and N status, such as soil dissolved organic C (DOC), available mineral N (AMN, sum of the soil NH4+ and NO3−), microbial biomass C (MBC), and microbial biomass N (MBN) contents, as well as the ratios of MBN to MBC (MBC/MBN), and MBC to total organic C (MBC/TC) should be correlated with soil N2O emissions (Anderson and Domsch, 1989; Grace et al., 1993; Song et al., 2007). Although numerous previous literatures have been reported on the effect of residue incorporation either on crop production or environmental aspects but a limited information is available on the environmental costs associated with crop production (yield-scaled N2O emissions) for residue management practices (Van et al., 2010; Qin et al., 2012). Studies addressing the impact of crop residue incorporation replacing inorganic K fertilization in a cottonwheat system on seed cotton yield-scaled N2O emissions and its relationship to the soil C and N status are even limited. Keep in view the previous literature, the present study was hypothesized that crop residue incorporation would increase cotton yield and yield-scaled N2O emissions compared with inorganic K fertilization. Further N2O fluxes would be positively correlated with the soil C and N supply attributes. So a six-year field experiment on crop residue and inorganic K fertilizer was conducted to verify the former set hypothesis under the climatic conditions of Dafeng, China. 2. Materials and methods 2.1. Site description A stationary field experiment was initiated in early June 2011. The location of the experimental station at Dafeng Basic Seed Farm (Dafeng, 33°24′ N, 120°34′ E) is in the Yangtze River delta in China. The location of the Dafeng site has a subtropical monsoon climate, with a hot and rainy summer and a cold and humid winter. The daily mean temperature and precipitation during the cotton growing seasons from 2014 to 2016 are shown in Fig. 1. The soil of the experimental site was sandy loam (6.8% clay and 57.0% sand) with the following topsoil (0–20 cm) properties before cotton transplantation in 2011: 1.18 g kg−1 total nitrogen (TN, sum of the organic and inorganic fraction of soil nitrogen), 26.4 mg kg−1 AMN, 12.1 g kg−1 soil organic carbon, 22.2 mg kg−1 Olsen phosphorus, 60.6 mg kg−1 water-soluble K, 255.8 mg kg−1 available K (NH4Ac extract), 1.1 g kg−1 nonexchangeable K, 18.4 g kg−1 structural K, 1.44 g cm-3 bulk density and pH 7.9 (soil:water = 1:2.5).
2.3. Seed cotton yield and yield components At harvest, 20 plants in the central row of each plot were harvested to measure seed cotton yield and yield components (number of bolls and seed cotton weight per plant). The seed cotton yield per hectare was calculated as the seed cotton weight per plant × number of bolls per plant × plant density. The percentage increase in seed cotton yield over the CK treatment was calculated by the following formula: (1) (YK - Y0) / Y0 × 100%
2.2. Experimental design and management
where YK is the seed cotton yield under different sources of K fertilization and Y0 is the seed cotton yield under the K control treatment.
The experiment was conducted with widely planted cultivars Gossypium hirsutum L. Siza 3 and Triticum aestivum L. Yangmai 16. Cotton seeds were sown on April 15, 2011 in a nursery bed and then transplanted to fields (after wheat harvesting) with 1.0 m and 0.3 m
Table 1 Chemical fertilizer rates and timing during the cotton growing season in 2015 and 2016. Chemical fertilization rates (N:P2O5:K2O, kg ha−1)
120:150:0 120:0:0 60:0:0
Fig. 1. Daily mean air temperature and precipitation in the cotton growing season from 2014 to 2016. 2
Application time (dd-mm) 2015
2016
15-Jun. 01-Aug. 04-Sep.
17-Jun. 01-Aug. 01-Sep.
