Keynote paper presented at the International Leucaena Conference, 1‒3 November 2018, Brisbane, Queensland, Australia.

1. Leucaena leucocephala (leucaena) has been reported to improve topsoil fertility in
hedgerow silvopastoral systems (Radrizzani et al. 2011; Conrad et al. 2017) and to increase
livestock productivity (Radrizzani and Nasca 2014). In the Chaco region there is no
published information on changes in soil organic carbon (OC) and total nitrogen (TN)
levels and their fractions (particulate and associate forms) under grazed leucaena pastures.
INTRODUCTION
OBJECTIVE
CONCLUSIONS
- Introduction of leucaena into a grass pasture promoted substantial capture of OC in the
subsoil (20-100 cm), especially the most stable form (AOC), which has minimal
susceptibility to mobilization in the deepest horizon (50-100 cm depth), attributed to a
greater abundance of leucaena roots deeper in the soil profile than of the grass.
- Leucaena introduction also enhanced N concentration in the topsoil (0-20 cm),
particularly the most labile form (PON) that promotes improvement in grass growth and
quality, attributed to N-rich leucaena leaf deposition, leaf recycled via animal feces and
nodule-N turnover from biological N fixation.
- Accordingly, the establishment of hedgerow leucaena silvopastoral systems, apart from
increasing cattle production, can improve soil fertility and hence N availability to grass
growth and can be utilized as a long-term greenhouse gas mitigation strategy.
REFERENCES
Bremner JM; Keeney DR. 1982. Nitrogen-total. In: Page AL; Miller RH; Keeney DR, eds. Methods of Soil Analysis, Part 2. 2nd Edn,
p. 595–624. ASA and SSSA, Madison, WI. doi:10.2134/agronmonogr9.2.c32
Cambardella CA; Elliott ET. 1992. Particulate soil organic-matter changes across a grassland cultivation sequence. Soil Science
Society of America Journal 56:777–783. doi:10.2136/sssaj1992.03615995005600030017x
Conrad KA; Dalal RC; Dalzell SA; Allen DE; Menzies NW. 2017. The sequestration and turnover of soil organic carbon in
subtropical leucaena-grass pastures. Agriculture, Ecosystems and Environment 248:38-47. http://www.publish.csiro.au/SR/SR18016
Di Rienzo JA; Casanoves F; Balzarini MG; Gonzalez L; Tablada M; Robledo CW. 2016. InfoStat versión 2016. Grupo InfoStat,
F.C.A., University Nacional de Córdoba, Argentina. http://www.infostat.com.ar
Nelson D; Sommers L. 1996. Total Carbon, Organic Carbon, and Organic Matter. In: Bigham JM; et al. eds. Soil Science Society of
America and American Society of Agronomy. Methods of Soil Analysis. Part 3. Chemical Methods-SSSA Book Series 5. Chapter 34.
p. 1001-1006. Madison, WI. doi:10.2136/sssabookser5.3.frontmatter
Radrizzani A; Shelton HM; Dalzell SA; Kirchhof G. 2011. Soil organic carbon and total nitrogen under Leucaena leucocephala
pastures in Queensland. Crop and Pasture Science 62:337-345. doi.org/10.1071/CP10115
Radrizzani A; Nasca JA. 2014. The effect of Leucaena leucocephala on beef production and its toxicity in the Chaco Region of
Argentina. Tropical Grasslands-ForrajesTropicales 2:127-129. http://www.tropicalgrasslands.info/index.php/tgft/article/view/155/102
ACKNOWLEDGMENTS
We are grateful to Jeremías Luchina, Martín Requena Cerra and Enrique Oviedo for his assistance in field sampling. This
work was supported by the Animal Research Institute of the semi-arid Chaco region (IIACS) of the Federal Institute of
Agricultural Technology (INTA), and by the School of Agricultural and Animal Sciences (FAZ) and the Science and
Technology Secretary of the National University of Tucumán (UNT), which is greatly appreciated.
The study aimed to evaluate the quantity and vertical distribution of OC and TN stocks and
their fractions (particulate and associate forms) in the soil profile (0-100 cm) of a 4-year-
old leucaena-grass pasture, and compare it with the adjacent pure tropical grass pasture in
the Chaco region of Argentina.
