sustainability of mixed pastures
3. Results And Discussion
3.5. Contribution of biological N 2 fixation to the mixed grass/legume pastures
The proportion of N in the legume derived from BNF was determined from the ratio of uriede-N to (uriede+nitrate)-N in boiling water extracts as described by Alves et al (2000b). Most studies on tropical legumes find that the proportion of N derived from BNF usually is between 70 and 80 % (Cadisch et al., 1989; Peoples and Baldock, 2001). In a mixed legume-grass pasture it would be expected that the grass would be highly competitive for available soil N and hence the legume dependence on BNF would be high (Viera-Vargas et al.
(1995). However, in the case of D. ovalifolium in this study, the %Ndfa never exceeded 65 % and in the cool months of July and August was as low as 30 to 40 % (Fig. 2). This low proportional contribution of BNF to D. ovalifolium was also noted in the pot experiments performed by Alves et al. (2000a, 2000b).
The evaluations of %Ndfa of the legume started only in mid-1995, but it was assumed that the values of %Ndfa recorded for January to July in 1996 would have been similar for the same months in 1995.
% Ndfa
0 20 40 60 80 100
Rainfall (mm)
0 20 40 60 80 100
1995 1996
% Ndfa Rainfall
Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul
Figure. 2. Change in the proportion of N derived from the air (%Nfa) via BNF in the legume Desmodium ovalifolium in the forage on offer. Samples taken at monthly intervals from August 1995 and July 1996. %Ndfa was determined using the ureide abundance technique as described by Alves et al. (2000b).
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To estimate the total contribution of biological N2 fixation to the grazed pasture system it was necessary firstly to estimate the total turnover of N in the system which was derived from the data on litter dynamics, N content of the litter samples, the proportion of legume in the samples and the concentration of N in the legume residues deposited in the litter and in the consumed forage.
The proportion of legume in the forage on offer (ignoring standing dead material) was consistently higher in the lower SR (2 animals ha-1) treatment and there was a higher proportion of legume in the forage on offer at the higher SR (4 animals ha-1) than in the SR 3 treatment (Fig. 3).
Jan Feb Mar 1995 Apr
May
Jun Jul Aug
Sep Oct Nov
Dec
% legume in forage on offer
0 20 40 60 80 100
SR 2 animal ha-1 SR 3 animal ha-1 SR 4 animal ha-1
Figure 3. Variation during the year of the proportion of legume (D. ovalifo-lium) DM in the green forage on offer, evaluated manually. Data from Rezende et al. (1999).
From analysis of the bolus samples for 13C abundance, it was possible to determine the proportion of legume C in the acquired diet of the cattle. At the lowest stocking rate the proportion of legume C consumed was moderate, estimated only twice during the year with a mean of 33 % legume C. However, for the higher SRs (3 and 4 animals ha-1) the means were higher at 51 and 56
%, respectively. These results are rather unexpected. Desmodium ovalifolium is known for its high tannin content which is thought to make it unpalatable for cattle and hence intake of this legume is thought to be generally very low. In fact the data presented here show that at low SR where the animals have more
choice to select the grass, they consumed less of the legume than when SR is increased and their options become more limited.
To estimate the % legume carbon in the deposited litter the samples were analysed for 13C abundance (Fig. 4). For these calculations it was assumed that litter from B. humidicola and D. ovalifolium had 13C abundance values of -10.66 and -25.31‰, respectively (Cantarutti et al., 2002). The proportion of legume in the litter was found to be far higher at the lowest SR (2 animals ha-1) which was almost certainly due to the fact that the cattle were rejecting it in favour of the B. humidicola.
1995 Jan Feb Mar
Apr May
Jun Jul
Aug
Sep Oct Nov
13C abundance (‰)
-22 -20 -18 -16 -14 -12 -10
Jan Feb Mar 1995 Apr
May Jun Jul Aug Sep Oct Nov Dec
% legume in litter
0 20 40 60 80 100
SR 2 animal ha-1 SR 3 animal ha-1 SR 4 animal ha-1
A
B
Figure 4. Variation during the year of (A) 13C abundance of, and (B) the pro-portion of legume carbon in, the litter deposited in 14-day intervals.
The proportion of legume C derived from the legume was based on the 13C abundance data, assuming that pure B. humidicola and D.
ovalifolium litter had 13C abundance values of -10.66 and -25.31‰, respectively. Data from Cantarutti et al. (2002).
