The Characteristics Of Cementing Material And Stability Of Soil Aggregates Under Straw Returning In Rice–Rape Cropping System

The rice–rape cropping system is one of the most important paddy–upland rotation systems in the subtropical regions of China.The cropping system has contributed substantially to food grain production and its sustainability is of utmost importance for ensuring regional food security.This system alternates between paddy and dry farming,which also causes the soil to alternate between wet and dry seasons.The dramatic variations of hydrothermal conditions during the cropping seasons cause changes in soil physical,chemical and biological characteristics.Aggregate is the basic unit of soil structure.Aggregate stability is an important property of maintaining soil structure and function,and it has important ecological significance in the fields of cultivated land quality,soil and water conservation and biological diversity.The internal causes（the soil itself）of soil aggregate formation and stability include the soil-forming environment and the basic physi-chemical properties of the soil,especially the cementation of cementing material;the external causes are mainly the disturbance of human activities,such as tillage practices,fertilization,straw returning and wetting-drying cycle and so on.The straw returning as an agricultural practice to improve soil organic carbon,structure and soil fertility has been widely applied.However,little is known about the characteristics of cementing material and stability of aggregates in response to long-term straw returning under rice–rape cropping system.In the present study,the paddy soils drived from the quaternary clay sediments in Wuxue City and the recent river alluvium in Jingzhou City,Hubei Province,are taken as the research objects.By analysising the content of organic carbon and iron oxide in the soil aggregates under conventional tillage without straw returning（T1）;conventional tillage with straw returning（T2）;no-tillage without straw returning（T3）and no-tillage with straw returning（T4）at Wuxue experiment site（for 9 years）,mineral nitrogen and phosphorus（F1）;mineral nitrogen,phosphorus and potassium（F2）;mineral nitrogen and phosphorus with straw returning（F3）;and mineral nitrogen,phosphorus and potassium with straw returning（F4）at Jingzhou experiment site（for 5 years）.Meanwhile,using scanning electron microscopy（SEM）,solid ¹³C nuclear magnetic resonance（NMR）,Fourier infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）and X-ray diffraction（XRD）and other methods to study the chemical composition of cementing material（organic carbon,iron oxides and clay minerals）in soil aggregates.By selective dissolution cementing material,combined with particle size analysis and determination of organic carbon and iron oxide content,showed the main function of the cementing material in the aggregate size,to explore the main reasons and mechanisms for the stability of soil aggregates under the rice-rape rotation system,and provide a scientific basis for reasonable cultivation and fertilization of farmland soils.The main results are as follows:（1）The percentage of>5 mm aggregates was highest in the all fraction（with values of 31.3–53.5%and 35.6–66.8%in paddy soils drived from the quaternary sediments and river alluvium,respectively）,followed by 2–0.25 mm aggregates,and the percentage of micro-aggregates（0.25–0.053 mm）and silt+clay（<0.053 mm）was lowest in the topsoil（0–20 cm）;the percentage of 2–0.25 mm aggregates was highest in the all fraction in the subsoil（20–40 cm）,accounting for above 35%of the total mass.Compared with the conventional tillage,conventional tillage with straw returning increased the percentage of>5 mm in the topsoil drived from Quaternary sediments,and reduced the percentage of micro-aggregates and silt+clay;the no-tillage treatment increased the percentage of>2 mm aggregates and reduced the percentage of micro-aggregates and silt+clay.The effect of no-tillage with straw returning treatment on the distribution of aggregates was most obvious.The chemical fertilizer in combination with straw returning increased the percentage of>5 and 5–2mm aggregates and reduced the percentage of micro-aggregates and silt+clay.In the subsoil,the effect of straw returning on the distribution of aggregates is more obvious,mainly by reducing the percentage of silt+clay.Seasonal changes could significantly affect the distribution of water-stable aggregates in the 0–40 cm soil layer,mainly affecting the percentage of macro-aggregates（>0.25 mm）and silt+clay.（2）In the topsoil,the mean weight diameter（MWD）and geometric mean diameter（GMD）of soil aggregates were 3.06–4.77 mm and 1.01–1.57 mm in paddy soils drived from the quaternary sediments,and 3.29–5.81 mm and 0.97–1.93 mm in paddy soils drived from river alluvium,respectively.The MWD and GMD of aggregates in the topsoil were higher than in the subsoil.Straw returning and no-tillage increased the MWD and GMD of aggregates in the topsoil,suggested that the straw returning and no-tillage measures could increase the stability of aggregates and improve the soil structure.In the subsoil,the effect of straw returning on the stability of aggregates was better than tillage treatment.In the topsoil,MWD and GMD of aggregates after rice harvest were higher than those after rape harvest in paddy soils drived from the quaternary sediments,which was opposite to the trend in paddy soils drived from river alluvium.（3）The surface of soil aggregates was smoother in paddy soils drived from the quaternary sediments than in paddy soils drived from river alluvium.Many small-sized particles were attached to the surface of the macro-aggregates,and more obvious pores after straw returning.Meanwhile,we observed that the aggregates contained some undecomposed straw residues.The pores were formed by the flocculent cohesive matter and humus are clearly visible in the soil micro-aggregates.The surface of the micro-aggregates combines many fine particles,which are arranged regularly and tightly,with a smoother surface and more developed pores after the straw returning.