The effects of increasing cropping and soil compaction on aggregate stability
and dry-sieved aggregate-size distribution, and their relationship to total
organic C (TOC) and the major functional groups of soil organic carbon, were
investigated on 5 soils of contrasting mineralogy. All soils except the
allophanic soil showed a significant decline in aggregate stability under
medium- to long-term cropping. Mica-rich, fine-textured mineral and humic
soils showed the greatest increase in the mean weight diameter (MWD) of dry
aggregates, while the oxide-rich soils, and particularly the allophanic soils,
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 the
humic soil showing the greatest recovery. Aggregate stability and dry
aggregate-size distribution patterns show that soil resistance to structural
degradation and soil resilience increased from fine-textured to
coarse-textured to humic mica-rich soils to oxide-rich soils to allophanic
soils.
Coarse- and fine-textured mica-rich and oxide-rich soils under pasture
contained 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
high in the humic mica-rich and allophanic soils, and levels did not decline
appreciably under medium- and long-term cropping.
13C-nuclear magnetic resonance evidence also indicates
that all major functional groups of soil organic carbon declined under
cropping, with O-alkyl C and alkyl C showing the fastest and slowest rate of
decline, respectively. On conversion back to pasture, both WSC and AHC
returned to levels originally present under long-term pasture. TOC recovered
to original pasture levels in the humic soil, but recovered only to
60–70% of original levels in the coarse- and fine-textured 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 electron microscopy indicated a loss of the binding agents around
aggregates under cropping. The effect of the loss of these binding agents on
soil structure was more pronounced in mica-rich soils than in oxide-rich and
allophanic soils. The very high aggregate stabilities of the humic soil under
pasture was attributed to the presence of a protective water-repellent lattice
of long-chain polymethylene compounds around the soil aggregates.
Experimental Sudies on the Formation of Humic Soil
