Impact of tillage erosion state on 14C labelled assimilate C distribution in a spring rapeseed (Brassica napus L.) soil system and soil C pools

Abstract

<p><strong>Keywords:</strong> <sup>14</sup>C pulse labelling, Tillage-erosion, SOC pool, C-undersaturation, C allocation, Climate change.</p><p>Tillage erosion is a dominant process on arable land which results in drastic changes in many properties of the topsoil, especially its organic carbon (SOC) content. Particularly on slope positions, some original topsoil is initially displaced toward depressions in the course of a tillage operation. The following tillage then causes nutrient and C-poor subsoil to be mixed into the remaining topsoil. Previous studies have shown that the resulting "C saturation deficit" can lead to increased accumulation of fresh assimilate-C in the soil, i.e., such eroded topsoil can act as a terrestrial carbon dioxide (CO<sub>2</sub>)-C sink. As a first step, we aim to determine the impact of tillage erosion state (i.e. extent of subsoil admixture) on C dynamics and storage by investigating the distribution of freshly assimilated C in the plant-soil-microbiota system and SOC pools. The soil under investigation was taken from the Ap and Bt horizon of an eroded Luvisol (Nudiargic Luvisol) located on the slopes at the CarboZALF-D site in Dedelow, Germany. This site is a Leibniz Centre for Agricultural Landscape Research (ZALF) experimental site where extensive studies on C dynamics and soil erosion are being carried out. A pot experiment with three simulated tillage erosion states, i.e. moderate, strong and topsoil with no admixture as a reference was carried out by incorporating 12% Bt, 24% Bt and 0% Bt material, respectively. Spring rapeseed (<em>Brassica napus L.</em>) was grown in the pots containing above treatments under controlled conditions as well as pots without plants, with bare soil that were used as control. The plants were pulse labelled with <sup>14</sup>CO<sub>2</sub> at the stem elongation stage, and the distribution of recently assimilated C into different compartments of plant-soil-soil gas system were assessed after 21 days. Sequential fractionation of the soil samples were carried out and <sup>14</sup>C distribution in SOC pools such as dissolved organic carbon (DOC), particulate organic carbon (POM), mineral associated organic matter (MAOM) as well as microbial mass carbon (C<sub>mic</sub>) were quantified. We hypothesize that, with increasing subsoil incorporation into the topsoil, the transfer of the freshly assimilated C into the soil and its incorporation into the MAOM pool will intensity.  </p>