Evaluating Nitrogen Use Efficiency (NUE) Indices on the Background of Mineral Status of the Seed Crop at Maturity: a Case Study of Maize

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1 Pol. J. Environ. Stud. Vol. 25, No. 5 (2016), DOI: /pjoes/61817 Originl Reserch Evluting Nitrogen Use Efficiency (NUE) Indices on the Bckground of Minerl Sttus of the Seed Crop t Mturity: Cse Study of Mize Witold Szczepnik* Deprtment of Agriculturl Chemistry nd Environmentl Biogeochemistry, Poznn University of Life Sciences, Wojsk Polskiego 71F, Poznn, Polnd Received: 22 September 2015 Accepted: 16 Februry 2016 Abstrct The first step in ssessing nitrogen fertilizer s impct on the environment relies on its productivity. Severl series of indices hve been developed to describe nitrogen use efficiency (NUE). Their usbility s potentil gro-environmentl tools depends, mong other things, on the degree of responsiveness to crop nutritionl sttus t its mturity. Selected NUE indices were evluted bsed on grin yield, the number of kernels per con (NKC), nd generl crop indictors such s i) totl nutrient uptke (TNU), ii) nutrient hrvest index (NHI), nd iii) unit nutrient uptke (UNU). This concept ws vlidted using originl experimentl dt on mize response to incresing nitrogen rtes (0, 100, 150, nd 200 kg h -1 ) on the bckground of long-term potssium fertilizing systems, differing in soil K fertility level (medium, high) nd K fertilizer ppliction (K0, K+). The most promising indices of dignostic vlues to describe both gronomic nd environmentl impcts of nitrogen fertilizer were the prtil fctor productivity of nitrogen fertilizer (PFP FN ) nd pprent nitrogen recovery (NR). The first one, directly describing productivity of the unit of pplied nitrogen fertilizer, showed high sensitivity to those nutrients, which significntly defined mize yield. In the studied cse it refers to potssium, whose low supply during the criticl window resulted in significnt decrese in the number of kernels per cob. Consequently, the reduced cpcity of the mize physiologicl sink during the grin-filling period resulted in the pprent excess of phosphorus nd mgnesium. This conclusion is corroborted by n N/P rtio of :1 nd n N/K rtio of :1. This ws the key reson for limiting productivity of nitrogen fertilizer. The NR index ws the slightly poorer indictor of nitrogen fertilizer mngement, but lso responded significntly both to nturl nd experimentlly induced fctors tht re decisive for production vribility. Other frequently used NUE indices did not llow for mking both simple nd relible evlution of the nitrogen fertilizer productivity under different mngement systems. Keywords: nitrogen productivity, totl nutrient uptke, nutrient hrvest index, unit nutrient uptke, mize *e-mil: witeczek@up.poznn.p

2 2130 Szczepnik W. Introduction The sustinbility concept s the rod mp of modern griculture is big chllenge for frmers. The key question is how to increse griculturl production to feed the world s continully growing popultion without negtively impcting the environment. The primry objective cn be chieved by incresing the re of rble soils nd/or by incresing yields. The first wy is limited, becuse ll fertile soils re lredy in griculturl use. Hence progress in food production depends principlly on yield increse. Nowdys, mize due to its high-yield potentil is considered key crop, hving production cpcity to cover the growing worldwide demnd for grin [1-3]. It is well recognized tht ny yield increment in mize is possible provided tht significnt input of resources, minly nitrogen, is the key fctor responsible for dry mtter production [4, 5]. The key problem of sustinble mngement of nitrogen in the soil-plnt system is low productivity of pplied nitrogen fertilizer, defined s nitrogen use efficiency (NUE) [1, 6]. This term includes two bsic physiologicl processes [7, 8]. The first defines processes responsible for nitrogen uptke from the soil. In intensive production, bsed on fertilizers, it refers to the contribution by the pplied N in totl or in the min product, for exmple grin. The NUE index, bsed on this ssumption, is known s pprent nitrogen recovery (R N ). The second index, termed s nitrogen physiologicl efficiency (PhE N ), describes set of metbolic processes tht re responsible for trnsforming previously ccumulted nitrogen into the hrvestble prt of the crop plnt. The product of both indices is known s gronomic nitrogen use efficiency (AE N ). The first nd the third index re frequently considered s indictors of potentil environmentl impct of nitrogen fertilizer [1]. Their usefulness results from the fct the only prt of the totl mount of nitrogen fertilizer (N F ) introduced into the soilplnt system is tken up by the crop (N U ): N F = N U + N S + N L (1) Prt of the non-consumed N by plnts undergoes fixtion (N S ) both by microbes nd/or directly to soil prticles. This prt, termed residul nitrogen, is potentilly vilble to crops, but only in the subsequent cropping seson. The prt of the pplied nitrogen fertilizer mrked N L is lost from the cropping system, resulting in contmintion of wter (underground, surfce) nd the tmosphere [9]. The efficiency of nitrogen fertilizer pplied by frmers globlly is pltry, rnging from 20 to 40% [1, 6]. In turn, the low recovery rte of nitrogen fertilizer cretes big thret to the environment nd to humn helth. Therefore, the key objective of modern griculture, oriented on sustinbility, is to diminish losses of nitrogen fertilizer into the environment by incresing its use by currently grown crops [10, 11]. Due to its rpid expnsion in temperte regions of the world, including Polnd, mize requires much ttention by frmers, environmentlists, nd scientists. There is substntil yield gp between the potentil of modern vrieties nd ctul yields [2, 12]. The key physiologicl principle of the effective in-seson mngement of nitrogen fertilizer in mize is strictly relted to its dynmics during the crdinl stges of yield formtion [4]. The first pek of N requirements is reveled t lef stge 5-7. During this prticulr stge, the flower initils re estblished [5]. Therefore, ny fctor impcting nitrogen uptke increses the probbility of the fulfilling crop yield potentil. This is the cse of zinc nd mgnesium when they re pplied to mize before this stge [13, 14]. The second crdinl stge of N uptke by mize extends from tsselling to the wtery stge of kernel development. The rte of nitrogen uptke in this prticulr stge is driven by blnced uptke of other nutrients, such s potssium, phosphorus, nd mgnesium [15-20]. In modern stygreen vrieties, the mount of nitrogen tken up by mize t the beginning of flowering constitutes round 50% of its finl uptke [4, 5, 21]. The mount of minerl nitrogen tken up by mize during the grin-filling period depends to gret extent, irrespective of nutrient supply, on crop cpcity to its utiliztion [22-24]. The remining prt, termed s residul soil minerl nitrogen (RSMN), cretes potentil thret for the environment. The increse of NUE is the first step in decresing the mount of residul soil nitrogen s the prerequisite for lowering its potentil leching [25, 10]. Productivity of nitrogen fertilizer is highly ffected by the course of wether during the growing seson. Polnd, bsed on climtic criteri, belongs to the continentl zone, whose min ttributes re frequent drought during the vegettive seson with frequently occurring high tempertures [26]. Therefore, crop plnts re thretened by biotic stresses. Potssium supply is nutritionl fctor significntly ffecting yield of seed crop, especilly mild under-wter stress, s frequently reported in Centrl Europe [27]. It is therefore ssumed tht wter shortges cn be meliorted, t lest prtly, by the dequte mngement of potssium, s the key nutritionl fctor improving nitrogen productivity [17, 28]. The mjor objective of this study ws to evlute set of indices describing nitrogen use efficiency (NUE) bsed on mize minerl sttus t mturity. The minor objective of the study ws to evlute the sensitivity of both sets of indictors to the potssium fertilizing systems. Mterils nd Methods Studies on indices of nitrogen use efficiency s groenvironmentl indictors were vlidted bsed on field experiments conducted during the three consecutive growing sesons of 2004, 2005, nd 2006 t RGD Brody (Poznn University of Life Sciences Experimentl Sttion; E nd N). The experimentl long-term tril ws estblished in 1991 on Albic Luvisol originting from lomy snd underlined by light lom. The field tril rrnged s the two-fctoril split-block design, replicted

