INTRODUCTION
The growing global concern over the environment stimulates the adoption of conservationist management systems, and the use of cover crops in the succession of vegetable crops is an interesting practice to preserve the soil quality without preventing high yields in crops of economic interest, such as corn (Chivenge et al., Reference Chivenge, Vanlauwe and Six2011; Nyamangara et al., Reference Nyamangara, Nyengerai, Maasvaya, Tirivavi, Mashingaidze, Mupangwa, Dimes, Hove and Twomlow2014). To this end, factors like the organic matter production and chemical composition of cover crops should be taken into account for the succession of vegetable crops, as they influence the release and cycling of nutrients (Koné et al., Reference Koné, Tondoh, Angui, Bernhard-Reversat, Loranger-Merciris, Brunet and Brédoumi2008). In this regard, the concentrations of C, N and P, cell wall components (hemicellulose, cellulose and lignin) and their interrelationships have been evaluated in studies of cover crops because they are the most important regulators of decomposition of crop residues (Matos et al., Reference Matos, Cardoso, Souto, Lima and Sá Mendonça2011; Mendonça and Stott, Reference Mendonça and Stott2003).
High concentrations of nutrients (especially N and P), C/N ratios below 25:1, and low lignin contents in crop residues promote their decomposition and lead to faster release of nutrients, which fosters their direct nutritional effect as green fertilizers (Mafongoya et al., Reference Mafongoya, Giller and Palm1997; Rosolem and Calonego, Reference Rosolem and Calonego2013). On the other hand, low-quality crop residues have an indirect nutritional effect because they modify the physical, chemical and biological properties of the soil (Tian and Kang, Reference Tian and Kang1998). The fast decomposition of crop residues may be undesirable for the succession of vegetable crops in regions where winters are dry, like the Brazilian Southeast, where production and maintenance of crop residues are more difficult (Silva et al., Reference Silva, Crusciol, Soratto, Costa and Ferrari Neto2010). Therefore, it is essential to better know the chemical composition of cover crops for each region (Tian and Kang, Reference Tian and Kang1998).
The organic matter production and chemical composition of cover crops vary according to species, region, climate and management (Thönnissen et al., Reference Thönnissen, Midmore, Ladha, Olk and Schmidhalter2000). Therefore, fertilizing cover crops can be an alternative to increase organic matter production (crop residues) in regions with dry winters aiming to optimize the cultivation of corn in succession and generate more profit to producers (Foloni and Rosolem, Reference Foloni and Rosolem2008). However, little information exists on the chemical composition of cover crops as a function of application of NPK fertilizers in tropical regions with dry winters. Thus, our objective was to evaluate the effect of NPK fertilization on the organic matter production and chemical composition of legumes (white lupine), brassicas (forage radish) and grasses (oat, millet and forage sorghum) utilized as cover crops in succession to the corn crop established in an Oxisol soil in Southeast Brazil.
MATERIAL AND METHODS
Location and soil-climatic conditions
The field experiment was carried out in Alfenas-MG, Brazil (21º25’S and 45º56’W; altitude of 888 m). The climate of the region is classified as tropical humid and dry (rainy in the summers and dry in the winters), according to the Köppen classification (Köppen and Geiger, Reference Köppen and Geiger1928), with an average annual precipitation of 1,590 mm. The climatic conditions recorded during the study are shown in Figure 1. The soil of the studied area was classified as an Oxisol (EMBRAPA, 1999), with the following chemical properties (Silva, Reference Silva1999) in the top 0–20 cm layer: pH in H2O = 6.4; P-Mehlich = 19 mg dm−3; K+ = 252 mg dm−3; Ca2+ = 4.9 cmolc dm−3; Mg2+ = 0.8 cmolc dm−3; Al3+ = 0.0 cmolc dm−3; H+Al = 1.9 cmolc dm−3; sum of bases (SB) = 6.4 cmolc dm−3; CEC = 8.3 cmolc dm−3; base saturation (V%) = 77; aluminum saturation (m%) = 0; organic matter (OM) = 35 g kg−1 and remaining P= 15 mg L−1. The organic matter content was considered medium, and the phosphorus and potassium contents were considered very high, according to the Soil Fertility Committee of Minas Gerais (Comissão de Fertilidade do Solo do Estado de Minas Gerais, CFSEMG, 1999).
Figure 1. Monthly average accumulated precipitation, temperature and relative humidity of 3 the air from November 2011 to May 2012 in the city of Alfenas, Southeast Brazil.