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Fig. 2. Effect of crop residue incorporation and inorganic K fertilization on seed cotton yield increment (%) over the control treatment (CK).from 2011 to 2016. Different letters under a same treatment in different years indicate significant difference according to the shortest significant ranges (SSR) test (P < 0.05). W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively; the seed cotton yield increments (%) over the control treatment (CK) from 2011 to 2013 were cited from our previous research (Sui et al. 2014; Yu et al. 2016b).
and SPSS (ver. 23.0, IBM software). The data were normally distributed and one-way analysis of variance (ANOVA) was used to test the significant difference among treatments for soil TC, TN, AMN, DOC, MBC, MBN, Olsen-P, and Av-K content as well as the ratios of MBC/MBN and MBC/TC, N2O fluxes, seasonal N2O emissions, and yield-scaled N2O emissions. Pearson correlation analysis was used to verify whether there are association between the N2O fluxes and all the above tested parameters of the soil. Post hoc mean separation analysis was performed using the multiple-range test of Duncan’s SSR at P < 0.01 and P < 0.05 levels of significance.
2.4. N2O sampling and analysis N2O samples were collected using static chambers and stored in cold storage before the gas chromatography analysis (Zou et al., 2005). A chamber with a fixed PVC frame and size of 0.8 m × 0.8 m × 0.8 m or 0.8 m × 0.8 m × 1.6 m, depending on the cotton plant height, was installed in each replication of all six treatments. At the top edge of the frames, 5 cm deep grooves filled with water were installed to seal the chamber. The chambers in our experiment were equipped with fans to ensure that the gas in the chambers was evenly mixed. Additionally, to minimize the changes in air temperature the chambers were covered with insulation and aluminum foil layers. During the two cotton growing seasons in 2015 and 2016, N2O samples were collected from 9:00-11:00 am every 10 days. However, in case of precipitation, fertilization, and midseason drainage events, N2O samples were collected more frequently. After 5, 10, 15, and 20 min of the chambers being covered, N2O samples were collected from the chamber with a 60 ml syringe and then analyzed in a gas chromatograph (Agilent 7890A, Shanghai, China) equipped with an electron capture detector (ECD). The temperature for the column was set at 40℃, and the ECD detector was maintained at 300℃. N2O fluxes and standard errors on seasonal N2O emissions during the cotton growing season in 2015 and 2016 were the average of the triplicate plots. Unless a linear regression value of R2 > 0.90 is obtained, the sample sets would be rejected. The seasonal N2O emissions were analyzed by the methods of Zou et al. (2005). The yield-scaled N2O emissions in 2015 and 2016 were obtained by dividing seasonal N2O emissions by seed cotton yield (Venterea et al., 2011). The soil temperature at approximately 0.1 m depth was recorded on every N2O sampling day using soil thermometers throughout the entire cotton growing season in 2015 and 2016.
3. Results 3.1. Seed cotton yield and yield components The seed cotton yield and yield components from each treatment from 2011 to 2013 were previously reported by Sui et al. (2014) and Yu et al. (2016b). All the yield and yield components data are given in Table 3. The residue incorporation and inorganic K application treatments yielded statistically similar seed cotton from 2011 to 2014. While in 2015, the yields of the inorganic K treatments were superior to that of residue retained treatments (Table 3). However, in 2016, only K300 produced relatively higher seed cotton yield compared either to residue retained treatments or to K150 treatment. Compared with the CK treatment, different sources of K fertilization significantly (P < 0.05) increased boll numbers and seed cotton yields from 2011 to 2016 (Table 3 and Table 6). Over the six years, the highest improvement in seed cotton yield by crop residue incorporation (20.7%–38.4%) over the control treatment (CK) was recorded in 2013 (the third year of residue incorporation) and then decreased gradually in the subsequent three years (Fig. 2). Meanwhile, among the residue treatments, the W9000 + C7500 treatment highest improved the seed cotton yield over CK by 1.9%–38.4% during the first five years (2011–2015). However, in 2016, more increase in seed cotton yield (13.9%) was observed in W9000 + C0 compared with rest of the residue application treatments.
