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Signal Grass Litter Decomposition Rate. Increases with Inclusion of Calopo. Hiran Marcelo Siqueira da Silva, José Carlos Batista Dubeux, Jr., Mércia Virginia ...
RESEARCH

Signal Grass Litter Decomposition Rate Increases with Inclusion of Calopo Hiran Marcelo Siqueira da Silva, José Carlos Batista Dubeux, Jr., Mércia Virginia Ferreira dos Santos, Mário de Andrade Lira, Mário de Andrade Lira, Jr., and James P. Muir*

ABSTRACT Low N availability is a major limitation in tropical and subtropical pasture systems and one of the main causes for system degradation. Including legumes in forage mixes may enhance soil-N presence and cycling, therefore mitigating the problem. To test this theory, signal grass [Brachiaria decumbens (Stapf) R. D. Webster] litter chemical composition and decomposition after inclusion of calopo (Calopogonium mucunoides Desv.) at 0, 50, or 100% of litter mass was evaluated. Litter samples were collected from both species and incubated by litter bag technique for 0, 4, 8, 16, 32, 64, 128, or 256 d in 2007 and 2008. Biomass decomposition was described by a simple exponential organic matter (OM) decay model (p < 0.0001; Y predicted [the remaining biomass at a given time of decomposition, predicted by the single exponential model] = 91.11–0.00451t and 94.16 –0.00217t for year 1 and 2, respectively). Remaining biomass was lower (p < 0.05) in 2007 (28%) than 2008 (54%) following 256 d incubation at least in part because of greater lignin concentrations in 2008 litter. Pure signal grass litter C:N values were 74 to 76% greater (p < 0.05) than pure legume while the inclusion of 50% calopo reduced (p < 0.05) grass ratios by 62 to 64%. Net annual N mineralization increased (p < 0.05) from 27% without legume to 38% with legume inclusion at 50% of the grass litter, a nutrient cycling acceleration of 16% (p < 0.05). This research indicated that the inclusion of calopo will ameliorate N deficiency in soils of a signal grass pasture.

H.M.S. da Silva, J.C.B. Dubeaux, M.V.F. dos Santos, and M. de Andrade Lira, Jr., Federal Rural University of Pernambuco – UFRPE. Av. D. Manoel de Medeiros, s/n, Recife, Pernambuco; M. de Andrade Lira, Pernambuco State Agricultural Institute – IPA, Av. General San Martin, 1371, Recife, Pernambuco, 50761-000; J.P. Muir, Texas AgriLife Research, Stephenville TX 76401. *Corresponding author ( j-muir@ tamu.edu). Abbreviations: ADIN, acid detergent insoluble N; LIG, lignin; OM, organic matter.

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asture ecosystems are one of the most economic and abundant options for animal protein production in the tropics (Marcelino et al., 2006). Frequently, however, inadequate management leads to pasture degradation (Soares and Restle, 2002; Peron and Evangelista, 2004). This degradation usually starts 4 to 5 yr after pasture establishment in fertile soils and is mainly linked to low soil fertility, particularly low N availability (Lira et al., 2006). While N fertilizer use is not common in tropical pastures such as those found in Brazil for economic reasons, incorporation of N-fi xing legumes is an option for increasing N availability to grazing ruminants in the pasture (Macharia et al., 2010; PirhoferWalzl et al., 2011), leading to improved pasture soil fertility. Increasing N availability in grass–legume pastures occurs through nutrient cycling, which is a series of transfers between soil, plant, and animal. Plants transfer nutrients to soil and later to plants again through litter, defined as plant tissue deposited directly on the soil (Dubeux et al., 2006b) as well as through root death and decomposition. Both quantity and quality of litter affect nutrient cycling rates through mineralization rates (Borkert et al., 2003).

Published in Crop Sci. 52:1416–1423 (2012). doi: 10.2135/cropsci2011.09.0482 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

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In pasture ecosystems, litter decomposition during the growing season continually affects plant nutrient supply (Dubeux, Jr., et al., 2007). These systems usually consist of several species, and the litter is therefore a mixture of those species. Litter quality is modified and the different components may have nonlinear interactions, both positive and negative, when compared to single species litter (Xiang and Bauhus, 2007). The objective in this trial was to evaluate signal grass [Brachiaria decumbens (Stapf ) R. D. Webster] litter chemical composition and decomposition rates compared to those of calopo (Calopogonium mucunoides Desv.) and a mixture of 50% legume and 50% grass.

