Soil enzymes, nematode community and selected physico-chemical properties as soil quality indicators in organic and conventional olive oil farming: Influence of seasonality and site features

Abstract

There is growing interest in the application of soil enzymes and nematode community indices as indicators of changes in soil quality under contrasting management practices. Although an abundant literature on this subject has arisen during the last 10 years, most of the studies have focused on measuring many soil quality indicators at a single or a few sampling times. However, soil enzyme activities show a natural temporal variability which could mask the variability due to the type and timing of soil management practices. In this study, we compared soil enzymes, nematode communities and physical–chemical soil properties in three pairs of organic and neighbouring conventional olive orchards. Dehydrogenase, β-glucosidase, arylsulfatase, acid and alkaline phosphatases activities, and potential nitrification were studied during an annual cycle, and variability due to sites, replicates within a site, management practices and seasonality has been accounted for. In addition, several nematode community indicators were also studied on one occasion. The geometric mean of enzymes activities (GMea), used as an integrating soil quality index, was validated through an independently performed principal component analysis (PCA). Seasonal variability of individual soil enzymes ranged from 29 to 71%, without a consistent temporal trend. Management system explained, on average, a maximum of 26.3 and 15% of the variability found for soil enzymes and nematode community indicators, respectively. Most of the variability found in both sets of indicators was due to different localities (up to 58 and 45% for soil enzyme and nematode community indicators, respectively) and replicates within a plot (up to 51 and 86%, respectively). Organic management resulted in significantly higher soil enzyme activities. However, differences were dependent on site and sampling. For nematode community indicators, the organic farms showed higher values only for one site. These results reveal the need for extensive comparative assessments to draw clear conclusions on the improvement of soil quality under sustainable management practices. The GMea was significantly higher in organic than in conventional managed plots, independently of the sampling and, moreover, showed significant correlation with the first axis of the PCA. In addition, the GMea, and scores on the first axis were highly correlated with some of the nematode indices. Therefore, the GMea was a suitable tool to condense the whole set of soil enzyme values in a single informative numerical value, which was more sensitive to management practices than nematode community indicators.