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notatum Flügge) pastures growing on Pomona and Smyrna sands. Treatments were three management intensities: Low (40 kg N ha21yr21 and 1.3 animal units ...
Litter Decomposition and Mineralization in Bahiagrass Pastures Managed at Different Intensities J. C. B. Dubeux Jr., L. E. Sollenberger,* S. M. Interrante, J. M. B. Vendramini, and R. L. Stewart Jr. and greater availability of soil nutrients in fertilized systems, may increase litter turnover resulting in greater nutrient supply to the pasture (Lupwayi and Haque, 1999). Stocking rate may also affect litter decomposition rates by altering soil nutrient availability (Thomas, 1992) and by modifying sward structure that creates a different microclimate (Hirata et al., 1991). Therefore, management practices affect nutrient dynamics in pasture ecosystems, but little attention has been given to this topic in grazing trials (Mathews et al., 1994). Greater understanding of these processes is needed to address the problems of pasture degradation in extensively managed systems and of excessive nutrient loss to the environment in intensively managed systems. The objective of this study was to evaluate the effect of pasture management intensity, defined in terms of N fertilization and SR, on aboveground plant litter decomposition and mineralization.

Reproduced from Crop Science. Published by Crop Science Society of America. All copyrights reserved.

ABSTRACT Plant litter is an important nutrient pool in grassland ecosystems. Management practices affect litter quality and may affect nutrient dynamics in pastures by altering the rates of nutrient mineralization and immobilization. The effect of management intensity on litter decomposition and nutrient disappearance was evaluated in a litter bag study on continuously stocked ‘Pensacola’ bahiagrass (Paspalum notatum Flu¨gge) pastures growing on Pomona and Smyrna sands. Treatments were three management intensities: Low (40 kg N ha21yr21 and 1.3 animal units [AU, one AU 5 500 kg live weight] ha21 stocking rate [SR]), Moderate (120 kg N ha21 yr21 and 2.7 AU ha21 SR), and High (360 kg N ha21 yr21 and 4.0 AU ha21 SR). Litter relative decomposition rate (k) was greater for High (0.0030 g g21 d21) than Low (0.0016 g g21 d21). Litter N, acid detergent insoluble N (ADIN), and lignin concentrations were greater for High than the other intensities at the end of the 168-d incubation period because of faster decomposition of soluble compounds. Across management intensities, approximately one-half of litter N remaining at the end of the incubation period was bound to acid detergent fiber (ADF). Net N mineralization through 128 d of incubation was only 200 to 300 g kg21 of total N. Increasing management intensity resulted in faster litter turnover and greater nutrient release, but nutrient release from litter was small and significant quantities of nutrients were immobilized even under the most intensive management.

L

MATERIALS AND METHODS Experimental Site A grazing experiment was performed at the University of Florida Beef Research Unit northeast of Gainesville, FL, at 298439 N lat on Pensacola bahiagrass pastures that had been established at least 10 yr before initiation of the study. Soils are mainly of the Pomona (sandy siliceous, hyperthermic Ultic Alaquods) and Smyrna (sandy siliceous, hyperthermic Aeric Alaquods) series of sandy Spodosols. Soil characteristics were described by Dubeux et al. (2006).

is often characterized on the basis of its concentration of C, N, P, lignin, polyphenols, and their ratios (Heal et al., 1997; Thomas and Asakawa, 1993), and these litter quality indicators are related to the nutrient mineralization and immobilization processes (Palm and Rowland, 1997). Litter of C4 grasses often is low in quality, resulting in N immobilization (Thomas and Asakawa, 1993), which in turn may lead to pasture degradation in low N-input systems (Rezende et al., 1999). In contrast, litter may play an important role in immobilizing nutrients and reducing nutrient losses to the environment in highly fertilized pastures (Wedin, 1996). Nitrogen fertilization and stocking rate may affect not only the amount of litter produced (Dubeux et al., 2006) but also its decomposition rates (Beare et al., 2005). Greater litter quality, because of higher nutrient uptake ITTER QUALITY

Treatments and Design Treatments were imposed during 2001, 2002, and 2003, but litter responses were measured only during 2002 and 2003. Treatments were three management intensities, defined in terms of combinations of stocking rate and N fertilization. The management intensities were Low (40 kg N ha21 yr21 and 1.2 AU ha21 target SR), Moderate (120 kg N ha21 yr21 and 2.4 AU ha21 target SR), and High (360 kg N ha21 yr21 and 3.6 AU ha21 target SR). Because initial heifer liveweight was greater than anticipated, actual SR were 1.4, 2.8, and 4.1 AU ha21 in 2002, and 1.3, 2.6, and 4.0 AU ha21 in 2003 for Low, Moderate, and High management intensities, respectively. A randomized complete block design was used and each treatment was replicated twice. Rationale for treatment selection, description of animal management, pasture fertilization, and facilities were reported by Dubeux et al. (2006).

