RESEARCH WORK

Response of mulberry (Morus alba) to increasing rates of organic nitrogen¹

J. A. Elizondo-Salazar

Facultad de Ciencias Agroalimentarias. Universidad de Costa Rica. San José, Costa Rica

E-mail: jorge.elizondosalazar@ucr.ac.cr

ABSTRACT

A trial was conducted at the Experimental Station «Alfredo Volio Mata» of the University of Costa Rica, in order to evaluate the application of increasing nitrogen rates (from an organic fertilizer) on the biomass production and crude protein of mulberry. The experimental period was 12 months. A mulberry plantation with 12 years of establishment, planted with spacing of 0,9 x 0,40 m (27 777 plants/ha), was used. The design consisted in completely randomized blocks with four treatments: 0, 150, 300 and 450 kg N/ha/year. The plants were pruned at 0,6 m above the soil level at the beginning of the trial and then every 90 days. The organic manure was applied in two equal dosages during the rainy season. The leaves and stems were separated and analyzed to determine the dry matter and crude protein content. No significant differences were found in the DM content or DM yield among treatments. Similar performance was found in the crude protein content of the whole plant and the stem; however, the CP content was higher with the highest nitrogen application, which significantly differed (p<0,05). It is concluded that the applied organic nitrogen rates neither showed remarkable responses in total biomass production, nor important alterations were observed in the structural proportions of the plants manifesting a change in the yield of leaves and stems.

Key words: Morus alba, organic fertilizers

INTRODUCTION

In some areas of Costa Rica and other parts of the world, forage availability and quality for grazing, sometimes, are not enough to satisfy the nutritional requirements of the animals, for which tree forages are widely used as complement for the diet.

Mulberry (Morus alba) is one of these forage species, which shows excellent palatability and intake characteristics in cattle (Boschini, 2000) as well as goats (Elizondo, 2004), besides showing high agronomic versatility and excellent biomass yields and nutritional quality (Boschini, 2006; Elizondo, 2007).

Diverse studies conducted with mulberry highlight the high protein content of the leaves, its production per area unit (Sánchez, 2002), as well as the need to restore to the soil the nitrogen extracted by the plants (Boschini et al., 1999). Thus, in order to obtain good forage yields with adequate protein levels, without compromising the soil reserves, high dosages of chemical fertilizers are required, which increases production costs and the risk of environmental contamination (Elizondo, 2006).

To reduce production costs and, in turn, use less contaminating technologies, a large number of farmers have chosen the use of organic fertilizers. Among the most used ones is compost, which is obtained by means of an aerobic biological process, in which microorganisms act on the rapidly-biodegradable matter (Van Kessel and Reeves, 2002). The efficiency with which crops utilize the nitrogen present in this type of manure depends on many factors related to the soil, plant, climate and management (Ladha et al., 2005); however, little is known about its use in crops of high forage yield, such as mulberry. For such reason, this work was conducted, in order to evaluate the application of increasing nitrogen rates (from an organic fertilizer) on the biomass production and crude protein of mulberry.

MATERIALS AND METHODS

Location. The field work was conducted at the Experimental Station «Alfredo Volio Mata» of the University of Costa Rica, located in the Cartago province at 1 542 masl.

Climate and soil. Mean annual rainfall is 2 050 mm, distributed from May to November. Mean relative humidity is 84% and mean temperature 19,5ºC. The soil, of volcanic origin, is classified as Typic Distrandepts (Vázquez, 1982) and has moderate depth, with good natural drainage and moderate fertility (table 1). Ecologically, the zone is typified as Low Humid Mountain Forest (Tosi, cited by Vázquez, 1982).

Characteristics of the area. An area of 1 000 m2 was used, of a mulberry plantation with 12 years of establishment and a planting density of 27 777 plants/ha (0,40 m between plants and 0,90 m between rows).

Experimental design and treatments. The total area was divided into three large blocks. Each block was subdivided into four plots for the different treatments and a completely randomized block design was used (Kuehl, 2002). Three organic fertilization rates (150, 300 and 450 kg N/ha/year) and a control without fertilization were used.

