Seasonal variation of microbial activity as affected by tillage practice and sugar beet foam amendment under Mediterranean climate

Enhancement of soil organic matter content by no-tillage practice and alleviation of soil acidity by Ca-amendment application has been recognized as important strategies to combat soil degradation. Based on the limited information about possible interactions between the two commonly used techniques, the aim of this study was (1) to evaluate the long-term effect of no-tillage and Ca-amendment on soil biological activity in a split-plot design established on degraded acid soil in SW Spain, and (2) to analyze the microbial activity fluctuation during a year in a Mediterranean climate with marked seasonality. The studied treatments included traditional tillage without Ca-amendment application (TT), no-tillage without Ca-amendment application (NT), amended traditional tillage (TT-A) and amended no-tillage (NT-A). The used Ca-amendment was the mixture of sugar beet foam (SF) and red gypsum (RG). A refresh dose of Ca-amendment was applied in October 2014 and was incorporated into the soil in the TT plots and let on soil surface in NT. Soil samples were collected in January, April, July and October 2015. The Ca-amendment increased the soil pH and the dehydrogenase, beta-glucosidase and acid phosphomonoesterase activity, but decreased microbial biomass carbon (MBC), probably as a result of the neutralizing effect on soil pH. The pH rise was also linked with the increase of microbial biomass nitrogen (MBN) and could be interpreted as a shift in the microbial population. The long-term effect of NT led to a higher organic carbon accumulation in the top soil layer which maintained higher soil moisture content during the water deficient months. The higher availability of organic carbon and the amelioration effect of NT to extreme conditions in the previous months increased the enzymatic activities, MBC, MBN and basal respiration in the NT plots, especially in July when the soil moisture was minimal and temperatures were high. The combination of both treatments did not show any significant synergistic interaction in any biological parameter. Therefore, we can evaluate the application of Ca-amendment on soil surface in NT fields as feasible alternative to lime application and incorporation from the biological point of view. In addition, the less extreme drought conditions in the NT plots and the consequent higher microbial activity during the summer can be considered as a strategy to increase the biological soil resistance against the drought events in the scenery of climate change. The variable response of the different agricultural practices to climate fluctuation should be included in future research in order to evaluate the response of biogeochemical cycling and organic matter dynamics to predicted future climate change.