3 Evluting Nitrogen Use Efficiency... four times, comprised the following set of fctors: 1. Potssium fertilizing system: four tretments, 2. Rtes of nitrogen fertilizer of 0, 100, 150, nd 200 kg N h -1. The tested systems of potssium fertiliztion re the combintion of: i) medium K fertility plot with nd without fertilizer potssium ppliction (cronym MK0, MK+, respectively), ii) high soil K fertility plot with nd without fertilizer potssium ppliction (HK0, HK+). Mize (vriety Eurostr, FAO 240) ws used s the test plnt. All fertilizers were pplied in spring, just before sowing. Rtes nd forms of used nutrients were s follows: i) P 2 O 5-80 kg h -1 s SSP (single superphosphte), ii) K 2 O 160 kg h -1 s KCl, nd iii) nitrogen s mmonium nitrte (34%). The first rte of 100 kg N h -1 ws pplied before sowing. The remining N rtes were dded in ccordnce with the experimentl schedule t the stge of 3(5) mize lef. All other grotechnologies were in ccordnce with stndrd prctice. The individul plot size ws 22.4 m 2. At mturity, crops were hrvested from the re of 11.2 m 2. Totl grin yields were djusted to 14% moisture content. Plnt smples for yield, biomss, nd cob structure determintion were tken up from n re of one m 2 t BBCH 89. Nitrogen concentrtion in plnt smples ws determined by the stndrd mcro-kjeldhl procedure. Plnt mteril for minerl element determintion ws minerlized t 640ºC nd the obtined sh ws next dissolved in 33% HNO 3. The phosphorus mesurement ws conducted using the vndium-molybdenum method by the Specord 2XX/40 t 436 nm wve. Potssium nd clcium were determined by flme-photometry nd mgnesium by flme tomic bsorption spectrometry. Results re expressed on dry mtter bsis. The generl indictors of mize nutritionl sttus were clculted bsed on dt on nutrient concentrtion nd crop biomss. The index, termed s nutrient totl uptke (TNU), ws clculted by multiplying ech nutrient concentrtion nd the respective biomss. The nutrient hrvest index, NHI, defining percentge of given nutrient in the min yield, ws determined using eqution: NHI = (G Nu /TNU) 100 [%] The index, termed s unit nutrient ccumultion (UNU), ws clculted s the rtio of TNU for given nutrient nd the respective yield of grin, s presented below for nitrogen: UNU = TNU/Y, kg grin kg -1 N where G Nu is the mount of given nutrient in grin (kg h -1 ), TNU is the mount of given nutrient in the whole crop biomss (kg h -1 ), nd Y is yield of grin (t h -1 ). Two min sets of indices of nitrogen use efficiency hve been developed bsed on: 1. Yield of grin: ) Prtil fctor productivity of nitrogen fertilizer, 2131 PFP FN = Y/N r, kg grin kg -1 N b) Agronomic nitrogen use efficiency, AE N = (Y N Y 0 )/ N r, kg grin kg -1 N c) Physiologicl nitrogen use efficiency, PhE N = (Y N Y 0 )/ (NU N NU 0 ), kg grin kg -1 N 2. Biomss: ) Totl productivity of nitrogen fertilizer, TP FN = B/N r, kg biomss kg -1 N b) Totl nitrogen use efficiency, TE N = (B N - B 0 )/N r, kg grin kg -1 N c) Apprent N recovery, R N = (NU N NU 0 )/N r 100, % d) Totl physiologicl nitrogen use efficiency, TPhE N = (B N B 0 )/ (NU N NU 0 ), kg grin kg -1 N where Y N is yield of grin hrvested from the plot with pplied nitrogen (kg h -1 ), Y 0 is yield of grin hrvested in control (kg h -1 ), B N is biomss produced by mize on the plot with pplied nitrogen (kg h -1 ), B 0 is biomss produced by mize in control (kg h -1 ), N r is nitrogen fertilizer rte (kg N h -1 ), NU N is totl nutrient uptke in crop biomss in the plot with pplied nitrogen (kg h -1 ), nd NU 0 is totl nutrient uptke in crop biomss in control (kg h -1 ). The experimentlly obtined dt were subjected to conventionl nlysis of vrince using the computer progrm STATISTICA 10. The differences between tretments were evluted using Tukey s test. Tbles nd figures present results of the F test (***, **, * indicte significnce t P < 0.1%, 1%, nd 5%, respectively). In the second step of NUE indices vlidtion, stepwise regression ws pplied to define the optiml set of vribles. In the computing procedure, consecutive vrible ws tken from the multiple liner regressions in the step-by-step mnner. The best regression model