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Cui Siyuan, Cao Guangqiao*
(1. Nanjing Institute of Agricultural Mechanization, Ministry of Agriculture and Rural Affairs, Nanjing 210014, China；
2. Key Laboratory of Modern Agricultural Equipment, Ministry of Agriculture and Rural Affairs, Nanjing 210014, China)
Abstract: Exploring the effects of straw returning years on soil aggregates and the distribution of organic carbon and total nitrogen in rice-wheat cropping fields is of great significance for improving its soil quality.Field experiment was conducted in 2010 with 9 treatments: no straw returning(NR), 1 year of straw returning(SR1), 2 years of straw returning(SR2), 3 years of straw returning(SR3), 4 years of straw returning(SR4), 5 years of straw returning(SR5), 6 years of straw returning(SR6), 7 years of straw returning(SR7), and 8 years of straw returning(SR8).The results showed that with the increase of straw returning years, the proportion of >2 mm aggregates increased, the proportion of 0.25-2 mm and <0.053 mm aggregates decreased, while the proportion of 0.053-0.25 mm aggregates had little change.The contributing rate of total soil organic carbon and total nitrogen was the highest in >0.25 mm macro-aggregates.Contents of soil organic carbon and total nitrogen were significantly positively correlated with the proportion of >2 mm and >0.25 mm aggregates, while negatively correlated with the proportion of 0.25-2 mm, <0.053 mm and <0.25 mm aggregates.In conclusion, continuous straw returning has positive effects on improving the soil aggregates fraction and promoting the transfer of soil organic carbon and total nitrogen to large aggregates.
Keywords: years of straw returning; water-stable aggregates; organic carbon; total nitrogen; rice-wheat cropping; contribution rate
Soil aggregates are important for soil structure, which play important roles in the accumulation and protection of soil organic matter, the optimization of soil water and air status, and the storage and utilization of plant nutrients[1].Large and stable aggregates are less susceptible to erosion than small and weak aggregates.Soil aggregates can limit the decomposition and utilization of soil organic carbon(SOC)in aggregates by microorganisms[2-3].Jastrow et al.[4]found that nearly 90% of the soil organic matter exists in the aggregates, and only 13% of the particulate organic matter is free outside the aggregate.Soil aggregates reduce the contact between enzymes and microorganisms and SOC in aggregates[5], limit the diffusion of gases, and then reduce the decomposition intensity of organic carbon in aggregates[6].In addition to physical isolation, the combination of minerals and SOC in aggregates also protects SOC by chemisorptions[7].
Studies showed that straw returning significantly increased the number and stability of soil aggregates, and this trend was more obvious with the increase of years of straw returning[8-9].Wang et al.[10]suggested that straw returning increased the content and stability of soil coarse aggregates based on a 3-a experiment.Zhang et al.[8]suggested that straw returning significantly increased the number and stability of soil aggregates, but the effect was not significant in the short term(within 5 years).Fang et al.[11]showed that no significant effect was found on aggregate distribution after 25 years of straw returning, and the relevant results needed further study.Zhang et al.[12]suggested that under the same fertilizer operation in rice-wheat cropping system, straw returning increased the contribution rate of soil macro-aggregate to SOC and total nitrogen(TN).These differences may be related to specific research sites, soil types, cropping systems and climate conditions, etc.The newly added organic carbon in soil undergoes a process from large aggregates to small ones[13].Under the background of continuous straw returning, the distribution of inputting carbon from straw returning in the aggregate should also change with different returning years.
The rice-wheat cropping system is a main cropping system in eastern China.The conventional agriculture system in the rice-wheat rotation field is gaining concern due to the high labor cost of rice and wheat planting.Studies have shown that straw returning could effectively improve the structure of soil aggregates.However, the study on effects of different years of straw returning on the carbon distribution among aggregates in rice-wheat cropping system was not fully, which was much less on aggregate nitrogen.Therefore, the scope of this experiment is to evaluate the effect of straw returning years on soil aggregate fractions and their SOC/TN distribution to provide theoretic basis of improving soil quality in rice-wheat fields of eastern China.
1.1 Experimental site
The field experiment was conducted in Yangzhou University(32°23′N, 119°25′E), Jiangsu Province, China.This area is located in the middle-lower Yangtze plain with a subtropical monsoon climate.The total accumulated temperature was 2 398 ℃, total precipitation was 445 mm, and total sunshine duration was 1 261 h from late October 2017 to late May 2018(a whole wheat growth period).The soil of the test site is light loam.This area is the main rice-wheat double cropping area in China.
