Resumen de Pig slurry and mineral nitrogen fertilisation ob maize under irrigated mediterranean conditions: effects on yield and the environment
Maize (Zea mays L.) is one of the most important field crops in the irrigated areas of the Ebro Valley (NE Spain) and its profitability is greatly influenced by N fertilisation. Fifty per cent of the maize crop land is fertilised only with mineral N fertiliser. In the rest of the area, manure - mainly pig slurry (PS) - is applied before maize sowing and mineral N fertiliser is applied at sidedress. Intensive pig (Sus scrofa domesticus) production is also an important economic activity in this area which generates important amounts of PS. As farmers usually apply N rates that are higher than maize requirements, pollution due to N leaching has appeared and some areas of the Ebro Valley, which have been declared nitrate vulnerable. In addition to nitrate pollution, continuous PS fertilisation could also cause soil contamination by Cu and Zn accumulation.
In order to investigate ways of improving maize N fertilisation, two field trials were conducted from 2002 to 2005 and continued for soil Cu and Zn determination from 2006 and 2007. The first trial applied six different rates of mineral N (0, 100, 150, 200, 250 and 300 kg N ha-1). The second trial involved three PS rates (0, 30 and 60 m3 ha-1) combined with three rates of mineral N (0, 100 and 200 kg N ha-1).
In both the PS and mineral N fertilisation trials, the grain yield response to N fertilisation varied considerably from year to year and was influenced by initial soil N content and by the rate of N fertilisation (organic or mineral). When establishing N fertilization recommendations, initial soil N content should always be taken into account.
In the N fertilisation trial, optimal N rates for obtaining maximum grain yields never exceeded 160 kg ha-1 during the 4 years of the study. The amount of N available to the crop (N applied with fertilisation plus initial soil N content) required to achieve maximum grain yields was 258 kg N ha-1. In the trial combining PS with mineral N fertilisation, in all years it was possible to achieve satisfactory grain yields using only PS (60 m3ha-1) or using 30 m3 ha-1 of PS combined with 100 kg ha -1 of mineral N.
N leaching in PS treatments was lower or similar than mineral N fertilised plots, suggesting that is possible fertilise maize with PS with less or equal N leaching risk than mineral N fertilisers.
After 6 years of continuous PS applications, there was an observed increase in both soil Cu and Zn, yet plant Cu and Zn concentrations did not increase with PS applications. Soil contamination by Cu and Zn applied by PS seems not to represent a significant hazard for the near future, but it needs to be taken into account for the more distant future.
Optimal N rates for maize production seemed to coincide with minimum environmental impacts and higher N efficiencies, suggesting the possibility of achieving maximum grain yields with minimum environmental risk.