MATERIALS AND METHODS The experiments were conducted at the Itambé ExperiFigure 1. Thirty-year average and yearly rainfall for 2007 and 2008 at mental Station of the Pernambuco Agronomical Institute, Itambé Experimental Station, Pernambuco, Brazil. Pernambuco State, Brazil located at 7°25! S, 35°06! W, and 190 m altitude and with a 51-yr average annual precipitation 45% leaves while calopo litter contained 47% leaves for the fi rst of 1200 mm and mean temperature of 25°C (CPRH, 2003). year, with the remaining in both cases being stem. For the secYearly precipitation during the experiment was 1320 and 1103 ond year, the leaf proportions were 43 and 48%, respectively. mm for 2007 and 2008, respectively (Fig. 1) (data obtained from Nylon bags with a 75 µm mesh measuring 15 by 30 cm were Instituto de Tecnologia de Pernambuco [http://www.itep.br/ oven dried at 65°C for 72 h, weighed, and fi lled with 11.25 g of LAMEPE.asp] in July 2011). The experiment was conducted litter, keeping the leaf:stem proportion on a OM basis observed on a Red-Yellow Argissoil, Acrisol according to FAO-WRB for each species and year cut into 2-cm pieces to simplify manor Oxisol according to soil taxonomy (EMBRAPA, 2006). agement (Dubeux, Jr., et al., 2006a). This allowed a litter density Soil characteristics at the beginning of this experiment were of 25 mg per cm–2. The incubated litter was not ground to keep pH(water) [pH measured in water] = 5.2, Mehlich-I P = 2.9 g the surface exposure similar to that observed for plant litter in m–3, Na+ = 5.0 molc m–3, K+ = 1.0 molc m–3, Mg+ = 17.0 molc the field. The bags were put on the soil surface with no overlap m–3, Ca+ = 23.0 molc m–3, Al+3 = 4.0 molc m–3, H + Al = 65.0 and covered by a fine layer of natural litter from the same plot. molc m–3, and organic matter (OM) = 49.4 g kg–1. Empty bags were incubated at each plot and time of incubation The experiment was conducted from 26 Mar. 2007 to 7 to monitor bag weight change during the incubation period. Dec. 2007 and repeated from 29 Apr. 2008 to 9 Jan. 2009. SigAt each harvest the bags were collected and hand brushed nal grass litter decomposition was measured in mixtures with to remove soil particles, oven dried at 65°C for 72 h, weighed calopo at 0, 50, and 100% proportions incubated in the soil for and bag weights adjusted by empty bag weight. The remain4, 8, 16, 32, 62, 128, or 256 d with five replicates for each time. ing material was used to determine OM (Dirceu and Queiroz, The replicates were conducted in different field exclusions in 2006) and N concentration according to AOAC (1990), C cona signal grass field that contained calopo. Each exclusion meacentration according to Bezerra Neto and Barreto (2004), ligsured 9 m 2, and the forage was cut every 28 d to keep an avernin (LIG) and acid detergent insoluble N (ADIN) according to age height similar to the area outside the exclusion, with the cut Van Soest et al. (1991) and Pell and Schofield (1993), and fibermaterial being removed. linked N according to AOAC (1990). All data was determined Senescent material was collected manually from the pasand reported on an OM basis to reduce data variability due to ture, dried in a forced circulation oven for 48 h at 65°C, and varying soil-residue levels in the bags. Initial litter values are later separated into leaf and stem. Signal grass litter was collected shown in Table 1. from the soil surface while calopo material was collected on the Data was analyzed using SAS MIXED procedure (SAS plant before actual leaf abscision. Signal grass litter consisted of Institute, 1994). Factorial arrangement between grass:legume Table 1. Nitrogen, C, lignin (LIG), and acid detergent fiber N (ADIN) concentrations on an organic matter (OM) basis for signal grass and calopo mixes in 2007 and 2008 before soil incubation at Itambé Experimental Station of the Pernambuco Agronomical Institute, Pernambuco State, Brazil. 2007 Calopo %

