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520 Agronomy Journa l • Volume103 , I s sue2 • 2011

ReducedNitrogenFertilizationtoCornfollowingAlfalfainanIrrigatedSemiaridEnvironment

SebastiánCela,*MontserratSalmerón,RamónIsla,JoséCavero,FranciscaSantiveri,andJaimeLloveras

Published in Agron. J. 103:520–528 (2011)Published online 7 Feb 2011doi:10.2134/agronj2010.0402Copyright © 2011 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Alfalfa and corn are important crops grown in rota-tion in many areas of the world. The beneficial effects of

growing legumes in rotation with cereals have been recognized for centuries (Columella, circa 70) and include weed control, breaking up diseases cycles, improving soil organic matter, and providing considerable amounts of N to the subsequent cereal crop, among others (Bruulsema and Christie, 1987; Bullock, 1992; Danso and Papastylianou, 1992). In spite of this, farmers underestimate the potential N value of alfalfa and seldom reduce N fertilization rates to corn succeeding alfalfa (El-Hout and Blackmer, 1990). Surveys conducted under irrigated semiarid conditions in Spain revealed that farmers usually apply more than 300 kg ha−1 of N fertilizer to corn to maximize yields; and only one out of four farmers reduce N inputs to corn grown after alfalfa (Alvaro-Fuentes and Lloveras, 2003; Sisquella et al., 2004). However, as N prices increase, producers will look more closely at the N contribution of preceding legume crops like alfalfa.

Several studies evaluated N fertilization of first-year corn after alfalfa in the rainfed Midwest (Fox and Piekielek, 1988; El-Hout and Blackmer, 1990; Bundy and Andraski, 1993; Morris et al., 1993). Nevertheless, the management of alfalfa and corn

is different in the rainfed Midwest compared to the irrigated semiarid areas of southern Europe. First of all, annual rainfall amounts in the Midwest can be variable and can lead to different crop responses on wet or dry years (Morris et al., 1993). Second, while alfalfa is harvested three to four times per year in many rainfed areas of the United States (yielding 6–13 Mg ha−1 per year, Fox and Piekielek, 1988), it is cut five to seven times per year in the irrigated areas of the Ebro Valley in Spain (yielding 14–22 Mg ha−1 per year, Lloveras et al., 2008). Higher alfalfa production can lead to higher amounts of N2 fixation (Gault et al., 1995) and potentially more N mineralized from the alfalfa residues. Third, regarding corn, grain yields (10–17 Mg ha−1) and N requirements (200–400 kg N ha−1) in irrigated semiarid areas (Ferrer et al., 2003; Cavero et al., 2008; Berenguer et al., 2009) are on aver-age higher than those reported in previous studies conducted in the rainfed Midwest. Finally, there are significant differences in soil types between the Midwest (predominantly Mollisols and Alfisols) and southern Europe (predominantly Inceptisols and Entisols). Consequently, the optimum N rates to corn after alfalfa under irrigated semiarid conditions may be different than those reported in literature. To our knowledge, there are few studies of this kind in irrigated semiarid conditions (Carter et al., 1991; Ball-esta and Lloveras, 2010). Carter et al. (1991), working in furrow-irrigated lands in Idaho, reported that N requirements of first-year corn following alfalfa (without tillage) could be met by decomposi-tion of alfalfa roots. In turn, Ballesta and Lloveras (2010), working in flood-irrigated lands in the Ebro Valley, compared continu-ous corn with corn after only 2 yr of alfalfa. However, alfalfa in irrigated areas can produce high forage yields during 4 to 5 yr, and alfalfa stands usually are kept that long before rotating to corn.

