1542 ISSN 1064-2293, Eurasian Soil Science, 2019, Vol. 52, No. 12, pp. 1542–1554. © Pleiades Publishing, Ltd., 2019. Organic Carbon Fractions, Aggregate Stability, and Available Nutrients in Soil and Their Interrelationships in Tropical Cropping Systems: A Case Study R. R. Ratnayake a, *, T. Roshanthan b , N. Gnanavelrajah b , and S. B. Karunaratne c a National Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka b Faculty of Agriculture, University of Jaffna, Jaffna, Sri Lanka c Hawkesbury Institute for the Environment, Western Sydney University, Richmond NSW, Australia *e-mail: renuka.ra@nifs.ac.lk Received January 9, 2019; revised March 3, 2019; accepted May 22, 2019 Abstract—Tropical agricultural soils have been claimed as a source of carbon. As agricultural systems in the trop- ics are highly diverse, it is useful to study soil organic C (SOC) of different agricultural systems. We quantified the SOC fractions, available nutrients, and aggregate stability in eight different tropical agricultural systems, including annual crops under different management scenarios, such as organic, inorganic, and combined fertil- izer applications. Annual crops treated with organic fertilizer only (A–OF), inorganic fertilizer only (A–IF), both organic and inorganic fertilizers (A–O/IF), perennial crops (PC), home gardens (HG), and abandoned home gardens (AHG) in Eutrustox soils and annual crops with organic fertilizer only (A–OFS) and unculti- vated land on Quartzipsamments soil (USR) were studied. The links between SOC fractions, available nutrients, and aggregate stability in these soils were analyzed. Regression models were fitted for SOC fractions and avail- able nutrients. Our results indicated that the different land use types exhibited significant variations in organic carbon fractions, aggregate stability, and available nutrients in soils. The available macro and micronutrients, except for nitrogen, showed a significant positive correlation with either total organic C (TOC) or carbon frac- tions indicating the synergy between them. The differences in soil C stocks clearly reflected the differences in litter fall and soil disturbance, as indicated by the highest C stocks in AHG. The dry weight of collected litter showed that AHG accumulated the highest litter content (97.38 g/m 2 ) compared to the lowest (37.63 g/m 2 ) in A–I/F. Organic matter addition to soil also increased the C stocks, even in annual crops. Aggregate stability showed a positive correlation with C fractions. The regression models developed in this study can be used to pre- dict available nutrients by measuring TOC or C fractions in similar land use types in the tropics. This study con- firmed that tropical agricultural systems that include annual crops have potential for storing and maintaining C in soils, if appropriately managed. The beneficial influence of SOC on available nutrients and aggregate stability could be a driving force to increase carbon stock in tropical agricultural systems. Keywords: soil C fractions, annual crops, perennial crops, home gardens, organic fertilizer DOI: 10.1134/S1064229319120123 INTRODUCTION Soil organic carbon (SOC) plays an important role in maintaining ecosystem services that are vital for sustainable agriculture [45]. Though decline of carbon stock of agricultural lands [20, 10] have been reported depending on the agricultural and forestry manage- ment practices and climatic scenarios, stored SOC can act as a greenhouse gas source or sink [47]. Enhancing carbon storage in soil is of great interest, as the soil itself contain approximately 2500 Pg (10 15 g) of C at the global level [48]. Therefore, understanding the behavior of SOC is critical for managing it in a way that minimizes the increase in greenhouse gasses. It is estimated that improving agricultural management practices could sequester between 400 and 800 Mt C per year worldwide [49]. Soil organic C consists of different SOC fractions that play different ecological roles and have various biological influences [43]. Soil organic C fractions vary in turnover rates and resistance to decomposition [21, 30, 43]. Therefore, when studying C sequestra- tion, it is important to explore not only these fractions, but also total SOC. Different systems are used to clas- sify these SOC fractionations. Soil itself is a matrix of these fractions that is combined by the formation of both macro and micro aggregates [22]. These aggre- gates will provide protection and allow the SOC to remain in soils for a long period. Additionally, SOC fractions are important for initializing process based SOC models, such as the Roth C carbon model, by SOIL CHEMISTRY