N

C

2008 LIG

ADIN

—————————————— g kg –1 OM —————————————— 0 50 100

0.42 0.97 1.48

30.26 28.18 28.30

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5.69 6.73 8.54

0.29 0.44 0.73

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N

C

LIG

ADIN

—————————————— g kg –1 OM —————————————— 0.33 0.83 1.16

31.75 30.13 30.19

3.36 7.67 12.89

0.13 0.38 0.66

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ratio and incubation time was analyzed in a complete randomized block design. Fixed effects included grass:legume ratio, incubation time, year, and their interactions. Random effects were block and its interactions with the fi xed effects. When significant (p ≤ 0.05) effects were observed, means were compared by SAS LSMEANS PDIFF adjusted by Tukey and, when incubation period and its interactions showed a significant effect, nonlinear models were applied to explain the data trend. When differences (p ≤ 0.05) were found among treatments, models were fitted to each treatment, with a single model used when no significant differences were found. Unless specified, differences were considered significant at p ≤ 0.05. The simple exponential decay model (Wagner and Wolf, 1999) was used for biomass OM disappearance and remaining N concentration as well as C:N, LIG:ADIN, LIG:N, and ADIN:N ratios according to the equation exemplified for remaining biomass:

Remaining biomass = B 0 × e –kt in which B 0 is the disappearance coefficient, k is the relative decomposition rate, and t is time in days. Lignin concentration response to days of incubation was evaluated by a linear plateau model as described by McCartor and Rouquette, Jr. (1977).

RESULTS AND DISCUSSION The simple exponential decay model was adequate to explain biomass decomposition (p < 0.0001) with a time × year interaction (Fig. 2A). Biomass loss, in general, was greater in 2007 than in 2008, particularly at the end of the incubation period (p < 0.0001), possibly due to greater precipitation (Fig. 1), combined with greater N concentrations in 2007 compared to 2008 (Table 1). The

Figure 2. Remaining biomass (g kg –1 organic matter [OM]) in 2007 and 2008 (A) and C:N ratio of signal grass litter mixed with different levels (0, 50, and 100%) of calopo (B) (pooled over years) after incubation at soil level at Itambé, Pernambuco Brazil.

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2007 results are similar to the 15% biomass loss found by Dubeux, Jr., et al. (2006a, b) for Bahiagrass (Paspalum notatum Flüggé) litter. Both 2007 and 2008 (Fig. 2B) models indicated ongoing decomposition up to 256 d, suggesting a longer period would be necessary to achieve stability. There was an interaction between legume inclusion level and years for remaining biomass (p = 0.0005), with an increase in overall decomposition due to legume inclusion in 2007 but not in 2008 (Table 2). While both N and LIG concentrations were greater with increasing legume inclusion rates for both years, 2008 had lower N and greater LIG concentrations than 2007, both of which could explain the absence of a legume inclusion effect in 2008. Since the year effect was stronger for LIG than for N concentration (from 4.2 to 14.8 g N kg–1 OM and 56.9 to 85.4 g LIG kg–1 OM in 2007 and 3.3 to 11.6 g N kg–1 OM and 33.6 to 128.9 g LIG kg–1 OM in 2008 with the increase in calopo inclusion from 0 to 100%), LIG is likely the main reason for the reduced decomposition in 2008. There was also an interaction between calopo inclusion levels and year for final N concentration (p < 0.0001), with 2007 presenting greater concentrations (Table 2), most probably due to the greater initial values (Table 1). The results are similar to those observed on Brachiaria humidicola (Rendle) Morrone & Zuloaga litter N concentration after inclusion of Desmodium heterocarpon (L.) DC. subsp. ovalifolium (Prain) H. Ohashi [syn. Desmodium ovalifolium (Prain) Wall. ex Merr.] (Cantarutti et al., 2002). The C:N ratio decomposition was different between years for pure signal grass litter (Table 2). The inclusion of calopo resulted in greater reduction in C:N ratios for signal grass compared to pure grass, probably due to the initially greater C:N ratio. This reduction likely reflects an initial C drop without an accompanying N release due to microbial demand for N when initial C:N ratios are high. At the same time, litter containing legume had almost no reduction in the C:N ratio, likely because initial N available to microbes was high. At the beginning of the experiment (Fig. 2B), these ratios were already in the 20 to 30 range so that less change in the ratio should be expected relative to material containing only signal grass (Mundus et al., 2008). The C:N ratios at the end of the trial confirm stabilization at the 20 to 30 range (Table 2) for both treatments that included calopo. Litter without legume still had ratios far in excess of that range (77 and 93 for 2007 and 2008, respectively), indicating that net immobilization of N was likely still occurring in signal grass litter after 256 d of incubation. Nitrogen release was 14 and 32% greater (p = 0.03) for the treatments with 50 and 100% calopo litter, respectively (Fig. 3A). Final values of 50, 61, and 73% N remained at the end of the experiment for 100, 50, and 0% calopo inclusion, respectively. This was seen throughout the experiment with a much greater decomposition rate for 100% calopo litter compared to 100% signal grass litter CROP SCIENCE, VOL. 52, MAY– JUNE 2012