ws chosen bsed on the highest F-vlue for the entire model nd significnce of ll independent vribles. Results nd Discussion Three groups of generl crop indictors of mize minerl sttus t mturity were used to vlidte NUE indices. The first, describing the totl nutrient uptke (TNU), informs bout the mount of prticulr nutrient in hrvested crop biomss. The second one, nutrient hrvest index (NHI), is dimensionless indictor of given element prtition in the mture crop, i.e., between the principl product, for exmple, in cerels, grin nd the residul biomss known s strw. The third index termed s the unit nutrient uptke (UNU) defines the mount of prticulr nutrient per unit of min product nd its mount in the respective mount of vegettive biomss. In the first step of the NUE indices vlidtion, ll these indictors were evluted by two yield chrcteristics. The first ws the number of kernels per cob (NKC), nd the second grin yield (GY). Growth Conditions nd Grin Yield The period extending from one week before nd three weeks fter flowering of mize is termed s the criticl window. It is decisive period for seed set up. Two

4 2132 Szczepnik W. Tble 1. Totl ccumultion of nutrients (g m -2 ). Min fctor Level of the fctor N P K Mg C Yers b b 1.85 b 3.29 c b b b 2.68 c c 3.00 b F *** *** *** *** *** Potssium fertilizing systems MK MK b 2.42 b bc HK b 2.32 b b HK b 2.32 b c F 4.89 ** 2.72 * ** Nitrogen rtes b 2.31 b b 1.73 b 2.85 b c 2.44 b c 1.84 b 3.28 c c 2.46 b c 1.88 b 3.30 c F *** 11.9 *** *** *** *** numbers mrked with the sme letter re not significntly different; ***, **, * : probbility levels t 0.001, 0.01, nd 0.05, respectively processes re responsible for the finl number of kernels per cob (NKC) [29]. The first is fertiliztion of flowers nd the second the rte of seed initils nturl bortion. The NKC under fvorble conditions cn rech 500. Any internl nd/or externl conditions disturbing fertiliztion of ovules or growth of seed initils leds to its bortion, in turn decresing the NKC. In rin-fed griculture, both processes re highly sensitive to wter shortges during the criticl window [30]. In the studied period, the totl sum of wter during the period extending from Jnury to the end of September mounted to 413 mm in 2004, 529 mm in 2005, nd 400 mm in 2006 (Tble 1). Amounts of precipittion during mize vegettion were 229, 361, nd 269 mm, respectively. However, decisive for the seed setup ws the criticl window period, covering the second nd third decdes of July nd the 1 August. In 2004, the totl mount of wter ws 36 mm nd the NKC rnged from 338 in the potssium unfertilized plot (MK0) to 402 in the high-potssium fertile plot with currently dded potssium (HK+). In 2005 the mount of rinfll wter in this prticulr period reched 97 mm. The NKC ws high, rising under these fvorble conditions from 460 in the MK0 to 480 in plnts grown in K fertile plots (HK). The worst conditions for seed set up were in The mount of precipittion during the criticl window ws 106 mm, but in July only nine mm. This resulted in drstic drop in NKC. It mounted to 224 in the MK0, rising up in other tretments to 275. The recorded response of NKC to wter nd potssium supply in 2004 nd 2006 corrobortes so fr published observtions [17, 28]. The hypothesis concerning the meliortive impct of n dequte supply of potssium on the formtion of yield components by seed crops ws proven to be under mild yer s recorded in 2004, but not in very dry Mize yield ws driven by interction of yers nd potssium mngement systems (Fig. 1). In 2004 specific impct of potssium systems on grin yield ws observed. A slightly higher yield ws n ttribute of both the medium fertile plot with dded K fertilizer (MK+) nd the high fertility plot, but without freshly dded K fertilizer (HK0). The yield gin due to effective supply of potssium compred with the K control (MK0) mounted to 0.9 nd 0.4 t h -1. The phenomenon observed in 2004 cn be explined by mize s bility to use effectively the nonexchngeble potssium resources [16, 31]. This finding Fig. 1. Effect of potssium tretments on the number of kernels per cob (NKC) nd grin yield (GY) in consecutive yers of the study. numbers mrked with the sme letter for grin yield re not significntly different.