The method of irrigation can influence corn response to N fertilization (Schepers et al., 1995; Pang et al., 1997; Diez et al.,

ABSTRACTNitrogen fertilization of corn (Zea mays L.) following alfalfa (Medicago sativa L.) can be reduced due to the considerable amounts of N released by this legume, but most research on this subject has been conducted in rainfed areas. The objective of this study was to determine the response of corn to N fertilization following high-yielding alfalfa in irrigated semiarid conditions. Seven field experi-ments were conducted between 2006 and 2008 in the Ebro Valley, Northeast Spain. Fields were sprinkler, furrow, or flood irrigated. Treatments comprised six N rates (0, 50, 100, 150, 200, and 300 kg N ha−1) applied to corn as two sidedressings. The N contained in the alfalfa roots and crowns ranged from 54 to 212 kg N ha−1, depending on the field. Alfalfa provided enough N to the subsequent corn to achieve from 10.3 to 16.7 Mg ha−1 of grain yield without N fertilization. Nitrogen fertilization was not required to maximize corn yields in three of seven fields. In the other four sites, the optimum nitrogen rates (ONR) that maximized corn yields ranged from 115 to 196 kg N ha−1. The highest ONR generally were observed in flood-irrigated fields than in sprinkler-irrigated fields. Net economic return from N fertilization was maximized at N rates between 0 and 150 kg N ha−1, suggesting that N applications for corn succeeding alfalfa can be greatly reduced compared to rates normally applied in irrigated semiarid areas (300 kg N ha−1).

S. Cela, F. Santiveri, and J. Lloveras, Dep. of Crop and Forest Sciences, Univ. of Lleida (UdL). Av. Rovira Roure, 191, 25198 Lleida, Spain; M. Salmerón and R. Isla, Soils and Irrigation Dep. (EEAD-CSIC Associated Unit), Agrifood Research and Technology Centre of Aragón (CITA), Aragón Government, Avda. Montañana 930, 50059 Zaragoza, Spain; J. Cavero, Dep. Suelo y Agua, Estación Experimental de Aula Dei (CSIC), Avda. Montañana 1005, 50059 Zaragoza, Spain. Received 24 Sept. 2010. *Corresponding author (sebastian.cela@pvcf.udl.cat).

Abbreviations: ONR, optimum nitrogen fertilization rate; PSNT0–30cm, pre-sidedress nitrate test (0–30 cm); SMN, soil mineral nitrate nitrogen.

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2000). Three irrigation systems (flood, sprinkler, and furrow irrigation) are present in the Ebro valley, the first two systems being the most common ones. Flood irrigation systems were built during late 19th–early 20th century and, in general, result in low uniformity of irrigation and low irrigation efficiency (Lecina et al., 2005). On the other hand, sprinkler irrigation systems are modern and usually have high irrigation efficien-cies (Power et al., 2000; Cavero et al., 2003). The main factors that affect the irrigation efficiency and consequently could affect corn response to N fertilization are the irrigation depth applied in each irrigation event (on average higher under flood than under sprinkler irrigation) and the frequency of irrigation (every 7–15 d under flood and furrow irrigation, depending on the irrigation channel, and every 1–4 d under sprinkler irrigation). Sprinkler irrigation has been associated with low N leaching losses, whereas flood irrigation has been associated with high N leaching losses (Causapé et al., 2006).

A better knowledge of the response of corn to N fertilization after alfalfa in irrigated semiarid conditions may help reduce N fertilizer inputs, improve the profitability of the crop, and minimize the environmental impact of surface and groundwa-ter pollution with nitrate. The main objective of this research was to establish the optimum N fertilization rate to corn after a 4- to 5-yr-old alfalfa crop under irrigated semiarid conditions.

MATERIALS AND METHODSSeven field experiments were conducted in the Ebro Valley,

Northeast Spain, between 2006 and 2008. This region has a semiarid climate characterized by low precipitations and high summer temperatures. Irrigation is needed to grow corn. The fields included in this study were Almacelles (ALM), Binefar (BIN), Castejon del Puente (CAS), El Tarros (ELT), Gimenells (GIM), La Tallada d’Emporda (LAT), and Zaragoza (ZAR). Table 1 summarizes the main characteristics of each field.

