Sugar beet yield and processing quality in relation to nitrogen content and microbiological diversity of deep soil layer

The aim of this study was to estimate the effects of different soil factors on sugar beet yield and processing quality. The parameters investigated included: (i) nitrogen (N) availability; (ii) overall microbial diversity and (iii) abundance of bacterial genes involved in key functions of the N cycle. These traits have been evaluated throughout the entire soil layer (0-2.5 m) explored by the sugar beet root system. Soil samples were taken from two nearby sites (A and B) in the Eastern Po Valley, Italy. At each site, three soil profiles were sampled every 0.5 m and the main soil physical and chemical characteristics were evaluated. ARISA (Amplified Ribosomal Intergenic Spacer Analysis) and Real Time PCR analyses were performed on genomic DNA extracted from soil samples for the estimation of microbial diversity and the quantitative presence of genes for ammonium monooxygenase (amoA-Archaea) and nitrite reductase (nirK). To assess the root length density, observations were made by means of minirhizotrons. At site B profile, organic matter as high as 13 % and min-N around 28 mg kg(-1) were found between 2 and 2.5 m depth. Sugar beets harvested at this site showed less sugar content and processing quality than at site A (soil similar to B but without deep N accumulation). Soil collected below 0.5 m displayed a more species-rich composition of microbial communities than in the upper layer. The presence of amoA (nitrification) and nirK (denitrification) genes was found in all layers explored (down to a depth of 2.5 m). These findings highlight the contribution of the deep layers to key processes of the soil N cycle and its availability for deep-rooted crops. The usefulness of sampling soil to the depth reached by the roots to assess N fertilizer requirements is discussed.