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By Montemurro, F Maiorana, M

The aim of this research was to study different ways of nitrogen (N) fertilization and soil tillage management to ensure high yield, quality and correct usage of the available resources. Therefore, a two-year field experiment (2001 and 2003) was carried out on a sugarbeet crop grown in a Mediterranean environment to evaluate the effects of two soil tillage depths (10-15 and 40-45 cm) and of a municipal solid waste (MSW) compost application, compared to mineral N fertilization. The following fertilization treatments were compared: MSW compost at 100 kg N ha^sup -1^ (Ncom); mineral N fertilizer at 50 kg ha^sup -1^ (N50) and at 100 kg ha^sup -1^ (N100); MSW compost combined with mineral N fertilizer (Nmix) (50 kg ha^sup -1^ as organic N plus 50 kg ha^sup -1^ as mineral N); slow release organic-mineral fertilizer (Nslow) and an unfertilized control (Contr). In each trial year, growth parameters, plant and soil N indicators, yield, quality, N uptake and N efficiency, soil chemical characteristics and N mineral deficit in the soil were determined. The findings of this research highlighted that MSW compost was an important N source for sugarbeet, especially when MSW compost was combined with mineral N fertilizer. In fact, the Nmix treatment did not determine any significant reduction in root (32.5 vs. 36.41 ha^sup -1^ of N100) and sucrose yields (3.56 and 3.65 ha^sup -1^). Furthermore, it presented a significantly lower amount of one of the most important qualitative parameters, the alfa amino N (1.74 and 1.97 meq 100 g^sup -1^ of N100), and ensured the least soil N deficit in 2003. The application of MSW compost significantly increased the extracted organic carbon in both 2001 (37% of increase for compost treatment in respect to unfertilized control) and 2003 (24%). Higher values were also recorded for total organic carbon in 2001 (20% of increase) and for humified organic carbon in 2001 (27%) and in 2003 (16%), whereas its application did not increase the total content of heavy metals. Finally, among plants and soil N indicators tested, the petiole nitrate content was the only one significantly and positively correlated with root and sucrose yields, indicating that it was an effective tool to monitor N supply. Introduction

Under Mediterranean climatic conditions, temperate winters and springs allow sugarbeet (Beta vulgaris L.) to be sown in autumn or in early spring and harvested in the summer. In these environments, sucrose accumulation is at its maximum in the period late June to early September, in which the addition of nitrogen (N) and the N uptake by plants cause the decrease in sucrose accumulation and production at harvest. Increasing fertilizer N levels in soil results in a high moisture content and high biomass of sugarbeet roots, but a low in sucrose concentration at the harvest (Carter and Traveller 1981). Furthermore, some authors (Adams et al. 1983; Anderson and Peterson 1988) report that the optimum N levels in the soil for sucrose production are usually lower than for root yield, due to its negative effects on sucrose concentration. Compared with corn and tomato, deeprooted sugarbeet scavenges the soil residual N, recovering considerably more soil N and relying less on fertilizer N (Hills et al. 1983). Carter and Traveller (1981) point out that sugarbeet root quality has been steadily declining since the increased use of N fertilizer and that excessive and late N applications decreased the extractive ability of stored sucrose, which reduced the refined sucrose production. The optimum agricultural management (tillage, type and level of N fertilizer) are necessary to reduce the environmental impact of agronomical practices and to increase profitability in crop production. Such information will be useful for developing economically and environmentally sound N fertilizer recommendations and agronomical management strategies for sugarbeet production. In this matter, Binford et al. (1992) suggest that crop analysis can be used to formulate recommendations on N use, since the plant responses depend on the integration of many factors (presence and availability of soil mineral N, weather and crop management). However, N estimation based on crop analyses at early stages could be unable to fully reflect the N supply by the soil (Schroder et al. 2000). Furthermore, applying N fertilizer at pre-sowing or during the season increases N uptake and N concentration in both tops and roots, but a greater rate of the N taken up by plants is used for the top growth. One possibility to obtain high yield and good quality of sugarbeet, with a lesser amount of mineral fertilizers, is to apply organic material sources, since they release N for longer time during the growing season than mineral supplies. Furthermore, in Mediterranean conditions (characterized by high temperatures in the summers), a higher mineralization rate and consequent depletion of organic matter in the soil, often occur. As a consequence, the amendment functions of an organic fertilizer are more important than just addition of nutrients, as suggested by Montemurro et al. (2004). Furthermore, many municipalities have examined composting as an alternative to landfilling for the management of organic solid waste materials. In the coming years, these materials will be land applied and therefore some knowledge of nutrient availability will be necessary to optimize crop yield and minimize environmental risks (Wolkowski 2003).