Corn was always planted following alfalfa in rotation. The previous alfalfa crops were 4- to 5-yr old, presented a good plant stand, and were plowed in the fall before corn sowing. Corn was seeded between late March and early May at a density of 8 to 8.7 plants m−2. The hybrids used are listed in Table 1. The experi-mental design was similar for all experiments and consisted of a randomized complete block design with three or four replicates (Table 1). The treatments tested were six N rates (0, 50, 100, 150, 200, and 300 kg N ha−1) applied to corn as ammonium nitrate (33.5% N). The plots were hand fertilized in two equal sidedress applications (V3–V6 and V5–V12 growth stages), with no N starter application at planting. Plot size ranged from 56 m2 (5.6 by 10 m) to 151 m2 (5.6 by 27 m), depending on the field. Mineral P and K fertilization was applied before corn planting at rates of 60 and 200 kg ha−1, respectively, to avoid any crop deficiency. Crop management (except N, P, and K fertilization) was done by the farmers following their conventional practices. The irrigation systems varied with the fields and are listed in Table 1. The amounts and the frequency of irrigation water were managed by each farmer following the typical schedule of the region (Table 1). Sprinkler irrigation system used was solid-set in all fields, so the application rate (mm hr−1) was known. With the application rate and the number of irrigation hours provided by farmers, we estimated the volume of water applied at each irriga-tion event. In the ZAR location, the volume of irrigation applied

was measured with an electromagnetic flow meter (Promag 50, Endress+Hauser, Reinach, Switzerland). In flood and furrow irrigated fields, the amount of irrigation applied in each irriga-tion event depends on the field characteristics (leveling, size) and type of soil, so we estimated the irrigation water applied by measuring the irrigation time and the channel flow in each field.

Soil and Plant Analysis

The number of residual alfalfa roots (roots plus crowns, with-out any regrowth or live vegetative tissue) and their biomass in the upper 30 cm of the soil was determined in 0.36-m2 samples replicated eight times per field after chisel and disk plowing. The alfalfa roots were collected and oven-dried at 60°C for 48 h to determine the dry weight. The dry roots were ground (1-mm sieve) and the N content was measured by the dry combustion method with a CN analyzer (TruSpec CN, LECO, St. Joseph, MI, USA). The total amount of N in alfalfa roots in the upper 30 cm of the soil (plowdown N yield) was calculated as the product of alfalfa root biomass by the N concentration in alfalfa roots.

Corn grain yield was measured in late September to early October at physiological maturity by harvesting the two central rows of each plot (representing 15–38 m2 depending on the field, Table 1). Grain moisture was determined for each plot from a 300-g sample, and grain yield was adjusted to 14% moisture con-tent. Relative grain yield was calculated for each field as the ratio between corn yield in a given plot and the average yield of the 300 kg N ha−1 treatment in the same field. Corn aboveground biomass (plant biomass) was determined at harvest by cutting the plants in 4 m of a central row from each plot. Moisture content was determined by chopping three entire plants per plot and oven-drying them at 60°C for 48 h. The dry plants were ground and the N content was determined by dry combustion at CAS and ZAR, and by near infrared (NIR) spectroscopy, using a previously calibrated 500 Infrared Analyser (Bran+Luebbe, Nor-derstedt, Germany) at the other locations. Whole plant-N uptake (plant plus grain, without roots) was calculated by multiplying corn aboveground biomass by its N concentration.

Soil mineral nitrate nitrogen content (SMN) was determined in each field before corn planting by taking eight soil samples to a depth of 90 cm in 30-cm increments. The soil was also sampled for presidedress nitrate test (PSNT0–30cm) determination just before the first sidedress N application. The soil was sampled by taking three cores per plot in the 0- to 30-cm layer in the central part of each plot. The amount of N available to corn in the root-ing zone before the active N uptake period was calculated as the sum of PSNT0–30cm and the N fertilizer applied to corn (Ferrer et al., 2003). Residual SMN was determined after corn harvest in each plot of each location by extracting three cores per plot to a depth of 90 cm in 30-cm increments. At the CAS and ZAR fields, the soil was fresh sieved to pass a 2-mm sieve, 10 g were extracted with 2 M KCl, and nitrate and ammonium concentra-tions were determined colorimetrically with a continuous flow analyzer (AA3, Bran+Luebbe, Norderstedt, Germany). At the rest of the fields, soil nitrate was extracted using deionized water and was measured using test strips with a Nitrachek device (KPG Products Ltd., Hove, East Sussex, UK) calibrated according to the standard procedure (Bischoff et al., 1996). Concentration of ammonium in the soil was measured at the beginning of the experiment (Table 1) and was considered negligible compared

