Nitrate leaching under maize cropping systems in Po Valley (Italy)

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Abstract

Intensive crop production in Po Valley (Northern Italy) is associated to high risk of nitrate leaching. A multi-year monitoring of soil solution nitrogen was conducted at 6 sites under the ordinary farm management of maize crop (Zea mays L.) in order to assess NO3–N leaching. The amount of N fertilizer (organic + mineral) varied from 209 to 801 kg N ha−1 year−1. Maize biomass ranged from 15 to 32 t ha−1 and N removal from 150 to 400 kg ha−1. Soil water solution was sampled at five depths along the soil profile (from 0.3 to 1.5 m) at time intervals of 7–30 days using suction cups. Soil water content (SWC) was measured daily by TDR at the same depths of suction cups. Soil water NO3–N concentrations varied from 0 to 110 mg L−1, with the highest concentrations measured after fertilizer application. Once validated on measured SWC data, SWAP model was applied to estimate the drainage flux. Annual leaching was calculated by multiplying drainage flux by soil water NO3–N concentration. N Leaching ranged from 14 to 321 kg ha−1 year−1, according to fertilization, crop N removal, rainfall, irrigation management, and it was mainly affected by N surplus.

Highlights

• A multi-year monitoring of NO3–N and soil water content were measured under maize cropping at 6 sites. • High soil nitrate concentrations were scored after fertilizer supply and ranged from 20 to 110 mg L−1. • Mean annual leaching varied from 14 to 321 kg ha−1 year−1. • High nitrate loss was primarily caused by N supply which largely exceeded crop demand. • The double cropping system of silage maize and a catch crop reduced up to 90% the N leaching.

Introduction

Europe with the Water Framework Directive (2000/60/EC, 2000) compels Member States to respect mandatory standards, such as the maximum permissible nitrate concentration in groundwater (50 mg L−1). It is therefore strictly important to monitor soil solution at field scale in order to assess the risk of nitrate pollution from agricultural lands. A in situ measurement of the soil solution NO3–N concentration is important to evaluate the actual nitrate leaching through unsaturated soil. A network of monitoring sites, at field scale, helps in the assessment of the impact on groundwater quality of different agronomic management of fertilization and irrigation. In fact, the nitrogen amount which exceeds plant demand may be lost as nitrate by water drainage, reaching the groundwater.

Lysimeters are often employed in the assessment of groundwater nitrate pollution (Prunty and Montgomery, 1991, Thomsen, 2005, Peu et al., 2007), although such approach can never fully reflect a full-scale field management (Trankler et al., 2005). Moreover, several other sources of errors can occur, mainly side wall flow (Corwin, 2000) and differences in drainage between lysimeter and field condition, due to the different bottom boundary conditions.

In the field, leaching is frequently computed by multiplying the nitrate concentration of soil water extracted by porous cups with estimated drainage water flux. Different authors evaluated the potential risk of nitrate pollution under controlled conditions by employing porous cups (Lord and Shepherd, 1993, Poss et al., 1995, Zotarelli et al., 2007). Hydrological dynamic simulation models or simple algorithms can be used to calculate drainage flux (Jackson, 2003, Askegaard et al., 2005, Gaur et al., 2006, Verbist et al., 2009).

With regard to nitrate leaching, several authors reported data collected throughout field trials, where primary experimental factors were soil type (Hack-Ten Broeke, 2001, Sibley et al., 2009), type of organic manure (Mantovi et al., 2006, Askegaard et al., 2005, Mirschel et al., 2007), cropping system (Booltink, 1995, Johnson et al., 1997, Mirschel et al., 2007), and irrigation (Zotarelli et al., 2009).

Beaudoin et al. (2005) quantified nitrogen leaching over 8 years below the rooting zone under different soil conditions, crop rotations and actual farming practices at 36 monitoring sites. They calculated N leaching by multiplying soil mineral nitrogen, measured in soil cores sampled at 1.2 m depth 3 times per year, and simulated drainage water. Nitrate concentration was mainly affected by water holding capacity of soil, ranging from 31 mg L−1 in deep loamy soils to 92 mg L−1 in shallow sandy soils. Mean calculated amount of leached nitrogen below the rooting depth was 8–45 kg NO3–N ha−1 year−1. Hack-Ten Broeke (2001) measured nitrate concentration over 4 years in sandy soil conditions in Netherlands under silage maize, grassland and Italian ryegrass. The measurements were carried out from porous cups at 1 m depth once a month and the mean annual nitrate concentration at 1 m depth was 67 mg L−1. Mantovi et al. (2006) measured soil nitrogen concentration in Po Valley by using ceramic cups under different levels of organic fertilization. They found nitrogen concentrations of up to 300 mg NO3–N L−1. In western Po Valley Grignani and Zavattaro (2000) studied mineral nitrogen concentration in soil solution that showed large temporal variations in the range of 1–150 mg NO3–N L−1.