2.5. Soil sampling Five randomized soil cores (3 cm diameter) from the uppermost soil layer (0-0.2 m) of each plot were sampled during the cotton flowering and boll setting stage on August 19 in 2015 and August 14 in 2016. Then, the soil samples were sieved through a 2-mm mesh and divided into two parts for 4 ℃ refrigerator storage and air-drying. The soil AMN content is the sum of the soil ammonium- N (NH4+-N) and nitrate- N (NO3−-N) content and was determined by the method of Lu (1999). The soil TC and DOC content was measured by the method of Lu (1999) and Jiang et al. (2006), respectively. Soil TN, Olsen-P, and Av-K content was measured by the method of Lu (1999). The MBC and MBN were determined using the fumigation-extraction method of Vance et al. (1987) and Brookes et al. (1985), respectively.
3.2. N2O fluxes and seasonal N2O emissions N2O fluxes were significantly (P < 0.05) affected by crop residue incorporation across the two cotton growing seasons in 2015 and 2016 (the fifth and sixth years of residue incorporation, Table 6). The data in Fig. 3 and Table 5 show the variation of N2O fluxes of different treatments during the two cotton growing seasons. According to the presented data, the W9000 + C7500 showed the highest N2O fluxes with values of 0.16 mg N2O m−2 h-1 and 0.14 mg N2O m−2 h-1 in month of August in 2015 and 2016, respectively. Seasonal N2O emissions ranged from a high of 2.06 kg N2O ha-1 in the W9000 + C7500 treatment to a low of 1.25 kg N2O ha-1 in the K300 treatment in 2015. But in 2016,
2.6. Statistical analysis All statistical analyses were performed with Microsoft Excel-2016 3
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Fig. 3. Seasonal variations of N2O fluxes in cotton growing seasons in 2015 and 2016; the vertical bars represent the standard deviations of the means. W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively.
Fig. 4. Ratios of soil microbial biomass carbon (MBC) to soil microbial biomass nitrogen (MBN) and MBC to total carbon (TC) (%), the vertical bars represent the standard deviations of the means. Different letters within the same plot in a same year indicate significant difference according to the shortest significant ranges (SSR) test (P < 0.05). W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively.
seasonal N2O emissions ranged from a high of 1.73 kg N2O ha-1 in the W9000 + C7500 treatment to a low of 1.01 kg N2O ha-1 in the K300 treatment. Compared with the CK, K150, and K300 treatments, treatments with crop residue incorporation had higher N2O fluxes and higher seasonal N2O emissions in both years. Moreover, compared with the W0 + C7500 treatment, the W9000 + C0 treatment released 13.7% and 19.4% more N2O during the cotton growing seasons in 2015 and 2016, respectively. (Fig. 4)
those under the inorganic K treatments in 2015 and 2016, respectively. 3.4. Soil fertility The soil TC, TN, DOC, MBC, AMN, MBN, Olsen-P and Av-K contents in the cotton flowering and boll setting stage in 2015 and 2016 are presented in Table 4. Compared with the CK, K150, and K300 treatments, the soil TC, TN and DOC contents were significantly (P < 0.05) improved by crop residue application (Table 6). The soil TC contents ranged from a high of 17.0 g kg−1 in W9000 + C7500 treatment to a low of 13.1 g kg−1 in the CK treatment in 2015 and ranged from a high of 17.4 g kg−1 in W9000 + C7500 treatment to a low of 13.8 g kg−1 in the CK treatment in 2016. The soil DOC contents ranged from a high of 101 mg kg−1 in the W9000 + C7500 treatment to a low of 74.2 mg kg−1 in the CK treatment in 2015 and ranged from a high of 106 mg kg−1 in the W9000 + C7500 treatment to a low of 75.5 mg kg−1 in CK treatment in 2016. The soil TN contents ranged from a high of 1.33 g kg−1 in the W9000 + C7500 treatment to a low of 1.22 g kg−1 in the CK treatment in 2015 and ranged from a high of 1.47 g kg−1 in the W9000 + C7500 treatment to a low of 1.35 g kg−1 in the CK treatment in 2016. Across the two cotton growing seasons, the wheat straw and cotton residue alone or in combination returning to the soil significantly (P < 0.05) decreased the MBC/TC ratio (Fig. 3). The ratios of MBC/TC
