An investigation of pasture legume root and shoot properties that influence their rate of decomposition in soil : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand
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Date
2021
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Massey University
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Abstract
Agriculture is the largest source of GHG emissions (47.8 %) in New Zealand. Emissions are increasing annually, driven by increasing relative productivity. Irrespective of the climate regime, grassland soils have historically sequestered large amounts of atmospheric C into SOM (soil C) raising interest in the potential for agricultural emissions to be mitigated through acceleration of soil C sequestration. Soil C sequestration is a direct result of the rate of deposition (excreta, plant litter, and roots) exceeding the rate of decomposition and can be raised by: 1) increasing the rate of input (manipulating the drivers of vegetation); or 2) increasing the longevity of C in the system.
This PhD study tests the hypothesis that C sequestration in pasture soils can be accelerated, by selecting pasture species that contribute slower decomposing litter to soil.
A series of laboratory incubation studies were conducted to measure the decomposition rate (CO₂ emissions) of plant shoots and roots with high (Lotus pedunculatus) and low (Trifolium repens) tannin contents. In addition the effects of residue management (fresh and freeze dried), application to soil (fresh - surface, freeze dried - surface, and freeze dried - mixed) and rate of application (2, 5, 10 mg C. g⁻¹ soil) were evaluated. The effect of species, plant management, plant part, and rate of application on C emissions were all statistically significant (P < 0.05), with large variance in CO₂ emissions associated with all treatments.
Plant species and plant part influenced the amount of C retained in the soil, although not entirely as expected. Lotus pedunculatus shoot material retained significantly more C than Trifolium repens shoot material at all rates of application (2, 5, 10 mg C. g⁻¹ soil); whereas Trifolium repens root material retained significantly more C than Lotus pedunculatus root material at all rates of application (P < 0.05). Notably plant roots and particularly Trifolium repens roots had slow decomposition rates compared to shoot materials.
Research showed that soil and plant residue preparation greatly influenced the total amount of C retained for both shoot and root treatments, with more C retained under conventional incubation techniques (dried - mixed application) than with fresh applications. This indicates that CO₂-C retention in a field situation may be overestimated if predicted using conventional laboratory incubation techniques. However from a research perspective it is infinitely easier to work with pre-dried incubation materials (timing, handling, chemical analysis) so it is highly likely that this style of incubation practice will continue to be the preferred method of research. Care must therefore be taken when extrapolating the results from such incubation studies.
A four compartment (2 soil C pools, persistent and labile; and 2 plant C residue pools, fast and slow) computer simulation model was developed and provided an excellent explanation of the CO₂ emissions from the incubation of fresh shoot and root material. The measurement of the metabolisable energy (ME) or lignin contents of plant shoot and root were successful in parameterising (allocating C to) the fast and slow plant residue pools. Plant tannin content was not able to explain CO₂ emission rates. The experimental and modelling studies provide evidence that grazed pasture rotations in mixed farming systems could be manipulated, by careful plant pasture species selection, to accelerate soil C sequestration. Litter and root metabolisable energy (ME) or lignin contents could be useful in species selection, but further research into other pasture species and pasture management techniques is required. Field studies should focus on the role of clover (Trifolium repens) roots in building pasture soil C content.
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Big trefoil, New Zealand, White clover, Plant litter, Biodegradation, Soils, Carbon sequestration