1.2 Experimental design
The field experiment was conducted using completely random block design from the autumn of 2010.There were 9 treatments in 2017—2018: no straw returning(NR), 1 year of straw returning(SR1), 2 years of straw returning(SR2), 3 years of straw returning(SR3), 4 years of straw returning(SR4), 5 years of straw returning(SR5), 6 years of straw returning(SR6), 7 years of straw returning(SR7), and 8 years of straw returning(SR8).Each treatment had 3 replicates and the plot was 4 m×3 m.After rice harvesting, the straw was mechanically cut into about 10 cm pieces and spread evenly in the plot.The straw was mixed into 0-15 cm soil by rotary tillage.The amount of straw returning was 9 000 kg/hm2.After wheat harvesting, the straw was removed from the field.The total amount of nitrogen applied during wheat growth period was 240 kg/hm2, and the rate of basal fertilizer∶tillering fertilizer∶jointing fertilizer∶booting fertilizer was 5∶1∶2.The amount of phosphate fertilizer(P2O5)and potassium fertilizer(K2O)was 90 kg/hm2and 150 kg/hm2, respectively, which were applied twice according to the proportion of 50% of base fertilizer and 50% of jointing fertilizer.The wheat variety is Yangfumai 4.
1.3 Soil sampling and analysis
Undisturbed soil cores were taken from 0-20 cm depths of each treatment plot after the wheat harvesting in May 2018, after which they were broken by hand gently, sieved through an 8 mm mesh, and air-dried.For aggregates fraction, the wet-sieving method was used following a procedure modified by Elliott[14].SOC concentration was determined using the external heating method of potassium dichromate(K2Cr2O7), and soil TN content was determined by Sei-micro Kjeldahl method[15].
1.4 Statistical analyses
Least-significant difference(LSD)was applied to multiple comparisons.Correlation in correlation analysis was expressed by Pearson index.Statistical calculation and analysis were performed by Excel 2010 and SPSS 17.0.
2.1 Soil aggregate fraction
As shown in Table 1, the proportion of >2 mm, 0.25-2 mm, 0.053-0.25 mm and <0.053 mm aggregate under different treatments at 0-20 cm soil layer was 16.72%-28.95%, 39.04%-47.91%, 20.62%-23.56%, 10.90%-15.71%, respectively.The aggregates in each treatment was dominated by >0.25 mm macro-aggregates, accounting for 63.00%-67.99%.With the increase of straw returning years, the proportion of different aggregate classes showed different trends, in which >2 mm aggregates decreased first and then increased, while 0.25-2 mm aggregates were opposite, increasing first and then decreasing.For 0.053-0.25 mm aggregates, the variation range was small, showing a trend of slow increase at first and then slow decrease, while for <0.053 mm aggregates, it gradually decreased.
Table 1 Aggregate-size distributions under treatments of different straw returning years %
2.2 Distribution of SOC in soil aggregates
In each treatment(Figure 1), the contents of SOC in >2 mm, 0.25-2 mm, 0.053-0.25 mm and <0.053 mm aggregates were 18.38-24.58 g/kg，20.98-23.47 g/kg，19.37-22.24 g/kg，13.48-19.38 g/kg, respectively.With the decrease of aggregate size, the aggregates SOC content of NR-SR3 and SR6 increased first and then to decrease, which of SR4, SR5, SR7 and SR8 decreased gradually.In general, the content of SOC in >0.25 mm macro-aggregate was higher than that in <0.25 mm micro-aggregate, indicating that SOC was mainly distributed in the macro-aggregate.With the increase of straw returning years, the SOC content in 0.053-0.25 mm aggregates showed a trend of increase-decrease-increase, while in other aggregates classed it increased generally.
Figure 1 Contents of organic carbon in water-stable aggregates under treatments of different straw returning years
As shown in Table 2, the contributing rates of SOC in >2 mm, 0.25-2 mm, 0.053-0.25 mm and <0.053 mm aggregates were 16.30%-31.19%, 39.75%-52.71%, 19.85%-23.15% and 9.26%-12.83%, respectively.Most of the SOC was distributed in >0.25 mm macro-aggregates, which contributed 66.01% to 71.60% of the total SOC.The contributing rate of 0.25-2 mm aggregates in each treatment was the highest, followed by >2 mm under SR4-SR8 treatments, and by 0.053-0.25 mm under NR-SR3 treatments.With the increase of straw returning years, the SOC contributing rate of >2 mm aggregates increased, which of the other aggregates classes decreased.
Table 2 Contributing rates of water-stable aggregates in soil organic carbon %
Table 3 Contributing rates of water-stable aggregates in soil total nitrogen %
2.3 Distribution of TN in soil aggregates
TN contents in >2 mm, 0.25-2 mm, 0.053-0.25 mm and <0.053 mm aggregates were 1.17-1.50 g/kg，1.23-1.54 g/kg，1.21-1.48 g/kg，1.09-1.34 g/kg, respectively(Figure 2).With the decrease of aggregate size, the TN contents under each treatment increased firstly and then decreased.Under NR, SR2, SR4-SR8, the TN content in 0.25-2 mm aggregate was the highest, while under SR1 and SR3, the highest was in 0.053-0.25 mm and >2 mm aggregate, respectively.In general, soil TN mainly distributed in >0.25 mm macro-aggregates.With the increase of straw returning years, the TN content in each aggregates classes increased, but the TN content in 0.053-0.25 mm and <0.053 mm aggregates classes decreased under SR8.