3.3. Yield-scaled N2O emissions Different K fertilization sources significantly affected the seasonal yield-scaled N2O emissions across two cotton growing seasons (Table 6). The yield-scaled N2O emissions from each treatment in the cotton growing seasons in 2015 and 2016 are presented in Table 5. K fertilization from residues significantly (P < 0.05) increased the yieldscaled N2O compared with the CK, K150, and K300 treatments during both seasons. Yield-scaled N2O emissions ranged from a high of 0.60 g N2O kg−1 seed cotton in the W9000 + C7500 treatment to a low of 0.31 g N2O kg−1 seed cotton in the K300 treatment in 2015 and ranged from a high of 0.34 g N2O kg−1 seed cotton in the W9000 + C7500 treatment to a low of 0.18 g N2O kg−1 seed cotton in the W0 + C7500 treatment in 2016. The highest yield-scaled N2O emissions under the residue treatments were 31.7%–93.5% and 1.7%–88.9% higher than 4
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alone (W9000 + C0 and W0 + C7500) were significantly lower in N2O fluxes and seasonal N2O emissions in the two cotton growing seasons of 2015 and 2016 (Fig. 3). This is because various crop residues consist of different C and N content with varying C/N ratios, which can affect soil N2O emissions in different ways (Shan and Yan, 2013). Higher C/N ratios or more exogenous C provision into the soil by residue application can enhance soil N immobilization and reduce soil available N for the microbial denitrification process and finally result in a higher soil N2O emissions (Baggs et al., 2010; Rezaei et al., 2017). Furthermore, residue returning duration, quantity of the residue returning and their interactions can affect seasonal N2O emissions (Table 6). The general trend of N2O fluxing was the same among the six treatments in both years, but significantly (P < 0.05) higher N2O emissions were observed during the cotton growing season in 2015 compared to 2016. This can be attributed to the higher mean air temperature in the cotton growing seasons of 2015 (Fig. 1). Because the higher temperatures can stimulate soil N2O emissions (Hu et al., 2016; Koponen and Martikainen, 2004; Zou et al., 2009). Zou et al. (2004) suggested an exponential relationship (y = 0.042e0.137x, R2 = 0.834, P < 0.001) between the normalized N2O emission and air temperature according to the datasets built from the non-waterlogged period of the rice-growing season.
were decreased under the W9000 + C7500 treatment compared with the inorganic K fertilizer treatments, lower than the W9000 + C0 treatment and the W0 + C7500 treatment in the 2015 and 2016 cotton growing seasons. The ratios of MBC/TC were 14.4%–15.9% lower under the W9000 + C7500 treatment compared to the inorganic K fertilizer treatments. We also found that wheat straw and cotton residue alone or in combination returning to the soil had significant effects on the ratios of MBC/MBN across the two cotton growing seasons (Fig. 3). The MBC/MBN ratios were decreased by 7.2%–16.4% under the W9000 + C7000 treatment compared to the inorganic K fertilizer treatments in the cotton growing seasons in 2015 and 2016. 3.5. Correlation analysis Soil TC, TN, MBC, MBN, Olsen-P, and Av-K contents did not show a significant correlation with soil N2O fluxes at flowering and boll setting stage of cotton in 2015 as well as in 2016. However, soil DOC contents showed a significant (P < 0.01) and positive correlation with N2O fluxes. Conversely, the soil AMN contents were significantly (P < 0.05) and negatively correlated with the soil N2O fluxes (Table 7). 4. Discussion 4.1. Seed cotton yield and yield components