Plant material and growth conditions
Before the implementation of the experiment, the area was planted with corn hybrid P30F53, which received 450 kg ha−1 of NPK (08-33-12) at planting, and had a fresh matter yield of 50 t ha−1 after 110 days of cultivation. After the corn was ensiled, the area was tilled manually to incorporate residues from the corn crop; to eliminate the Brachiaria decumbens Stapf. plants that occupied the corn interrows; for the seeding (±3 cm of depth) of cover crops (described below); and to incorporate the fertilizer into some treatments. The experiment was implemented according to the treatments, which consisted of the cultivation of five cover crops: oat (Avena sativa L. cv. IAC 7), white lupin (Lupinus albus L. cv. common), millet (Pennisetum glaucum L. cv. BN-2), forage radish (Raphanus sativus L. cv. IPR-116) and forage sorghum (Sorghum bicolor L. cv. AG 2002), using 80, 75, 30, 10 and 20 kg ha−1 of seeds at planting, respectively. The cover crops were cultivated with (200 kg ha−1 of NPK [08-28-16], applied during manual tilling) and without fertilization at planting, composing the 5 × 2 factorial arrangement. The fertilization dose used at the planting (200 kg ha−1 of NPK) of cover crops was half of the amount recommended for the cultivation of corn in succession to cover crops (Raij et al., Reference Raij, Cantarella, Quaggio and Furlani1996), since fertilized cover crops are expected to provide another part of nutrients (Silva et al., Reference Silva, Muraoka, Buzetti, Espinal and Trivelin2008a). A randomized-blocks experimental design with three replicates was adopted. Experimental plots had an area of 15 m2 (5 × 3 m). No treatments or irrigation were applied during the growth of the cover crops. The cover crops were harvested 5 cm above the soil surface (86 days after seeding), when the plants were in the following phenological stages: A. sativa L. (booting stage), L. albus L. (vegetative stage), P. glaucum (pre-flowering stage), R. sativus L. (vegetative stage), and S. bicolor L. (EC2 stage). After being harvested, the material was sent for analysis.
Evaluations of mass production and chemical composition
Fresh matter production was determined as the average of four subsamples harvested in each plot using a metal frame with an area of 0.25 m2 (0.5 × 0.5 m) thrown over the pasture, at random. The dry matter (DM) production was obtained after drying the plant material in a forced-air oven at 60 ºC for 72 h. Subsequently, the organic matter production results were extrapolated to 1 ha. The roots (0–20 cm depth) of the cover crops (chemical composition of the cell wall) were also evaluated after 10 plants were collected in each plot, using a mattock. After collection, the soil adhered to the roots was removed with running water and sieves to prevent losses of material. Next, the samples of root were placed in a forced-air oven at 60 ºC for 72 h. Later, the dry plant material (shoots and roots) was ground in a Wiley mill and transferred to the laboratory for analyses.
The DM contents were determined according to the method described by Association of Official Analytical Chemists (AOAC, 1990), while neutral detergent fibre (NDF, hemicellulose + cellulose + lignin) and acid detergent fibre (ADF, cellulose + lignin) concentrations were obtained by the methods described by Goering and Van Soest (Reference Goering and Van Soest1970). The hemicellulose contents were obtained as the difference between NDF and ADF. To determine the concentration of N, the plant material was subjected to sulfuric acid digestion, followed by distillation and titration and to determine the concentrations of P and K, the material was subjected to nitric-perchloric digestion. The concentration of K was determined by using a flame photometer, and P, by colorimetry (Malavolta et al., Reference Malavolta, Vitti and Oliveira1989). The accumulation of nutrients was obtained as the product between DM production and the concentration of the respective nutrient, and the nutrient use efficiency was calculated as the division between the DM production and the accumulation of the respective nutrient. Next, the results of accumulation and nutrient use efficiency were extrapolated to 1 ha.
Statistical analyses
The obtained results were subjected to analysis of variance, with means compared by Tukey's test (p ≤ 0.05) on SAS (Statistical Analyses System, 2008) software. Subsequently, Pearson's correlation studies were carried out among organic matter production, DM content, cell wall components and concentrations of N, P and K in the cover crops to determine the effect of fertilization.
