Nitrogen mineralization in sandy loam soils under an intensive double-cropping forage system with dairy-cattle slurry applications

Nitrogen (N) mineralization and soil mineral N contents were measured at 2-week intervals over a 2-year period (June 1994-May 1996) on two different sites in the North West region of Portugal. The experiment was established in fields, which had for many years been under a double-cropping forage system with maize from May to September and a winter crop (mixture of cereals and Italian ryegrass) during the rest of the year. In addition to N fertilizers, dairy-cattle slurry was applied regularly at the sowing of each crop. On this intensive forage system, quantification of N released from slurry, crop residues and soil organic matter becomes important when better N use efficiency and reduced environmental impact from agricultural practices are required. Net N mineralization rates of the 0-10 cm soil layer fluctuated considerably between consecutive incubation periods and ranged from - 0.88 to 1.87 mg N kg(-1) day(-1) with annual average rates of between 0.41 and 0.65 mg N kg(-1) day(-1). The total N mineralized in the 10 cm depth soil layer reached values between 122 and 224 kg N ha(-1) year(-1), showing that mineralization was a very important N source for the crops. The amounts of N released during the cold season (November-February) were equivalent to 27-48% of the yearly total. Regression analysis indicated that seasonal variation in N mineralization was only poorly explained by soil moisture and temperature. The changing balance during the year between soil moisture and temperature will contribute to the relatively constant N mineralization rates. Soil mineral N contents during the maize crop were high and exceeded the nutrient requirements for the optimum yield of this crop. Under the climatic conditions of the region and due to the poor development of the winter crop plants at the time, the mineral N left in the soil after the maize crop and released by mineralization during the cold season is particularly vulnerable to nitrate leaching losses. (C) 2001 Elsevier Science B.V. All rights reserved.