5 Evluting Nitrogen Use Efficiency... is in ccordnce with Dmm, et l. [32], who observed fster growth rte of mize roots under conditions of semi-drought, providing fresh ppliction of fertilizer potssium. In the second, 2005, chrcterized by n mple wter supply to growing plnts, yields were very high. The highest of 9.9 t h -1 ws gin n ttribute of MK+ plot. The yield gin due to dded K fertilizer mounted to 0.65 t h -1. The lowest yields, irrespective of K mngement systems, were recorded in 2006, with drought in July. The key reson for the yield (Y) drop ws the insufficient size of the physiologicl sink, i.e., NKC. This conclusion ws corroborted by the developed regression model: Y = 0.011NKC for n = 12, R 2 = 0.78 nd P This eqution implicitly corrobortes the hypothesis on seed density s the key yield vrible for grin yield. Therefore, ny stress fctor, like wter shortge, tht leds to kernel bortion results in yield drop [15, 21]. Totl Nutrient Uptke The mount of prticulr nutrient in the crop biomss t hrvest, considered the primry chrcteristic of mize minerl sttus, ws significntly ffected by the course of wether during the growing seson. The rnge of nutrients verged over experimentl tretments in descending order ws s follows: N ( ) > K ( ) > C (26-33) > P (20-27) > Mg (12-20) kg h -1 The effect of potssium fertilizing systems on totl nutrient uptke (TNU) ws significnt for nitrogen, phosphorus, nd potssium (Tble 1). The effect of nitrogen fertilizing rtes on totl nutrient ccumultion ws positive up to 100 kg N h -1 for phosphorus nd mgnesium, nd to 150 kg N h -1 for nitrogen nd potssium. The effect of potssium systems on nutrients uptke ws reported for nitrogen nd phosphorus in the MK+ plot. The wter stress in 2006, considerbly depressed potssium uptke, but not the ccumultion of other nutrients. Potssium is considered the primry fctor ffecting nitrogen uptke during wter shortge [17, 28]. The impct of TNU indices on yield performnce ws complex. The stepwise nlysis llowed developing regression models, discriminting the effect of prticulr nutrients on the NKC nd GY. The respective equtions re: NKC = TPU 197.4TMgU for R 2 = 0.94 GY = TMgU TCU for R 2 = 0.83 (2) (3) 2133 These two reltionships clerly corroborte the hypothesis tht excess phosphorus (TPU) nd mgnesium (TMgU) in mize t its physiologicl mturity cn be considered indictors of growth disturbnce [16]. These two equtions evidently inform us tht mounts of ccumulted phosphorus nd mgnesium were in excess with respect to NKC nd GY. At the sme time, the content of clcium becme fctor limiting yield. The excess of clcium in seeds cn be considered n indictor of wter stress. The effect of K fertilizing systems on nitrogen, phosphorus, nd potssium whole uptke ws significntly modified by the course of wether during ech of the growing sesons. However, the significnt reltionship with grin yield hs been found only for TKU nd GY: GY = KA TKU for n = 12, R 2 = 0.78 This eqution implicitly indictes tht potssium content reched the sturtion level in the mize cnopy. For the clculted TKU optimum of 166 kg K h -1, the mximum yield ws 9,259 kg h -1. The theoreticl yield ws very close to the yield hrvested on the plot with the medium K soil fertility level nd pplied potssium fertilizer (MK+). This fct indirectly indictes the presence of other fctors, limiting potssium uptke from plots rich in potssium (HK). Nutrient Hrvest Index The studied hrvest indices, except for clcium, were significntly ffected by the course of wether (Tble 2). The most striking impct of the yer-to-yer vribility ws reported for nitrogen nd phosphorus. Their vlues declined in the order: 2005 > 2004 > The potssium fertilizing system ffected nitrogen, phosphorus, nd potssium prtition the most in the HK+ plot, incresing its content in vegettive biomss in expense of grin. These indices only in 2005 were t the level reported for the high productive systems [21]. A positive impct of nitrogen fertilizing rtes on nutrient hrvest indices reched the mximum t N rte of 100 kg h -1, decresing therefter. The mgnesium hrvest index showed the sme yer-toyer vribility trend s found for nitrogen. The impct of the potssium mngement system on MgHI reched n elevted level in the K-rich plot but without current ppliction of fertilizer (HK0). The effect of nitrogen incresing rtes on its vlues ws s rule negtive. The yield-forming effect of studied indices on grin yield ws highly complex, s results from developed regression models. It hs been found tht yield of mize significntly depended on PHI: GY = PHI for R 2 = 0.86 (4) (5)

