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Sequestering soil carbon in the low input farming systems of the semi-arid tropics – does litter quality matter?

  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
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