Root and crop responses of sweet pepper (Capsicum annuum) to increasing N fertilization

Rooting is the mechanism by which roots explore soil resources to nourish and anchor the plant to the ground. In vegetable crops, nitrogen (N) application exceeds crop demand due to over fertilization, thereby contributing to N losses through nitrate (NO₃⁻) leaching. To improve N fertilization, knowledge of the response of rooting behaviour and root dynamics to N fertilization will be very useful. In this study, the effect of rates of N application on rooting were assessed in two sweet pepper (Capsicum annuum) crops grown in an artificially layered soil, with sand mulch, in Almería (south-eastern Spain). The treatments were very deficient, conventional, and very excessive in terms of N application. Yield, crop N absorption and dry matter of the shoot part were determined. Statistically significant differences were found in shoot dry matter between the very deficient N, compared to conventional and very excessive N. Root length density decreased with increasing application of N, with significantly higher density in the very deficient N application. In relation to depth, root length density in the very deficient N was nearly double (in the 2016 crop) and triple (in the 2017 crop) than in conventional N in the sand mulch layer (0–0.10 m depth). In contrast, root length density in the very deficient N treatment was in general lower than in conventional and very excessive N application in the 0.10–0.20 m layer. In the deeper layers, 0.20–0.30 and 0.30–0.40 m, no effects of N treatments on root length density were found. In relative terms, plants subjected to very deficient N treatment allocated relatively more roots in the sand mulch layer and less roots in the 0.10–0.20 m layer than when subjected to conventional and very excessive N. Root length density was negatively correlated with shoot dry matter, crop N absorption, yield and residual soil mineral N at the end of the crops. Overall, results of the present work suggest that conventional and very excessive N application maximized the development of the shoot part and crop yield and diminished root length density, particularly in the sand mulch layer (0–0.10 m depth). A higher root length density was not sufficient under very deficient N in terms of matching dry matter and yield of the conventional N treatment.