Therefore, the objectives of this work were: i) to evaluate the effects of two soil tillage depths on sugarbeet performance; ii) to predict the N fertilizer needs of sugarbeet, so that farmers can obtain optimum root and sucrose productions and good level of quality; iii) to compare organic and mineral N fertilization strategies. To accomplish these objectives, yield, quality, N uptake, N utilization, mineral soil N deficit, plant and soil N indicators were determined in a two-year field experiment on sugarbeet crop.

Material and Methods

Site

The research was carried out during the springsummer 2001 and 2003 in Foggia (Southern Italy), on the Experimental Farm of the Agronomic Research Institute. The climatic conditions were typical of the Mediterranean environment. The mean monthly temperatures and the rainfall of 2001 and 2003 were compared with the long-term average (1952-2000). During the 2001 and 2003 sugarbeet growing seasons (February - September), the total rainfall was almost the same, but substantially lower than the long-term (235.8,227.4 and 315.8 mm, respectively). Conversely, the mean temperature in 2001 was substantially higher than both 2003 and the long-term average (19.05, 17.68 and 17.63 [degrees]C, respectively).

The soil was silty-clay of alluvial origin, classified by Soil Taxonomy-USDA as Fine, Mesic, Typic Chromoxerert. Soil samples (0- 50 cm depth) were taken from each elementary plot and the mean main starting characteristics are as follows: total N: 1.22 g kg^sup - 1^, determined by the Kjeldhal digestion and distillation method; available P: 27 mg kg^sup -1^, extracted by 0.5 M NaHCO^sub 3^ and determined by Olsen and Sommers method (Olsen and Sommers 1982); exchangeable K: 1018 mg kg^sup -1^, extracted by 1M ammonium acetate and determined by Thomas method (Thomas 1982); organic matter: 20.7 g kg^sup -1^, by Walkley-Black method (Nelson and Sommers 1982); pH: 8.83, on 1 : 2.5 soil water suspension (McLean 1982).

Experimental Setup and Compost Characteristics

The experimental design was a split-plot with three replications. The main plot was assigned to the soil tillage, whereas the split- plot to the N fertilization treatments. The following two tillage depths were compared: shallow tillage (10-15 cm), indicated as ST, and deep tillage (45-50 cm), indicating as DT. The DT treatment included moldboard plowing (one month before sowing) and two disk harrowing to prepare proper seedbed, while ST treatment consisted of two disk harrowing, just before sowing.

In the split-plot, the following N fertilizing strategies were compared: Municipal Solid Waste (MSW) compost application with 100 kg ha^sup -1^ of organic N (Ncom); mineral N fertilization with 50 kg N ha^sup -1^ (N50); mineral N fertilization with 100 kg N ha^sup - 1^ (N100); compost and mineral application with 50 kg N ha^sup -1^ of organic N, as MSW compost, and 50 kg N ha^sup -1^ of mineral N (Nmix); slow release organic-mineral fertilizer with 100 kg N ha^sup -1^ (Nslow); and unfertilized control (Contr).