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All litter samples of both existing litter and that deposited in 14 days (pooled for 3 samples per paddock) were analysed for total N as described in the Materials and Methods (Section 2.2.3.1.). Using exactly the same equations as those used to calculate total litter DM deposited in the 12 month period, but substituting the total N (mg N m-2) for MS for the existing and deposited litter, it was possible to calculate the total N deposited in litter over the 12 months (Table 5). Even in the treatments where there was no legume, the quantity of N recycled via deposition of litter is very considerable. For the lowest SR (2 animals ha-1) the value was estimated at 170 kg N ha-1 yr-1 falling to 105 kg N ha-1 yr-1 for the highest SR (4 animals ha-1). However, it should be stressed that these are not values of N inputs in the manner of N fertilizer inputs, this is N that is recycled. The mean rates of N deposition is the litter are high, ranging from 287 to 466 g N ha-1 d-1, but the rate of growth of the B. humidicola is also fast and this N is rapidly mineralized and reutilized.
There was an extremely large impact of the presence of the legume on the N recycled through the litter pathway (Table 5). N recycled via litter decom-position was almost double at the lowest SR (2 animals ha-1) from 170 to 325 kg N ha-1 yr-1. The reason for this is not only the large input of N from BNF into the pasture system (see below), but also the presence of the legume in the litter decreased its C:N ratio. This almost certainly was responsible for the increased rate (not significant at P<0.05) of decomposition as witnessed by the increased magnitude of the decomposition constant ‘k’ in this treatment. The
‘k’ values for the higher SRs were lower, which agrees with the observation that in these treatments the proportion of legume in the deposited litter was much lower than for SR1.
Table 5. N recycled in litter in grass-only Brachiaria humidicola and mixed B. humidicola/ Desmodium ovalifolium pastures grazed by crossbred Brahman steers at 3 stocking rates. Data for the 12 months of 1995.
Adapted from Cantarutti et al. (2002).
Mean of N in litter Decomp. Total N deposited in litter (12 months) Pasture Stocking rate Existing Deposited in
28 days rate (k) Estimate*
14 days Corrected+ B.humidicola
an. ha-1 --- mg N m-2 --- g g-1 day-1 --- kg N ha-1 year-1
----2 705 852 -0.0661 111.0 170.2
3 614 748 -0.0691 97.5 151.1
4 483 548 -0.0600 71.3 105.4
Mean 601 716 0.0651 93.2 142.2
Legume-
grass 2 953 1352 -0.0936 176.3 325.0
3 801 958 -0.0688 125.0 193.1
4 736 794 -0.0567 103.4 149.0
Mean 830 1034 -0.0730 134.9 222.4
Coef. Variation (%) 28.2 26.7 23.7 26.7 34.2
Analysis of variance
Factor: Pasture(P) * ** ns ** *
Stocking rate (L) ns * ns * *
Interaction P x L ns ns ns ns ns
a Litter N deposited in 14 days x 2. This value was utilized to calculate ‘k’ according to equation 03 – Materials and Methods, Section 2.2.3.2.
b Calculated from ((litter N deposited in 14 days)/14) x 365.
c Calculated using Equation 9 - see Materials and Methods
d ns, *, ** indicate respectively that differences between means were not significant, or significant at P=0.05 or 0.01.
The 13C abundance data enabled the estimation of the quantity of legume C as a proportion of all C in both the litter and in the consumed forage (Fig.4 and Table 3, respectively). To calculate the quantity of N derived from BNF in the litter and in the consumed forage it is first necessary to know the C:N ratio of the legume in these materials. For this purpose regressions were plot-ted between the proportion of legume N in the litter, and in the bolus samples, and the concentration of N in the same samples (Fig. 5). The regression of N concentration with % legume in the litter allowed the determination of the concentrations of the N in grass and legume by extrapolation to 0 and 100 % legume, respectively, and these values were 6.8 and 13.4 g N kg litter-1 (Fig.
5A). The similar regression of N concentration with % legume in the diet (as
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130
determined from bolus samples) indicated that the grass (B. humidicola) in-gested by the animals in the mixed pasture had a mean N concentration during the year of 12.5 g kg-1 and the legume (D. ovalifolium) 23.5 g kg-1. The grass in the bolus samples taken from the cattle grazing on the B. humidicola in monoculture (mean of 11,8 g N kg DM-1) was only very slightly lower in N concentration than of that estimated for the grass in the bolus samples in the mixed grass/legume pasture (12.5 g N kg-1). This suggests that there was little direct or indirect (e.g. via litter) transfer of N from the legume to the grass.
