Belowground carbon input and translocation potential of fodder radish cover-crop

We compared the soil C input potential of a common catch-crop (fodder radish) established in 6-year-old direct-drilled (DD) plots with adjacent conventionally tilled (CT) plots on a Danish sandy loam soil by use of C-14-isotope labelling techniques. Intact monoliths of soil with actively growing fodder radish seedlings were extracted in Autumn of 2008 from DD and CT field plots and labelled with (CO2)-C-14 at different time intervals during fodder radish growth. Labelled monoliths were then sampled 6 and 100 days after termination of labelling by clipping above-ground biomass at soil level and separating below-ground components into macro-roots and macro-root-free soil at 0-10, 10-25 and 25-45 cm soil depth. Using fodder radish C-14 data and the preceding spring barley biomass yield data we estimated C input from the spring barley-fodder radish cycle in addition to evaluating the effect of the removal of spring barley harvestable straw on soil C input. Potential soil C input under straw removal scenarios with and without an established fodder radish crop was also evaluated. Relative to other depths, over 70% of labelled below-ground C was found in the 0-10 cm soil depth in both DD and CT treatments for each of the two samplings. For both macro-root and macro-root-free soil and in both tillage treatments, labelled C decreased significantly with depth (P < 0.05). A decline of labeled C in macro-root but an increase of labeled C in macro-root-free soil was observed from day 6 to day 100 for both tillage treatments. Over the autumn-winter growing period, total below-ground C input by fodder radish within the 0-45 cm soil depth was approximately 1.0 and 1.2 Mg C ha(-1) for CT and DD, respectively. We used data from 100 days after labelling, which coincided with the incorporation of the field fodder radish biomass, to estimate that the total fodder radish contribution to below-ground C after biomass incorporation would range between 1.6 and 1.7 Mg C ha(-1) for DD and CT, respectively. The figures for spring barley straw removal with fodder radish establishment would be between 4.9 and 5.1 Mg C ha(-1), while with no fodder radish establishment, C input to the soil would range between 3.2 Mg C ha(-1) and 3.4 Mg C ha(-1), which is approximately 0.6 Mg C ha(-1) lower than the 4 Mg C ha(-1) biomass C input required to maintain long-term soil organic C. In comparison, under straw retention and fodder radish catch-crop establishment the total spring barley and fodder radish C input would be approximately 6.1 and 6.5 Mg C ha(-1) for DD and CT, respectively. We conclude that fodder radish catch-crops have a potential for mitigating against soil C depletion resulting from export of cereal straw to other uses.