MSW compost was uniformly broadcasted for Nmix and Ncom (50 and 100 kg N ha^sup -1^, respectively) in one solution about 1 month before sowing, calculating the amount of these two treatments on the basis of its total N content. For N100, mineral N (as ammonium nitrate) was applied in two equal amounts at sowing (March, 15, 2001 and April, 3, 2003) and at 75 and 67 days after sowing (DAS) in 2001 and 2003, respectively. For Nmix, mineral N was supplied during plant growth stage (75 and 67 DAS for 2001 and 2003, respectively). For N50, mineral N (as ammonium nitrate) was applied at sowing. The Nslow, made by ILSA Factory, was applied iolution at sowing and consisted of: total N: 29%; organic N: 5%; mineral N (as urea): 24%; total organic carbon: 18%; organic matter: 32%. The phosphorus fertilizer (50 kg P^sub 2^O^sub 5^ ha^sup -1^ ) was spread during main soil plowing, whereas no K fertilizer was applied in both years, due to high initial exchangeable potassium soil content (1018 mg kg^sup -1^). Irrigation was scheduled whenever the cumulative evaporation from class "A" pan reached 90 mm. The irrigation volume was equal to 80% of the water stored in the soil layer 0-50 cm. Insects and diseases were controlled following local methods. The same MSW compost used in both years was obtained by Cupello Engineering (Italy) through the aerobic transformation of MSW by selective collection. The main chemical characteristics were as follows: total N: 1.47 g kg^sup -1^; Cu: 330 mg kg^sup -1^; Zn: 751 mg kg^sup -1^; Pb: 670 mg kg^sup -1^; Ni: 217 mg kg^sup -1^; Cd: 1.3 mg kg^sup -1^; total organic carbon (TOC): 13.75 g kg^sup -1^1; total extracted organic (TEC): 7.67 g kg^sup -1^1; humified organic carbon QHA+FA): 2.51 g kg^sup -1^1; C/N: 9.55. Total N was determined using the Kjeldhal digestion and distillation method. TOC, TEC and C(HA+FA) were determined according to the Springer and Klee method, modified by Sequi et al. (1986), whereas heavy metals content was extracted by a mixture of HCl and NNO^sub 3^ (9 : 3) and determined by atomic absorption spectrometry (Page et al 1982).

Determinations and Statistical Analysis

During cropping cycles and at maturity stage, occurred 147 DAS in 2001 and 139 DAS in 2003, one square meter of sugarbeet (cv. Azzurra) plants was randomly taken from each plot. The leaves, crowns and roots were washed, weighed fresh, shredded, sub sampled, dried (70 [degrees]C till constant weight), weighed again and analyzed, to determine total N content (CHN elemental analyzer - Fison EA 1108), which allows the calculation of N uptake (N content per dry weight). At the end of each cropping cycle the root yield, sucrose yield (sucrose concentration per dry matter weight) and alfa amino N content in the roots were determined. Sucrose concentration was obtained using the cold digestion method and a polarimeter, while the alfa amino N by ninhydrin method, as suggested by Halvorson et al. (1978). The following N parameters were calculated according to Delogu et al. (1998): 1) N utilization efficiency (NUtE, expressed in kg kg^sup -1^), as the ratio between root yield and total N uptake; 2) N harvest index (NHI in %), as the ratio between root N uptake and total N in plants at maturity. Furthermore, soil mineral N (NO^sub 3^-N + NH^sub 4^-N) at 0-50 cm soil depth at both the beginning and the end of each trial year was also determined. On the basis of these determinations, the mineral N deficit in the soil was calculated by the difference between the available N during growth seasons (mineral N in the soil at the beginning of each year plus N supply) and the sum of N uptake and mineral N at the end of cycles (Montemurro et al. 2002). At the end of 2001 and 2003, the heavy metals (Cu, Zn, Pb and Ni, determined using the same methods applied for the MSW compost analysis) and the total, extracted and humified organic carbon contents were determined in the control, NlOO and Ncom treatments. For making these determinations three soil samples (0-50 cm depth) were taken from each split plot, pooled in one sample, air dried, ground to pass a 2-mm sieve and then analyzed using the same procedure as MSW compost.

Finally, the following N indicators were determined: pre-sowing soil mineral N (NO^sub 3^-N + NH^sub 4^-N) at 0-50 cm soil depth; petiole nitrate content and SPAD (Soil Plant Analysis Development) readings at root swelling, at ripening and as a mean of the entire cycle (5 determinations occurred at the main phenological sugarbeet stages); leaf total N content (CHN elemental analyzer) recorded at swilling and at ripening. SPAD readings were made using a hand-held chlorophyll meter (SPAD 502; Minolta) at mid-length on the most recently matured leaves of the 5 randomly selected plants from each elementary plot. On the sap from the petiole of leaves, the nitrate content (Nitrachek reflectometer; Merck) was measured to determine the NO^sub 3^-N concentration.

Statistical analysis was carried out using the General Linear Model of the SAS software package (SAS Institute 1990). Differences among the treatments were evaluated with the Least Significant Difference (LSD) and the Duncan Multiple Range Test (DMRT) at P<0.05, for two or more values respectively, whereas Pearson correlation coefficients were used to correlate yield and protein content with N indicators. Finally, the regression analysis was fitted to study the relationship between yield and the significant N indicators and between yield and N uptake.

