This study tested whether repeated applications of low or high quality leaf litters would lead to higher soil carbon sequestration in a rainfed tropical rice system in northeast Thailand over 9 seasons. The results showed that lower and higher quality litters resulted in similar small increases in soil organic carbon and other indicators, disproving the hypothesis. Intermediate quality residues resulted in significantly higher apparent recovery of sulfur, indicating better synchrony in nutrient release. Overall, small and repeated leaf litter applications significantly increased productivity and sustainability of the low-input paddy system, showing benefits for similar systems.

1. Sequestering soil carbon in the low input
farming systems of the semi-arid tropics –
does litter quality matter?
Anthony Whitbread
Rod Lefroy
Graeme Blair
Yothin Konboon (Late)
Kunnika Naklang (Late)

2. Introduction
• Limited evidence from the drylands that soil C in arable systems
can be ‘sequestered’ under normal management.
• Apart from the aims of mitigation, managing soil C has many
other benefits (soil structure, nutrient supply capacity, ‘soil
health’) which may not payoff for several years.
• Long term trials and multiple measures of systems performance
are needed to assess a change in the system.
• In low input systems, plant residues can be used to manage soil
fertility but few examples of residue ‘quality’ being a factor.
• Simple proxies of quality (Palm et al. AGEE, 2001) are available.
• The hypothesis tested in this paper: “Will repeated applications
of low as compared to high ‘quality’ leaf litters lead to higher C
sequestration?”

3. Methodology
• This paper presents a unique long term application of leaf litters
with varying qualities to a rice system:
• Rainfed tropical rice system in north-east Thailand (URRC)
• Paddy field experiments (9 seasons) Split plot design
• Measured -nutrient dynamics, SOM dynamics and crop yields.
4 leaf litters+ no-leaf litter control × 2 fertiliser rates × ± rice stubble
Naklang et al. Plant&Soil (1999)
Whitbread et al. Plant&Soil (1999)
Whitbread et al. Ag.Eco.Env(2003)
CT CL Lability CMI
(mg/g)
Forest 23.0 4.6 0.25 100
Paddy 3.5 0.4 0.14 9

4. Methodology
Shrubs surrounding field supplied the leaf litters of varying qualities.
Ubon Ratchathani Rice Research Centre (URRC) DoA

5. Methodology
• Assessment of decomposition characteristics
• Cumulative C release from leaf litters as determined by the
perfusion technique over 60 days
0
10
20
30
40
50
60
70
0 10 20 30 40 50 60
Days
ResidueCrelease(%)
C. cajan
S.saman
P. taxodifolius
A. auriculiformis
5%l.s.d.
12
23
25
19
C:N
Lefroy et al. (1995)

6. Methodology
Separation of Labile Fractions of Soil Carbon
• Total carbon (CT) by catalytic combustion
• Labile carbon (CL) by oxidation with 333mM KMnO4
(Blair et al. 1995)

7. Results Figure: Labile carbon (mg/g)

8. Results Figure: Total carbon (mg/g)

9. Results Figure: Grain yield 1992-2000

10. Results Grain yield 1992-2000
1992 1993 1994 1995 1996 1997 1998 1999 2000
Low N:P:K 1167 1207 1441 1318 1834 1604 1748 1413 1846
High N:P:K 1431 1455 1806 1558 2109 1953 2074 1633 2218
significance *** *** *** *** ** *** *** ** ***
Straw - 1241 1266 1598 1419 1896 1761 1861 1477 1935
Straw + 1356 1395 1649 1456 2046 1796 1961 1569 2129
significance ns * ns ns * ns ns ns *
* p≤0.05, ** p≤0.01, *** p≤0.001

11. Results Figure: Apparent recovery of S

12. Conclusions
• The hypothesis, ‘will repeated applications of low as compared to
high ‘quality’ leaf litters lead to higher C sequestration’ was not
proven.
• Lower and higher quality litters resulted in similar small increases
in soil organic C and in other indicators (yield).
• Apparent recovery of S was sig. higher with intermediate ‘quality
residues’ an indication of better synchrony in release of S.
• Small and repeated leaf litter applications did significantly increase
overall productivity and sustainability of a low input paddy system.
• Results are therefore very significant for low input systems.

13. The reality of managing soil as one component of the farm.
• Decisions made by farm households are made in context of many factors.
• While the maintenance of soil is the basis of sustainability, short term
considerations dominate decisions.
• Small and repeated leaf
litter applications may be
practical at small scale
with ‘trees on bunds’.
• Large scale – Zero Till,
residue retention,
rotations, living mulch,
etc. all require
mechanisation to be
practical