J.C.B. Dubeux, Jr., Dep. de Zootecnia/UFRPE, Av. Dom Manoel de Medeiros, S/N, Dois Irma˜os, 52171-900, Recife-PE, Brazil; L.E. Sollenberger and S.M. Interrante, Agronomy Dep., University of Florida, Gainesville, FL 32611-0300; J.M.B. Vendramini, Soil and Crop Science Department, Texas A&M University, Overton, TX 75684; R.L. Stewart, Jr., Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0306. This research was sponsored in part by USDA CSREES Tropical and Subtropical Agricultural Research Program Grant 34135-12348. Received 19 Aug. 2005. *Corresponding author ([email protected]).

Litter Decomposition and Nutrient Disappearance Litter decomposition and nutrient disappearance were estimated by a litter bag technique. For the purposes of these measurements, litter was defined as the senescent leaves still

Published in Crop Sci. 46:1305–1310 (2006). Forage & Grazinglands doi:10.2135/cropsci2005.08-0263 ª Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: ADF, acid detergent fiber; ADIN, acid detergent insoluble N; AU, animal units; DM, dry matter; k, relative decomposition rate; NDF, neutral detergent fiber; OM, organic matter; SE, standard error; SR, stocking rate.

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attached to the plant. The reason for this approach was to avoid collecting litter on the ground that was already degraded to an unknown extent. The litter was obtained by cutting standing herbage from each of the six experimental units during May of each grazing season. Herbage from each experimental unit was kept separate from the others and was oven-dried (608C for 72 h) but not ground so that surface area remained as similar as possible to the original litter. Green and senescent herbage was hand-separated thereafter, and the senescent fraction (6 g per bag) was placed into polyester bags with 75-mm mesh size and measuring 15 3 20 cm. The bags were heat-sealed and incubation times were 0, 4, 8, 16, 32, 64, and 128 d. Each incubation time, with the exception of Day 0, was replicated six times within each experimental unit, resulting in 36 bags per experimental unit. Empty bags were also incubated for the different periods to correct the bag weight after incubation. Litter bags were placed on the ground in sets of six, one for each incubation time, and covered with existing litter from that experimental unit. The sites where bags were placed were chosen to represent the average herbage mass of the pasture, on the basis of disk settling height. Cages were placed over the sites where each set of six bags was located to protect them from grazing animals. Thus, a total of six cages per pasture were used for the litter bag experiment, one cage for each complete set of incubation times. Herbage inside the cage was clipped biweekly throughout the 128-d period to maintain the herbage height inside the cage as close as possible to the average canopy height of the pasture, and the clipped material was removed. The 128-d periods were from 22 July to 27 Nov. 2002 and 23 July to 28 Nov. 2003. At the end of each incubation time, the six litter bags per pasture were collected, oven-dried (608C for 72 h), and composited within an experimental unit. The composited samples were milled to pass a 1-mm screen and analyzed for dry matter (DM), organic matter (OM), C, N, P, neutral detergent fiber (NDF), ADF, lignin, and ADIN. Dry matter and OM analyses were performed by the procedure described by Moore and Mott (1974). Carbon, N, and ADIN analyses were done by dry combustion with a Carlo Erba NA-1500 C/N/S analyzer (HaakBuchler Instruments, Saddlebrook, NJ). Phosphorus was determined by micro-Kjeldahl digestion and read in the autoanalyzer using a colorimetric procedure. The NDF, ADF, and lignin analyses were run in an ANKOM fiber analyzer (ANKOM Technology, 2003a, 2003b, 2003c). The percentage of remaining nutrient was calculated on the basis of the content of each nutrient before and after the incubation period.

Statistical Analyses Nonlinear models were used to fit the decay curves by Proc NLIN from SAS (SAS Inst., 1996). Before choosing the model, each data set was plotted to observe the pattern of distribution. Decay curves usually followed the double or single exponential functions, and nutrient concentration data followed the two-stage model. The double exponential model was used first to explain the decay curves, and when it was not significant (P . 0.10), the single exponential decay model was used and fit the data well. This happened when nutrient immobilization occurred to a greater extent at the beginning of the incubation periods, as in the total N decay curve. The double exponential model (Weider and Lang, 1982) was used for biomass decay and P loss curves, and it is described by Eq. [1]:

x 5 Ae2k1 t 1 (12A)e2k2 t

[1]

where x 5 proportion of remaining biomass at time t, A 5 constant k1, and k2 5 decay constants.

