| The 5-year field experiment was conducted from October, 2011 to June, 2016 using land in a winter wheat-summer maize rotation at the Research Station of Yanzhou Academy of Agricultural Sciences, Yanzhou, Shandong, China. A widely planted cultivar, Jimai 22, was selected as the experimental material. The main plots were combinations of tillage and total aboveground residue returned from the summer maize crop; the residues of winter wheat were removed from the field after harvest in the five years (5-year rotary tillage with residues removed, RT; 5-year rotary tillage with residues applied, RS; 5-year deep plowing tillage with residues applied, DS; 2-year rotary tillage followed by deep plowing tillage in the 3rd year with residues applied, RS/DS). The subplots were three N treatments: recommended N (225 kg N ha-1,N225),reduced N (165 kg N ha-1,0.7×Nm,N165),and increased N (300 kg N ha-1,1.3×Nm,N300). The responses of grain yield,nitrogen (N) utilization and soil quality to tillage practice and N rate. The main results are shown as follows:1. The grain yield and nitrogen utilization as affected by tillage practice and N rateDuring the study duration of 5 years, downward tendencies of grain yields were observed from the 3rd year under the continuous rotary tillage treatments, especially within residues removal, and the grain yields in 2015-2016 were 1.85-6.33% lower than those in 2011-2012.Increasing N rate could reduce the decreases of grain yield under continuous rotary tillage treatments. The DS and RS/DS treatments could persistently increase grain yield in the 5 years with the average annual rate of 4.51% and 3.63%, respectively. In the 2011 to 2012 and 2012 to 2013 seasons, there were no differences among treatments with residues return, but lower yields were observed in RS, compared to RS/DS in the 2013 to 2014, 2014 to 2015, and 2015 to 2016 seasons.In RT treatment, the average grain yield of N165 was 5.45% lower than that of N225; there were no significantly differences of grain yield between N225 and N300. Under the N rate of N165, grain yields of RS/DS and DS were 8.94 t ha-1 and 9.00 t ha-1 in 2015-2016,respectively, and there were no significantly differences with N225. It was suggested that moderately reduced the N rate within appropriate tillage method could produce high grain yield.There was a significant and positive relationship between grain yield and total dry matter accumulation (DMA) of above-ground, and it indicated that high DMA was the material foundation to obtain high grain yield. In 2013-2014, 2014-2015 and 2015-2016 seasons,compared to RT and RS, DS and RS/DS reduced the harvest index (HI), and there was a negative relationships between grain yield and HI. It was suggested that the increase of grain yield mainly owe to the increase of DMA.Residues return significantly increased the above-ground nitrogen uptake (AGN), nitrogen accumulation in grain,and PFPN,and there was a significant and positive relationship between AGN and grain yield. In the first three years, treatments with residues return did not show higher nitrogen utilization efficiency (NUE), and it was mainly due to high AGN. In 2014-2015 and 2015-2016 seasons,compared to RT,residues return significantly increased the NUE. The micro-plot experiment indicated that, compared to RS, DS and RS/DS treatments significantly increased the distribution of residue N in plant and grain.2. The soil quality as affected by tillage practice and N rateContinuous 5 years’ residues return reduced the soil bulk density (SBD) and total porosity at the 0-10, 10-20, and 20-30 cm soil depths, suggesting that residues return could effectively improve soil structure and aeration. In 0-10 and 10-20 cm soil layers, increasing N rate could significantly reduce SBD and increase soil total porosity; however, the N rate had no significantly effect on SBD and porosity.Residues return significantly increased the soil organic carbon (SOC) and soil total N (TN)content. The SOC and TN contents increased with the increase of N rate, whereas, there was no significantly difference between N225 and N300 on SOC content.The soil microbial diversity was signific,antly increased within residues return treatments at the 0-10, 10-20, and 20-30 cm soil depths and the microbial community composition obviously changed within different tillage practices. In the three soil layers, residues return resulted in significant increases in the relative abundances of Proteobacteria, Nitrospirae, and Actinobacteria and significant decrease in that of Acidobacteria. Compare to RT, treatments of residues return significantly increased the activity levels of soil invertase, protease, urease,and nitrate reductase at the 0-10, 10-20, and 20-30 cm soil depths. Soil invertase and protease activity levels were increased with the N rate at the three soil layers; whereas, the responses of soil urease and nitrate reductase activity levels to N rate were significantly different in the three soil layers. The correlation analysis showed that there were significant and positive relationships between soil microbial diversity and enzyme activities (P < 0.01).Compared to DS and RS/DS, in our study, RS resulted in greater soil C and N content,enzyme activity, and microbial diversity, at the 0-10 cm depth. However, the values were less at the 10-20 and 20-30 cm soil depths, indicated that the tillage methods affected the vertical distribution of soil nutrients and microbial biomass. The different effects suggested that long-term residues return with rotary tillage was not the appropriate approach to improve the soil quality of plough layer; residues return with continuous or timely deep plowing tillage could balance the distribution of soil character in the plough layer to improve soil quality.3. The soil aggregate and changes of SOC and TN pools as affected by tillage practice and N rateIn the 0-10, 10-20, and 20-30 cm soil depths,compared to RT, residues return significantly increased the mass proportion of macro-aggregate by 17.16%, 19.41%,and 15.89%, respectively. It suggested that could prevent soil erosion. The mass proportion of macro-aggregate of the four tillage practices showed various tendencies at different soil layers.In the 0-10 cm soil layer,the mass proportion of macro-aggregates of different treatments were ranged by RS > RS/DS > DS > TS; however, ranged by DS > RS/DS > RS > RT in the 10-20 cm and 20-30 cm layers.After 5-years continued crop residues input and soil tillage, the levels of SOC and TN pools with residues input treatments were significantly higher than those of no residues input treatments. The soils under RS and RT treatments gathered more SOC and TN to the 0-10 cm layer compared to DS and RS/DS treatments. In this study, SOC and TN pools under DS and RS/DS treatments were significantly increased, approximately 1.83 and 1.56 t C ha-1, and 0.11 and 0.10 t N ha-1 were sequestrated from 2011-2016, respectively. However, the soil of 0-30 cm under RT were occurred SOC and TN lost, and approximately 0.33-0.49 t C ha-1 and 0.09-0.24 t N ha-1 were lost each year; increasing N application rate could reduce the loss of soil N pools. The SOC pools in RS treatments were increased 0.13 t ha-1 each year; however,the TN pools under N225 and N165 were lost by 0.56 and 0.25 t ha-1 in the 5 years,respectively, and it was mainly due to the decreased in 20-30 cm layers; TN pools of 0-30 cm soils under N300 were increased by 0.16 t N ha-1, but the TN pool of 20-30 cm soil layer were decreased.From the perspective of soil aggregate, the change of SOC pool mainly owed to the change of soil macro-aggregate associated C pool. The soil macro-aggregate associated TN pools under RT treatments were significantly decreased in the 5 years by 1.55—2.34 t ha-1. Residues return treatments significantly increased the macro-aggregate associated TN pools and decreased the micro-aggregate associated TN pools, indicating that residues return was beneficial to the formation of soil macro-aggregate. In the RT treatments, the decreases of TN pool were mainly due to the significant reduction of soil macro-aggregate associated TN pool.On the contrary, the change of micro-aggregate associated TN pool was the major factor of total TN pool. In the treatments of DS and RS/DS, the changes of soil aggregate associated TN pools were significantly different under different N rate. |
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