Carbon and nitrogen release from cover crop roots is poorly predicted by their chemical composition

Root-derived carbon (C) and nitrogen (N) release in relation to chemical composition has rarely been quantified in field studies. The objectives of this study were to evaluate C and N release from six summer cover crop roots and to correlate it with their chemical composition. Root decomposition and N release from velvet bean (Mucuna aterrima), pearl millet (Pennisetum americanum), dwarf pigeon pea (Cajanus cajan), sunn hemp (Crotalaria juncea), showy rattlebox (Crotalaria spectabilis), and jack bean (Canavalia ensiformis) were evaluated over 2 years under no-till subtropical conditions. There was no difference in C release rates for either the labile or recalcitrant C compartments in the first (average: k₁ = 0.0577; k₂ = 0.0017) or the second year (average: k₁ = 0.1657; k₂ = 0.0029). Root C remaining after 140 days did not differ in the first year (average 46.4%), but it was higher in the second year for pearl millet (65.3%) compared to the other species (40.2%). N release was the most intense during the first 21 days and decreased drastically afterward. After 140 days, the N remaining in pearl millet and velvet bean roots was higher (77.7%) than in the other species (47.1%) in the first year, while in the second year, pearl millet contained more N (50%) compared to velvet bean (38%) and jack bean (28.1%). The C and N release rates were poorly correlated to the chemical composition of the summer cover crop roots. Our results reinforce the agronomic recommendation to sow main crops immediately after cover crop management to maximize N recovery from roots.