Particulate organic matter, carbohydrate, humic acid contents in soil macro- and microaggregates as affected by cultivation

Cultivation is known to strongly affect not only soil structure but also organic substances responsible for aggregation. This research was conducted to study the effect of cultivation on the distribution of soil macro- and microaggregates as related to changes in soil organic matter content in a Typic Haplustoll, located in central Cordoba, the principal peanut (Arachis hypogaea L.) growing area of Argentina. Samples were taken from A or Ap horizons at (1) an undisturbed and (2) a cultivated site for determination of the aggregate fractions by the wet sieving method. The mineral associated organic carbon (MOC) and particulate organic carbon (POC), in soils and their aggregates, were separated by dispersion and sieving procedures. Carbohydrate content in soil and in aggregate fractions was determined by dilute acid (CHda) and hot water (CHhw) extraction, whereas humic (HA) and fulvic acid (FA) were extracted by the NaOH method. The results indicated that the total macroaggregates (> 250 μm) content was 1.7 times lower in the cultivated than in the undisturbed soil. The large macroaggregates (2800–2000 μm) were the most affected, decreasing a 92% due to cultivation compared with undisturbed. In contrast, the microaggregate (250–53 μm) content was twice that high in the cultivated than in the undisturbed soil. The concentrations of OC, POC, CHda, CHhw were all reduced substantially by cultivation, with the microaggregates showing an almost complete loss of its POC content. The destruction of these transient organic cementing agents was assumed to have contributed to the collapse of the macroaggregates. This has resulted in exposure of POC, making it more available to rapid oxidation and microbial attack. There were indications suggesting POC and CHhw contents to be valuable as indicators of soil structure degradation due to exhaustive cultivation practices. Although it is well know that humic substances are chemically and structurally much more stable than nonhumic substances but our results showed a surprising decrease humic substances under continuous cultivation.