After solving the above equation, the output parameters (A, k1, and k2) of each experimental unit were used to calculate their respective k values using Eq. [2] (Weider and Lang, 1982):

k5

2k1 Ae2k1 t 2 k2 (12A)e2k2 t Ae2k1 t 1 (12A)e2k2 t

[2]

The time used to calculate k was 128 d, which corresponds to the total length of each incubation trial. The single exponential model (Wagner and Wolf, 1999) was used for total N decay and C to N ratio, and it is described by Eq. [3]:

x 5 B0 e2kt

[3]

where x 5 proportion of remaining biomass at time t, B0 5 constant, k 5 decay constant. The two-stage model described by McCartor and Rouquette (1977) was used to fit nutrient concentration over time. Pearson correlation coefficients were calculated for all models applied, correlating the observed data with the predicted data from the models. After fitting the appropriate model for each experimental unit within each grazing season, the output parameters were analyzed by Proc Mixed from SAS (SAS Inst., 1996) with year considered a fixed effect. Means were compared by the LSMEANS procedure of SAS.

RESULTS AND DISCUSSION Litter Relative Decomposition Rate The k value for litter biomass during the 128-d incubation trial increased with management intensity. Relative decomposition rate was 0.0016, 0.0021, and 0.0030 g g21 d21 for Low, Moderate, and High, respectively, with High being greater than Low (P 5 0.085), and Moderate not different from either Low or High (P . 0.170). Considering the k values obtained after 128 d of incubation, the litter half-life in the Low management intensity was 433 d while the litter half-life in High was 231 d. Nitrogen fertilization has been reported to increase residue mineralization rate (Kalburtji et al., 1997; Lupwayi and Haque, 1999). Increasing SR increases the proportion of nutrients returning to the pasture via excreta (Thomas, 1992), and those nutrients are more available to pasture plants than those returned via C4 grass litter (Haynes and Williams, 1993). Therefore, litter decomposition rates are also expected to be greater when higher SR is adopted. The k value depends on litter quality, soil temperature, and soil moisture (Heal et al., 1997). This includes the proportion of the total C remaining in the litter, as k is greater at the beginning of the incubation period (Gijsman et al., 1997). In the current study, litter biomass loss over the 128-d incubation followed a double exponential model (Fig. 1), and although litter relative decomposition rates for DM varied among management intensities, the output parameters from the double exponential model were similar (P . 0.10). Loss was rapid at the beginning of the incubation; approximately 15% of the litter biomass was lost after only 8 d. The k value averaged 0.0148 g g21 d21 during the first 14 d vs. 0.0022 g g21 d21 over the entire 128 d of incubation. The faster rate of decay early in the period results from the

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Fig. 1. Litter biomass remaining on Pensacola bahiagrass pastures managed at a range of intensities during 2002–2003. Pearson correlation coefficient 5 0.91.

decomposition of more soluble compounds, but the k value tends to stabilize, or decrease slowly, after the more soluble compounds are decomposed (Heal et al., 1997). Decay rate slowed after this initial period, and biomass loss after 128 d of incubation ranged from 40 to 60%. These values are similar to those reported by Deshmukh (1985) using the litter bag technique to estimate C4 grass litter decomposition in Kenya. Sollenberger et al. (2002) reviewed k in the literature and found values for different tropical grasses ranging from 0.0020 g g21 d21 in dictyoneura [Brachiaria dictyoneura (Fig. & De Not.) Stapf] (Thomas and Asakawa, 1993) to 0.0174 g g21 d21 in ‘Aruana’ guineagrass (Panicum maximum Jacq.; Schunke, 1998). The k values for tropical legumes ranged from 0.0017 g g21 d21 in desmodium (Thomas and Asakawa, 1993) to 0.0603 g g21 d21 in Arachis repens Handro (Ferreira et al., 1997). Data from these trials were gathered in the summer rainy season; however, different incubation periods, different approaches to gathering litter, and varied environmental conditions contribute to large variability across experiments.

