Spatial distribution characteristics and influence factors of degree of compaction in dry-farming Huang-Huai-Hai Plain of China

Soil compaction is a major cause of physical degradation in agricultural fields. The Degree of Compaction (DC) is a useful parameter to characterize the response of crops to different soils. Taking the degree of compaction as the research object, this study aims to explore the characteristics of soil compaction in the dry-farming Huang-Huai-Hai plain of China. 255 soil samples were collected in 2017. Mann-Kendall mutation test, geostatistics, and redundancy analysis were used to investigate the spatial distribution characteristics of the plough and compacted layer in the study area, particularly the spatial variation and influencing factors of the degree of compaction. The influence of compaction on grain yield was determined to preliminarily propose the optimal range for the degree of soil compaction. The results showed that the depth of both ploughed and compacted layers increased from north to south, where the maximum depth of the ploughed layer reached 22.50 cm, and the minimum was only 10.21 cm, whereas, the maximum depth of the compacted layer was 17.50 cm, and the minimum was 7.50 cm. There were significant differences in the degree of soil compaction in different regions. Specifically, the degree of compaction in the compacted layer was significantly higher than that of the ploughed layer. Nevertheless, there was spatial consistency in the degree of compaction of the ploughed and compacted layers. In the ploughed layer, the higher value area of the degree of compaction was distributed mainly in the eastern of Henan Province, the northern of Anhui Province, and the northern of Hebei Province, where the maximum was 87.68%, whereas, the lower value area was in the northwest of Shandong Province, and the southern of Hebei Province. There was a significant impact of the degree of compaction in the ploughed layer on grain yield (P<0.01), where the yield was higher when the degree of compaction was in the range of 70%-80%. The degree of soil compaction depended on both natural and human factors. The contribution of each influencing factor to the degree of soil compaction was ranked in a descending order: the average annual precipitation, average annual temperature, total power of agricultural machinery, sand content, silt content, clay content, and soil organic matter content. Among them, the average annual precipitation, average annual temperature, and total power of agricultural machinery presented extremely significant effects on the degree of soil compaction (P<0.01). In addition to uncontrollable natural factors, mechanical tillage was an important human factor for soil compaction. In general, soil compaction was improved by deep plowing with large agricultural machinery, but much attention should also be paid to soil compaction caused by machinery. The findings can provide a sound theoretical reference to improve the soil compaction in farmland, thereby formulating the management measures in dry-farming Huang-Huai-Hai regions.