Using ultrasonic energy to elucidate the effects of decomposing plant residues on soil aggregation Zhaolong Zhu a,b, *, Denis A. Angers c , Damien J. Field b , Budiman Minasny b a State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F University, Yangling Shaanxi 712100, China b Faculty of Agriculture and Environment, The University of Sydney, Biomedical Building C81, 1 Central Avenue, Australian Technology Park, Eveleigh, Sydney, NSW 2015, Australia c Quebec Research and Development Centre, Agriculture and Agri-Food Canada, 2560, Hochelaga Blvd, Québec, G1V 2J3 QC, Canada A R T I C L E I N F O Article history: Received 27 February 2016 Received in revised form 3 September 2016 Accepted 2 October 2016 Available online xxx Keywords: Aggregate stability Organic matter Ultrasonic energy Relative soil strength A B S T R A C T 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 CO 2 C g 1 C for total respiration rate) being greater than barley amended samples (0.178 g CO 2 C 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 mm and 731 mm 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. ã 2016 Elsevier B.V. All rights reserved. 1. Introduction Soil structure is a key attribute in the soil’s ability to provide functions including its ability to store water and influence its movement, hold nutrients and buffer contaminants, sequester carbon, provide a habitat supporting biodiversity, as well as prevent degradation due to erosion (Bronick and Lal, 2005; McBratney et al., 2014; Six et al., 2000). The decline in soil structure compared to its natural state is mainly caused by changes in land use and intensive soil and crop management practices (Chan et al., 2003), and can be measured by the amount of soil aggregates and their relative stability. Interventions such as maintenance of cover crops, stubble retention, minimum tillage and organic materials addition have been shown to improve soil structure and soil aggregation (Bhattacharyya et al., 2009). The effects of organic inputs on soil aggregate stability are well known and have been studied extensively (e.g. Diacono and Montemurro, 2010). A review by Abiven et al. (2009) evaluated a classical conceptual model by Monnier which considered the temporal effects of organic amendments on aggregate stability. The review found that the aggregate stability depends on the nature of the organic inputs where easily decomposable products have an intense and transient effect on aggregate stability while more recalcitrant products have a smaller but longer term effect. Most studies leading to these generalisations are based on the * Corresponding author at: State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F University, Yangling Shaanxi 712100, China. E-mail address: zhu_zl@nwsuaf.edu.cn (Z. Zhu). http://dx.doi.org/10.1016/j.still.2016.10.002 0167-1987/ã 2016 Elsevier B.V. All rights reserved. Soil & Tillage Research 167 (2017) 1–8 Contents lists available at ScienceDirect Soil & Tillage Research journa l homepage: www.e lsevier.com/locate/st ill