Studies on the dynamics of organic sulphur and carbon in pastoral and cropping soils : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
Soil organic matter (SOM) can be depleted or regenerated by altering land management practices. Soil tests capable of reporting the size of dynamic SOM fractions may be useful for indicating the environmental cost of landuse and management practices. Information on the effect of land management practices on soil organic S content and turnover is scarce. This study evaluated the ability of a sequential chemical fractionation procedure to characterise changes in soil S and C organic fractions on a range of pasture and cropping soils with different management histories. The fractionation involved an initial extraction with ion exchange resins followed by dilute (0.1 M NaOH) and concentrated (1 M NaOH) alkali. In addition, recently rhizodeposited 14C (root+exudate derived) produced during a short-term (one week) 14CO2 pulse-labelling study of intact soil cores growing ryegrass/clover pastures, was used to trace the fate of root-derived C in both chemical and density fractionation procedures. In pasture and cropped topsoils, the major amounts of soil S and C were either extracted in 0.1 M NaOH (49-69% S and 38-48% C) or remained in the alkali-insoluble residual fraction (17-38% S and 46-53% C). These two fractions were more sensitive to change caused by different landuse and management practices than the resin and 1 M NaOH fractions. With a large amount of dynamic soil C remaining in the residual fraction it was concluded that increasing strengths of alkali were not capable of sequentially fractionating S and C in SOM into decreasingly labile fractions. The chemical fractionation allocated recent root and root-released 14C amongst all the fractions. Again, most root 14C appeared in the 0.1 M NaOH and residual fractions. Although small in amount, C of higher specific activity (more recently synthesised root C) was preferentially extracted by resin and 1 M NaOH extracts. Density separation was not capable of recovering recent root and root-released 14C in a single fraction. Root-derived 14C was distributed between light (mostly fibrous root debris) (42%) and heavy (organics attached to clay and silt) (45%) fractions. The dispersing reagent soluble fraction recovered <13% of the 14C. An anaerobic incubation and various acids and oxidising agents were tried, in order to recover a greater proportion of root and root-released 14C as a single identity. These were not very successful in either extracting or increasing the alkali solubility of the root C fraction. A 30% H2O2 pretreatment of soil plus roots, or hot 1 M HNO3 treatment of the residual fraction, were more efficient extractants of the root C fraction and should be investigated further to check their ability to better characterise soil organic S and C fractions with a change in management practices. The 14CO2 pulse labelling study of pasture swards showed a greater allocation of recently photo-assimilated 14C to the topsoil layer with a greater proportion of 14C recovered in roots than in the soil. An in situ soil solution sampling technique with mini Rhizon Soil Moisture samplersTM effectively monitored the rapid appearance of a 14CO2 pulse in soil water at various depths. A comparison of the 14CO2 pulse labelling study under light and dark conditions indicated that, in the light lysimeters, 14CO2 photo-assimilation/translocation/rhizosphere respiration was the main pathway for CO2 generation at various soil depths. In the dark lysimeters, 14CO2 diffusion was the main mechanism and 14C assimilation (either photo-assimilation or assimilation by chemolithotrophs in rhizosphere soil) was small. The 14CO2 activity in soil water from four soil depths of dark and light soil cores, and a CO2 diffusion model, were used to identify the 14CO2 contribution from rhizosphere respiration in the light lysimeters. A model was developed, but the unknown geometry of the air-filled pore space in the undisturbed soil cores made it impossible to precisely calculate the contribution made by root respiration to soil water 14CO2 activity.