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Grassland set-asides (GLSA) are fields that are taken out of intensive annual crop production and seeded with a mixture of grasses and legumes for one to four years to improve soil quality. The objectives of this study were to evaluate (i) the relationships among soil organic carbon (SOC), permanganate oxidisable C (POXC), dilute-acid extractable polysaccharides (DAEP) and aggregate stability to determine if they may be used as proxies for one another, (ii) whether these indicators could be used to predict aggregate stability, (iii) if differences in soil quality after shortterm GLSAs, detected with aggregate stability, could instead be detected with POXC or DAEP and (iv) potential use of diffuse Fourier transform spectroscopy (FT-MIR) to predict POXC, DAEP and aggregate stability in the Fraser River Delta region of British Columbia, Canada. There were strong relationships among SOC, POXC and DAEP, but the relationship between DAEP and SOC ([R.sup.2] = 0.60, P < 0.0001) was less strong than that observed between POXC and SOC ([R.sup.2] = 0.71, P < 0.0001). All three soil C fractions were significantly predicted with the 2-6 mm aggregate size fraction but the correlations for DAEP ([R.sup.2] = 0.43) and POXC ([R.sup.2] = 0.36) were stronger than that for SOC ([R.sup.2] = 0.29). Predictions of soil quality indicators using FT-MIR produced [R.sup.2] = 0.92 for POXC, [R.sup.2] = 0.93 for DAEP and [R.sup.2] = 0.62 for the 2-6 mm aggregate size fraction. These results suggest that FT-MIR holds promise as a low-cost method to determine labile soil C fractions that are better proxy soil quality indicators for aggregate stability than SOC. Additional keywords: agriculture, labile carbon fractions, permanganate oxidisable carbon, perennial grass systems, soil polysaccharides, soil structure. Received 5 July 2019, accepted 3 February 2020, published online 31 March 2020, Introduction Soil quality is a concept commonly used to evaluate sustainable and beneficial land management practices in agricultural systems, and the most widely acknowledged indicator of soil quality is soil [...]