Changes in soil quality in response to the application of different organic amendments to abandoned croplands under semi‐arid conditions

Choosing the most suitable organic amendment to restore abandoned croplands in semi-arid conditions is difficult. This is due to the complexity of soil processes and the high number of soil properties modified by the substrates. This work evaluates the variations in the main physical, chemical and biochemical properties, and the quality in terms of nutrient enhancement and microbiological activity (assessed by the soil quality index) of two abandoned agricultural soils treated with four composted organic amendments throughout 18 months in Andalusia (Southern, Spain) compared to untreated and natural sites. The tested organic amendments were vermicompost, compost from greenhouse crop residues, compost from poultry manure, certified organic compost produced from a mixture of chicken and sheep manure, and plant residues. The organic amendments noticeably changed the contents in organic matter (up to 170% in soils treated with vermicompost) and nutrients (up to 40-fold for phosphorus after application of poultry manure) as well as some enzymatic activities compared to untreated soils. These changes were different according to the applied compost and the treated site, and the monitored soil properties were also affected by a significant variability (p < .001) but not monotonic over time. The short-term effects of treatments were different in the study areas. The treatments with animal compost had better effects on soil properties (increases in soil quality index by 90%-120% compared to untreated soils) in the first site. In the second site, the compost from greenhouse plants was the most effective at increasing the soil quality (+50%). This was due to the significant increases in organic carbon, nitrogen, phosphorus and enzymatic activity of soil. Overall, the study indicates that the application of organic soil amendments will have a different behaviour at a specific site and therefore the selection of the most suitable amendment in terms of the restoration of soil quality effectiveness of abandoned soil croplands.