Nitrogen-15 labelling and tracing techniques reveal cover crops transfer more fertilizer N to the soil reserve than to the subsequent crop

Abstract

Estimating the capacity of cover crops (CC) to capture soil nitrogen (N) and to transfer captured N to subsequent crop remains a challenge because CC biomass production, N content and decomposition rate vary depending on environmental conditions and management practices. The objectives of this study were (1) to compare biomass production and N accumulation by a mixture of red and white clovers intercropped with barley, and oat, field pea and oilseed radish grown as catch crops after barley harvest, (2) to determine the capacity of different CCs to catch and transfer fertilizer N (mineral or pig slurry) to subsequent wheat and the soil reserve, and (3) to determine the influence of CC type on yield of spring wheat in the next year. A 2-y crop rotation (barley – wheat) was repeated twice on a sandy loam using 15N-labelled fertilizers applied at the time of CC establishment. Fertilization increased above-ground biomass yield in oat and oilseed radish, but had little effect on legume CCs. The legume CCs accumulated more total N in their biomass (63–127 kg N ha−1) than non-legume CCs (47–68 kg N ha−1). However, oat and oilseed radish were more efficient at capturing the applied fertilizer 15N (33–51 %) than legume CCs (2–36 %). Intercropped clovers resulted in the largest increases in subsequent wheat yields (23–92 %), followed by oilseed radish and field pea. At the end of the 2-y rotation (wheat harvest), the recovery of fertilizer 15N by subsequent wheat was < 10 %, whereas 26–60 % were still present in the 0−30 cm soil layer. We conclude that non-legume CCs have a good capacity to capture applied fertilizer N, whereas legume CCs (especially clovers) had a limited capacity. Nevertheless, clovers CC resulted in the greatest wheat yields in the following year, likely due to efficent transfer of symbiotically fixed N and improved soil quality, whereas fertilizer N captured by non-legume CCs was mainly routed to the soil N reserve, with potential to supply N to subsequent crops through a legacy effect.