Table 2. Characteristics of signal grass litter after 256 d of decomposition with different inclusion levels of calopo in 2007 and 2008 at Itambé Experimental Station of the Pernambuco Agronomical Institute, Pernambuco State, Brazil. Inclusion level (%)

2007

2008

—————— Remaining biomass (%) —————— 0 50

76.6aB† 71.1bB

82.3aA 83.3aA

100 SE

70.9bB 1.2

84.3aA 1.2

p = 0.009 p = 0.10

p = 0.12 p = 0.97

Linear effect Quadratic effect

————— g N kg –1 litter organic matter ————— 0 50

6.3cA 14.2bA

4.7bA 18.2aA

100 SE

22.4aA 1.7

17.3aA 1.7

p < 0.0001 p = 0.78

p = 0.007 p = 0.04

Linear effect Quadratic effect

—————————— C:N ratio ——————————— 0 50 100 SE Linear effect Quadratic effect

77.0aB 29.3bA 18.8cA 1.9 p < 0.0001 p < 0.0001

92.9aA 33.2bA 24.0cA 2.0 p < 0.0001 p < 0.0001

———— g lignin kg –1 litter organic matter ———— 0 50 100 SE Linear effect Quadratic effect

132cA 151bB 173aB 2.8 p < 0.0001 p < 0.0002

94cB 171bA 223aA 2.9 p < 0.0001 p < 0.0001

————————— Lignin:N ratio ————————— 0 50 100 SE Linear effect Quadratic effect

24.7aA 10.5bB 7.7cB 0.7 p < 0.0001 p = 0.0002

20.2aB 13.5bA 13.0bA 0.8 p = 0.0001 p = 0.01

——— Acid detergent insoluble N:N ratio ——— 0 50 100 SE Linear effect Quadratic effect †

67.5aA 50.0bB 56.8bB 2.8 p = 0.003 p = 0.0006

64.4aA 60.7aA 67.6aA 3.0 p = 0.21 p = 0.18

Values with similar uppercase letters on a line or lowercase letter in a column under the same heading do not differ (p > 0.10) according to a least square difference multiple range separation.

(k values of 0.00248 and 0.00029 g g–1 d–1, respectively), with the mixed litter showing intermediate decomposition (k values of 0.00132 g g–1 d–1). There was still net N release, despite the fact that C:N ratios would suggest that there should have been immobilization not mineralization (Fig. 2B). These results are similar to those observed by

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Figure 3. Percentage of N remaining in signal grass litter mixed with 0, 50, and 100% calopo (A) (pooled over years) and percentage of litter N remaining during the 2 yr of the trial (B) (pooled over three signal grass and calopo mixes) after incubation at soil level at Itambé, Pernambuco Brazil.