% Legume C in bolus samples
0 20 40 60 80 100
N concentration (g N kg-1) 10 12 14 16 18 20 22 24 26
10 12 14 16 18 20 22 24 26
N concn in diet = 12.54 + (0.110 x % legume in diet) r2 = 0.92 P<0.001
B
% legume C in deposited litter
0 20 40 60 80 100
N concentration (g kg-1) 6 7 8 9 10 11 12 13 14 15
6 7 8 9 10 11 12 13 14 15
N concn in litter = 6.76 + (0.066 x % legume in litter) r2 = 0.59 P<0.001
A
Figure 5. Regression of (A) % legume in deposited litter and (B) % legume in diet with N concentration. The samples for the ingested diet were taken from cattle fitted with esophageal fistulae (bolus samples). In both cases % legume C was determined using 13C abundance measurements.
The values of N in the legume and grass in the bolus samples are consi-derably higher than those in the green grass or legume on offer determined by Cantarutti et al. (2002) in this same study. The green grass on offer had a mean N concentration of 9.1 g N kg-1 and the legume 17.0 g N kg-1, considerably lower than the bolus samples. In both cases this indicated that the cattle were capable of selecting a diet of higher protein content from the sward than that estimated in the material taken in samples of material on offer. This selectivity of such animals grazing Brachiaria and mixed Brachiaria/legume swards has been noted in many previous reports (e.g. Carulla et al, 1991; Pereira et al., 1992).
Using the concentration of N in the legume, the concentration of C in the legume and grass and the estimates of the litter deposited in each monthly pe-riod it was possible to calculate the N recycled via legume litter at each of the three SRs (Table 6). Total C content of the B. humidicola and D. ovalifolium litter was 399 and 375 g C kg DM, respectively (Cantarutti et al., 2002). With these data and the estimates of the proportion of N derived from BNF using the ureide abundance technique (Fig. 2), it was possible to calculate the annual input to the pasture (soil/plant/animal) system. At the lowest SR, where there was low animal consumption of the legume but a much higher content in the forage on offer and the deposited litter, the legume D. ovalifolium was estimated to contribute a total 17 Mg of DM to the total NAPP of 37 Mg ha-1 (Table 4).
At the higher SRs (3 and 4 animals ha-1) the total contribution of the legume to the litter was much lower (3 to 5 Mg ha-1 yr-1), but the quantity of legume consumed, between 4.7 and 6.6 Mg ha-1 was much higher than in SR2 (2.1 Mg ha-1). The concentration of N in the legume (13.4 g N kg DM-1) deposited as litter was far lower in than in the ingested forage (23.5 g N kg DM-1). Hence, although the quantity of legume consumed by the cattle in SR4 was less than half of that deposited in the litter in SR2, the total legume N and the total N derived from BNF was not that much lower, 108 kg N ha-1 compared to 135 kg N ha-1 of biologically fixed N in the litter deposited in SR2.
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Table 6. The annual contribution of legume DM and N to the pasture system through deposited litter and animal intake in a mixed D. ovalifolium/B.
humidicola pasture pastures grazed by crossbred Brahman steers at three stocking rates. Data for the 12 months of 1995. Data from Rezen-de et al (1999), Cantarutti et al (2002), BodRezen-dey et al. (2004), Pereira et al. (2009), and other unpublished data of the authors.
Stock-ing rate Deposited litter Animal intake Litter + Animal
intake An. ha-1 Total DM legume Total
DM1
Total le-gume N
Total N from
BNF2 Total
DM Total legume
DM1 Total legume
N
Total N from
BNF2 Total legume
N
Total N from
BNF --- Mg DM ha-1
yr-1 --- --- kg N ha-1 yr-1 --- --- Mg DM hayr-1 --- -1 --- kg N ha-1
yr-1 --- ---- kg N ha-1 yr-1
----2 33.1 14.5 195.4 107.5 6.7 2.10 49.3 27.1 244 135
3 26.0 5.0 67.3 37.0 9.6 4.70 110.4 60.7 178 98
4 23.6 3.1 42.0 23.1 12.2 6.61 155.2 85.4 197 108
Mean 27.6 7.5 101.6 55.9 9.5 4.5 105.0 57.7 206 114
1 Total legume N estimated using the 13C natural abundance technique. See Materials and Methods (section 2.2.2.)
2 Proportion of N derived from BNF estimated using the ureide abundance technique (Alves et al., 2002).