Combined effects of biochar amendment, leguminous cover crop addition and snow removal on nitrogen leaching losses and nitrogen retention over winter and subsequent yield of a test crop (Eruca sativa L.)

•Leguminous cover crops increased N availability, but increased soil N losses over winter.•Biochar addition increased over-winter soil N retention and increased crop yield.•Biochar pyrolysis temperature was negatively correlated with microbial biomass N.

Soil nitrogen (N) losses over winter and early spring are often substantial in northern temperate regions as a result of soil freeze-thaw. In agricultural fields, both biochar amendment and the use of cover crops have been proposed as strategies to increase crop N availability, but it is unclear how they may interact to affect soil N retention over winter, particularly when leguminous cover crops are used. We performed a soil mesocosm experiment to examine the interactions of three factors: biochar amendment (with biochar pyrolysis temperatures ranging from 250 to 550 °C), addition of leguminous cover crop material (red clover; Trifolium pratense L.), and snow removal (to increase soil freezing) on soil N retention over winter and subsequent plant yield. We installed a resin bag at the bottom of each mesocosm to examine the leaching of inorganic N over winter. We also measured N leaching and microbial biomass N (MBN) at spring melt and the yield of an arugula (Eruca sativa L.) test crop sown in the spring. Inorganic N losses increased ∼4 × over winter with cover crop residue addition; this increase was reduced by biochar amendment, but only by 9%. Biochar pyrolysis temperature, which was associated with increased soil pH, was negatively correlated with MBN. Cover crop addition increased arugula yield greatly (>4 × ), while biochar addition and snow removal had minor positive and negative effects (10-20%) on arugula yield. Overall, there were few interactive effects among treatments. While our results confirm that biochar amendment can mitigate soil N losses over winter and support greater plant yield the following spring, the biochar made a relatively small contribution to N cycling relative to the addition of the leguminous cover crop residue, which increased both N losses and plant yield substantially.