Litter N Disappearance Total N disappearance followed a single exponential model, and there were differences due to year but not among management intensities (Fig. 2A). Net N mineralization varied from 200 to 300 g kg21 after 128 d of incubation, resulting in a small contribution of N from the litter pool to the pasture. A perspective on the importance of the litter pool in terms of N immobilization and mineralization was obtained by linking litter deposition (Dubeux et al., 2006) and the N-release curves. Considering an average rate of litter deposition of 27 kg ha21 d21 for 2002 and 2003, and litter N concentration of 12.7, 14.3, and 21.6 g kg21 for Low, Moderate, and High (Dubeux et al., 2006), respectively, the amount of N returned through the litter

pool was estimated for a period of 140 d, calculated in 10 cycles of 14 d. Nitrogen released during this period by the litter pool was estimated by the decomposition parameters for N in 2003 (B0 5 0.9338 and k 5 0.00287, which are the single exponential model parameters). Because litter first deposited had 140 d to decompose while the litter deposited during the 10th cycle had only 14 d, different extents of decomposition were accounted for when the final amount of N released was estimated. As management intensity increased from Low to High, N immobilized increased from 49 to 83 kg N ha21 and N mineralized increased from 12 to 20 kg N ha21. This level of N mineralization supports the observation that in low N input C4 grass systems (e.g., the Low management intensity in this study), low litter quality plays a role in N immobilization and may contribute to pasture degradation (Fisher et al., 1994; Cantarutti, 1996). It also suggests a role of litter as a buffering pool (Wedin, 1996), potentially reducing N losses to the environment in highly fertilized pasture systems (e.g., the High management intensity in this study). The immobilization potential of C4 grass litter has been observed in other studies. For example, in green panic (Panicum maximum Jacq. var. trichoglume) pastures, net N mineralization did not occur until 50 to 100 d after litter deposition. Even after a year, only 200 to 300 g kg21 of all litter N was released in the soil, primarily because of microbial immobilization (Robbins et al., 1989). In another study, during the first week of incubation of soil samples with herbage of creeping signalgrass [Brachiaria humidicola (Rendle) Schweick.], 600 to 800 g kg21 of all soil mineral N was immobilized in the microbial biomass, and 300 to 500 g kg21 remained immobilized after 150 d (Cantarutti, 1996). Concurrently, an increase of N in the microbial biomass of 12 to 36% was measured, indicating that a large proportion of soil mineral N was effectively immobilized.

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deposition (27 kg OM ha21 d21; Dubeux et al., 2006), the amount of P returned through this above-ground litter was approximately 5.7 kg ha21 during a 140-d period. If an average of 50% of this P was released, only 2.9 kg ha21 would be made available to the pasture from litter during this period. This may be an overestimation because of the shorter time period available for degradation of litter P deposited later in the grazing season. Therefore, the above-ground litter contribution to P supply of these pastures was very limited. The potential for P immobilization, however, particularly by the below-ground litter is great. Gijsman et al. (1997) reported root C:P ratio up to 1780 in creeping signalgrass grown on an Oxisol, while microbial C:P ratio in these soils ranged from 34 to 50. When considering C:P ratio, values below 200:1 result in mineralization predominating, whereas above 300:1 immobilization is greatest (Dalal, 1979; McLaughlin and Alston, 1986; Novais and Smyth, 1999). Considering that the P concentration in the litter DM on Day 0 was 1.5 g kg21 and the C concentration was 430 g kg21, the average C:P ratio on Day 0 was 287, and increased with length of the incubation period.

Litter C-to-N Ratio

Fig. 2. N disappearance during incubation of litter from Pensacola bahiagrass pastures managed at a range of intensities during 2002 and 2003 (A). Pearson correlation coefficient in 2002 5 0.59; Pearson correlation coefficient in 2003 5 0.77. P disappearance of litter averaged across 2002 and 2003 (B). Pearson correlation coefficient 5 0.88. Litter C to N ratio from 2002 and 2003 (C). Pearson correlation coefficient in 2002 5 0.62; in 2003 5 0.71.