4.3. Yield-scaled N2O emissions
In the current study, it was hypothesized that the seed cotton yield would increase with mid-long term incorporation of crop residues replacing inorganic K fertilizer, and the data in Fig. 2 and Table 3 confirms this. On substituting for inorganic K fertilizer, crop residues enhanced the soil K supplying ability in a similar manner resulting in a higher crop K content and better photosynthetic performance, which ultimately increased crop productivity in comparison to the K control treatments (Hu et al., 2015; Rebafka et al., 1994; Yu et al., 2016a). In the present study, incorporation of crop residue (as a replacement for inorganic K fertilization) significantly (P < 0.05) increased the soil Av-K content and the seed cotton yield during the six-year compared to the CK treatment (Fig. 2). Moreover, crop residue incorporation, similar to inorganic K fertilization, mainly improved boll numbers and therefore, improved cotton yield, which is consistent with the results of previous reported studies (Sui et al., 2014; Yu et al., 2016a). Furthermore, the effects of residue incorporation on crop production depend greatly on the environmental factors which are constantly changing (Hiel et al., 2016). Therefore, the effects of residue incorporation on the seed cotton production would change at different time span. In the present study, the highest improvement in seed cotton yield over the CK treatment was found in the third year of residue incorporation (2013), but decreased gradually in the subsequent years (Fig. 2). Meanwhile, crop residue incorporation always increased the seed cotton yield during the six-year period. The highest improvement in seed cotton yield by residue incorporation over the CK treatment was under the W9000 + C7500 treatment during the first five years (2011–2015), but in the sixth year (2016) of residue incorporation, seed cotton yield was the highest under the W9000 + C0 treatment (Fig. 2).
The yield-scaled N2O emissions provide useful information for evaluating the crop production and influence of greenhouse gases (Van et al., 2010). We hypothesized that incorporation of crop residues replacing inorganic K fertilizers would significantly increase yield-scaled N2O emissions, and the data in Table 5 confirms this. Across the two cotton growing seasons (2015 and 2016), wheat straw and cotton residue applied to the soil alone or in combination significantly (P < 0.05) increased the yield-scaled N2O emissions in both years. Treatments with residue incorporation showed higher yield-scaled N2O emissions than the CK, K150, and K300 treatments. The yield-scaled N2O emissions were obtained by dividing seasonal N2O emissions by seed cotton yield (Van et al., 2010; Qin et al., 2012). Compared to the CK, K150, and K300 treatments, with crop residue incorporation, the increase was greater by 0.83–1.19 times for seed cotton yield and by 1.01–1.71 times for seasonal N2O emissions (Table 5). The increase in seed cotton yield by crop residue incorporation was not enough to counterbalance the increase in seasonal N2O emissions, which finally resulted in higher yield-scaled N2O emissions in the present study, consistent with the results of previous studies (Plaza-Bonilla et al., 2014). In addition, compared to inorganic sources of fertilizers, exogenous N provided to the soil by crop residue was reported to promote N2O fluxes and seasonal N2O emissions simultaneously, which resulted in higher yield-scaled N2O emissions (Table 2, Pittelkow et al., 2013;Plaza-Bonilla et al., 2014). Table 2 Accumulated carbon (C), nitrogen (N), phosphors (P), potassium (K) and ligin (kg ha−1) brought in by crop residues incorporation to the soil before cotton transplanting in 2015 and 2016.
