Broccoli response to application of natural nanobiochar amendment, deficit irrigation and nitrogen fertigation

Due to several factors, including anthropogenic global warming, drought is increasing in many arable areas which is one of the major threats to global food security. Abiotic stresses such as water deficit severely reduce crop productivity. Using strategies subsurface irrigation with porous clay capsules and natural nanobiochar amendment could be pivotal in reducing water and nitrogen fertilizer inputs as well as the effects of abiotic stresses on plants. Biochar addition to soil may improve soil physical and chemical characteristics. Soil water retention in the root zone enhances under the same irrigation schedule following biochar application. Biochar could improve plant access to soil nutrients and pivotal in reducing fertilizer inputs to agricultural soils.

Greenhouse experiments were conducted in two years (2019 and 2020) on broccoli planted in sieved and non-sieved samples of a loam soil amended with natural nanobiochar (i.e., 0 and 10% by weight) under three irrigation regimes (i.e., 100, 75, and 50% of full irrigation) and two levels of nitrogen fertilizer (i.e., 225 and 300 kgha-1 N). The irrigation system was porous clay pitcher subsurface irrigation. The full irrigation (FI) treatments were watered to refill the pore space in the rooting zone to the field capacity (100% FI), while in deficit irrigation treatments plants received 75% of the full irrigation (75% FI) or 50% of the full irrigation (50% FI). Fertigation treatments included two levels of N fertigation (i.e., 225 kg N ha-1 and 300 kg N ha-1) in the seedling, vegetative growth and green head stages were applied. Physiological and morphological traits of broccoli including secondary head fresh and dry weights, secondary head density, concentration of nutrients, total chlorophyll and leaf proline were investigated.

Plants exposed to deficit irrigation and nitrogen fertigation presented a significant decrease in head fresh and dry weights, head density and also total chlorophyll, while an increase in the leaf proline content was observed. The application of NNB in soil mitigated the drought and nutritional stresses effects on the morphological traits. The results showed significant effects of the natural nanobiochar application in sieved and none-sieved soils on head fresh and dry weight, head density, total chlorophyll and leaf proline for both seasons (2019 and 2020). Natural nanobiochar addition to both sieved and non-sieved soils improved the head fresh and dry weights, head density, and total chlorophyll content. Moreover, it significantly reduced the leaf proline of broccoli plant under deficit irrigation and nitrogen fertigation. The highest values of fresh and dry weight, head density and calcium, magnesium and chlorophyll concentrations were 99.26 g, 39.79 g, 0.92 g cm-3, 5.96 mg g-1, 3.52 mg g-1 and 2.52 mg g-1 in SNAS+NR2+FI, NNAS+NR2+FI, NUS+NR2+FI, SNAS+NR2+FI, SNAS+NR2+FI and NUS+NR2+FI treatments, respectively. As well as, the lowest value of leaf proline was 0.08 µmol g-1 in NNAS+NR2+FI treatment.

Our study demonstrated some of the beneficial effects of natural nanobiochar as an organic amendment for promoting crop productivity. Hence, the integration of natural nanobiochar addition along with deficit irrigation and low N fertigation condition could be considered as a viable and optimum approach in terms of water and fertilizer saving, yield and cost. Thus, the combination of this technique and sub-surface irrigation with the porous clay pitcher system will particularly be useful in arid regions that frequently suffer with water shortages condition.