Soil rock fragments: Unquantified players in terrestrial carbon and nitrogen cycles

Abstract Soils with a significant proportion of rock fragments (RFs, with diameter >2 mm) are widely distributed in terrestrial ecosystems. However, a lack of quantitative information about RF characteristics restricts our ability to explain and predict the related soil carbon (C) and nitrogen (N) processes, and induces biases of C and N investigations in areas where soils contain significant RFs. In this paper, we reviewed the direct and indirect effects of RFs on soil C and N cycles, including via affecting soil C and N stocks, hydrology, temperature, and via chemical weathering that releases C, N, cations and anions, consumes CO2 and regulates pH. Based on this, we discussed current challenges and potential solutions in investigating C and N cycles of high RF content soils. First, we proposed a series of approaches to quantifying the effects of RFs on C and N stocks and soil hydrology across scales. Second, we emphasized the evolutions of soil RFs’ properties over time, and recommended including RF physicochemical properties in the routine soil inventory. Third, we suggested that both the effects of RFs and the evolutions of soil RFs over time should be considered in model simulation. Finally, we provided a framework by coupling monitoring, mapping and modelling to investigate the roles of RFs on soil C and N cycles from earth’s critical zone to global scale. This review will improve our understandings of RFs and terrestrial C and N cycles, and their responses to global change.