4.2. Seasonal N2O emissions We further hypothesized that residue incorporation taking the place of inorganic K fertilization would significantly increase N2O fluxes and seasonal N2O emissions during the fifth (2015) and sixth (2016) cotton growing seasons. The differential impacts of crop residue of different types and quantities on N2O fluxes and seasonal N2O emissions were also hypothesized. Compared with the CK, K150, and K300 treatments, the residue incorporation treatments showed higher N2O fluxes and higher seasonal N2O emissions. Moreover, compared to the treatment with both wheat straw and cotton residue incorporation (W9000 + C7500), treatments with wheat straw or cotton residue
Year
Treatment
C Ligin (× 103 kg ha−1)
N P (kg ha−1)
K
2015
W0 + C7500 W9000 + C0 W9000 + C7500
14.9 21.0 36.0
5.6 7.2 12.8
351 416 767
73 86 159
883 785 1669
2016
W0 + C7500 W9000 + C0 W9000 + C7500
18.6 25.0 43.6
6.7 8.7 15.4
434 463 897
87 100 187
995 914 1910
W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively. 5
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Table 3 Effect of crop residue incorporation and inorganic K fertilization on seed cotton yield (kg ha−1), boll number (m-2), boll weight (g) and seed cotton yield nitrogen (kg ha−1) from 2011 to 2016. Year
Treatment
Seed cotton yield (× 103 kg ha−1)
Boll number (m−2)
Boll weight (g)
2011
CK W9000 + C0
4.90 a 5.18 a
90.4 a 95.0 a
5.4 a 5.5 a
2012
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
5.04 4.91 5.01 5.15 5.19 5.44
a a a a a a
88.4 88.4 93.1 93.7 92.4 99.3
a a a a a a
5.7 5.6 5.4 5.5 5.6 5.5
a a a a a a
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
4.87 5.89 6.73 6.76 6.38 6.64
b ab a a a a
104 126 140 143 133 138
b ab a a a a
4.7 4.7 4.8 4.7 4.8 4.8
a a a a a a
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
3.29 4.00 4.35 4.45 4.29 4.48
b ab a a a a
64.0 78.4 85.5 86.5 83.0 86.2
b ab a a a a
5.1 5.1 5.1 5.1 5.2 5.2
a a a a a a
2015
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
2.87 2.94 3.39 3.41 3.77 4.12
d d c c b a
62.4 66.4 75.4 74.3 84.3 90.8
d d c c b a
4.6 4.4 4.5 4.6 4.5 4.5
a a a a a a
2016
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
4.80 5.37 5.47 5.10 5.42 5.57
c ab ab bc ab a
87.1 c 100 ab 101 ab 94.3 bc 99.8 ab 101 a
5.5 5.4 5.4 5.4 5.4 5.5
a a a a a a
2013
2014
Table 5 Effect of crop residues incorporation and inorganic K fertilization on soil N2O flux in the cotton flowering and boll setting stage, seasonal N2O emissions and yield-scaled N2O emissions after five and six years crop residues incorporation in 2015 and 2016. Treatment
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
Soil N2O fluxes (mg N2O m−2 h-1)
Seasonal N2O emissions (kg N2O ha−1)
Yield-scaled N2O emissions (g N2O kg−1 seed cotton)
2015
2015
2015
0.08 0.12 0.14 0.16 0.12 0.07
2016 c b ab a b c
0.07 0.11 0.13 0.14 0.10 0.05
c b a a b d
1.37 1.61 1.83 2.06 1.55 1.25
2016 e c b a d f
1.06 1.29 1.54 1.73 1.29 1.01
d c b a c d
0.48 0.55 0.54 0.60 0.41 0.31
2016 bc ab ab a c d
0.22 0.24 0.28 0.34 0.24 0.18
bc bc ab a bc c
Different letters within the same column in a same year indicate significant difference according to the shortest significant ranges (SSR) test (P < 0.05). W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively.
4.4. Soil fertility After the fifth and sixth years of continuous crop residue incorporation, in 2015 and 2016 respectively, most of the soil fertility parameters were significantly (P < 0.05) improved by crop residue incorporation compared to the CK treatment (Table 4), which is consistent with the results of previous studies (Banger et al., 2009; Hao et al., 2001; Tan et al., 2017). However, the soil MBN contents rather than the soil MBC contents were significantly increased by crop residue incorporation compared with the CK, K150, and K300 treatments in 2016. This finding is contradictory with the observations of Powlson et al. (1987) and Kaur et al. (2010), who reported that the soil MBC and MBN contents were both higher in soils with crop residue incorporation. Moreover, the ratios of MBC/TC and MBC/MBN indicate soil organic matter quality and provide an extra information on soil C and N dynamics (Anderson and Domsch, 1989). In the present study, the lowest ratios of MBC/MBN were found in the control and W9000 + C0 soils in 2015 and 2016, respectively. Similarly, Ebhin et al. (2006) reported that the MBC/MBN ratio was 3.87 times higher with farmyard manure + inorganic fertilizers. The MBC/TC ratio was lowest in the W9000 + C7500 soil in both years, which is in contrast to results from a study conducted by Kaur et al. (2010). The reduced MBC/MBN and MBC/TC ratios might be due to the change in microbial species and
Different letters within the same column in a same year indicate significant difference according to the shortest significant ranges (SSR) test (P < 0.05). W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively; data from 2011 to 2013 were cited from our previous research of Sui et al. (2014) and Yu et al. (2016b).