Cantarutti and Boddey (1997) for D. heterocarpon inclusion in B. humidicola litter. There was a difference in remaining N decomposition between years (Fig. 3B). There were both greater remaining N and greater decomposition rates for 2007 than 2008, most likely due to greater rainfall (Fig. 1) in 2007. However, in both years the N concentration was similar in the 256 d litter residue (Table 2). Lignin concentration increased with calopo inclusion in the signal grass litter (Table 1). A linear plateau model (Fig. 4A) best describes all treatments, with lignin concentration increasing up to 31 d to approximately 160 g LIG kg–1 signal grass litter OM when no calopo was added compared to 200 and 220 g LIG kg–1 signal grass litter OM for 50 and 100% calopo litter. While stable LIG concentrations 1420

were similar for 2007 and 2008 (210 and 180 g LIG kg–1 for 50 and 100% calopo litter, respectively) there was a marked difference for the time necessary to stabilize those values (32 and 17 d for 2007 and 2008, respectively). There was an interaction between year and legume inclusion for final LIG concentration (Table 2). While in both years LIG concentration increased with legume inclusion compared to pure signal grass litter, pure grass litter LIG concentration was greater in 2007 than in 2008 while the reverse was true after calopo inclusion. In the latter case, the greater concentration of LIG in calopo likely masked lower signal grass LIG concentrations in 2008. The LIG:N ratio was not affected by time, although there was an interaction between year and legume inclusion (Table 2; p < 0.0001). Results from both years showed

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Figure 4. Lignin concentration in litter containing 0, 50, and 100% calopo mixed with signal grass (A) and lignin (LIG) concentration in litter during 2 yr of the trial (B) after incubation at soil level at Itambé, Pernambuco Brazil.

a reduction in the ratio with increased legume inclusion but a stronger decrease for 2007 than 2008 (from 25 to 8 in 2007 and from 20 to 13 in 2008 for 0 and 100% calopo). This was due to the greater N and lower LIG litter concentrations with legume inclusion in 2007. While both Aita and Giacomini (2003) and Dubeux, Jr., et al. (2006b) found LIG:N ratio to be a good predictor for decomposition rates, this was not the case in this trial since there was no change in this ratio over time. There was an interaction (p = 0.01) between legume inclusion rate and year for ADIN:N ratio (Table 2). With the exception of pure signal grass litter, the ratios were greater in 2008 than in 2007. This data confirms the lower nutritive value of the 2008 legume material because LIG is largely indigestible and slows passage rate in ruminants CROP SCIENCE, VOL. 52, MAY– JUNE 2012

(Wilson, 1994). Dubeux, Jr., et al. (2006a, b), working with P. notatum ‘Pensacola’, found ADIN:N ratios after 128 d of decomposition of approximately 50, greater than those found in our data after 256 d. The LIG:ADIN ratio showed an interaction between time, year, and legume inclusion rate (p = 0.0002; Fig. 5A and 5B). This was apparently due to the lower LIG and greater N concentrations (Table 2; Fig. 4B) found in 100% signal grass litter in 2007 in comparison with 2008. Decomposition rates over time were generally similar for the treatments with legume inclusion in the litter but greater for pure signal grass litter in 2007 (Fig. 4A). Because there was little likelihood of LIG disappearance after 32 d, the change in LIG:ADIN ratio must be due to an increase in ADIN concentration. This result confirms that during

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Figure 5. Ratio of lignin and acid detergent fiber N (LIG:ADIN) of signal grass litter composed of 0, 50, and 100% calopo during 2007 (A) and 2008 (B) after incubation at soil level at Itambé, Pernambuco Brazil.

decomposition, N adheres to LIG, resulting in lower availability of N to microbes (Berg and McClaugherty, 2010)

greater insight into stability of these systems as well as the animal production we can expect from these mixtures.

CONCLUSIONS

References

Calopo inclusion in signal grass litter increased decomposition rates vis-à-vis litter containing only signal grass. It also reduced N immobilization, increasing overall nutrient cycling throughout the system. This supports the practice of including legumes in grass pastures to benefit not just the animal directly but the grass indirectly. Further research should indicate minimum ratios of calopo:signal grass required to achieve these results, a necessary step because legumes are often less productive and less aggressive than accompanying grasses in perennial tropical systems. Long-term studies looking at soil fertility as well as grass production and nutritive value should provide

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