Fertilisation of irrigated maize with pig slurry combined with mineral nitrogen

Abstract

Maize (Zea mays L.) is a very important crop in many of the irrigated areas of the Ebro Valley (NE Spain). Intensive pig (Sus scrofa domesticus) production is also an important economic activity in these areas, and the use of pig slurry (PS) as a fertiliser for maize is a common practise. From 2002 to 2005, we conducted a field trial with maize in which we compared the application of 0, 30 and 60 m³ ha⁻¹ of PS combined with 0, 100 and 200 kg ha⁻¹ of mineral N at sidedress. Yield, biomass and other related yield parameters differed from year to year and all of them were greatly influenced by soil NO₃⁻-N content before planting and by N (organic and/or mineral) fertilisation. All years average grain yield and biomass at maturity ranged from 9.3 and 18.9 Mg ha⁻¹ (0 PS, 0 mineral N) to 14.4 and 29.6 Mg ha⁻¹ (60 m³ ha⁻¹ of PS, 200 kg ha⁻¹of mineral N), respectively. Grain and total N biomass uptake average of the studied period ranged from 101 and 155 kg ha⁻¹ (0 PS, 0 mineral N) to 180 and 308 kg ha⁻¹ (60 m³ ha⁻¹ of PS, 200 kg ha⁻¹of mineral N), respectively. All years average soil NO₃⁻-N content before planting and after harvest were very high, and ranged from 138 and 75 kg ha⁻¹ (0 PS, 0 mineral N) to 367 and 457 kg ha⁻¹ (60 m³ ha⁻¹ of PS, 200 kg ha⁻¹of mineral N), respectively. The optimal N (organic and/or mineral) rate varied depending on the year and was influenced by the soil NO₃⁻-N content before planting. For this reason, soil NO₃⁻-N content before planting should be taken into account in order to improve N fertilisation recommendations. Moreover, the annual optimal N rates also gave the lowest soil NO₃⁻-N contents after harvest and the lowest N losses, as a consequence they also could be considered as the most environmentally friendly N rates.

Introduction

Intensive swine (Sus scrofa domesticus) production is an important economic activity in several European countries. Spain with 25 million heads is the second largest producer in the EU (Ministerio de Agricultura, Pesca y Alimentación, 2004) but more than 40% of its swine production is concentrated in the Ebro Valley (north-east of the country). Intensive swine rearing produces considerable amounts of pig slurry (PS) and its disposal represents an important environmental and economic problem.

Pig slurry in the Ebro Valley has been traditionally spread on the soil surface before sowing winter cereals or maize (Zea mays L.). However, PS is usually considered by farmers as a waste product rather than as a source of N and nutrients. For this reason, maize producers do not normally reduce mineral fertilisation when they apply PS, and total rates of N application (PS plus mineral N fertilisation) are usually higher than 400 kg N ha⁻¹ year⁻¹ (Sisquella et al., 2004).

Environmental problems caused by N over fertilisation have long been detected in many areas of the EU. Consequently, the EU prompted a directive of protection of waters against nitrate pollution (European Union, 1991). Concerning this directive, some areas of the Ebro Valley have been declared nitrate vulnerable areas. In this areas, no more than 350 kg N ha⁻¹ year⁻¹ can be applied, of which no more than 180–210 kg N ha⁻¹ year⁻¹ should derive from organic materials (Diari Oficial de la Generalitat de Catalunya, 1998, Diari Oficial de la Generalitat de Catalunya, 2004). In the case of PS, this amount is equivalent to approximately 30–60 m³ PS ha⁻¹ year⁻¹. Due to the great intensification of swine production, nitrate vulnerable areas are growing and becoming important in the Ebro Valley.

As it happens, in many irrigated areas of the Mediterranean, intensive maize production is one of the most important field crops. In the Ebro Valley, grain yields range from 11 to 15 Mg ha⁻¹ and plant nitrogen uptake is over 250–300 kg ha⁻¹. Nitrogen fertilisation is one of the most important factors influencing maize production and has an important effect upon profitability. Fertilising maize using PS, which is cheap and abundant in many areas, could offer an interesting way to reduce production costs and also help to solve the PS disposal problem (Buman, 1998).

In addition to adversely affecting agricultural sustainability and crop profitability, over fertilisation with N, from both mineral and organic sources, may have undesired side effects on other resources such as water, air and energy. Soil nitrate leaching may reduce water quality, while nitrous oxide released from the soil may contribute to the greenhouse effect (Blackmer, 1992). Moreover, a significant amount of the energy utilised by agriculture is employed to manufacture mineral N fertilisers (Blackmer, 1992). Consequently, the use of organic fertilisers derived from agricultural activities could also offer an alternative way to save energy.

Several studies conducted in different areas of the world have reported on the use of PS as a fertiliser with numerous field crops and different climatic and soil conditions. Some of these have shown that appropriate PS applications can help to achieve suitable grain yields when partially or completely substituting mineral fertilisers (Motavalli et al., 1993, Bocchi and Tano, 1994, Zebarth et al., 1996, Nevens and Reheul, 2005). However, only a few studies have been carried out under irrigated Mediterranean conditions characterised by high grain yields (Domingo and Bosch, 2001, Daudén and Quílez, 2004, Daudén et al., 2004). These previous studies tested PS rates that were higher than those stipulated by the EU directives. They utilised surface irrigation in which water losses and nitrate leaching were almost unavoidable due to the low efficiency and lack of uniformity of application (Pang et al., 1997, Spalding et al., 2001). As a consequence, to produce good grain yields surface irrigation may require higher rates of PS than more efficient irrigation systems (e.g. sprinkler irrigation).

The objectives of this study were to evaluate the effects, in high yielding sprinkler irrigated systems with continuous maize production, of applying two rates of PS (30 and 60 m³ ha⁻¹) and its combination with two mineral N fertilisation at sidedress (100 and 200 kg N ha⁻¹) on: (1) maize growth and yield, (2) grain and plant N uptake, (3) the evolution of soil NO₃⁻-N concentrations, and (4) the estimation of N losses. The studied rates were similar to those allowed by current EU directives because another main objective of this study was to obtain the optimal N rates (the minimum rates that produce maximum yields) and at the same time to fulfil the EU directives.