Table 4 Effect of crop residue incorporation and inorganic K fertilization on soil total carbon (TC), total nitrogen (TN), dissolved organic carbon (DOC), microbial biomass carbon (MBC), available mineral nitrogen (AMN), microbial biomass nitrogen (MBN), Olsen P (Olsen-P) and available potassium (Av-K) contents of the top 20 cm soil layer of the experimental field in the cotton flowering and boll setting stage at 19th August in 2015 and 14th August in 2016. Year
Treatment
TC (g kg−1)
TN
DOC (mg kg−1)
MBC
AMN
2015
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
13.1 13.1 14.0 17.0 14.5 14.9
b b b a b b
1.22 1.26 1.32 1.33 1.29 1.26
2016
CK W0 + C7500 W9000 + C0 C7500 + W9000 K150 K300
13.8 14.9 15.7 17.4 14.1 14.5
c bc b a c c
MBN
d cd ab a bc cd
74.2 c 79.0 bc 87.2 b 101 a 80.5 bc 80.6 bc
500 534 554 579 562 573
b ab a a a a
56.5 55.6 61.2 63.2 60.2 60.6
b b ab a ab ab
82.0 90.8 88.6 95.0 79.7 75.0
1.35 c 1.37 bc 1 .45 a 1.47 a 1.36 bc 1.39 b
75.5 c 82.6 bc 89.6 b 106 a 87.1 b 86.5 b
570 582 603 647 620 630
c c bc a ab ab
58.3 58.0 64.0 67.1 60.8 64.6
c c abc a bc ab
93.5 b 96.6 a 106 a 110 a 95.1 bc 94.1 b
a a a a a a
Olsen-P
Av-K
23.7 24.2 24.4 25.2 23.9 24.2
a a a a a a
180 199 190 215 216 237
c bc bc abc ab a
26.1 24.0 24.1 27.6 26.5 25.2
ab b b a ab ab
164 199 202 215 218 260
c b b b b a
Different letters within the same column in a same year indicate significant difference according to the shortest significant ranges (SSR) test (P < 0.05). W0 and W9000 stand for wheat straw applied at 0 and 9000 kg ha−1 before cotton transplantation, respectively and C0 and C7500 stand for cotton residue applied at 0 and 7500 kg ha−1 before wheat sowing, respectively. 6
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Table 6 ANOVA showing the effects of year (Y), treatment (T), and their interactions (Y × T) on soil total carbon (TC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), total nitrogen (TN), water-soluble inorganic nitrogen (WIN), microbial biomass nitrogen (MBN), Olsen P (Olsen-P) and available potassium (Av-K) contents of the top 20 cm soil layer from the experimental fields, seasonal N2O emissions (Es), seed cotton yield (Yil) and yield-scaled N2O emissions (Yil-Es). Source of variance Y T Y×T
TC
TN ns
1.45 3.88 * 0.54 ns
276 ** 26.3 ** 1.70 ns
DOC
MBC ns
2.55 9.14 ** 0.11 ns
F values and significance levels (**P < 0.01, *P < 0.05 and
13.7 ** 1.80 ns 0.13 ns ns
WIN 1.48 2.64 0.10
MBN ns ns ns
21.1 ** 31.2 * 0.40 ns
Olsen-P 3.84 0.99 0.64
ns ns ns
Av-K ns
0.51 17.1 ** 1.11 ns
Es
Yil
Yil-Es
4.68 * 39.0 ** 31.1 **
230 ** 5.44 ** 1.36 ns
179 ** 12.7 ** 2.60 ns
P ≥ 0.05).