Results and Discussion

Effects of Two Years, Soil Tillage and N Strategies On Sugarbeet Yield and Quality

Figures 1 and 2 showed the root and sucrose yields of sugarbeet as affected by years, soil tillage and N strategies. Significant (P<0.05) higher root yield (40.1 t ha^sup -1^) was found in 2003 as compared to that (31.71 ha^sup -1^) of 2001. Similar trend was found for sucrose yield (3.97 t ha^sup -1^ of 2003 vs. 2.95 t ha^sup -1^ of 2001). These large differences have been obtained even when the soil total available and fertilizer N were similar, possibly as a result of climatic factors. In particular, considering that the sugarbeet was normally and regularly irrigated in both years, the differences recorded were mainly due to the temperature, which was substantially higher in 2001, reducing normal plant growth and, consequently, the yield. No significant difference (P

FIGURE 1. Root yield of sugarbeet as affected by years (2001 and 2003), soil tillage depths (Deep Tillage and Shallow Tillage) and N strategies (mineral fertilizer and MSW application). Different letters are significantly different according to Least Significant Difference (years and soil tillage depths) and Duncan Multiple Range Test (N strategies) at P

FIGURE 2. Sucrose yield of sugarbeet as affected by years (2001 and 2003), soil tillage depths (Deep Tillage and Shallow Tillage) and N strategies (mineral fertilizer and MSW application). Different letters are significantly different according to Least Significant Difference (years and soil tillage depths) and Duncan Multiple Range Test (N strategies) at P<0.05.

FIGURE 3. Alfa amino N content as affected by years (2001 and 2003), soil tillage depths (Deep Tillage and Shallow Tillage) and N strategies (mineral fertilizer and MSW application). Different letters are significantly different according to Least Significant Difference (years and soil tillage depths) and Duncan Multiple Range Test (N strategies) at P<0.05.

A significantly higher (P

Therefore, the results of this research pointed out that the mixed application of N fertilizer (half amount of organic source as MSW compost and half of mineral N applied during sugarbeet cropping cycle) reduced the value of alfa amino N, thus ensuring a correct compromise between yield and root quality. Similar results were observed in other crops such as tomato and maize (Maynard 1995; Eriksen et al. 1999; Montemurro et al. 2005b).

Effects of Two Years, Soil Tillage and N Strategies On N Uptake and N Indicators

Significant difference (P

Effect of years (2001 and 2003), soil tillage depths (Deep Tillage and Shallow Tillage) and N strategies (mineral fertilizer and MSW application) on N uptake and N utilization efficiency parameters of sugarbeet grown in Foggia (Southern Italy) area.

Significant and positive correlations were found between total N uptake and both root yield (correlation coefficient: 0.63) and sucrose yield (0.57) (Table 2), suggesting that this parameter had a great importance in beet production. The results of this research were in agreement with the finding of Bilbao et al. (2004). Conversely, it is possible that nutrient uptake alone may not be responsible for the yield responses that we observed, but uptake efficiency from improved soil physical, chemical and biological properties may interact to increase yield in crops produced on compost-amended soil (Singer et al. 2004). Furthermore, our results confirmed that the regression between total N uptake and root yield (R^sup 2^ = 0.57) was positive and linear (Figure 4a). Even if with less absolute value (R^sup 2^ = 0.33), a linear relationship between sucrose yield and total N uptake was also found (Figure 4b). These results indicated that total N uptake is an important parameter influencing the yield of sugarbeet cropped in Mediterranean conditions.

TABLE 2.

Correlation coefficients among yields and quality with N indicators, N uptake and N use efficiency parameters of sugarbeet grown in Foggia (Southern Italy) area.

Root yield was positively correlated with the petiole nitrate concentration recorded at root swelling and at ripening (Table 2). Furthermore, positive correlation occurred with the mean value of petiole nitrate concentration measured during the entire cycle. Similar behavior was also observed in psucrose yield, except the correlation at root swelling, but the highest values were recorded at ripening (R^sup 2^ = 0.69 and R^sup 2^ = 0.73 for root and sucrose yields, respectively). Therefore, the results of our study indicated that this plant N indicator measured in June could be used to estimate N fertilizer needs for maximum sucrose yield with optimum efficiency in accordance with Shock et al. (2000). Linear relationships between the petiole nitrate concentration at ripening and both root yield (R^sup 2^ = 0.48) and sucrose yield (R^sup 2^ = 0.56) were found (Figure 5).

FIGURE 4. Regression of the total N uptake with root and sucrose yields.

FIGURE 5. Regression of the nitrate content at ripening with root and sucrose yields.

