Using ultrasonic energy to elucidate the effects of decomposing plant residues on soil aggregation

- Soil aggregate stability is controlled by the quality and quantity of C added to the soil.
- Decomposition of barley residues produces stronger soil aggregates than alfalfa.
- The effect of C inputs on aggregate formation can be elucidated by ultrasonic energy.

Water-stable soil aggregates are generally formed following the addition of organic materials in soils. This process is mediated by the interaction between microbes and soil organic matter in ways that are still not completely understood. To get insight into the effects of decomposing plant residues on aggregate dynamics, a clay soil with an inherently low soil organic carbon (SOC) content, was amended with two different sources of organic matter (alfalfa, C:N = 16.7 and barley straw, C:N = 95.6) at different input levels (0, 10, 20, & 30 g C kg−1 soil). These were incubated for a period of 3 months over which soil respiration was assessed using the NaOH capture method, water aggregate stability was determined with the mean weight diameter (MWD) by wet sieving, and the relative strength of aggregates exposed to ultrasonic agitation was modelled using the aggregate disruption characteristic curve (ADCC) and soil dispersion characteristic curve (SDCC). As expected, the quality and quantity of organic matter added controlled the respiration rate, with alfalfa (0.457 g CO2C g−1 C for total respiration rate) being greater than barley amended samples (0.178 g CO2C g−1 C) at any C input rate. Both residue quality and quantity of organic matter input also influenced the amount of aggregates formed and their relative strength. The MWD of soils amended with alfalfa residues was greater than that of barley straw at lower input rates and early in the incubation (e.g. at 28 days of incubation and at a rate of 10 g C kg−1 soil, MWD was 575 μm and 731 μm for barley straw and alfalfa, respectively). However, in the longer term (84 days of incubation), the use of ultrasonic energy revealed that barley straw resulted in stronger aggregates, especially at higher input rates despite showing similar MWD as alfalfa. The use of ultrasonic agitation, where we quantify the energy required to liberate and disperse aggregates allowed us to differentiate the effects of C inputs on the size of stable aggregates and their relative strength.