Since nitrate losses occur in intensively cropped areas, then it is of primary interest to measure leaching and nitrate concentration in the unsaturated zone under ordinary practices of farmers. Field monitoring is particularly important in the case of intensive arable farming systems where large amount of nitrate may be drained to groundwater altering its quality. Po Valley (Northern Italy) is characterized by such high N input farming systems and accounts for 7 million livestock units (LSU, equal to 500 kg), and a density of about 1.7 LSU ha−1 of utilized agricultural area (UAA). Furthermore it has one of the largest aquifer in Europe and 67% of the UAA is defined as Nitrate Vulnerable Zone (ISTAT, 2010). The most diffused crops are grain and silage maize (Zea mays L.) being key crops of intensive agricultural systems (Grignani et al., 2007). Continuous maize cropping has a high potential risk of nitrate leaching particularly when a large supply of nitrogen and water is applied (Acutis et al., 2000).

The objective of this study was to quantify the nitrate leaching and its relation with ordinary management in 6 fields with contrasting pedoclimatic conditions under grain and silage maize over a period of 2–5 years in Po Valley.

The collected data sets are also suitable to be used in modelling application, being representative of the studied area.

Section snippets

Sites description

Experimental data sets were collected over a maximum of 5 years at 6 sites, mainly sown with maize: Caviaga (LO, province of Lodi, 45.31°N, 9.50°E, 72 m a.s.l.), Cerese (MN1 and MN2, province of Mantova, 45.12°N, 10.79°E, 20 m a.s.l.), Landriano (PV, province of Pavia, 45.28°N, 9.27°E, 84 m a.s.l.), Ghisalba (BG, province of Bergamo, 45.69°N, 9.75°E, 178 m a.s.l.), Luignano (CR, province of Cremona, 45.17°N, 9.9°E, 57 m a.s.l.), all located in Lombardy plain (Po Valley). The monitoring was conducted

NO3–N concentrations in suction cups

Mean, minimum and maximum concentrations of NO3–N in soil water solution (mg L−1) at different depths are reported in Table 3. Large differences in time and in depth of the NO3–N concentrations were recorded at the monitoring sites, with values from near zero up to 110 mg L−1. In Fig. 2 a contour plot (obtained by using Kriging with linear variogram model) of soil water solution NO3–N concentration over time through soil profile is shown at each monitoring site. High values were measured where N

Nitrogen surplus effect on nitrate leaching

The annual N surplus, defined as the difference between N fertilization and crop N removal (Grignani and Zavattaro, 2000), was compared to the estimated value of NO3–N leaching losses, year to year at each monitoring site. A significant linear regression (p < 0.01) resulted between N surplus and NO3–N leaching (Fig. 5). Since the slope of the regression line resulted equal to 0.87, then leaching appeared to be the main source of nitrogen loss of N that exceeded the crop demands. As an overall

Conclusion

At several sites in intensive agriculture lands of Po Valley, which lays on one of the largest European aquifer, concentration of nitrate in soil solution and leaching was measured at a field scale. The whole set of measurement highlighted an high risk of leaching with concentration that exceeded drinkability threshold of 50 mg L−1 of nitrate. In five out of six of the monitored sites the N supply strongly exceeded the crop demand. This large N supply involved high nitrate loss and appeared to be

Acknowledgements

This study forms part of the regional project ARMOSA, where soil water and nitrate dynamics are measured and analyzed under different cropping systems at monitoring sites in arable farms. Such project is currently ongoing and co-ordinated by the Department of Plant Production of the University of Milan, the Regional Agency for Agricultural and Forestry Development of Lombardy Region, the Institute for Mediterranean Agricultural and Forestry Systems – National Research Council of Ercolano,

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