No significant correlation was observed between SPAD readings and root and sucrose yields (Table 2), showing that this N indicator was a less sensitive measurement of plant N status than a petiole nitrate concentration, confirming the results of Sexton and Carroll (2002). Finally, no significant correlation was found among the plant total N content, soil pre-sowing mineral N, and root and sucrose yields. In any case, Anderson and Peterson (1988) suggest that the combination of petiole analysis with soil tests can give the best estimate of increments for fertilizer N needed to achieve optimum efficiency.

Effects of Two Years, Soil Tillage and N Strategies On Soil Characteristics and N Mineral Deficit

Nitrogen balance is a useful tool in studying the different flows of N in an agricultural productive system because it summarizes the principles of the various transformations and biological processes of a soil (Keiner et al. 1997). The results of this research showed a soil mineral N deficit (which is a simple and effective example of N balance) in all the treatments, in both years (Figure 6). In particular, the N deficit was significantly higher (P

FIGURE 6. Soil N mineral deficit (mean of the two soil tillage depths) for two-year experiment as affected by N strategies. Within each year the values with different letters were significantly different according to Duncan Multiple Range Test at P

TABLE 3.

Effect of N strategies (mineral fertilizer and MSW application) on chemical soil characteristics recorded before experiment and at the end of each experimental year of sugarbeet grown in Foggia (Southern Italy) area.

The total organic carbon significantly increased from 2001 to 2003 (11.64 and 14.35 g kg^sup -1^, respectively) (Table 3), probably due to the large amount of crop residues produced by the sugarbeet and incorporated into the soil with the tillage. MSW application did not overcome this effect and, therefore, no significant variation was observed among the treatments tested. For extracted organic carbon, significant increases (P

The ability of plants to accumulate and to translocate metals and micronutrients to edible and harvested parts depends on the soil characteristics, climatic factors, plant genotype and agronomic managements (McLaughlin et al. 1999). The results of this research indicated that no significant increase in the total content of heavy metals was found in the Ncom treatment at the end of the two-year experiment (Table 3). As suggested by Montemurro et al. (2005b), the lack of heavy metals accumulated could be due to the dilution in the soil of the elements contained in the MSW compost after its application. Even if the prevention of heavy metal accumulation in the soil is one of the most important requisites for a sustainable agricultural production, the findings of this research confirmed that the presence of trace elements in MSW compost should not affect soil application (Logan et al. 1999), at least in the short-term period (two consecutive distributions). Conversely, Zhang et al. (2006) suggest that repeatedly application of compost on the same land could lead to accumulation of metals in the soil. Finally, the amount of these elements in the arable soil could be due to both natural occurrence and anthropogenetic sources, but only the latter are manageable and, as a consequence, to reduce heavy metal accumulation, the amount of external materials should be minimized (Moolenaar and Beltrami 1998).

Conclusions

The results obtained from our two-year research experiment on MSW compost applied to sugarbeet crop highlighted that its application can be considered an important N source, especially when it was distributed in association with mineral N fertilizer (Nmix treatment). In particular, sucrose yield reached greater values in the soils amended with this organic material, in comparison with the unfertilized control, and the alfa amino N showed the least value in Nmix treatment. The trace elements in the soil did not increase in the short-term period (two compost applications). Therefore, the recommendation for the MSW compost use should consider either the optimal application rate for crop production, or the number of distributions that a land can receive for not overcoming the limits imposed by the law. The application of MSW compost significantly increased the extracted organic carbon in the soil in both 2001 (37.01% of increase for Ncom treatment in respect to the unfertilized control) and 2003 (23.75% of increase) years. Although with no statistical significance, the results also indicated an increase in total organic carbon for 2001 (20.30%) and in humified organic carbon for both 2001 (26.89%) and 2003 (15.78%). In addition, the use of MSW compost as a partial substitution of mineral fertilizer (Nmix treatment) appeared to be a very useful practice to sustain sugarbeet quality. Therefore, the findings of this research showed the possibility of integrating mineral fertilization with compost to improve soil fertility and, consequently, its application could be very important when the mineralization rate is high.

Finally, the petiole nitrate content, highly and positively correlated in this study with root and sucrose yields, can be usefully adopted to schedule N fertilization during sugarbeet growth.

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F. Montemurro and M. Maiorana

Istituto Sperimentale Agronomico, Ban, It

Copyright J.G. Press Inc. Spring 2007

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