5. Conclusions
Table 7 Pearson correlation coefficients among N2O fluxes (mg m−2 h-1) and soil total carbon (TC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), total nitrogen (TN), water-soluble inorganic nitrogen (WIN), microbial biomass nitrogen (MBN), Olsen P (Olsen-P), available potassium (Av-K) contents, ratios of MBC/MBN and MBC/TC of the top 20 cm soil layer from the experimental fields. Soil C, N correlate parameter
N2O fluxes (mg N2O m−2 h-1)
TC DOC MBC TN WIN MBN MBC/MBN MBC/TC Olsen-P Av-K
0.479 ns 0.626 * −0.034 ns 0.230 ns −0.812 ** 0.394 ns −0.157 ns 0.394 ns 0.073 ns −0.123 ns
Crop residue as a K source can promote seed cotton yield as similar to inorganic K fertilizer, but the effects varied over the six years. A consecutive crop residue incorporation simultaneously increased the soil N2O flux and seasonal N2O emissions compared with the inorganic K fertilizer applicant. However, the increase in seed cotton yield by residue incorporation was not enough to counterbalance the increase in seasonal N2O emission, which finally resulted in a higher yield-scaled N2O emissions. Furthermore, the findings of this study suggest that the crop residue as a K source is beneficial for increasing cotton productivity but could lead to a rise in soil N2O fluxes, seasonal N2O emissions, and yield-scaled N2O emissions. Acknowledgement This work was funded by the Special Fund for Agro-scientific Research in the Public Interest (201503136), National Key Research and Development Program of China (2017YFD 0201900), China Agriculture Research System (CARS-18-14) and Jiangsu Collaborative Innovation Center for Modern Crop Production (JCIC-MCP).
Pearson correlation coefficient and significance levels (**P < 0.01, *P < 0.05 and nsP ≥ 0.05).
populations (Joergensen et al., 2010). On the one hand, treatments without organic sources fertilizer adversely affected the N storage of total biomass (Joergensen et al., 2010). The other possible reason of down-regulated of the MBC/TC ratio might be due to the inputs of lowquality litter (higher C: N ratio) immobilizing N (Deng et al. 2016).
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4.5. Correlation analysis The soil N2O fluxes showed positive and significant correlations with soil C pools and these results are in accordance with the findings of Bhattacharyya et al. (2012). In the present study, however, a significant positive (P < 0.05) relationship was found only between the soil N2O fluxes and soil DOC contents. Meanwhile, a significant but negative (P < 0.05) relationship was observed between the soil N2O fluxes and soil AMN contents (Table 7). This is contrary to findings of Verchot et al. (2006), who reported that there were no notable correlations between the soil N availability and the soil N2O fluxes. During the cotton flowering and boll setting stages in 2015 and 2016, the soil TC, TN, Olsen-P, and Av-K content was not significantly (P < 0.05) correlated with the N2O fluxes. Crop residue incorporation and inorganic fertilizer application, however, significantly impacted the soil MBC and MBN contents. Previous studies have also shown that soil N2O fluxes are significantly and positively (P < 0.05) associated with the soil MBC and MBN content (Lou et al., 2007). However, the present study found no significant correlations between the N2O fluxes and the soil MBC and MBN contents. Higher MBC/MBN and MBC/TC ratios in the soil receiving crop residues showed a higher activity of soil C and N metabolism (Carter, 1991; Ebhin et al., 2006). The weak correlation between soil N2O fluxes and MBC/MBN and MBC/TC ratios indicate the nitrification and denitrification processes that produce N2O in the soil may not be directly affected even though the soil C metabolism is more active in residue incorporated soils.
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