Harnessing massive application of lignocellulosic inputs to fallow and regenerated soils

Soil management practices have led to a generalized decarbonization of agricultural soils' organic matter and carbon contents. Whereas mulching application of large amounts of organic matter (OM) on the soil surface, the incorporation of massive amounts of organic matter into the soil profile (e.g. 40-120 m³ ha^-1) has been much less common, and has only been studied in a limited capacity, with effects varying depending on soil type, climate and origin of the incorporated lignocellulosic material. Large amounts of carboniferous materials applied to soil can provoke nutritional disequilibria, and potential nutritional deficiencies for plants. However, it has been shown that sweet potato (Ipomea batatas) may be able to overcome N limitation, though the exact mechanism is not clear.

            In a field experiment in Catalonia, we applied an equivalent of 150 t ha^-1 ramial chipped wood (RCW) simultaneously with sweet potato cultivation. The experiment was set up in two adjacent in arable fields with a cold semi-arid climate (yearly precipitation ~ 400 mm) with contrasting previous management: one field had been abandoned for 15 years, while the other had been managed with regenerative farming techniques including organic manure application, limited tillage, and green cover since 2006. RCW was applied in March, sweet potato was planted in May, and the plants were harvested in October. Using a combination of techniques including mass balances of C and N in fine earth and large (> 2 mm) organic fractions, plant nutritional analysis, and ¹⁵N stable isotope natural abundance method, we examined C and N dynamics in the soil and plant nutrition.

            RCW incorporation increased both C and N in the plots where applied (Figure). However, absolute and relative gains were much greater in the regenerated plots, and the recovered C and N was much greater in the regenerated plots. Sweet potato N nutrition was not seen to be influenced by soil chemical properties (N fractions), pointing to other non-identified nutrition sources. Sweet potato leaf 𝛿¹⁵N also changed dramatically between samplings (March and October), indicating a change in N source.

            These first results give some indications about the potential for rapid soil recarbonization in soils under different management and crop appropriateness to make this transition.