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with nitrate concentrations, in line with previous studies con-ducted in the area (Villar-Mir et al., 2002).

Net returns from N fertilization were calculated for four scenarios, including two corn grain prices (120 and 240 € Mg−1 grain) and two N fertilizer prices (650 and 1300 € Mg−1 N) by the following equation:

Net return (€ ha−1) = (yield Ti – yield T0) × grain price – (N applied Ti × N price) [1]

where yield is corn grain yield in treatment i (Ti) and at the 0 N rate (T0), and N applied Ti is N fertilizer applied to corn in treatment i.

Nitrogen balance was calculated for each plot at each location. Mineralization (Nmin) was estimated from the

unfertilized treatment by applying the equation Nmin = residual SMN + plant (aboveground biomass) N uptake – initial SMN (Sexton et al., 1996) and assuming that N losses from unfertilized plots were unimportant and that fertilizer N addition did not alter soil N mineralization. Nitrogen losses (N lost) from the soil profile during the corn growth period were estimated from the N balance for the fertilized plots (Angás et al., 2006; Berenguer et al., 2009) by the following expression:

N lost = residual SMN + N uptake − initial SMN − Nmin − N fertilizer [2]

which considers N lost as the sum of N lost by leach-ing, volatilization, denitrification, and unaccounted-for

Table 1. Fields, soils, and crops information for the seven trials. Values of variables between brackets are the standard error.

Abbreviation ALM BIN CAS ELT GIM LAT ZARField Almacelles Binefar Castejon del Puente El Tarros Gimenells La Tallada d’Emporda ZaragozaYear 2007 2008 2007 2008 2006 2008 2008

Latitude 41°43´N 41°51´N 41°57´N 41°42´N 41°39´N 42°40´N 41°43´NLongitude 0°26´E 0°17´E 0°09´E 1°10´E 0°23´E 3°30´E 0°48´WAltitude,m 247 286 330 275 260 20 225Meantemperature,°C 14.1 13.7 13.6 13.4 13.8 14.8 13.9Annualrainfall,mm 364 457 243 330 364 651 444

IrrigationTypeofirrigation Sprinkler Sprinkler Flood Flood Flood Furrow SprinklerAmount,mm 650 600 1200 1000 600 500 870Frequency,d 2–4 1–2 7–14 14 14 7–9 3–4

Soils (at planting)Type Typic Typic Typic Typic Petrocalcic Xerofluvent Typic

Xerochrept Xerofluvent Xerofluvent Calcixerept Calcixerept Oxiaquic XerofluventDepth,cm

>90 >90 >90 >90 <60 >90 >120

Texturalclass loam loam loam siltloam loam sandyloam siltloampH(0–30cm)

7.8 8.4 8.3 8.2 8.3 8.3 8.4

OM(gkg–1,0–30cm)

26.0 22.2 22.2 22.5 28.0 16.8 18.9

NO3–N(kgha–1,0–90cm)

197(18) 130(21) 202(22) 187(26) 134(23) 189(13) 67(6)

NH4–N(kgha–1,0–30cm)

9 8 4 5 11 5 8

P†(mgkg–1,0–30cm)

75 23 5 8 35 15 20

K‡(mgkg–1,0–30cm)

207 104 76 112 162 65 193

PSNT(kgha–1,0–30cm)

210(11) 73(3) 160(7) 77(3) 143(5) 154(6) 40(2)