RESULTS
Organic matter production and composition of the cell wall
The organic matter production from the shoots and the concentrations of DM (only shoots), NDF, ADF and hemicellulose from the shoots and roots were changed significantly (p ≤ 0.05) by the treatments (Table 1). The production of fresh matter by all cover crops was higher when there was fertilization with 200 kg ha−1 NPK (08-33-12) at planting as compared with the crops without fertilization; L. albus L. plants had the largest fresh matter of all plants. However, the DM content of A. sativa L., P. glaucum L., and R. sativus L. without fertilization was higher than that of the fertilized plants. Regarding the cell wall components, fertilization provided a 9% higher NDF content in the shoots of A. sativa L.; 22% more hemicellulose in P. glaucum L.; and 48, 36 and 377% higher NDF, ADF and hemicellulose contents, respectively, in the R. sativus L. plants as compared with the treatment without fertilization. In the roots, the NDF and ADF contents of A. sativa L., R. sativus L. and S. bicolor L. fertilized at planting was greater than that of the unfertilized plants, but the NDF of L. albus L. and P. glaucum L. was 7 and 12% higher in fertilized plants than in unfertilized plants, respectively. L. albus L. showed higher NDF and ADF contents than the other cover crops with and without fertilization (Table 1).
Table 1. Mass production from the shoots and chemical composition of the cell wall (shoots and roots) of the cover crops cultivated with and without NPK fertilization after the ensilage of corn.
Means ± standard error of the mean (SEM) followed by different uppercase letters in the row and different lowercase letters in the column differ by Tukey's test (p ≤ 0.05). FMP – fresh matter production; DMP – dry matter production; DM – dry matter; NDF – neutral detergent fibre; ADF – acid detergent fibre; LSD – least significant difference.
Concentrations, accumulations and use efficiency of N, P and K
Concentrations, accumulation and nutrient use efficiency of N, P and K in the shoots of the cover crops were changed significantly (p ≤ 0.05) by the treatments (Table 2). When fertilization was applied, shoot N concentration of R. sativus L. and S. bicolor L. increased by 33 and 135%, respectively, as compared to unfertilized plants. Interestingly, shoot N concentration in unfertilized plants of A. sativa and P. glaucum was 16 and 54% higher than in fertilized plants, respectively. S. bicolor L. plants showed the highest concentration of N and the lowest concentration of P and K as compared with the rest of the cover crops when fertilization was applied. In general, the highest accumulations of N, P and K by the cover crops were found when there was fertilization at planting. The L. albus L. plants showed the greatest accumulation of N, whereas the greatest accumulations of P were found in A. sativa L., L. albus L. and R. sativus L., and the greatest accumulation of K was observed in A. sativa L. and L. albus L. The use of N by A. sativa L. and P. glaucum L. and of P and K by S. bicolor L. was 16, 54, 82 and 20% more efficient, respectively, when fertilization was applied, as compared with the crops without fertilization. However, the use of N by L. albus L., R. sativus L. and S. bicolor L. and the use of K by A. sativa L., P. glaucum L. and R. sativus L. were more efficient in unfertilized plants (Table 2).
Table 2. Concentration, accumulation and nutrient use efficiency of N, P and K by the shoots of the cover crops cultivated with and without NPK fertilization after the ensilage of corn.
Means ± standard error of the mean (SEM) followed by different uppercase letters in the row and different lowercase letters in the column differ by Tukey's test (p ≤ 0.05). N – nitrogen; P – phosphorus; K – potassium; LSD – least significant difference.
Pearsons’ correlations
The N and the DM contents of the A. sativa L. plants with and without fertilization at planting were positively correlated, but the concentration of P in unfertilized plants was negatively correlated with the hemicellulose content, and K was also negatively correlated with NDF (Table S1 in Supplementary Material available online at http://dx.doi.org/10.1017/S001447971600034X). Conversely, the concentrations of K and NDF in the unfertilized plants of L. albus L. were positively correlated, but there was a negative correlation between P and NDF, and between N and ADF. The P. glaucum L. plants fertilized at planting displayed a positive correlation between the concentrations of N and NDF, as well as between the K and ADF. However, there was a negative correlation between the concentration of K and the DM and hemicellulose contents. When fertilization was not applied at planting, the concentration of K and the hemicellulose content of P. glaucum L. were positively correlated. Fresh matter production by R. sativus L. showed a positive correlation with the concentration of P, with and without fertilization at planting. The concentration of K in the R. sativus L. plants that were fertilized was negatively correlated with the cell wall components. The concentrations of P and K in S. bicolor L. displayed a negative correlation with the hemicellulose content when there was fertilization at planting. When the plant was not fertilized, the concentrations of N and P were positively correlated with the hemicellulose and NDF contents, respectively (Table S1 in Supplementary Material).
