Cover crop N effects are different for subsequent sugar beet and maize.

Abstract

Cover crops (cc) are supposed to decrease the soil mineral N content (Nmin) during winter and increase the N supply to subsequent main crops due to mineralisation of N previously prevented from leaching. However, data on N supply from cc grown before sugar beet and maize have rarely been reported for Central European conditions. Therefore, our study aimed to provide information for cc differing in frost resistance and biomass quantity.  In 2018/19 and 2019/20, field trials were conducted on three Luvisol sites in Germany, comprising a sequence of either fallow or autumn sown cc (oil radish, saia oat, spring vetch and winter rye) followed by sugar beet and maize main crops sown next spring. Immediately before sowing, cc plant material was incorporated by tillage to 10-25 cm soil depth. The N effect of cc on sugar beet and maize was calculated as the difference in plant N uptake after a specific cc compared to bare fallow for distinct phases of the growing season.  Winter rye and oil radish cc revealed the greatest potential for scavenging nitrate from the soil profile while reductions caused by frost-sensitive saia oat and spring vetch were more variable. The amount of N in the cc biomass was negatively correlated with soil Nmin in autumn. Thus, for environmentally effective cover cropping in Central Europe, species with a sufficiently high frost tolerance should be chosen.  Despite cc N uptake of up to 170 kg N ha^-1 and C:N ratios were < 20, for sugar beet a positive N effect was found between March and July only and amounted to 20 kg N ha^-1 at maximum, while between August and September, net immobilization was predominant with up to 40 kg N ha^-1. In contrast, for silage maize the N effect was close to zero from sowing until July but clearly positive with values up to 60 kg N ha^-1 in the second half of the growing season. Differences among cc species were not consistent across the site/years investigated. Sugar yield was lowest after rye and correlated positively with Nmin in spring. Maize total plant dry matter yield revealed no clear differences among cc. Correlation between yield and cc N effect was low and not improved by a multiple regression approach.  Thus, factors other than in-season N supply from cc apparently impacted yield to a larger extent. For sugar beet, we hypothesize that the rapid depletion of the upper 30 cm soil layer from nitrate by the crop might cause substantial N immobilization during cc biomass decomposition in the summer months. In contrast, under maize N uptake is delayed, fostering microbial breakdown of and N release from the cc biomass.