CornCultivar PR33P67 PR34N43 PR34N43 Mitic Helen PR33B51 PR34N43FAOCycle 600 500 500 600 700 600 500Numberofreplicates 4 4 3 3 4 4 3Areaharvested,m2 38 38 15 38 38 38 15

AlfalfaAge,yr 4 5 5 5 4 5 4Plantdensity,plantsm–2 60(6) 19(3) n/a§ 60(5) 39(4) 48(5) n/aRootN,gkg–1 23.8(1.0) 26.3(0.8) n/a 26.5(0.6) 23.4(0.5) 26.3(0.8) n/aRootN,kgNha–1 201(25) 54(10) n/a 212(27) 69(7) 71(12) n/a†P:Olsenmethod.‡K:Ammoniumacetatemethod.§n/a:notapplicable.

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processes. A negative value of N lost indicates a N loss from the soil–plant system.

Statistical Analysis

Regression analysis was used to evaluate the response of the variables measured to the N rates applied (Jandel Scientific, 1992). The regression analysis was performed at each field and a linear

response plus plateau model (LRP) was used for the corn grain variable. The threshold value at which grain yield is maximized was defined as the ONR. A LRP model was also fitted to the response of relative corn yield to available N (PSNT0–30cm+N fertilizer applied), and simple linear regressions were fitted to the response of corn N uptake, residual SMN, and N losses to available N.

Fig. 1. Relationship between the corn grain yield and the N applied as fertilizer in the seven fields. The linear response plus plateau model and the optimum nitrogen rate (ONR, kg N ha–1) was presented when the model was significant (*p < 0.05; **p < 0.01). The fields were Almacelles (ALM), Binefar (BIN), Zaragoza (ZAR), La Tallada d’Emporda (LAT), Castejon del Puente (CAS), El Tarros (ELT), and Gimenells (GIM). Bars represent the standard error.

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RESULTSPrevious Alfalfa Crop

The total N contained in the 4- to 5-yr-old alfalfa roots present in the upper 30 cm of the soil ranged from 54 to 212 kg N ha−1, depending on the field (Table 1). Because mean N concentration of alfalfa roots varied by only 12% among fields, the differences in total N in alfalfa roots were mainly related to differences in alfalfa stand density (from 19–60 plants m−2) and in average alfalfa-root weight (from 5.6–13.3 g root−1) (Table 1).

Corn Yield and Net Economic Return Depending on Nitrogen Rate

Corn grain yields ranged from 10.3 to 16.8 Mg ha−1, depend-ing on the field and the N rate applied (Fig. 1). At three fields (ALM, ZAR, and LAT) corn yield was not affected by the N rate. However, at the other four fields (BIN, CAS, ELT, and GIM) there was a significant response of grain yield as the N applied increased, with ONR ranging from 115 to 196 kg N ha−1 (Fig. 1). Alfalfa provided enough N to produce from 10.3 to 16.7 Mg ha−1 of corn grain without additional fertilizer N. Grain yield responded to the N fertilizer rate in all flood irrigated trials, but responded to the N fertilizer applied in only one of three sprin-kler irrigated trials. The ONR that maximized yields appeared to be higher under flood irrigation systems (118–196 kg N ha−1) than under sprinkler irrigation systems (0–115 kg N ha−1).

Net economic returns from N fertilization of corn following alfalfa were calculated for four scenarios, including two corn grain prices (120 and 240 € Mg−1 grain) and two N fertilizer prices (650 and 1300 € Mg−1 N). Net returns varied depending on the field and the grain/N price scenario (Fig. 2). In general, the most profit-able N rate was lower than 150 kg N ha−1, independent of the price scenario. In sprinkler-irrigated fields, N rates higher than 50 kg N ha−1 resulted in net economic losses, except for the scenario of high grain price and low N price at BIN and ZAR. In contrast, the most profitable N rate at three flood-irrigated fields was between 100 and 150 kg N ha−1, depending on the price scenario.

Corn N uptake varied between 150 and 457 kg N ha−1 and was significantly related to grain yield (R2 = 0.55, p < 0.001). Corn N uptake increased with increasing N fertilization rates at four out of seven locations, three of which were flood irrigated (Fig. 3).