Interrelationships between parameters that regulate the decomposition of cover crops
The C/N, C/P, C/K, N/P, N/K, P/K, NDF/N, ADF/N and hemicellulose/N ratios in the shoots of the cover crops were changed significantly (p ≤ 0.05) by the treatments (Table 3). The C/N ratio of the L. albus L., R. sativus L. and S. bicolor L. plants that received fertilization was 14, 33 and 136% lower than that of the unfertilized plants, respectively, although the N/P and N/K ratios were higher when fertilization was applied. On the other hand, N/P and N/K ratios of A. sativa L. and P. glaucum L. decreased with fertilization. The lowest C/K ratios of the A. sativa L., P. glaucum L. and R. sativus L. plants occurred when fertilization was applied at planting, but this treatment resulted in higher NDF/N, ADF/N and hemicellulose/N in the same plants. Unfertilized L. albus L. and S. bicolor L. plants, however, showed higher NDF/N, ADF/N and hemicellulose/N than fertilized plants. P. glaucum L. showed the highest C/N and NDF/N ratios of all plants when fertilization was applied, but without it, L. albus L. had the highest NDF/N ratio. The P/K ratio of R. sativus L. and S. bicolor L. reduced when there was fertilization at planting as compared with the treatment without fertilization. Fertilization provided a higher C/P ratio in the shoots of S. bicolor L. as compared with the unfertilized treatment (Table 3).
Table 3. Ratios between nutrients and cell wall components of the cover crops cultivated with and without NPK fertilization after the ensilage of corn.
*Concentration of carbon estimated at 450 g kg−1 dry matter. C – carbon; N – nitrogen; P – phosphorus; K – potassium; NDF – neutral detergent fibre; ADF – acid detergent fibre; Hem – hemicellulose; LSD – least significant difference.
Means ± standard error of the mean (SEM) followed by different uppercase letters in the row and different lowercase letters in the column differ by Tukey's test (p ≤ 0.05).
DISCUSSION
The supply of fertilizers is essential to increase organic matter production by plants due to the required amount and physiological functions performed by the nutrients (Marschner, Reference Marschner1995). This fact was demonstrated in this study, in which the production of mass (Table 1) of all cover crops increased with NPK fertilization, even with the high fertility of Oxisol. The obtained results are similar to those described by Tian and Kang (Reference Tian and Kang1998). With the increase in organic matter production, more nutrients were extracted by the plants (Table 2), which may provide greater cycling of nutrients (crop residues) and better use of chemical fertilizers by the successive crop as a result of the increased soil biological activity (Arf et al., Reference Arf, Silva, Buzetti, Alves, Sá, Rodrigues and Hernandez1999). However, the concentration of nutrients in the biomass does not necessarily increase with the use of fertilizers, as there may be some imbalance between the nutrients in the soil solution, resulting in lower uptake of an element over another, or greater production of mass, resulting in a dilution of the concentration of that nutrient (Marschner, Reference Marschner1995). For instance, the concentrations of N in A. sativa L. and P. glaucum L. and the concentrations of P and K in S. bicolor L. were higher without fertilization (Table 2), which changed the composition of the cell wall of cover crops differently among the treatments (Table 1).
Although the molecules of hemicellulose, cellulose and lignin do not have N, P and K in their structure, these nutrients affect directly and indirectly the expression of enzymes acting in their synthesis (Gille et al., Reference Gille, Cheng, Skinner, Liepman, Wilkerson and Pauly2011; Quang et al., Reference Quang, Hallingbäck, Gyllenstrand, von Arnold and Clapham2012). Fertilization with NPK increased the hemicellulose content of P. glaucum L. plants and the hemicellulose + cellulose + lignin (NDF) content of R. sativus L. and A. sativa L. plants (Table 1). The increase in the concentrations of hemicellulose over lignin is interesting in the utilization of cover crops, since the former is more easily degraded by the soil microorganisms than lignin (Espíndola et al., Reference Espíndola, Guerra, Almeida, Teixeira and Urquiaga2006). This allows the nutrients, especially N, to be released faster, which may provide a reduction in nitrogen fertilization in planting the successive crop (Silva et al., Reference Silva, Muraoka, Buzetti, Espinal and Trivelin2008a). In the roots, the cellulose + lignin (ADF) of A. sativa L., R. sativus L., and S. bicolor L. increased with the supply of NPK at planting (Table 1). The increase in these cell wall fractions in the roots may contribute to increasing the water infiltration and porosity of the soil, since the decomposition of the roots by the soil microorganisms will be slower because of the mechanical protection of lignin on cellulose, enabling the formation of channels throughout the soil profile (Rosolem et al., Reference Rosolem, Foloni and Tiritan2002).
