Soil carbon inputs from grain legumes based cropping systems with cover crops

A greener agricultural sector calls for a diet shift from meat to plant protein sources. The inclusion of grain legumes (GLs) such as faba bean (Vicia faba) and pea (Pisum sativum), and further cover crops in crop rotations likely helps the transition towards more sustainable food systems, for example in Denmark where only 3.6% of the organic agricultural land cultivated with GLs. Thus, most GLs in Denmark for human consumption are imported dry grains with only a small local production of green peas. Yet, early harvested GLs may offer environmental and economic benefits by facilitating the establishment and growth of cover crops, which can improve carbon (C) inputs to soils and also opening new possibilities for biorefining of green residues. However, there is no empirical data of C inputs, including phyllo- and rhizodeposition (ClvPR), in GLs-based cropping systems under Danish conditions. Here we performed a field experiment with multiple pulse labelling of ¹³CO₂, to quantify C yields of GLs based cropping systems (faba bean, pea, and the mixture of pea and spring barley) to 1-m soil depth, in comparison to a cereal (spring barley, Hordeum vulgare) based system. The multiple pulse labellingwas conducted with both main crops (faba bean, pea, spring barley, and the mixture) as well as the subsequent cover crops, to obtain an estimation of total C yields over a full growing season. The results showed that the total C yields of the main crops was similar among different GLs (4.4, 4.6, and 4.9 Mg C ha^-1 for faba bean, pea, and the mixture, respectively), which was slightly lower than that of spring barley (6.5 Mg C ha^-1). The same pattern was seen for ClvPR of main crops. Further, the preceding main crops had little effect on the total C yield of subsequent cover crops (2.9, 2.4, 2.3, 2.5 Mg C ha^-1 for cover crops following fababean, pea, the mixture, and spring barley, respectively). Thus, no complementary effect of mixed pea and spring barley was found to increase the C yield. However, GLs based cropping systems received no nitrogen (N) fertilizer as opposed to spring barley, which was fertilized with 100 N ha^-1 (in slurry). This reduces the reliance on external inputs, and might minimize negative environmental impacts including greenhouse gases emissions, which needs future studies.