Effects of land use and reducing conditions of paddy fields on soil quality and high energy moisture characteristic structural stability indices in North of Iran

The present paper is aimed at investigating the effects of land use (particularly reducing conditions of paddy fields) on soil structural stability as characterized by the high energy moisture characteristic (HEMC) method and finding the relationship between soil structural stability and available water (AW) in Amol city in Mazandaran province, Iran. Land use conversion led to a change in the soil organic matter (OM) content. The highest OM was found in the forest and pasture land uses, being significantly different from those of citrus garden and paddy field land uses. The high values of bulk density in the citrus garden and paddy fields can be attributed to intensive machinery traffic and puddling, respectively. However, bulk density was low in pasture and forest land uses because of high OM and macropores. Forest and the paddy fields had higher soil structural stability due to high OM/Clay ratio. The HEMC indices including volume of drainable pores ratio (VDPR) and stability ratio (SR) were calculated using the modified van Genuchten model, and the ratio of slopes at the inflection point of HEMCs of fast-wetted to slow-wetted soil samples ( S i R) was determined using the van Genuchten model. The HEMC indices showed positive and significant correlations with OM and OM/Clay ratio. By increasing the OM/Clay ratio, greater clay became complexed with organic matter, and thus soil structure stability increased. A strong correlation was observed between VDPR and OM/Clay in citrus garden and paddy fields. Furthermore, there was a significant correlation between VDPR and AW. The highest field capacity and AW were observed in the forest and paddy fields due to high OM and the presence of smectitic clays under reducing conditions, respectively.