From global to local: trends and drivers of C:N ratios in soils and particle-size fractions

Abstract

The balance between soil organic carbon (SOC) and N – assessed by C:N ratios – serves as a common indicator of soil organic matter (SOM) quality and decomposition. The potential mineralization of SOM also relates to the partition of total SOM across soil particle size fractions – sand, silt, clay - and its eventual association with such mineral components, which in turn can be affected by varying edaphic and environmental controls. Investigating the controlling factors of C:N ratios in soil and in soil particle-size fractions can provide insight into SOM stability and turnover, and thus into the potential for nutrient supply in agroecosystems, SOC sequestration and greenhouse gases emissions. Here, we aimed to assess the main trends and controlling factors of soil C:N ratios, by means of compiling global and local data on C:N ratios in bulk soils and in sand, silt, and clay fractions. For the global assessment, we compiled data from 74 studies and 30 countries. The selected studies contained data on C:N ratios in soils and in particle-size fractions (sand: Ø > 50 μm, silt: 2 < Ø < 50 μm, and clay: Ø < 2 μm), or SOC and N concentrations. Information on latitude, longitude, altitude, temperature and mean annual precipitation (MAP), Köppen climate classification, soil depth, land use (grassland, forest, and agriculture), soil pH, soil texture were also collected for modelling the C:N ratios. Global mean C:N ratio in soils was 13.2. Sand-sized SOM had the highest average (18.1) and broadest range of variation, while silt had a mean value of 14.3 and clay had the lowest mean (9.9) and the most narrow variation. C:N ratios in soils and in particle-size fractions were lower under arid climate, and higher under forests, particularly in temperate climates. Random Forest (RF) modelling suggests that sand content is the primary driver of global soil C:N ratios due to wide variability in C:N ratios of SOM associated with sands at a global scale. Köppen climate, sand content, and altitude were the most important explanatory variables for sand C:N ratios, whereas MAP, Köppen climate, and soil pH were the most relevant variables to explain variations in the global clay C:N ratios. In the local assessment, the effects of soil texture, mineralogical composition and fertility on C:N ratios in soils and in particle-size fractions were assessed under native vegetation in tropical highlands near Lavras, Brazil. Soil samples were collected in soils formed on eight contrasting parent materials at the 0-5, 30-40, and 90-100 cm depths. C:N ratios did not vary with depth (except for sand fractions), decreased with increasing silt contents, sum of exchangeable bases, and Mn oxide contents, and increased with clay, exchangeable Al3+ and H+Al contents. The inverse relationship between C:N ratios and soil fertility indicators in such soils is probably driven by higher retention of ammonium N forms either as cations or parts of organic molecules, particularly in the most fertile soil on itabirite. In contrary to the global trends, clay fractions were the most variable in C:N ratios, thus acting as the main driver of bulk soils C:N ratios, which was ascribed to the wide variation in clay mineralogy and activity sampled. Our findings evidenced the variables affecting C:N ratios in soils worldwide and, in the Lavras area, how C:N ratios are affected by soil texture and fertility. Such knowledge can support modeling the response of agroecosystems to changes in land use and climate at the global or regional scales. Locally, it can help designing management practices towards improved SOM accumulation and sustainable agriculture.