（4）Long-term straw returning and no-tillage increased the soil organic carbon（SOC）content in the topsoil.Straw returning also increased the SOC content in the subsoil.The content of SOC was higher after rice harvest（the average SOC content were 23.2 and 19.9 g/kg in paddy soils drived from the quaternary sediments and river alluvium,respectively）than after rape harvest（the average SOC content were 20.7and 17.6 g/kg in paddy soils drived from the quaternary sediments and river alluvium,respectively）.With the decrease of aggregate size,the content of SOC decreased first and then increased in paddy soil drived from the quaternary sediments,the SOC concentration was the highest in>5 mm aggregates（17.9–28.8 g/kg）and lowest in micro-aggregates（7.52–15.6 g/kg）.With the decrease of aggregate size,the content of organic carbon increased first and then decreased in paddy soil drived from the river alluvium,the SOC concentration was the highest in 5–2 and 2–0.25 mm aggregates,which were 11.7–24.2 g/kg and 11.1–26.6 g/kg,and lowest in silt+clay.Straw returning and no-tillage could increase the content of SOC in each aggregate,especially macro-aggregates.（5）Among all aggregates sizes,O-alkyl C was the dominant SOC functional group,accounting for 38%–46%and 41%–48%of total SOC in paddy soils drived from the quaternary sediments and river alluvium,respectively.The relative proportion of alkyl C followed the order of（0.053–0.25 mm）>（<0.053 mm）>（>0.25mm）,whereas that of O-alkyl C showed the opposite order.Straw returning increased the relative proportion of alkyl C and O-alkyl C in paddy soil drived from the quaternary sediments and river alluvium,respectively.In addation,straw returning treatment increased the relative proportion of aromatic C in the silt+clay fraction of paddy soils drived from the quaternary sediments,and in aggregates（>0.25 and0.053–0.25 mm）of paddy soils drived from river alluvium.Long-term straw returning could promote the enrichment of clay in aggregates.Straw returning could increase SOC contents by 27.0%–95.0%in clay fractions within aggregate-size classes.The relative intensities of C-H and C-O in large aggregates（5–2 mm）were higher than in micro-aggregates（0.25–0.053 mm）,C=C showed the opposite trend.The relative intensities of aliphatic-C（C–C/H）and alcohol-C（C–O）in clay fractions within aggregate-size classes were decreased,wheres,aromatic-C（C=C）and carboxylic-C（C（O）O）in near-surface of clay fractions within aggregate-size classes were increased under straw returning.（6）The content of free（Fed）and amorphous（Feo）iron oxides gradually increased as the aggregate size decreased,and the content of Fed and Feo in 0.25–0.053 mm aggregates was lower than that in 2–0.25 mm aggregates in paddy soil drived from the quaternary sediments.The Fep content was higher in macro-aggregates than in small-and micro-aggregates.The content of Fep was the highest in the>5 mm aggregates and lowest in micro-aggregates.The content of Fed and Feo were highest in 5–2 and 2–0.25 mm aggregates,respectively,the content of Fep was highest in>2 mm aggregates of paddy soil drived from river alluvium.The content of Fed in topsoil layer was lower than in subsoil,but the content of Feo and Fep had the opposite trend.The effect of straw returning on the content of iron oxide in topsoil was more significant than in subsoil.Straw returning reduced the content of Fed,but increased the content of Feo and Fep.The content of Fed was higher after the rice harvest（the average Fed content were 16.2 and 26.1 g/kg in paddy soils drived from the quaternary sediments and river alluvium,respectively）than rape harvest（the average Fed content were 14.4 and 24.7 g/kg in paddy soils drived from the quaternary sediments and river alluvium,respectively）.（7）Soil organic carbon was the most important cementing material,followed by Fep and Feo,whereas Fed was the weakest cementing material.SOC,Feo and Fep had direct contribution to aggregate stability.In our study,SOC and Feo improved aggregate stability indirectly through Fep.Feo was positively correlated with the abundance of O-alkyl C in aggregates,indicated that Feo could be combine with unstable organic carbon compound（O-alkyl C）to form a mineral-organic complex.The retention of Feo on carbonyl C and aromatic C is stronger in paddy soil drived from the quaternary sediments than drived from the river alluvium.（8）The iron oxides could increase in soil aggregate stability when SOC contents were low.The Fed had a greater effect on the stability of aggregates than Feo in high SOC–low iron oxides soil,but both had the same effect in low SOC–high iron oxides soil.SOC is the major cementing material for>2 and 2–0.25 mm aggregates.The SOC is a key agent influencing soil aggregation not only in macro-aggregates,but also in micro-aggregates in high SOC–low iron oxides soil.Fed is a crucial binding of micro-aggregate in the soil studied.The high SOC–low iron oxides soil had more organo-mineral complexes to promote the formation of aggregate.（9）Long-term returning straw increased the concentration of poorly crystalline ferrihydrite,while they reduced the concentration of hematite and goethite in clay fractions with soil aggregates drived from the quaternary sediments.The clay fractions within micro-aggregates contained more Fe（III）and ferrihydrite than that of large aggregates（5–2 mm）.The clay fractions within aggregate-size classes samples contain 1.4 nm minerals（HIV）,illite,kaolinite,chlorite,goethite and hematite.Kaolinite is the most abundant mineral in clay fractions within aggregate-size classes.The straw returning reduced the content of HIV and increased the content of vermiculite in each aggregate.With the decrease of aggregate size,the content of HIV decreased and then increased,and the content was the lowest in micro-aggregates.Compared to the aggregate fractions,the relative content of vermiculite in the silt+clay component is lower.The relative content of illite in micro-aggregates was higher than that of other sizes.（10）Straw returning could increase the content of organic carbon,Feo and Fep in the soil aggregates and the stability of the aggregates.Soil organic carbon is the most important cementing material for the test paddy soil aggregates,especially the large aggregates,followed by Fep and Feo,and Fed has the weakest cementation.Straw returning promoted the formation of organic-mineral complexes and the stability of agglregates by changing the chemical composition of organic carbon and mineral content in aggregates and clay particles.