Sequestering soil carbon in the low input farming systems of the semi-arid tropics –

does litter quality matter?Anthony Whitbread

Rod LefroyGraeme Blair

Yothin Konboon (Late)Kunnika Naklang (Late)

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?”

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

Methodology

Shrubs surrounding field supplied the leaf litters of varying qualities.

Ubon Ratchathani Rice Research Centre (URRC) DoA

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 60Days

Re

sid

ue

C r

elea

se (

%)

C. cajan

S.saman

P. taxodifolius

A. auriculiformis

5%l.s.d.12

23

25

19

C:N

Lefroy et al. (1995)

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)

Results Figure: Labile carbon (mg/g)

Results Figure: Total carbon (mg/g)

Results Figure: Grain yield 1992-2000

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

Results Figure: Apparent recovery of S

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.

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