ALVES Bruno J. R. (1), ZOTARELLI Lincoln (1), FERREIRA Elvino (1), OLIVEIRA Octávio C. de (1), BODDEY Robert M. (1), URQUIAGA Segundo (1)

Transcription

1 Scientific registration n : 2210 Symposium n : 14 Presentation : poster Decomposition and release of N, P and K of litter from grass and legume forage species either alone or mixed Décomposition et libération de N, P et K de litières d'espèces fourragères herbacées et légumineuses seules ou en mélange ALVES Bruno J. R. (1), ZOTARELLI Lincoln (1), FERREIRA Elvino (1), OLIVEIRA Octávio C. de (1), BODDEY Robert M. (1), URQUIAGA Segundo (1) (1) EMBRAPA - Agrobiologia, P.O.Box 74505, , Seropédica, RJ, Brazil. Introduction Beef cattle on tropical soils is usually managed with minimum fertiliser use and this situation has lead to serious limitations of N and P to plant growth, being assigned as one of the main causes to pasture decline and degradation (Oliveira et al., 1997). An annual P application is commonly used in many areas but it seems did not prevent pasture degradation that reinforce the idea that N should be also applied to enhance and keep pasture productivity. In most cases N fertilisation is economically prohibitive and the introduction of a legume as companion forage is considered as an alternative strategy to supply an extra N (from biological nitrogen fixation) to the pasture. Forage legumes could fix from atmosphere most of their N demanded even under poor soil fertility (Cadisch et al., 1989; Viera-Vargas et al., 1995). In a mixed sward the forage legume could represent an input of 40 to 60 kg/ha of external N, depending on its proportion in the sward. It is desirable that the amount of fixed N should balance the exported N as animal products and the N lost by leaching and volatilisation (Boddey et al., 1994). On the other hand the presence of the legume represents a competitive sink of other soil nutrients such as P and K. Thus, apart from the ability to fix N 2, the choice of a suitable forage legume should take into account the degradability of its residues and the consequent return of nutrients to the soil solution. According to Thomas (1992) litter recycling is the main form of release of nutrients to soil and differences in the decomposition patterns are very dependent on climate conditions and chemical quality of litter (Thomas and Asakawa, 1993). Low lignin/n litter of legumes such as Stylosanthes spp. and Arachis pintoi have decomposed two to three times faster than the high lignin/n litter of Andropogon gayanus and Brachiaria spp grasses, mainly during wet seasons (Thomas and Asakawa, 1993). This characteristic of some legumes could be determinant for grass production maintenance specially in terms of improvement of mineralization of soil organic N reserves (Thomas et al, 1995) but also through a synergistic effect by accelerating the decomposition of grass litter and consequently the nutrient release to the soil. This subject is the matter of study of this paper. 1

2 Material and Methods The experiment was conducted on a Panicum maximum pasture growing on an ultisol (Typic Hapludult) of the experimental area of EMBRAPA- Agrobiologia, Seropédica, Rio de Janeiro. Standing dead litter of Panicum maximum cv. Colonião and Stylosanthes guianensis cv. Mineirão were colected from plants of established stands. Each litter material were characterised for N and C content (C/N), and lignin and polyphenols (Follin-Denis) contents. The decomposition patterns of the forage litter were monitored using a covered litter method. Five grams of each litter material were placed on a litter-free soil surface and covered with a 20 x 20 cm nylon square (10 mesh size) firmly specked,. The mixed material was obtained by mixing 2.5 g of P. maximum and 2.5 g of S. guianensis (dry matter basis). The loss of litter mass by decomposition was monitored at the 15, 32, 62 and 144 days, by a destructive sample scheme. Sampled litter material was oven dried, weighed and ground for total N, P and K analysis. Data of litter dry matter and nutrient contents were expressed as a percentage of the amount present at the beginning of the incubation in the field. Percent of litter or nutrient remaining after a certain time were fitted to a single exponential equation in the form of X = X o.exp (-kt), were X and X o are, respectively, the percents of material remaining at a time t and potentially decomposable at a rate k (Thomas and Asakawa, 1993). From the equation it was possible to estimate the time that each plant material took to be half of the initial dry weight and half of N, P and K initially present (or the half-life of each parameter that is T 1/2 = 0.693/k). Results and Discussion All parameters of all litter material of the decomposition and nutrients release studies presented a good adjustment to the single exponential equation (Table 1). Chemical characterisation of the litter material of each forage revealed a lower C/N ratio (21) and a (lignin (6%) + polyphenols (1.1%))/N ratio of 3.5 to S. guianensis compared to the values observed to P. maximum (C/N = 43; (lignin (9%) + polyphenols (0.3%)) = 8.1). As expected by either the C/N ratio or the (lignin + polyphenols)/n ratio the decomposition of the legume litter were much faster than the grass litter (Table 1) and around 40 days after the start of incubation more than 50% of the legume litter desapeared from the system (Figure 1). The grass litter reached the same level of decomposition by 120 days of incubation. The decaying pattern observed for S. guianensis could credit this legume to be of potential value to form a mixed pasture with a grass due to its easier release of nutrients. However the presence of legume litter mixed with grass litter did not provoke any relevant acceleration on grass litter decomposition (Table 1). This affirmative is deduced from the values of predicted half-life that assumes no interaction effect between grass and legume materials. In this work these predicted values were almost the same to the observed half-life obtained from fitted equations. Potassium released from litter was generally very fast compared to the N and P (Table 1). Around 50% of total K of all litter was released to the soil after 10 days of incubation and this behaviour has been also observed in other reports (Thomas and Asakawa, 1993; Palm and Sanchez, 1990). Nitrogen and P release from litter followed the same pattern of dry matter decay observed to the legume and grass litter material, but the P release were faster. When both forage legume were mixed, instead of an expected 2