Litter P Disappearance Phosphorus decomposition was described by a double exponential decay model, and no management intensity or year differences were detected (Fig. 2B). After 128 d of incubation, approximately 600 g kg21 of net P mineralization had occurred. The average litter P concentration on Day 0 was 1.5 g kg21, and it decreased to 1.2 g kg21 by Day 128. Assuming the average rate of litter

Litter C to N ratio decreased across the incubation period. The single exponential model fit this response, with differences between years but not among management intensities (Fig. 2C). Decreasing C to N ratio over time is expected because the more soluble C compounds decompose rapidly, but N immobilization by the low quality residue and the N bound to the fiber reduce N losses. Residue C to N ratio in 2002 at the start of the incubation period was greater than in 2003 (Fig. 2C) and explains the slower rate and lesser extent of N mineralization in 2002 (Fig. 2A). Final C to N ratios were less than 20 in 2003, thus, mineralization of that litter should occur. The high lignin concentration at the end of the period (Fig. 3B), however, likely was controlling the decomposition rate. Although C to N ratio remains a critical variable in decomposition models, several studies have demonstrated important interactions with other factors including the form of the C in the plant cells as an energy source, the concentration of other nutrients, and the composition of various secondary plant compounds (Heal et al., 1997).

Litter N Concentration: Total N and ADIN Total N concentration in the litter increased during the incubation period for all management intensities (15–24 g kg21 for Low, 15–23 g kg21 for Moderate, and 17–31 g kg21 for High), but it increased to the greatest extent for High (P = 0.001). The character of litter N also changed over time. The ADIN concentration in the residue increased from beginning to end of the incubation (Fig. 3A), and ADIN as a fraction of total N increased from 200 g kg21 of total N at the start of incubation to between 400 and 500 g kg21 of total N by Day 40, and this concentration was sustained through the end of the incubation (data not presented). Thus, despite the

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time (Fig. 3B); therefore, the N-binding capacity was also likely to be greater in High.

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Litter Lignin and Lignin:N Ratio

Fig. 3. Acid detergent insoluble N (ADIN) (A), ash-free lignin concentration (B), and lignin:N ratio (C) in Pensacola bahiagrass litter from pastures managed at a range of intensities during 2002–2003.

Ash-free lignin concentration also increased during incubation, and similar to ADIN, it increased to the greatest extent for the High management intensity (Fig. 3B). Lignin plays an important role in the decomposition process because of all naturally produced organic chemicals, lignin is probably the most recalcitrant (Hammel, 1997). Heal et al. (1997) reported that litter decomposition is mainly controlled by the rate of lignin decomposition, and that this rate, in turn, is increased by high cellulose concentration and decreased by a high N concentration. Keyser et al. (1978) demonstrated that the ligninolytic system of lignin-decomposer fungi is synthesized in response to N starvation. Therefore, the greater lignin concentration for High (vs. Moderate, P 5 0.001; vs. Low, P 5 0.003) was likely not only because of greater decomposition rates resulting in more rapid decomposition of soluble compounds leaving lignin behind, but also because of lower lignin decomposition rates resulting from more N available in High. Lignin concentration 64 d after incubation initiated was greater than 250 g kg21 in the High management intensity. Lignin:N ratio also increased over the incubation period, but unlike ADIN and lignin concentrations, it was lowest for High (Fig. 3C). Lignin:N ratio is an indicator of residue decomposition rate, and is often negatively correlated with biomass loss (Thomas and Asakawa, 1993). Magid et al. (1997) suggested, however, that the lignin:N ratio is not a critical determinant of short- to medium-term decomposition rates, but it may be very important in governing long-term decay. Heal et al. (1997) pointed out that cereal and legume straws and litter from annual crops usually contain less than 100 to 150 g kg21 of lignin, and C-to-N ratios of 50 to 100 are reasonable predictors of decomposition rates because the higher ratios mainly reflect lower N concentration in tissues rather than changes in C form. When lignin is increasing over time, however, the lignin:N ratio may be a better indicator than C:N of C availability to microorganisms. Although lignin concentration was greater for the High management intensity, lignin:N ratio was lower (vs. Moderate, P 5 0.107; vs. Low, P 5 0.068), indicating a better quality litter resulting in faster relative decomposition rates for the litter at the High management intensity.

CONCLUSIONS increase in N concentration over time, almost half of this N was bound to the ADF; therefore, it had low availability for microbial decomposition. The availability of C and N, rather than their total concentration in the residue, plays a critical role in residue decomposition and nutrient release (Ruffo and Bollero, 2003). Whitmore and Handayanto (1997) suggested that increasing lignin concentration increases the protein-binding capacity of residues. The High management intensity residue had a greater increase in lignin concentration over

Increasing N fertilization and SR of bahiagrass pastures resulted in litter with faster relative decomposition rate and greater quantity of nutrient mineralization. Across a wide range of intensities, however, the quantity of nutrient released from litter during the growing season in which it was deposited was low and the quantity immobilized relatively high. This supports the conclusions of other researchers who have suggested that the relatively low quality of C4 grass litter plays an important role in N immobilization and contributes to

