Tillage-induced changes to soil structure and organic carbon fractions in New Zealand soils

The effects of increasing cropping and soil compaction on aggregate stabili ty and dry-sieved aggregate-size distribution, and their relationship to to tal organic C (TOC) and the major functional groups of soil organic carbon, were investigated on 5 soils of contrasting mineralogy. All soils except t he allophanic soil showed a significant decline in aggregate stability unde r medium- to long-term cropping. Mica-rich, fine-textured mineral and humic soils showed the greatest increase in the mean weight diameter (MWD) of dr y aggregates, while the oxide-rich soils, and particularly the allophanic s oils, showed only a slight increase in the MWD after long-term cropping. On conversion back to pasture, the aggregate stability of the mica-rich soils increased and the MWD of the aggregate-size distribution decreased, with t he humic soil showing the greatest recovery. Aggregate stability and dry ag gregate-size distribution patterns show that soil resistance to structural degradation and soil resilience increased from fine-textured to coarse-text ured to humic mica-rich soils to oxide-rich soils to allophanic soils. Coarse-and fine-textured mica-rich and oxide-rich soils under pasture conta ined medium amounts of TOC, hot-water soluble carbohydrate (WSC), and acid hydrolysable carbohydrate (AHC), all of which declined significantly under cropping. The rate of decline varied with soil type in the initial years of cropping, but was similar under medium- and long-term cropping. TOC was hi gh in the humic mica-rich and allophanic soils, and levels did not decline appreciably under medium- and long-term cropping. C-13-nuclear magnetic res onance evidence also indicates that all major functional groups of soil org anic carbon declined under cropping, with O-alkyl C and alkyl C showing the fastest and slowest rate of decline, respectively. On conversion back to p asture, both WSC and AHC returned to levels originally present under long-t erm pasture. TOC recovered to original pasture levels in the humic soil, bu t recovered only to 60-70% of original levels in the coarse- and fine-textu red soils. Aggregate stability was strongly correlated to TOC, WSC, and AHC (P < 0.001 ), while aggregate-size distribution was moderately correlated to aggregate stability (P < 0.01) and weakly correlated to AHC (P < 0.05). Scanning ele ctron microscopy indicated a loss of the binding agents around aggregates u nder cropping. The effect of the loss of these binding agents on soil struc ture was more pronounced in mica-rich soils than in oxide-rich and allophan ic soils. The very high aggregate stabilities of the humic soil under pastu re was attributed to the presence of a protective water-repellent lattice o f long-chain polymethylene compounds around the soil aggregates.