Figure 2 Contents of total nitrogen in water-stable aggregates under treatments of different straw returning years
As shown in Table 3, the contributing rates of TN in>2 mm, 0.25-2 mm, 0.053-0.25 mm and <0.053 mm aggregates were 15.28%-28.73%, 39.30%-48.38%, 20.07%-22.80% and 9.51%-14.19%, respectively.TN was mainly distributed in >0.25 mm macro-aggregates with a contributing rate of 63.66%-68.33%.The contri-buting rate of 0.25-2 mm aggregate under each treatment was the highest, followed by>2 mm under SR5-SR8 and 0.053-0.25 mm under NR-SR4, respectively.With the increase of straw returning years, the TN contribution rate of >2 mm aggregates increased gradually, which of 0.25-2 mm, 0.053-0.25 mm, and <0.053 mm aggregates decreased.
2.4 Correlation analysis
As shown in Table 4, the proportion of >2 mm and >0.25 mm aggregates were significantly positively correlated with SOC and TN content(P<0.01), which of 0.25-2 mm, <0.053 mm and <0.25 mm were significantly negatively correlated with SOC and TN content(P<0.01).
Table 4 Correlation between proportion of aggregates and TN/SOC
This study showed that with the increase of straw returning years, the change trends of proportion of different aggregates classes were different.
The proportion of >2 mm aggregates decreased firstly and then increased, which of 0.25-2 mm increased firstly and then decreased.The proportion of 0.053-0.25 mm aggregate had a small variation range, which of <0.053 mm aggregate increased slightly at first and then decreased, which was consisted of some studies[8, 16].The proportion of >2 mm aggregate was lower than that of NR under SR1 and SR2, but it also increased with the increase of straw returning years.As the straw had not been completely degraded in 1-2 years, incomplete degradation of straw could loose soil and prevent the aggregating of small particles, so the effect of straw returning on promoting soil particle polymerization was not obvious[16].With the increase of straw returning years, various kinds of humus generated by straw decomposition had strong cementation properties, which could promote the formation and stability of soil large aggregates[17-19]and increase the proportion of large aggregates.
Studies have shown that soil micro-aggregates store more organic carbon[20], which may be related to differences in soil type and planting system, etc., and the contributing rate of soil aggregate SOC is affected by the proportion of aggregate and aggregate SOC content.Hou et al.[21]suggested that straw returning significantly increased the SOC content in aggregates, and the effect increased with the increase of aggregate classes.Meng et al.[22]suggested that SOC content in each soil aggregates classes increased with the increase of straw returning amount.Wang et al.[23]suggested that straw returning significantly increased the SOC contributing rate of >2 mm aggregates.This study suggested that SOC was mainly distributed in macro-aggregates under each treatment.The contribution rate of SOC in macro-aggregates was 66.01%-71.60%, which was significantly higher than that in micro-aggregates, in consistent with previous researches[12, 24-25].The years of straw returning had a certain effect on the SOC content indifferent aggregates classes.Except for 0.053-0.25 mm aggregates, the SOC content in the other aggregates classes increased with the increase of straw returning years, indicating that straw returning increased the input of soil exogenous carbon and increased the SOC content in aggregates.The years of straw returning significantly affected the contributing rate of SOC in aggregate to total SOC.With the increase of straw returning years, only the contribution rate of SOC in>2 mm aggregates increased, indicating that increasing straw returning years could effectively promoted the transfer of SOC to >2 mm aggregate.
Straw returning significantly improved the stability of soil aggregates[26].An et al.[27]studied the effect of different amounts of corn straw returning from 0 to 12 000 kg/hm2on the distribution of nitrogen in aggregates, and the results showed that the aggregates nitrogen content was the highest under 9 000 kg/hm2treatment, then increasing straw returning amount would reduce the nitrogen content in aggregates.This study showed that soil TN was mainly distributed in >0.25 mm macro-aggregates with the contributing rate of 63.66%-68.33%, consisted with Zhang et al.[28].The TN content in each aggregates classes increased with the straw returning years, mainly due to the additional nitrogen input in the soil.However, the TN content in 0.053-0.25 mm and <0.053 mm aggregates decreased under SR8, mainly because straw returning increased soil particle agglomeration, and promotes the transfer of TN from micro-aggregates to macro-aggregates[29].With the increase of straw returning years, the contributing rate of TN in >2 mm aggregates increased gradually, which of other aggregates classes decreased, indicating that increasing straw returning years could promote nitrogen transfer to large aggregates.
With the increase of straw returning years, the proportion of >2 mm aggregates increased gradually, while that of 0.25-2 mm and <0.053 mm aggregates decreased.The content of SOC and TN in each soil aggregates classes increased with the increase of straw returning years.>0.25 mm macro-aggregates contributed the most to the total SOC and TN.The contributing rate of >2 mm aggregates to SOC and TN increased with the increase of straw returning years.Continuous straw returning has a positive effect on improving the soil aggregates fraction and promoting the transfer of SOC and TN to large aggregates.
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