Saturday, 15 July 2006

Soil Influence on Nitrogen Availability for Sugar Beet.

Rosa Marchetti1, Enrico Biancardi2, Piergiorgio Stevanato2, and Gilda Ponzoni1. (1) Agricultural Research Council - Agronomical Research Institute, Modena Section, Viale Caduti in Guerra, 134, Modena, I-41100, Italy, (2) Agricultural Research Council - Experimental Institute for Industrial Crops, Rovigo Section, Via Amendola, 82, Rovigo, I-45100, Italy

Sugar beet (Beta vulgaris subsp. vulgaris L.) is highly affected by Nitrogen (N) fertilization: low N availability reduces root yield, whereas excess N is detrimental to the root technological quality. It is therefore important to correctly estimate the amount of fertilizer N needed by the crop. In Italy, the Italian Beet Growers Association recommends N rates ranging between 40 and 120 kg ha1. These recommendations are mainly based on the inorganic N amount that can be measured in the cultivated soil layer (i.e., in the 0.3-0.4-m top soil) in the autumn preceding the sugar beet seeding. A technological quality decline of the harvested roots, apparently induced by N excesses, has sometimes been observed in sugar beet crop areas of Northern Italy, without the supply of high fertilizer N amounts. The aim of this study was to verify if any accumulation of N below the cultivated soil layer could occur to explain this decrease in sugar beet technological quality. Soil profiles of most common soil types of the Po river alluvial plain (North-Eastern Italy) were sampled in fields cropped to sugar beet, in 0.25-m increments, to a depth of 3 m. In fact sugar beet roots may grow to different depths, depending on soil type and water availability, and root development even to 3-m soil depth was observed in these soils, in non-irrigated management conditions. Forty-five profiles were sampled in total, in May or June of the 2000-2003 period, in 32 locations. Soils of the study area are flat and derive from alluvial deposits of the Quaternary Era; they are frequently characterized by a seasonally fluctuating water table. The soil samples were analyzed for soil moisture, inorganic N (nitrate N + nitrous N + ammonium N), organic C, Kjeldahl N, Olsen P, carbonates, sand, silt and clay contents. Soil cation exchange capacity and pH were also determined. Silty-clay loam, silt loam, and loam were the prevailing soil textural classes. In the 45 profiles considered, the inorganic N content in the cultivated top soil (to 0.5-m soil depth) was on average equal to 12.9 mg kg1 dry soil. Nitrate-N content was higher in the cultivated layer, where it equaled on average 92.9% of the inorganic-N content. It became progressively lower for increasing soil depths. Ammonium-N content was also lower, below the cultivated soil layer. Nevertheless we measured an increase of ammonium N in the 2- to 3-m soil layer, and, in 9 soil profiles from 5 sampling locations, we found a very high ammonium-N content (even more than 100 mg N kg1 dry soil). A Principal Component Analysis was applied to the average value of the log-transformed analytical data, relevant to the samples collected between 2- and 3-m soil depth. The first 2 principal components explained 68% of the data set overall variance, and allowed us to group the sampling locations on the basis of the soil analytical description. The sampling locations with the highest ammonium-N content in the deeper soil layers were those having also the highest content of organic C, Kjeldahl N, and clay, the highest cation exchange capacity, and the lowest pH values. These analytical characteristics are often associated with organic matter-rich soils at low redox potential. As it is known that crops may take up ammonium N as easily as nitrate N, those among them that are endowed with root systems capable of reaching deep soil layers may find there high amounts of available ammonium N. As far as sugar beet is concerned, crop roots may reach the deepest soil layers just in the latest growth phase, when N excesses are more likely to compromise the root technological quality. Ammonium-N high levels being associated with soils with high organic matter content in the subsoil, the presence of deep organic layers in soils could be regarded as an index of N availability below the cultivated soil layer. This information may help the extension services in fine-tuning to site-specific situations the fertilizer N recommendations, which, at the moment, are chiefly based on N availability in the top soil-layer.

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