Experimental procedure. The application of manure was fractioned in two equal parts during the rainy season. The compost-type organic manure was elaborated from garden residues (leaves, grass and shrubs). A representative sample of the manure was sent to the Agronomic Research Center of the University of Costa Rica for its chemical analysis (table 2).

At the beginning of the experiment, the mulberry plants were pruned in a uniform way at 60 cm above the soil level. After this homogenization, four consecutive cuttings every 90 days were programmed, for a total duration of 12 months (beginning: July, 2003; end: July, 2004).

At the end of each experimental regrowth period, the plots were totally harvested at the same height of the homogenization pruning. The fresh biomass production was weighed in the field, a random sample was extracted from 10% of the whole plants of each plot and they were separated into stems and leaves. Each component was weighed fresh and dried at 60ºC for 48 hours. The samples were ground in a Willey grinder, with a one-millimeter mesh. Afterwards, the dry matter and crude protein content was determined, following the methods approved by AOAC (2002). The yields of green biomass, dry biomass and crude protein per hectare were estimated in the whole plant, the leaves and stems from the samplings made.

The data obtained were analyzed with the PROC GLM of the statistical pack SPSS (2006), according to the following mathematical model:

Yijk= µ + αi + βj + γk + εijk

Where:

Y_ijk = Production in kg/ha/year

µ= General mean

α_i = Effect of the i-eth treatment

β_j = Effect of the j-eth block

γ_k = effect of the k-eth sampling

ε_ijk = Experimental error

The sources which were statistically different (P < 0,05) were subject to Duncan's test.

RESULTS AND DISCUSSION

Table 3 shows the total green matter, dry matter and crude protein yield of the whole mulberry plant, the leaves and stems, obtained from the sequential cuttings made throughout the experiment for each treatment. In general, no significant differences were found for the evaluated production variables. The total green matter yield was 106,9; 102,6; 120,1 and 123,0 t/ha/year for rates 0, 150, 300 and 450 kg N/ha/year, respectively. These values partially exceed the ones reported by Boschini et al. (1998) and Espinoza (1996), who in intensive exploitations located in high luminosity zones, on well-drained soils and with moderate or high rainfall obtained 70-119 t/ha/year of fresh biomass.

The total dry matter yield for each of the treatments can be observed in figure 1. The application of 0, 150, 300 and 450 kg N/ha/year produced 23,4; 23,6; 26,1 and 26,7 t/ha/year, respectively; these values are very similar to the ones reported in other trials. In Guatemala, Rodríguez et al. (1992) obtained from 0,4 to 6,8 t DM/ha/y culting with whole plants, harvested at 30 cm above the soil level, with pruning intervals of six to 12 weeks and fertilization from 0 to 80 kg N/ha/year.

In another study, when using nitrogen fertilization rates of 0-480 kg/ha/year during three years, Benavides et al. (1994) obtained 19-30 t DM/ha/year. On the other hand, Elizondo (2007) evaluated the application of 150 kg N/ha/year from two types of organic manures and obtained 21,6 and 21,5 t DM/ha/year for composting and vermi-manure, respectively.

Figure 2 shows the crude protein yields. The application of 0, 150, 300 and 450 kg N/ha/year produced 2,7; 2,6; 3,1 and 3,2 t/ha/year, respectively. As general average for the four treatments, the quantity of crude protein produced is equivalent to 2 961,85 kg/ha/year, which represents 473,9 kg N/ha annually removed.

This value is very similar to the one reported by Elizondo (2007), which was 437 kg. It is important to state that the continuous removal of this nitrogen quantity depletes the reserves of any soil, for which it would be important to evaluate the mulberry production along some years when N is not applied.

Table 3 shows the dry matter and crude protein content of the whole mulberry plant, the leaves and stems, obtained from the sequential cuttings made throughout the experiment for each treatment.

The dry mater content of the whole plant was 22,13%, as average for the different treatments; this value is higher than the one found by Boschini (2001), who reported an average of 17,3%.

