The effect of organic materials on soil aggregate stability: implications for soil erosion

The effect of cow dung, wheat straw, calcite, gypsum and their interactions on soil aggregate stability was determined after six months incubation at standard moisture (10 kPa suction) and temperature (25°C) conditions. The effect of cow dung and wheat straw on simulated soil erosion loss was also determined after one month incubation. Aggregate stability was measured by a wet sieving method and calculated as the mean weight of diameter (MWD) of aggregates. Soil erosion was assessed using simulated rainfall at a rate of 40 mm per hour, similar to rainstorm events in Central Sulawesi, Indonesia. Soil microstructure was investigated by polarising microscope examination after three months incubation and by scanning electron microscopy (SEM) after one month incubation. Total soil nitrogen and soil organic carbon contents were also measured to assess their relationships with MWD. A sensitivity analysis of the universal soil loss equation (USLE) was also conducted to assess the relative influence of the soil erodibility factor on soil erosion. Over a period of six months, soil aggregate stability was significantly increased following the addition of cow dung and wheat straw, with the straw addition resulting in a slightly higher MWD. On the other hand, both calcite and gypsum tended to decrease the MWD. Increased aggregate stability was associated with soil microorganism activity, particularly fungi, as revealed by SEM observations. Decreased MWD was associated with the breakdown of large aggregates into small aggregates. Total nitrogen and organic carbon contents of the soil were significantly increased following the addition of cow dung and wheat straw. A significant correlation was also found to exist between both total soil nitrogen and organic carbon contents, and MWD. Both polarising microscopy and SEM observations revealed an improvement of soil microstructure with the addition of cow dung and wheat straw. Compared to the addition of dung, the straw amendment resulted in lager porosity values and larger sizes of water-stable aggregates. The soil erosion assessment under simulated rainfall showed that the addition of straw significantly reduced the amount of soil loss, while cow dung did not significantly affect the amount of soil loss. The significant reduction in soil loss was associated with increased aggregate stability and improved soil microstructure leading to greater soil resistance to raindrop impact. While the non-significant effect of the cow dung addition was primarily due to the presence of fresh earthworm casts that are very susceptible to dispersion as a result of raindrop impact. The USLE was found to be very sensitive to combined changes of the crop management factor and the support practice factor compared to any other combinations or single changes. Changes in the soil erodibility factor, through improved soil structure form and increased organic matter content, was found to be successful in reducing soil erosion loss predictions. The implications of the findings in the present study are discussed, particularly with reference to conditions in Central Sulawesi, Indonesia