Comparing the effectiveness of radish cover crop, oilseed rape volunteers and oilseed rape residues incorporation for reducing nitrate leaching

The soil water and N dynamics have been studied during two long fallow peri ods (between wheat or oilseed rape and a spring crop) in a field experiment in Chalons-en-Champagne (eastern France, 48 degrees 50' N, 2 degrees 15' E ). The experiment involved frequent measurements of soil water, soil minera l N, dry matter and N uptake by cover crops. Water and N budgets were estab lished using Ritchie's model for calculating evapotranspiration in cropped soils and a model (LIXIM) for calculating water drainage, N leaching and N mineralisation in bare soils. During the first autumn and winter, a radish cover crop (grown from September 1994 to January 1995) was compared to a ba re soil. During the second period (July 1995 to April 1996), a comparison w as carried out between (i) oilseed rape volunteers, (ii) bare soil with two types of oilseed rape residues incorporated into the soil (R0 and R270 res idues) and (iii) bare soil without residues incorporation. R0 and R270 resi dues came from two preceding oilseed rape crops which received two rates of N fertilizer (0 and 270 kg N ha(-1)). Soil mineral N content was markedly reduced by the presence of radish cover crop or oilseed rape volunteers during autumn. The calculated actual evapo transpiration (AET) did not differ much between treatments, meaning that th e transpiration by the cover crop or volunteers was relatively low (100-150 L kg(-1) of dry matter). Consequently, nitrate leaching was reduced during the rest of the winter and spring as well as nitrate concentration in the percolating water: 45 vs. 91 mg NO3- L-1 for radish cover crop and bare soi l, respectively. The incorporation of oilseed rape residues to soil also ex erted a beneficial but smaller action on reducing the nitrate content in th e soil. This effect was due to extra N immobilisation which reached a maxim um of about 20 kg N ha(-1) in mid-autumn for both types of residues. Nine m onths after the incorporation of the oilseed rape residues, and comparing t o the control soil without residues incorporation, N rich residues induced a significant positive N net effect (+ 9 kg N ha(-1)) corresponding to 10% of N added whereas for N poor residues no net effect was still obtained at the end of experiment (-3 kg N ha(-1), not significantly different from 0). To reduce nitrate leaching during long fallow periods, it is necessary to p romote techniques leading to decrease mineral-N contents in the soil during autumn before the drainage period, such as (i) residue incorporation after harvest (without fertiliser-N) and (ii) allowing volunteers to grow or sow ing a cover crop just after the harvest of the last main crop.