On the other hand, the crude protein content in the leaves was significantly higher (P<0,05) in the treatments where the highest nitrogen dose was applied as compared to the control.

Rodriguez et al. (1992), when applying 0, 40 and 80 kg N/ha/cutting, reported acceptable yields, but observed a low response in the increase of crude protein in the leaves (17,5-18,0%). The values found in this study are higher than the ones obtained by Rodriguez et al. (1992) and similar to the ones reported by Elizondo (2007), which were 10,9-16,1%. However, Boschini (2001) reported an average of 16,5%.

No remarkable differences were found when using increasing nitrogen rates. Yet, it is important to state that the nitrogen contained in organic manures and the one in the soil are found in more than 95% in organic form (White, 2006). Inorganic nitrogen is obtained during the microbial decomposition of organic matter in the ammonification process, according to the following reaction:

Organic nitrogen (proteins, nucleic acids) NH₄ is oxidized to NO₃ in the nitrification process (influenced by soil temperature, humidity and aeration) and it is represented by the following reaction:

The two reactions above describe the process known as mineralization (White, 2006), which is accelerated with an increase in temperature, adequate humidity and good oxygen availability (Jarvis et al., 1995). Once the organic N has been mineralized, plants can use it. The mineralization of N is a slow microbial process, which is affected by such factors as soil type, temperature, pH, aeration and humidity, among others (Van Kessel and Reeves, 2002). The quantity of organic N that can be mineralized is the organic N multiplied by a mineralization factor which varies between 0,25 and 0,35 (Van Kessel and Reeves, 2000). For example, the availability of organic N has been estimated in 35; 12; 5 and 2% of the initial organic nitrogen for the first, second, third and fourth year after the application of cattle manure (Van Kessel and Reeves, 2002).

On the other hand, some authors indicate that the mineralization of organic N in the first year of application can vary between 0 and 50% (Chae and Tabatabai, 1986; Lupway and Haque, 1998). Because of the above-explained facts, the lack of response in mulberry to organic fertilization in this trial can be partially ascribed to a low mineralization level of the organic nitrogen contained in the manure, which limits the quantity of inorganic nitrogen for the plants to the one that is available in the soil.

The lack of response by mulberry to nitrogen fertilization was reported by Boschini et al. (1999), when using five nitrogen dosages (0, 150, 300, 450 and 600 kg/ha) from inorganic fertilizer (ammonium nitrate); these authors did not observe a noticeable effect in the dry matter production of the whole plant, leaves and stems, which was ascribed to the high total nitrogen contents (0,29%) in the soil, at the beginning of the trial.

Contrary to the reports in this trial and the one conducted by Boschini et al. (1999), Espinoza (1996) found significant responses in total dry biomass production and obtained average yields of 16,1; 21,6 and 24,1 t/ha/year for the application of 180, 360 and 540 kg/ha/year, respectively; this study was conducted in Costa Rica, in three different zones, with three mulberry varieties and three nitrogen fertilization rates.

Rodríguez et al. (1992), when evaluating pruning frequencies and nitrogen fertilization rates, also reported a positive response. Espinoza (1996) as well as Rodríguez et al. (1992) did not refer to the relation between the nitrogen content in the soil and the production obtained, which suggests they worked on soils with low nitrogen rates.

In this trial mulberry was planted on a soil with a relatively high N content (table 1) and a moderate value of total N (0,27%). Bertsch (1995) stated that the normal range of total nitrogen in the soil can oscillate between 0,02 and 0,40%, and that the higher rates appear only when the organic matter contents are high. Thus, the lack of response of mulberry to nitrogen fertilization can be also explained by the high nitrogen levels present in the soil, considering that the content of other minerals was also found in sufficient quantities so that plant growth was not limited.

Conclusions

The nitrogen rates applied to the soil did not show noticeable responses in total biomass production, and no alteration was manifested in the structural proportions of the plant which would indicate a change in leaf or stem yield. The lack of response was accounted for by the high nitrogen contents in the soil at the beginning of the trial and the low mineralization of N in the organic manure.