Organic Farming Practices Facilitate Soil Carbon Stabilisation Following Massive Application of Ramial Chipped Wood: A Case Study of Sweet Potato Cultivation

Abstract

Improving soil organic matter contents in semi-arid zones reduces the risk of soil degradation and improves climatic resilience.As a technique for the recarbonisation of abandoned or degraded soils, applications of massive amounts of exogenous biomass(EB) such as wood chips have not been widely tested, and agronomic integration is still lacking. While it has been shown thatsweet potato (Ipomea batatas) may be able to overcome nitrogen (N) limitations provoked by massive applications of EB, theexact mechanism is not clear. In a field experiment in Valencia (Mediterranean, semi-arid climate), we mixed in the soil profile(0–30 cm) an equivalent of 150 t ha−1 ramial chipped wood (RCW) before sweet potato cultivation, set up in two adjacent fieldswith long-term management histories (15 years) of abandonment versus organic farming. RCW incorporation in plots led toincreased concentrations of soil organic carbon (SOC) in the fine earth fraction, which was greater in organically managedplots; whereas average SOC increased from 13.3 to 17.2 g kg−1 in the abandoned plot, it increased from 16.0 to 27.4 g kg−1 in theorganically managed plot. Furthermore, despite greater initial SOC contents in organically managed plots, soil stabilisation of C(and N) was greater in relative terms as well, with an average SOC increase of 68%, as compared to only 29% in abandoned plots.Sweet potato leaf N content was not associated with measured soil N species; however, management history was relevant, sinceN nutrition was improved in the organic field by 0.44%–0.73% (w/w) over the growing season, and those plants in the abandonedfield contained about 20% more N derived from the atmosphere (Ndfa). Sweet potato leaf δ15 N also changed dramatically overthe sampling period, indicating a change in N source. The results show a potential for rapid soil recarbonisation with massiveapplication of lignocellulosic biomass in soils, but which is dependent on management. Also, we observed a particular capacityof sweet potato for accessing soil-bound organic N sources under conditions of high demand