Effect Of Machinery Tillage And Seasonal Freeze-thaw On Soil Structure In Black Soil Region

Agricultural machinery tillage and seasonal freeze-thaw cycles are two major processes affecting soil structure in black soil region of Northeast China. Combining practical machinery operation and natural freeze-thaw cycles with artificial machinery compaction in the field and artificial freeze-thaw cycles in the lab, the plus and minus benefits of machinery tillage, characterization of seasonal freeze-thaw cycles, and their effects on soil structure were studied based on typical black soil.Firstly, the effects of machinery type and antecedent water content on soil structure and soil available nutrient were investigated by measuring soil bulk density, soil strength, soil porosity, soil aggregate distribution and stability, three soil phases and availability of nitrogen, phosphorus, potassium. The results showed that(1)Machinery tillage had positive and negative influence on soil structure, soil in top cultivated layer can be loosened and ameliorated however the subsoil compaction was resulted. For heavy and medium machinery, subsoil compaction formed in the soil depth of 41～60cm and 31～40cm, respectively.(2)Heavy machinery did better in improving soil environment compared with medium machinery, soil bulk density under heavy machinery operation decreased significantly by 3.5% and 7.2% after harvesting and subsoiling and NCP/CP increased dramatically also(P<0.05);however during the soil depth of 17.5～30cm under medium machinery operation there was a new plow pan produced because of the depth difference between harvesting and subsoiling.(3)Heavy machinery harvesting and subsoiling could increase phosphorus availability by 20.07%～380.91% overall, and increase potassium availability by 10.04%～20.61%(P<0.05)in the subsoil; medium machinery operation presented a increase and decrease trend of phosphorus availability and increase potassium availability by 17.05%～27.52% (P<0.05)in the subsoil; both heavy and medium machinery had no significance on soil nitrogen availability.(4)Antecedent water content had a significant effect on soil structure under machinery operations. Higher water antecedent resulted in deeper subsoil compaction at 40cm, which was deeper by 10cm than lower water content. Soil compaction occurred at the first pass when water content was higher. (5)By analyzing aggregate distribution and stability indexes of PA0.25,MWD,MWSSA,D and PAD0.25 in top soil of 0～30cm, we could inferred that few traffic passes promoted soil aggregation however more traffic passes greater than 5 would reduce water stability and mechanical stability of soil aggregates.Secondly, we monitored natural seasonal freeze-thaw cycles in different positions along slop and imitated freeze-thaw cycles under different water condition. The effects of freeze-thaw cycles on soil structure were studied by measuring soil bulk density, soil strength, soil porosity, void ratio, soil saturation, soil aggregate distribution and stability. Results showed that (1)During seasonal freezing and thawing cycles of October,2008 to June,2009, soil on the higher elevation had more intensive freeze process with shallow snow depth; the biggest frozen depth in higher and lower elevation were 229cm and 191cm respectively. The speed of freeze and thaw processes were 1.17cm/d and 2.83cm/d in higher elevation position, which were 1.03cm/d and 2.60cm/d for lower elevation position.(2)Soil at depth of 50～70cm was more susceptive to seasonal freezing and thawing cycles(P<0.050; after seasonal freezing and thawing, dry aggregates in cultivated soil (0-30cm) layer dispersed and stability of water stable aggregates bigger than 0.25mm increased significantly(<0.05), soil strength in 0～10cm decreased by 84.11%(<0.05), bulk density were in the same value of 1.10～1.11g/cm.(3)Frequency, intensity and water condition could affect soil aggregates distribution and stability (0-10cm) significantly:dry aggregates bigger than 5mm and water stable aggregates bigger than>0.25mm increased when there was no extra water applied during freezing and thawing cycles; however soil aggregates dispersed more when applying water during freezing and thawing cycles, dry aggregates smaller than 2mm and water stable aggregates of 0.25～1mm increased significantly(P<0.05).(4)Seasonal freeze-thaw cycles could loosen and improve soil environment in cultivated layer compacted by tractor, but one seasonal freeze-thaw process would not make the compacted soil recover to the natural level.(5)During artificial freezing-thawing process, dry aggregates and water stable aggregates were influenced by the number of cycles significantly(P<0.01), antecedent water content could only affect water stable aggregates smaller than 5mm(P<0.01)。Dry aggregates were more susceptive than water stable aggregates, and the MWD of dry aggregates was affected by method of sampling soil(P<0.01).(6)From the image analysis results of CT, freeze-thaw cycles could increase the average number of pores of soil at depth of 0～40cm and decrease average number and Feret diameter of pores during soil depth of 40-80cm. For argillic layer (40-80cm), average number and area of pores with size bigger and equal to 5mm were influenced by antecedent water content significantly(P<0.05); The number of freeze-thaw cycles had more influence on the average number of pores compared with antecedent water content.Meanwhile, we constructed generalized soil structure index (GSSI)and soil three phases index (GSSI)based on the Cobb-Douglas production function, which can use to quantify and evaluate soil structure condition.Results from this research could offer the theory reference to further study quality and management of black soil and contribute realistic significance to guide machinery tillage and ameliorate soil structure and function of black soil.