3 synergistic effect it was found an antagonistic one. The half-lives associated to the release of N and P of the mixture was over the predicted values (Table 1). The only explanations to this behaviour is the existence of some substance that acts inhibiting the soil biomass or that acts directly trapping composts rich in N and P. The allellopatic effect of some plant species is well known but there is no information related to P. maximum. The other hypothesis could better explain the antagonistic phenomenon: polyphenols-like substances have the property of trapping N-compounds that could explain the diminished release of N and P from the mixed litter. Based on the half-lives (Table 1) it could be deduced that P. maximum litter was responsible for the antagonistic effect despite of legume litter had presented higher polyphenols content. As Scalbert (1991) highlighted polyphenols have different properties with respect to binding N- containing compounds that depend more on their molecular weights and structure than its quantity. Table 1. Coefficient of determination obtained from the adjustment of a single exponential equation to percent of remaining litter dry matter and total N, P and K; and the estimated and predicted half-lives of litter of P. maximum, S. guianensis and the mixture of both. Forage Litter R 2 Litter Half-life (days) 1 T 1/2 estimated 2 T 1/2 predicted P. maximum from k DM N P K S. guianensis DM N P K Mixture 3 DM N P K The estimate were performed based in the relation T 1/2 =0.693/k. 2 Prediction obtained by a weighed mean between initial content and half-lives of each material alone g of each forage (1:1, dry weight basis). 3

4 % Litter remaining Stylosanthes guianensis Panicum maximum S. guianensis:p. maximum Days Figure 1. Loss of dry matter from litter of P. maximum, S. guianensis and the mixture of both. Bibliography BODDEY R.M., RESENDE C. DE P., PEREIRA J.M., CANTARUTTI R.B., ALVES B.J.R., FERREIRA E., RICHTER M., CADISCH G. & URQUIAGA S. (1994) The nitrogen cycle in pure grass and grass/legume pastures: evaluation of pasture sustenability. In: Nuclear Techniques in Soil-Plant Studies for Sustainable Agriculture and Environmental Preservation. p Vienna, Austria: FAO/IAEA. CADISCH G., SYLVESTER-BRADLEY R. & NÖSBERGER J. (1989) 15 N-based estimation of nitrogen fixation by eight tropical forage legumes at two levels of P:K supply. Field Crops Research. 22: OLIVEIRA de Oliveira O.C., Pereira de Oliveira I., Urquiaga S., Boddey R.M., Ferreira E., Alves B.J.R., Ayarza, M., Miranda C.H.B. e Vilela L. (1997). A baixa disponibilidade de nutrientes como uma causa potencial da degradação de pastagens no Cerrado Brasileiro. In: III Simpósio Nacional de Recuperação de Áreas Degradas (III SINRAD), Ouro Preto, MG

5 PALM C.A & SANCHEZ P.A (1990) Decomposition and nutrient release patterns of the leaves of trhee tropical legumes. Biotropica, 22: SCALBERT A (1991) Antimicrobial properties of tannins. Phytochemistry, 30: THOMAS R.J. (1992) The role of the legume in the nitrogen cycle of productive and sustainable pastures. Grass Forage Sci. 47: THOMAS R.J. & ASAKAWA N.M. (1993) Decomposition of leaf litter from tropical forage grasses and legumes. Soil Biology and Biochemistry, 25: THOMAS R.J., FISHER M., AYARZA M.A & SANZ J.I. (1995) The role of forage grasses and legumes in maintaining the productivity of acid soils in Latin America. IN: Soil Management: Experimental basis for sustainability and environmental quality (Lal R. and Stewart B.A eds.). Advances in Soil Science VIERA-VARGAS, M.S., OLIVEIRA O.C. DE, SOUTO C.M., CADISCH G., URQUIAGA S. & BODDEY R.M. (1995) Use of different 15 N techniques to quantify the contribution of biological nitrogen fixation to legumes. Soil Biol. Biochem. 34: Keywords : Covered litter method, Panicum maximum, Stylosanthes guianensis Mots clés : litière de couverture, Panicum maximum, Stylosanthes guianensis 5