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pasture degradation in low-input systems (Fisher et al., 1994; Cantarutti, 1996). Our data also support the suggestion of a role of litter as a buffering pool (Wedin, 1996), especially for N, potentially reducing nutrient losses to the environment in heavily fertilized pastures. Because roots and rhizomes are an important nutrient pool in Pensacola bahiagrass pastures, additional investigation of below-ground litter quality and decomposition rates will enable better understanding of nutrient dynamics in the total system. REFERENCES Technology, A.N.K.O.M. 2003a. Method for determining acid detergent lignin in beakers [Online] http://www.ankom.com/09_ procedures/procedures4b.shtml (verified 11 February 2006). Technology, A.N.K.O.M. 2003b. Method for determining acid detergent fiber [Online] http://www.ankom.com/09_procedures/ procedures1.shtml (verified 11 February 2006). Technology, A.N.K.O.M. 2003c. Method for determining neutral detergent fiber [Online] http://www.ankom.com/09_procedures/ procedures2.shtml (verified 11 February 2006). Beare, M.H., D. Curtin, S. Thomas, P.M. Fraser, and G.S. Francis. 2005. Chemical components and effects on soil quality in temperate grazed pasture systems. p. 25–36. In S.C. Jarvis et al (ed.) Optimisation of nutrient cycling and soil quality for sustainable grasslands. Wageningen Academic Publishers, Wageningen, the Netherlands. Cantarutti, R.B. 1996. Dinaˆmica de nitrogeˆnio em pastagens de Brachiaria humidicola em monocultivo e consorciada com Desmodium ovalifolium cv. Itabela no sul da Bahia. D.S. Dissertation, Universidade Federal de Viçosa, Viçosa, MG. Dalal, R.C. 1979. Mineralization of carbon and phosphorus from carbon-14 and phosphorus-32 labeled plant material added to soil. Soil Sci. Soc. Am. J. 43:913–916. Deshmukh, I. 1985. Decomposition of grasses in Nairobi National Park Kenya. Oecologia 67:147–149. Dubeux, J.C.B., Jr., L.E. Sollenberger, J.M.B. Vendramini, R.L. Stewart, Jr., and S.M. Interrante. 2006. Litter mass, deposition rate, and chemical composition in bahiagrass pastures managed at different intensities. Crop Sci. 46: this issue, add page numbers. Ferreira, E., C.P. Rezende, R.M. Boddey, S. Urquiaga, and B.J.R. Alves. 1997. Decomposiça˜o da liteira de diferentes espe´cies forrageiras avaliadas no campo em diversas condiço˜es clima´ticas. (Arq496; CDROM) 26th Congresso brasileiro de cieˆncia do solo. SBCS, Rio de Janeiro, RJ, Brazil. Fisher, M.J., I.M. Rao, M.A. Ayarza, C.E. Lascano, J.I. Sanz, R.J. Thomas, and R.R. Vera. 1994. Carbon storage by introduced deeprooted grasses in the South American savannas. Nature 371: 236–238. Gijsman, A.J., H.F. Alarco´n, and R.J. Thomas. 1997. Root decomposition in tropical grasses and legumes, as affected by soil texture and season. Soil Biol. Biochem. 29:1443–1450. Hammel, K.E. 1997. Fungal degradation of lignin. p. 33–45. In G. Cadisch and K.E. Giller (ed.) Driven by nature: Plant litter quality and decomposition. CAB International, Wallingford, UK. Haynes, R.J., and P.H. Williams. 1993. Nutrient cycling and soil fertility in the grazed pasture ecosystem. Adv. Agron. 49:119–199. Heal, O.W., J.M. Anderson, and M.J. Swift. 1997. Plant litter quality and decomposition: An historical overview. p. 3–30. In G. Cadisch and K.E. Giller (ed.) Driven by nature: Plant litter quality and decomposition. CAB International, Wallingford, UK. Hirata, M., Y. Sugimoto, and M. Ueno. 1991. Litter decomposition in bahiagrass (Paspalum notatum Flu¨gge) swards under different cutting heights. Grassl. Sci. 36:458–463. Kalburtji, K.L., A.P. Mamolos, and S. Kostopoulou. 1997. Nutrient release from decomposing Lotus corniculatus residues in relation to soil pH and nitrogen levels. Agric. Ecosyst. Environ. 65:107–112. Keyser, P., T.K. Kirk, and J.G. Zeikus. 1978. Ligninolytic enzyme

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