Based on the results obtained in this study, it is clear that the effect of fertilization with NPK on the cell wall contents of cover crops still needs to be further studied, given that when the correlation between the levels of K and hemicellulose in P. glaucum L. was analysed, we observed that it was negative when NPK was supplied, and positive without fertilization (Table S1 in Supplementary Material). Just like the hemicellulose, cellulose and lignin contents, the interrelationship of between fractions and the concentrations of C, N, P and K modify the chemical composition of the plant residues and their decomposition rate (Lima et al., Reference Lima, Sakai and Aldrighi2012). The C/N ratios of L. albus L., R. sativus L. and S. bicolor L. decreased when NPK was supplied at planting (Table 3). Lower C/N ratios provide a rapid decomposition of the crop residues in tropical regions, but, in the winter, decomposition is slower due to the lower temperatures and availability of water, which is beneficial for the release of nutrients to take place at the beginning of the cultivation of corn in succession (Gonçalves and Ceretta, Reference Gonçalves and Ceretta1999). This fact suggests that the successive crop in summer should be planted shortly after the cultivation of cover crops to increase the use of nutrients released in the plant decomposition process (Arf et al., Reference Arf, Silva, Buzetti, Alves, Sá, Rodrigues and Hernandez1999). The N/P and N/K ratios of A. sativa L. and P. glaucum L. were lower when NPK was applied (Table 3), suggesting that these plants utilize large amounts of absorbed N for mass production (Table 1), since the concentration of N in the leaf tissue reduced and its use efficiency by the plant increased with fertilization (Table 2) (Masclaux-Daubresse et al., Reference Masclaux-Daubresse, Daniel-Vedele, Dechorgnat, Chardon, Gaufichon and Suzuki2010).
Other ratios such as NDF/N, ADF/N and hemicellulose/N are important to indicate the variability of mineralization or immobilization of N in the soil (Lima et al., Reference Lima, Sakai and Aldrighi2012). Thus, it was observed that the NDF/N, ADF/N and hemicellulose/N ratios of the A. sativa L. and P. glaucum L. plants increased when fertilization was applied (Table 3). This occurs as a result of the increased mass production (fibrous components) and dilution of the N absorbed and incorporated into the plant. This fact is undesirable, as mentioned by Cobo and Barrios (Reference Cobo and Barrios2002), who reported negative correlations between NDF, ADF and lignin concentrations and the C/N and lignin/N ratios of cover crops with N available in the soil that led to lower uptake of N by the rice crop two to eight weeks after planting. In this regard, the use of cover plants with higher NDF/N, ADF/N and hemicellulose/N ratios would be indicated for successive crops that have a higher demand for N, P and K after the first month of growth (Carvalho et al., Reference Carvalho, Bustamante, Sousa Sobrinho and Vivaldi2008; Francisco et al., Reference Francisco, Câmara and Segatelli2007). Usually, legume species are more largely utilized for green fertilization/rotation of crops as they have lower C/N, NDF/N, ADF/N and hemicellulose/N ratios and lower contents of fibrous components; however, based on the results obtained in this study, we noted that the grass species also have potential to be used in regions with dry winters, especially when fertilized with NPK at planting (Lima et al., Reference Lima, Sakai and Aldrighi2012; Tian and Kang, Reference Tian and Kang1998).
In addition to improving the chemical composition of cover crops and, indirectly, the physical, chemical and biological soil properties, the supply of NPK can be utilized to anticipate the fertilization of the succeeding crop (Foloni and Rosolem, Reference Foloni and Rosolem2008; Francisco et al., Reference Francisco, Câmara and Segatelli2007; Tian and Kang, Reference Tian and Kang1998). These strategies allow a faster decomposition of the crop residues and release of nutrients, which benefits the corn crop in succession and results in good yields and profit (Chivenge et al., Reference Chivenge, Vanlauwe and Six2011; Silva et al., Reference Silva, Silva, Suhre, Argenta, Strieder and Rambo2007, Reference Silva, Silva, Sangoi, Piana, Strieder, Jandrey and Endrigo2008b, Reference Silva, Silva, Minetto, Strieder, Jandrey and Endrigo2008c). Although the results obtained in this study are highly satisfactory, further research should be conducted to evaluate the decomposition of residues from cover crops (legumes, brassicas and grasses) fertilized with NPK in regions with dry winters. Additionally, more studies are necessary on decomposition rates in the field and the subsequent corn crop before specific recommendations can be made about fertilization and the choice of the cover crop.
SUPPLEMENTARY MATERIALS
For supplementary material for this article, please visit http://dx.doi.org/10.1017/S001447971600034X.
