Cover crops and organic amendments to prevent nitrate contamination under a wet climate

The efficiency of a rye cover crop as a preventive measure for nitrate contamination in water resources was assessed in field tests. The study region (Galicia, NW Spain), which is severely affected by this environmental impact, has an annual average precipitation markedly exceeding the evapotranspiration level. Two additional corrective measures (viz. buried wheat and lupin green manure) were also studied in parallel. A randomized complete block experimental model spanning a period of three years was used to systematically determine the organic matter, total N, NO3- -N and NH4+-N contents, and the C/N ratio, in the 0-20 and 20-40 cm soil layers. Based on the significance of the different effects examined in the variance explanation model (viz. the repeated measurements general linear model) as evaluated using multivariate comparisons, the rye treatment proved more efficient than the other two corrective practices at controlling NO3- leaching from soil. By contrast, the application of the two plant residues to the soil exposed a priming effect induced by the simultaneous incorporation of mineral N. The suppression of the individual effects of some variability sources revealed a clear fertilizing effect in the green manure and net immobilization resulting from the incorporation of straw into the soil.