Soil Nitrogen Dynamics

The SMN0–90cm before corn planting varied among fields, rang-ing from 67 kg N ha−1 at ZAR to 202 kg N ha−1 at CAS (Table 1), and averaged 158 kg N ha−1. The average initial SMN of the three sprinkler-irrigated fields (131 kg N ha−1) was slightly lower than the SMN of the three flood-irrigated fields (174 kg N ha−1). The PSNT0–30cm ranged from 40 kg N ha−1 at ZAR to 210 kg N ha−1 at ALM (Table 1). The average PSNT0–30cm of the three sprinkler irrigated fields (108 kg N ha−1) was similar to the average of the three flood-irrigated fields (127 kg N ha−1). Residual SMN0–90cm after corn harvest increased with increasing N fertilization at five out of seven locations (Fig. 3), and the two locations where residual SMN did not increase to applied N were both flood-irrigated. In general, the N lost estimated by the balance method increased with N-fertilization rates at all locations except at BIN (Fig. 3). In the sprinkler-irrigated fields, fertilizer N rates lower than 150 kg N ha−1 resulted in negligible N losses (Fig. 3).

Pooling data from different sites, there was not a clear relationship between the relative corn yield in the unfertil-ized plots and the PSNT0–30cm (Fig. 4). When no N fertilizer was applied, the relative corn grain yield at each field was at least 92% of maximum in the three sprinkler-irrigated fields and the furrow irrigated field, but it was lower than 90% of maximum in one of the flood irrigated fields and lower than 80% of maximum in the other two flood irrigated fields (Fig. 4). However, a clear trend was observed when available N at sidedress (PSNT0–30cm plus N fertilizer applied) was taken into account. Relative corn yield significantly increased as avail-able N levels increased in the three flood irrigated fields, with threshold values that maximized grain yield ranging from 234 to 319 kg N ha−1 (Fig. 5). On the contrary, there was a small or no response of relative corn yield to increases in available N levels in the sprinkler-irrigated fields, and the threshold values were lower than 185 kg N ha−1 (Fig. 5). This difference likely reflects higher N losses under flood than sprinkler irrigation.

DISCUSSIONNitrogen Content in Alfalfa Residues

The alfalfa roots were collected from the 0- to 30-cm soil depth, and deeper root residues were not recovered. For this reason, the plowdown N yield of the 4- to 5-yr-old alfalfa roots underestimates the total amount of N incorporated with alfalfa residues. The N yields of alfalfa roots in this study were similar to or higher than those published by Bruulsema and Christie (1987), who reported 66 kg N ha−1 contained in 1-yr-old alfalfa roots (0–15-cm depth), by Justes et al. (2001), who reported from 80 to 100 kg N ha−1 contained in 2-yr-old alfalfa roots (0–20-cm depth) and than those calculated from data of Heichel et al. (1984) (<53 kg N ha−1). The N concentration in alfalfa roots and crowns (≈25 g kg−1) was higher than the concentration reported by Justes et al. (2001) (15.9 g kg−1) and by Heichel et al. (1984) (≈from 11–19 g kg−1) and could be considered a good estimation, at least for our area, because it was similar among fields.

Corn Response to Nitrogen Fertilization after Alfalfa

The corn grain yields found in our experiments were normal for irrigated semiarid conditions of the Ebro valley, which depend on the soil and the irrigation system (Ferrer et al., 2003, Cavero et al., 2008; Berenguer et al., 2009). Yields in this study were higher than those reported in previous studies of corn succeeding alfalfa under rainfed conditions in the Mid-west (about 10–12 Mg ha−1; Fox and Piekielek 1988; El-Hout and Blackmer, 1990, Bundy and Andraski, 1993, Morris et al., 1993; Stranger and Lauer, 2008). The high corn yields observed in the unfertilized plots (from 10.3–16.7 Mg ha−1) could be partially explained by the N provided by mineralization of alfalfa residues, considering that up to 70% of the plowdown-N can mineralize during the first year after alfalfa (Fox and Piekielek, 1988). In agreement with this, Kurtz et al. (1984) reported that the fertilizer N recommendations of corn follow-ing alfalfa could be reduced up to 180 kg N ha−1 depending on the quality (plant density) of the previous alfalfa crop.

