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

1. Soil C and N in a shade coffee agroecosystem in
Hawaii
Adel Youkhana and Travis Idol
University of Hawaii-Manoa, 1910 East West Rd, Honolulu, HI 96822, USA. Email: idol@Hawaii.edu
Introduction
• Nitrogen (N)-fixing trees in agricultural systems
generally increase soil organic matter (SOM),
including C and N
• Pruning shade trees provides mixture of leaves
and woody material for use as mulch or green
manure to further improve SOM
• Removal of prunings for animal fodder,
fuelwood, etc. may counteract these benefits
• Mulch and litter inputs must be stabilized as
humified SOM to improve soil quality
Methods
• Leucaena hybrid ‘KX2-Hawaii’ (L. leucocephala x
L. pallida) planted with coffee on 2 x 2-m
spacing and pollarded every 6 or 12 months
• Mulch added back to tree-coffee plots or
removed to add as mulch to open-grown coffee
• Mulch decomposition and change in biochemical
fractions measured over 12 months
• Total soil C and N measured annually for 3 yr
• Physical and chemical organic soil C fractions
separated and compared over 2-yr period
Plate 4. Leucaena mulch decomposition from
various fractions in shade and full-sun (above).
Conclusions
• Mulch from periodic pruning of Leucaena trees
decomposed rapidly in all biochemical fractions
• Mulch addition significantly increased soil C
and N, but tree presence without did not
• Most soil C stability fractions increased with
mulch, including humified, more stable ones
• Increase in soil C was 50% of the mulch added
over 2 years
Plates 7 and 8. Changes in soil C stability
fractions (POM: unstable, SC: stable) over 2 years
with and without mulch addition. Absolute mass
(above). Proportion of total soil C (below).
Objectives
The objectives were to assess:
(a)Effects of a common N-fixing tree on soil C and
N with and without mulch addition
(b)Distribution of soil C into different physical and
chemical stability fractions
Findings
• Mulch decomposed rapidly but was greater
under shade trees than in full sun. There was
significant loss in all biochemical fractions.
• Soil C and N increased significantly over time
with mulch addition. Without mulch, soil C
change was negative, but this was due to
significant decline in soil bulk density with trees
References
Youkhana, A. and T. Idol. 2009. Tree pruning mulch increases soil C
and N in a shaded coffee agroecosystem in Hawaii. Soil Biol.
Biochem. 41:2527-2534.
Youkhana, A. and T. Idol. 2011. Addition of Leucaena-KX2 mulch in
a shaded coffee agroforestry system increases both stable and
labile soil C fractions. Soil Biol. Biochem. 43:961-966.
Plate 1. Pollarding Leucaena hybrid ‘KX2-Hawaii’
and chipping for mulch with trees present or in
open-grown coffee.
Plate 2. Collecting soil for organic matter
fractionation in an acid digester.
Shade
0
20
40
60
80
100
0 3 6 9 12
Time (mo)
Massremaining(%)
soluble hemicellulose cellulose lignin ash
Full-sun
0 3 6 9 12
Full-sun Shade
Year Soil C (Mg ha-1)
No
Mulch Mulch
No
Mulch Mulch
2006 45.6 45.6 57.4 57.4
2008 33.8 46.7 51.7 68.9 †
Change -11.8 1.1 -5.7* 11.5*
Soil N (Mg ha-1)
2006 2.5 2.5 2.6 2.6
2008 2.4 3.92 3.04 4.72 †
Change -0.1 1.42 0.4 2.12 *
Year
Dry
Mass Carbon Nitrogen
Mg ha-1 mg kg-1 Mg ha-1 mg kg-1 Mg ha-1
2005* 17.6 423 7.45 9.02 0.16
2006 16.1 414 6.67 8.6 0.14
2007 15.3 421 6.44 6.78 0.10
Table 1. Mulch dry mass, C and N production More findings
• With no mulch but tree cover, no change in soil
C fractions
• With mulch addition, all C fractions increased
except for the least stable (coarse POM)
• The only change in proportions was a shift from
coarse to fine POM with mulch additions
0
10
20
30
40
50
2006 2008 2006 2008
No Mulch
POM-C POM-F POM-P SC-H SC-N
mulch
Mulch
No-mulch
Plate 6. Full-sun and shade-grown coffee.
Table 2. Changes in soil C and N with shade and
mulch addition of Leucaena ‘KX-2 Hawaii’
Mulch
Mulch
0
20
40
60
80
100
2006 2008 2006 2008
No Mulch
POM-C POM-F POM-P SC-H SC-N
+
+
+
+
+
-