Nitrate Leaching after Organic Amendment under Soil Solarization in Two Different Soils and Its Effects on Yield and Health of Escarole and Pepper
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The use of high rates of manure application before solarization is an increasingly accepted practice by Spanish vegetable crops growers. Nevertheless the EU directive 91/676/CEE establishes a limit of 170 kg N/ha in the applied manure, to reduce nitrate leaching. This directive does not distinguish among different kind of soils, nor climates. In order to study the possible increase in nitrate leaching due to solarization combined with manure application, we set up an experiment on a greenhouse with a loamy-clay soil and also under tunnel with loamy sand soil at the IVIA Experiment Station of Carcaixent (Valencia, Spain). Every year, three manure application rates equivalent to 0, 170 and 340 kg N/ha were applied. Manure was incorporated to the soil and then solarisation was applied from the 1st of August to mid September. The statistical design was with complete blocks with three replicates. Soil temperatures were monitored during solarization at 10 and 20 cm depth. Fungicidal effect was determined by biological probes containing infested roots at 10, 20 and 30 cm depth. On 1st October an escarole crop was planted, and was grown until the end of December or beginning of January, followed by a pepper crop up to mid July. As the nitrate content in irrigation water was high (about 200 mg L-1) and soil levels of P2O5 and K2O were high, no mineral fertilizers were added during the study. In order to reduce the nitrate input in irrigation water, the crop was irrigated with rainwater whenever it was possible. Sclerotinia sclerotiorum was isolated every year from the escaroles plants whereas Meloidogyne incognita was isolated in pepper only on the third and forth year. In the loamy sand soil the healthier escaroles were frequently produced on the 340 kg/ha of N treatment. Generally there were not yield differences on pepper, except in 2008 when pepper plants suffered an attack by Meloidogyne incognita with a significant yield reduction in the check treatments with loamy sand soil. Check treatment, without manure, did not affect the incidence of Meloidogyne on pepper crop in the loamy sand soil, while higher rates of manure reduced gall number in roots. There were no clear differences in NO3-N content in soil (0-60 cm depth) throughout the experiment between treatments. However, soil nitrate levels in the loamy sand soil were much lower than in the loamy clay soil. No cumulative effects of treatments on soil nitrate levels in the 0-60 cm layer were found after 5 years, with a total N application of 340 kg/ha/year in the highest N rate treatment.