6 2134 Szczepnik W. Tble 2. Sttisticl evlution of nutrient hrvest indices. Min fctor Level of the fctor N P K Mg C Yers b 0.71 b b 0.05 Potssium fertilizing systems c 0.74 b 0.25 b c F 202.8*** *** *** *** MK b 0.68 b 0.26 b 0.46 b 0.05 MK b 0.68 b 0.24 b HK b 0.68 b 0.23 b 0.50 b 0.06 HK b 0.05 F 3.75* 3.36 * *** ** Nitrogen rtes b b 0.50 b 0.06 b b 0.69 b 0.26 b 0.48 b b b 0.24 b F 7.60*** 4.12 ** * *** ** numbers mrked with the sme letter re not significntly different; ***, **, * : probbility levels t 0.001, 0.01, nd 0.05, respectively The key yield-forming fctor, NKC, ws lso significntly dependent on PHI: NKC = PHI for R 2 = 0.86 These sets of equtions implicitly indicte the phosphorus minerl sttus of mize t mturity s fctor reflecting disturbnce in yield component performnce. It cn be ssumed tht too low PHI ws the result of the insufficient size of the physiologicl sink for phosphorus, i.e., NKC. Therefore, it cn be concluded tht the reduced number of kernels ws the key reson for the excess of non-productive phosphorus in mize t hrvest. Indices of Unit Nutrient Uptke The indices of the unit nutrient uptke (UNU), except for potssium, were driven by the course of wether during prticulr seson (Tble 3). With respect to this fctor the obtined rnges, presented in descending order, re s follows: N (19 31) > K (15 16) > C ( ) > P ( ) > Mg ( ) kg t -1 These rnges re, except for mgnesium, higher thn those published recently by Bender et l. [1]. The min reson for the differences is the level of grin yield. In the studied cse it reched eight t h -1, but in the cited pper on verge it ws 12 t h -1. The lowest vlues were recorded in 2005, the yer with the highest GY, wheres 2006 sw (6) the lowest yield. These differences indirectly indicte the potentil productivity of most nutrients, but especilly of nitrogen, phosphorus, nd mgnesium provided n mple supply of wter. The impct of potssium fertilizing systems on UNU ws found for nitrogen, phosphorus, nd potssium. All these indices responded significntly to current K fertilizer ppliction, irrespective of soil K fertility level. The highest vlue ws reported for the K-rich plot currently fertilized with potssium. The UNU indices for nitrogen, phosphorus, mgnesium, nd clcium were negtively correlted with yield nd NKC. The best regression models re s follows: GY = UPU 0.97UMgU for R 2 = 0.88 NKC = UMgU for R 2 = 0.95 These two equtions implicitly indicte n excess of phosphorus nd mgnesium. In fct, they cn be considered indictors of mize growth disturbnce during grin yield formtion due to insufficient size of the physiologicl sink, i.e., NKC. In gronomic prctice, three indices such s UNU, UPU, nd UKU require specil ttention due to their previling impct on plnt growth during the vegettive seson. The optimum N/P nd N/K rtios in mize t mturity for mximum yield of grin should rech 5:1 nd 1:1, respectively [4]. In the studied cse, the N/P rtio ws in the rnge :1. The rnge indictes on the (7) (8)

7 Evluting Nitrogen Use Efficiency Tble 3. Unit ccumultion of nutrients (kg t -1 grin + respective biomss of vegettive prt). Min fctor Level of the fctor N P K Mg C Yers b b 3.84 b b c 3.97 c c 4.39 c F *** *** *** *** Potssium fertilizing systems MK MK b 2.94 b b HK b 2.98 b b HK b 3.03 b c F 4.89 ** * *** Nitrogen rtes b b 3.55 b c b 3.85 b c b 3.89 b F *** *** 5.2 ** numbers mrked with the sme letter re not significntly different; ***, **, * : probbility levels t 0.001, 0.01, nd 0.05, respectively imblnce of nitrogen nd phosphorus unit uptke due to insufficient ccumultion of the second nutrient in grin. The bove-presented controversy hs been explined by studying the vribility of unit potssium uptke index. Its vlues were the result of interction of both potssium fertilizing systems nd nitrogen incresing rtes. As presented in Fig. 2, plnts grown on medium K fertile soil without freshly pplied potssium (MK0) incresed the UKU in ccordnce to the progressing N rtes. A much steeper increse, however, ws noted for the rich K soil lso without K fertilizer ppliction. Quite different trends took plce in K sub-plots, currently supplied with K fertilizer. In the medium fertility K soil, ny response to incresing N rtes ws noted. In the fertile K soil even decresed trend ws observed. Potssium tretments without freshly pplied potssium indirectly underline the importnce of K soil reserves s source of K for growing plnts, especilly for K to rech the soil. This finding corrobortes the concept tht cerel crops re ble to use efficiently soil K resources when provided with high supply of this nutrient during the growing seson [17-19, 28, 31]. The simple index, such s the N:K rtio, vried from 1.25 to 2.17:1. The highest yield of 9.65 t h -1 ws obtined in two tretments. The first one represents the MK+ plot nd the second the HK0. As shown in Fig. 3, the incresing N:K rtio resulted in decrese in the number of kernels. Therefore, the controversy concerning on the one hnd deficiency of phosphorus nd on the other hnd its excess t mize t mturity cn be explined only by the reduced cpcity of the mize physiologicl sink, i.e. the NKC. As consequence, mize requirements for some nutrients like phosphorus nd mgnesium significntly decresed, Fig. 2. Effect of potssium fertilizing systems nd incresing nitrogen rtes on the unit potssium uptke index. numbers mrked with the sme letter re not significntly different. Fig. 3. Number of kernels per cob s function of the N/K rtio.

