Litter decomposition is a fundamental ecosystem process. It is responsible for nutrient cycling and influences carbon (C) sequestration, and soil physical and chemical properties. In nature, litter is usually heterogeneous and may not decompose the way homogeneous litter does. For example, heterogeneous litter decomposition is frequently non-additive. This makes the rate of nutrient cycling as well as fluxes of C into and out of soil C pools impossible to predict. The most frequently proposed mechanisms for positive, non-additive decomposition include the supply of limiting mineral nutrients, the supply of available C (priming), and the improvement of micro-environmental conditions. However, all three mechanisms are controversial in the sense that no single mechanism accounts for all cases of non-additive decomposition. In mesic ecosystems, both soil microbes and soil fauna are the major causes of decomposition. Microbes decompose litter by producing extracellular enzymes. The comminution of litter by soil animals interacts with microbial activities by increasing substrate surface area. In our study, positive, non-additive decomposition of oat straw when mixed with clover was not due to enhancing microarthropod density in oat straw but associated with significantly increased microbial activity in oat straw. We further investigated the factors that contribute to positive, non-additive decomposition by testing several common hypotheses used to explain non-additive decomposition (increased water content, and the transfer of C and/or nitrogen (N) compounds from clover to oat straw). We also tested a new hypothesis, which is that C, N and other nutrients are simultaneously supplied by clover to stimulate the decomposition of oat straw. Our study indicated that the addition of water to oat straw did not increase oat straw decomposition and adding ammonium chloride only or glucose and ammonium chloride together to oat straw had no significant effect on oat straw decomposition. Glucose addition alone (Low concentration) increased oat straw decomposition but was not sufficient to predict the effect of clover litter. Either the addition of glucose, ammonium chloride and other minerals together to oat straw, or soil was in contact with oat straw and glucose and ammonium chloride were added, oat straw decomposition was stimulated as if clover were present. These results suggest that the limiting resources are some combination of C, N and other mineral nutrients and that soil itself may be a source of limiting nutrients in litter decomposition. In nature, some combination of high quality litter and soil itself may supply resources that stimulate the decomposing organisms’ activity on low quality litter and then the decomposition of low quality litter. Our research provides insight into the dynamics of heterogeneous litter decomposition and will allow us to better model nutrient cycling.