Previous studies conducted in rainfed conditions in the United States (Fox and Piekielek, 1988; Bundy and Andraski,

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1993; Morris et al., 1993; Lawrence et al., 2008; Stranger and Lauer, 2008) found either no response of first-year corn yield to N fertilization succeeding alfalfa or found low ONR (≈25–50 kg ha−1). Although we did not find response of grain yield to N fertilizer applied after alfalfa in three out of seven fields, we observed higher ONR in the other four fields (between 115 and 196 kg N ha−1) compared to those reported in the rainfed Midwest. This fact could be first explained by the higher grain yields and N uptake of corn in irrigated semiarid conditions. Moreover, depending on the irrigation management, irriga-tion can promote higher N losses and lower N use efficiency compared to rainfed conditions.

The irrigation system appears to have affected the response of corn to N fertilization. Thus, N application was needed to maximize corn yields after alfalfa in all the flood-irrigated

fields, but N fertilization only slightly increased corn yields in one out of three sprinkler-irrigated fields. In line with this, a higher amount of available N in the soil at sidedress (PSNT plus N fertilizer applied) was required to maximize relative corn yields in flood-irrigated fields (234–319 kg N ha−1) than in sprinkler-irrigated fields (<185 kg N ha−1) (Fig. 5). The higher corn response to N fertilization under flood irrigation than under sprinkler irrigation may be explained by the higher estimated N losses in flood-irrigated fields than in sprinkler-irrigated fields, especially at low N rates (50–100 kg N ha−1) (Fig. 3). Larger amounts of water were applied during the corn growing season at each irrigation event with flood irrigation than with sprinklers, which would help to explain the higher N losses under flood-irrigation than under sprinkler irrigation. The irrigation water depth applied was representative of the

Fig. 2. Net return from N fertilization for the different fields and N rates. Two extreme grain prices (120 and 240 € Mg–1) and two extreme N prices (650 and 1300 € Mg–1) were combined into four price scenarios. The fields were Almacelles (ALM), Binefar (BIN), Zaragoza (ZAR), La Tallada d’Emporda (LAT), Castejon del Puente (CAS), El Tarros (ELT), and Gimenells (GIM). Net return (€ ha–1) = (yield Ti– yield T0) × grain price– (N applied Ti × N price). Yield is corn grain yield in treatment i (Ti) and at the 0 N rate (T0), and N applied Ti is N fertilizer applied to corn in treatment i.

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amounts normally applied to corn by farmers in the irrigated areas of the Ebro valley (550–750 mm under sprinkler irriga-tion, and 800–1200 mm under flood irrigation) (Cavero et al., 2003; Dechmi et al., 2003; Lecina et al., 2005). In gen-eral, lower N losses have been found with sprinkler irrigation compared to flood irrigation (Power et al., 2000; Cavero et al., 2003; Causapé et al., 2006).

In the rainfed areas of the Midwest, the critical PSNT0–30cm that separates N-responsive from N nonresponsive fields has been established at 21 to 30 mg N kg−1 (≈90–125 kg N ha−1 assuming a soil bulk density of 1.4 g cm−3) in monoculture corn (Bundy and Andraski, 1993, Blackmer et al., 1989, El-Hout and Blackmer, 1990) and at 14 to 21 mg N kg−1 (≈60–90 kg N ha−1) in corn after alfalfa (El-Hout and Blackmer, 1990, Morris et al., 1993). We have insufficient data to establish a

Fig. 3. Corn N uptake (black symbols), residual soil mineral nitrate nitrogen content (SMN) at corn harvest (gray symbols), and estimated N lost (white symbols) for the different fields and N rates applied to corn after alfalfa. Linear equations were fitted to the data sets, slope units are kg kg–1 applied N. * and ** indicate significant effects of N applied at 0.05 and 0.01 levels. The fields were Almacelles (ALM), Binefar (BIN), Zaragoza (ZAR), La Tallada d’Emporda (LAT), Castejon del Puente (CAS), El Tarros (ELT), and Gimenells (GIM). Bars represent the standard error.