8 2136 Szczepnik W. Tble 4. Sttisticl evlution of indices of nutrient use efficiency. Min fctor Yers Potssium fertilizing systems Level of the fctor PFP FN AE N PhE N TFP FN BE N R N % TPhE N b b c b b b F *** *** *** *** MK b MK b HK HK b F * c c b b F *** *** numbers mrked with the sme letter re not significntly different; ***, **, * : probbility levels t 0.001, 0.01, nd 0.05, respectively in turn negtively ffecting the productivity of nitrogen fertilizer. Indices of Nitrogen Use Efficiency The most promising gro-environmentl NUE index, tking into ccount the key objective of this pper, seems to be PFP FN followed by TFP FN (Tble 4). The dvntge of this index over others results from its sensitivity both to the course of wether nd experimentl fctors. This index, lone mong others, showed significnt response to rtes of nitrogen fertilizer. As expected, its vlues decresed in the opposite wy to incresing rtes of nitrogen fertilizer. The PFP FN ws the highest, verged over other fctors, in tretment with 100 kg N h -1. The 1 kg N fertilizer resulted in 82 of grin. This vlue is high, but below the mximum in Polish conditions, rnging from 96 to 103 kg grin kg -1 N [27]. The min reson of the only good vlue of PFP FN ws the low NKC. As result, mize ws unble to tke dditionl mounts of nitrogen from soil during the grinfilling period. The respective vlues of TFP FN were lmost double compred to PFP FN ones. The TFP FN, s lone, showed significnt response to the potssium fertilizing system. The freshly dded potssium resulted in the TFP FN index incresed. Therefore, in yers with wter stress, the difference in unit productivity of nitrogen fertilizer is, s rule, much lower for biomss compred to grin. It simply mens low efficiency of nitrogen remobiliztion during the grin-filling period due to insufficient cpcity of the physiologicl sink [20, 24, 30]. The gronomic nitrogen efficiency index, AE N, ws wek indictor of nitrogen productivity in the highyielding mize system. Much higher sensitivity, but only to the course of wether, ws reported for the BE N. Its vlues were twice s high in the dry 2006 compred to other yers. The sme trend of response ws noted for nitrogen recovery (R N ). Its vlue in 2006 ws slightly bove 60%, wheres in the two other yers it did not exceed 35%. All other NUE indices did not show ny response to both the course of wether nd experimentl fctors (Tble 4). Both sets of NUE indices were evluted using three groups of predictors: yield nd NKC, TNU, nd UNU. The nlysis of the PFP FN predictive potentil hs been fully corroborted for grin yield, which is not surprising when considering the clcultion procedure. As presented below, the best regression model hs been chieved, introducing into the model lso the AE N : GY = PFP FN 0.043AE N for R 2 = 0.99 The predictive dominnce of PFP FN hs been corroborted by the regression model developed for NKC: NKC = PFP FN PhE N for R 2 = 0.99 (9) (10) These two equtions implicitly indicte reduced grin yield nd NKC due to insufficient productivity of nitrogen fertilizer. The reson is rooted in the imblnced nutritionl sttus of mize in stges of NKC performnce [29, 30]. It hs been well documented tht wter stress during the period extending from the tsselling to the wtery stges of kernel growth results in reduction in NKC [17]. It cn be concluded tht shortge of potssium during this

