Nonlinear turnover rates of soil carbon following cultivation of native grasslands and subsequent afforestation of croplands

Authors: HERNANDEZ-RAMIREZ, GUILLERMO - University Of Alberta; Sauer, Thomas; CHENDEV, YURY - Belgorod State University; GENNADIEV, ALEXANDER - Lomonosov University

Submitted to: Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/16/2021
Publication Date: 7/19/2021
Citation: Hernandez-Ramirez, G., Sauer, T.J., Chendev, Y.G., Gennadiev, A.N. 2021. Nonlinear turnover rates of soil carbon following cultivation of native grasslands and subsequent afforestation of croplands. Soil. 7(2):415-431. https://doi.org/10.5194/soil-7-415-2021 .
DOI: https://doi.org/10.5194/soil-7-415-2021

Interpretive Summary: Many soil properties change when land is converted from a natural grassland to crop cultivation and again when cultivated land is planted to trees. This study evaluated changes in soil organic matter (SOM) with time of cultivation or time since tree planting. SOM is an important soil quality characteristic as it affects many soil processes including soil structure formation and nutrient cycling. Different techniques were used to determine the plant source of SOM (native grass, crops, or trees) and follow changes in the amount of these different pools of SOM over time in the surface soil layers at sites in the Russian Steppe and U.S. Great Plains. The results generally showed a slow loss of SOM with time during cultivation of crops on former grasslands and a more rapid increase in SOM beginning about 5 years after tree planting on former cropland. Knowing these relative rates of changes in SOM are important for predicting the amount of carbon being lost or potentially being stored in soil following a change in land use. This research is important to growers, researchers, and policymakers interested in how land management affects soil quality and carbon storage.

Technical Abstract: Land use conversions can strongly impact soil organic matter (SOM) storage, which creates paramount opportunities for sequestering atmospheric carbon into the soil. It is known that land uses such as annual cropping and afforestation can decrease and increase SOM, respectively; however, the rates of these changes over time remain elusive. This study focused on extracting the kinetics (k) of turnover rates that describe these long-term changes in soil C storage and also quantifying the sources of soil C. We used topsoil organic carbon density and d13C isotopic composition data from multiple chronosequences and paired sites in Russia and United States. Reconstruction of soil C storage trajectory over 250 years following conversion from native grassland to continual annual cropland revealed a C depletion rate of 0.010 years-1 (first-order k rate constant), which translates into a mean residence time (MRT) of 100 years (R2= 0.90). Conversely, soil C accretion was observed over 70 years following afforestation of annual croplands at a much faster k rate of 0.055 years-1. The corresponding MRT was only 18 years (R2= 0.997) after a lag phase of 5 years. Over these 23 years of afforestation, trees contributed with 14 Mg C Ha-1 to soil C accrual atin the 0 to 15 cm depth increment. This tree-C contribution reached 22 Mg C Ha-1 at 70 years after tree planting. Over these 70 years of afforestation, the proportion of tree-C to whole soil C increased to reach a sizeable 79%. Furthermore, assuming steady state of soil C in the adjacent croplands, we also estimated that 45% of the prairie-C existent at time of tree planting was still present in the afforested soils 70 years later. As intrinsic of k modeling, the derived turnover rates that represent soil C changes over time are nonlinear. Soil C changes were much more dynamic during the first decades following a land use conversion than afterwards when the new land use system became mature approached equilibrium. Collectively, results substantiated that C sequestration in afforested lands is a suitable means to proactively mitigate escalating climate change within a typical person’s lifetime, as indicated by MRTs of few decades.