Fig. 4. Relationship between relative corn yield and pre-sidedress nitrate test (PSNT0–30cm) for the seven fields in the plots that received no N fertilization. White symbols correspond to flood-irrigated fields, black symbols correspond to sprinkler-irrigated fields and the gray symbol corresponds to the furrow irrigated field.

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threshold value to separate N responsive from N nonresponsive fields under irrigated semiarid conditions. However, our results suggest that the PSNT0–30cm values required to maximize relative corn yield would be higher under flood irrigation than under sprinkler irrigation (Fig. 4). Further research is needed to obtain accurate estimations of critical N levels for maximum corn yields following alfalfa under different irrigation systems.

Despite the large differences observed in the required PSNT, the maximum economic benefit was attained with N rates ranging from 0 to 150 kg N ha−1 regardless of the irriga-tion system and price scenario. Bearing in mind that most corn producers in irrigated areas of Spain apply more than 300 kg N ha−1 to corn following alfalfa (Sisquella et al., 2004; Cavero et al., 2003; Isidoro et al., 2006), our estimates indicate that N losses range up to 240 kg N ha−1 at this N rate (Fig. 3). Nitrogen fertilization of first-year corn succeeding alfalfa can and, based on our data, should be greatly reduced, especially

under sprinkler-irrigated systems, with low risks of yield penal-ties and economic losses.

CONCLUSIONSThe plowdown N yield of 4- to 5-yr-old alfalfa roots ranged

between 54 and 212 kg N ha−1 depending on the field. The previous alfalfa crops provided enough N to achieve from 10.3 to 16.7 Mg ha−1 of corn grain without any N application. In three out of seven fields, maximum corn yields following alfalfa were obtained in unfertilized plots. In the other four fields, the ONR that maximized corn yields following alfalfa were comprised between 115 and 196 kg N ha−1 and were higher than the ONR reported for corn following alfalfa in rainfed conditions. The irrigation system seems to have influenced corn response to N fertilization following alfalfa, given the lower ONR observed under sprinkler irrigation (0–115 kg N ha−1) than under flood irrigation (118–196 kg N ha−1). Net

Fig. 5. Relationship between the amount of available N (pre-sidedress soil NO3–N at 0 to 30 cm plus N fertilizer applied) and the relative corn grain yield for the seven fields. The fields were Almacelles (ALM), Binefar (BIN), Zaragoza (ZAR), La Tallada d’Emporda (LAT), Castejon del Puente (CAS), El Tarros (ELT), and Gimenells (GIM). Bars represent the standard error.

528 Agronomy Journa l • Volume103, Issue2 • 2011

economic return from N fertilization was always maximized with N rates comprised between 0 and 150 kg N ha−1, inde-pendently of the grain/N price ratio. Since most farmers in the study area fertilize corn with more than 300 kg N ha−1, this work shows that N rates applied to first-year corn after alfalfa could be greatly reduced in irrigated semiarid conditions, espe-cially in sprinkler irrigated fields. Further research is needed to accurately establish critical PSNT that separate responsive from nonresponsive sites of corn after alfalfa under different irrigation systems.

ACKNOWLEDGMENTS

To the personnel of the Laboratori de Cultius Extensius of the University of Lleida and the personnel of Soils and Irrigation Department of C.I.T.A. for their field and laboratory assistance. We would also like to thank the personnel of the Cooperatives of Almacelles, Binéfar, Transalfals-La-Vispesa, Gimenells, and the IRTA Lleida and the IRTA Mas Badia for their helpful assistance. This study was financed by the Spanish Ministry of Science and Innovation (Project no. AGL2005-08020-05 AGR) and by the European Project Qualiwater (INCO no. 015031).

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