Organic plum cultivation in the Mediterranean region: The medium-term effect of five different organic soil management practices on crop production and microbiological soil quality

Abstract

Organic farming practices recommended by European regulations offer an alternative to conventional soil management, but these practices should be analyzed in greater depth to assess their effectiveness on different plants and in different local conditions. Accordingly, this study evaluates the efficacy of five different organic management strategies on an organic plum farm in Southeastern Spain. This study was conducted for a period of six years and the five different organic treatments were applied in triplicate in 256m2 plots randomly distributed in three blocks (15 plots in total); each plot had 16 plum trees. Every year crushed pruning wastes were incorporated into the soil in all the experimental plots. One triplicated plot only received this aforementioned treatment (crop biomass treatment, CB). The rest of the plots also received one of the following treatments: (i) the addition of a commercial product for organic agriculture based on bacteria (biofertilizer), composed of Azospirillum brasilense (bacteria fixing N) and Pantoea dispersa (bacteria with a capacity for solubilising phosphates and stimulating plant growth) (biofertilizer treatment, B); (ii) the annual addition of 20t/ha of certified commercial organic compost (annual compost treatment, AC); (iii) the biennial addition of 20t/ha of certified commercial organic compost (biennial compost treatment, BC); and (iv) the sowing and subsequent incorporation of a mixture of 60% Avena sativa and 40% Vicia sativa (green manure treatment, GM). Each year the following indicators of ecosystem sustainability related to soil microbiological characteristics and carbon fixation were measured after harvest, in addition to plum yield: organic carbon (Corg), water soluble C, humic substances and humic acid C, microbial biomass C, respiration, ATP, dehydrogenase activity and hydrolase enzymes (alkaline phosphatase, urease and beta-glucosidase) activities. The two way ANOVA of the results indicated that all the above parameters were significantly affected (p≤0.001) by both, treatment and time. The interaction of treatment and time also significantly affected all of these parameters, except for yield and dehydrogenase activity. Composts not only produced better results than green manure cover crops and biofertilizer treatments in terms of plum yields, but they also brought about higher increases in the soil C pool and greater improvements in soil microbiological characteristics. In the six years of the study, compost-treated soils showed higher average plum yields and higher organic carbon, humic substance and humic acid contents than the rest of treatments. The size and activity of microbial population was also greater in the compost-treated soils, reflected in the higher values found in these soils for microbial biomass C, soil respiration, ATP (adenosine-tri-phosphate) content and dehydrogenase and hydrolases activity. Nevertheless, yields when compost was added biennially (BC) tended to be higher than in the annual compost treatment (AC) in most years as well as on average, although the differences were usually not significant. Yields in the sixth crop showed a high positive correlation with soil humic substances and humic C content (p≤0.01), as well as with Corg, microbial biomass C and phosphatase and urease activity (p≤0.05).