Abstract:

Aims The Longdong Loess Plateau in Gansu Province is one of the main apple producing areas in China. Plastic film-mulching is often applied to maintain soil moisture as well as water-saving in apple orchards. It is reported that long-term film mulching may cause the degradation of soil physical properties and inhibition of root growth. The objective of this study were to explore the effect of long-term mulching on the physical properties, stability of the surface (0-20 cm) and subsurface (20-40 cm) layer soil, and to investigate the changes of apple fine root growth characteristics in quantity, morphology, configuration and anatomical traits.

Methods Using soil profile and stratified sampling method, the changes of the physical properties and soil structural stability of the surface and subsurface layer soil was analyzed under film-mulching 2 years (2Y), film-mulching 4 years (4Y) and film-mulching 6 years (6Y), conventional tillage (CK) treatments, and roots of 18-year-old apple trees were collected at rapid growing period (days after fruit harvest and before defoliation) to investigate the spatial distribution by measuring the root length, surface area, specific root length, catheter diameter and catheter density. Principal component analysis was used to extract the main factors of root and soil changes under the condition of plastic film mulching, and to analyze the adaptation strategies for fine root growth of apple trees to the physical degradation of rhizosphere soil.

Important findings Short-term film mulching (2Y) treatment significantly improved the soil water content and total porosity in the subsurface soil layer, increased by 18.04%, 4.53%, respectively, and reduced the soil density by 2.36% than that of conventional tillage (CK) treatments. Growth of fine roots increased in subsurface soil, and the specific surface area was 151% of CK. Film mulching promoted the movement of clay particles to the subsurface soil resulting in obvious deposition and cementation. The physical clay in subsurface soil was higher than that of surface soil. The physical clay in subsurface soil under 2Y, 4Y and 6Y mulching were 115.64%, 115.58% and 114.21% of those in surface soil, which led to soil compaction. Soil texture, aggregate characteristics and organic matter content were selected as the main load factors, which dominated degradation process of subsurface soil, and inhibited the number and configuration characteristics of roots, apple fine roots of long-term film mulching (4Y or 6Y) concentrated in the surface layer of the soil. In the subsurface soil, fine roots were found to be shortened and coarsened with inhibiting elongation growth and increasing catheter diameter, indicating the “intensive” root construction strategy the offset the weakening of absorption function caused by the fine root quantity and weakening of morphological characteristics. In conclusion, the ‘invisible' degradation of subsurface soil physical property occurred in long-term film mulching orchard will have an influence on healthy roots growth and sustainable soil utilization. It is recommended that 2-year was the suitable for continuous film mulching years in Longdong area, and the mulching film should be removed periodically to promote root growth and optimize soil structure.

Key words: film mulching, fine roots, subsurface soil, soil texture, aggregate stability, principal component analysis