1Instituto de Investigación Animal del Chaco Semiárido (IIACS), Centro de Investigaciones Agropecuarias (CIAP), Instituto Nacional de Tecnología Agropecuaria (INTA). https://inta.gob.ar/iiacs
2Facultad de Agronomía y Zootecnia, Universidad Nacional de Tucumán. http://www.faz.unt.edu.ar/
Correspondence: Alejandro Radrizzani, Instituto de Investigación Animal del Chaco Semiárido, Instituto Nacional de Tecnología Agropecuaria, Argentina. Email: radrizzani.alejandro@inta.gob.ar
NATALIA BANEGAS1,2, ROBERTO CORBELLA2, EMILCE VIRUEL1,
ADRIANA PLASENCIA2, BELEN ROIG2 AND ALEJANDRO RADRIZZANI1
Leucaena leucocephala introduction into a tropical pasture in the
Chaco region of Argentina: effects on soil carbon and total nitrogen
MATERIALS AND METHODS
This study was carried out at the Animal Research Institute of the Semi-arid Chaco Region, operated by
the National Institute of Agricultural Technology (INTA), located at Leales, Tucumán (27º11´ S, 65º14´
W; 335 masl), in the west of the Chaco region, Northwest Argentina. The soil samples were collected
from 4 plots of 1 ha each: 2 plots with pure grass pasture (PP) and the other 2 plots with leucaena-grass
pasture (LP) from 12 transects 10 m in length. Along each transect, 5 soil cores (0-1 m deep) divided into
3 depths (0-20 cm, 20-50 cm and 50-100 cm) were collected every 2.5 m. The 5 soil samples collected at
each depth were mixed to form a single composite sample per depth for each transect. Organic carbon
(OC) concentration was determined by Walkley Black (Nelson and Sommers 1982). Total nitrogen (TN)
concentration was determined by Kjeldahl (Bremner and Keeney 1982). Fractions of OC and TN were
measured in 50 g of each composite sample through particle size analysis, following the technique
described by Cambardella and Elliot (1992); particulate organic carbon (POC) and nitrogen (PON), and
associate organic carbon (AOC) and nitrogen (AON) were determined.
Analysis of variance of soil fertility parameters and mean comparisons (Tukey, P<0.05) within
pastures were performed to assess the effects of leucaena introduction. All statistical analyses were
carried out using InfoStat software (Di Rienzo et al. 2016).
RESULTS AND DISCUSSION
Carbon: In both pastures, stratification of total OC was observed in the soil profile, with
higher levels in the topsoil (0-20 cm horizon) than in the subsoil (20-50 cm and 50-100 cm
horizons) (Figure 1A). This stratification was more pronounced in soil supporting PP than
in soil supporting LP, since OC concentrations continued to decline with depth in PP but no
differences were observed between subsoil depths in LP. For LP and PP soils, 53% and
43%, respectively, of the total OC in the first meter of soil was contained in the combined
subsoil horizons (20-100 cm depth)
Concentrations of POC were also stratified in both pasture soil profiles but stratification
was different from that for OC (Figure 1B). In contrast with OC concentrations, POC was
higher in PP than in LP in the topsoil horizon (61.5% vs 45.5%). Concentrations of AOC
were also stratified, but differences between pasture soils were restricted to the 50-100 cm
horizon where AOC was higher in LP than in PP (Figure 1C)
Nitrogen: Concentrations of TN followed a similar trend to OC (Figure 1D). However, TN
was higher in LP than in PP only in the topsoil horizon (0.141±0.0039% vs.
0.131±0.0035%, respectively).
Concentrations of PON were also stratified in both pasture soils but followed different
patterns from those for the TN concentrations in the subsoil (Figure 1E). In the 0-20 cm and
20-50 cm horizons, PON was greater in LP than in PP, showing that most of the TN in this
horizon was in the labile ON form. In contrast, PON was higher in PP than in LP in the 50-
100 cm depth. Concentrations of AON were also stratified as were TN and PON but the
relationships were the mirror images of those for PON. AON was higher in PP than in LP in
the 0-20 cm and 20-50 cm horizons, but in 50-100 cm, the highest value was found in LP.
C:N ratio: C:N ratio increased with depth in the leucaena pasture but decreased with depth
in the grass pasture, showing inverse relationships between C:N ratio and soil depth. In the
leucaena pasture, higher inter-row grass production and quality could be expected than in
the pure grass pasture.
In relation to the fraction ratios, the main contribution of leucaena was in the PON
(labile N form) in the first 50 cm of the LP soil profile (0-20 and 20-50 cm depths), with
lower POC:PON ratios than in the deep soil (50-100 cm).
Figure 1. Concentrations of organic carbon (OC) (A), particulate OC (POC) (B), associate OC (AOC), total nitrogen (TN)
(D), particulate organic nitrogen (PON) (E) and associate organic nitrogen (AON) (F) in relation to soil depth (0-20, 20-50
and 50-100 cm horizons) in soils for the leucaena pasture (grey bars) and the pure grass pasture (white bars) at the Animal
Research Institute of the semi-arid Chaco region-INTA. Means with different letters are significantly different (P<0.05); bars
represent standard error.
Instituto Nacional de Tecnología Agropecuaria