9 Evluting Nitrogen Use Efficiency... period results in the NKC decrese, in turn negtively ffecting nitrogen fertilizer productivity. The dvnced procedure of NUE indices evlution relies on the degree of their sensitivity to indictors of mize minerl sttus t hrvest. In the studied cse, the highest prognostic impct of ech NUE index ws n ttribute of mgnesium nd clcium. The predictive strength of these two nutrients (presented by the R 2 coefficient) decresed in the order: NR (0.85) PFP FN (0.83) > PhE (0.54) > AE (0.37) The respective equtions for two best NUE indices re: 2137 supported by the negtive signs of the unit productivity fctor in the eqution presented below: TFP FN = UPU for R 2 = 0.53 (15) The conducted divgtion lso showed tht the clssicl index, i.e., gronomic efficiency, ws only useful for describing the impct of nitrogen fertilizer on biomss productivity. The best model for this index cn be developed to include mgnesium nd clcium s predictors: R N = TMgU TCU for R 2 = 0.85 (11) BE = TMgU 34.28TCU for R 2 = 0.80 (16) PFP FN = TMgU TCU for R 2 = (12) Effective mngement of nitrogen in mize tissues, s presented by these two equtions, depends on reltionships between mgnesium nd clcium. The nlysis of the R N eqution implicitly corrobortes the conclusion chosen by Potrzycki [22]. This uthor clerly showed tht mgnesium cn significntly increse nitrogen recovery, provided reltively low rte of nitrogen fertilizer. This conclusion is fully corroborted by eqution No. 12. The sign of the mgnesium fctor indirectly informs the nonproductive excess of this nutrient in mize tissues t the stge of mturity. The importnce of mgnesium s the PFP FN predictor hs been reinforced, tking into ccount the UNU index s the denomintor. The best predictive model hs been obtined bsed on mgnesium nd phosphorus unit uptke indices: PFP FN = UPU 7.16UMgU for R 2 = 0.92 (13) The second set of indices, bsed on biomss produced by mize, showed slightly weker predictbility of TFP FN by both TNU nd UNU s denomintors. At the sme time, the best model operted on quite different sets of nutrients compred to the PFP FN index: TFP FN = K C for R 2 = (14) This model indirectly indictes reduced ccumultion of nitrogen in vegettive biomss t mize mturity due to shortges of potssium nd clcium, mening tht productivity of nitrogen fertilizer ws therefore relted, to some extent, to crop biomss insted to GY. The min reson ws too low performnce of the cob, leding to n excess of phosphorus nd mgnesium. This conclusion is As in the cse of TFP FN, these two secondry nutrients becme decisive for nitrogen productivity. Conclusion The conducted study implicitly showed tht indices describing nitrogen use efficiency presented different responses to growth conditions impcted by nturl nd gronomic fctors. The best indictor of nitrogen fertilizer productivity ws the prtil fctor productivity of nitrogen fertilizer (PFP FN ). This index cn be used s the predictor of both NKC nd GY. The obtined regression models implicitly indicte tht productivity of pplied nitrogen fertilizer is most governed by the number of kernels per cob. The reduced size of the physiologicl sink resulted in decresed requirements of the growing cob for nitrogen, phosphorus, nd mgnesium during the grin-filling period. Therefore, nitrogen fertilizer reched mximum unit productivity of 82 kg in the tretment with 100 kg N h -1. The PFP FN index ws highly sensitive to ny growth disturbnce, therefore it cn be considered useful tool for efficient mngement of nitrogen fertilizer, tking into ccount both i) its productivity nd ii) potentil thret of residul minerl nitrogen to the environment. The conducted study implicitly showed n excess of mgnesium nd phosphorus in mize tissues t physiologicl mturity. The pprent excess of both nutrients ws result of growth disturbnce during the criticl window due to insufficient supply of potssium to mize plnts. Deficiency in potssium supply resulted in lowered NKC. References 1. CASSMAN K.G., DOBERMANN A., WALTERS D.T. Agroecosystems, nitrogen use-efficiency, nd nitrogen mngement. Ambio 31 (2), 132, CIAMPITTI I.A., VYN T.J. Understnding globl nd

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