Development of a Critical Nitrogen Dilution Curve Based on Leaf Dry Matter for Two Spring Rapeseed (Brassica napus L.) Cultivars

Supplementing crop growth with nitrogen (N) can significantly increase yields. However, the excessive use of N in intensive cropping systems has led to lower N use efficiency, an increase in greenhouse gas emissions as well as water pollution. The critical N concentration (Nc) is the minimum N required for maximal growth and can be used as a tool for accurate N management during the growing season. Leaf dry matter (LDM) is an important indicator of crop growth potential and a measure of light-energy utilization, and yield formation in rapeseed. LDM increases as more N fertilizer is applied, although differences in LDM are small under high fertilization levels. Therefore, construction of a Nc curve based on LDM during the vegetative growth of spring rapeseed would be a worthwhile objective. Our aims in the present study were (i) to construct a leaf Nc curve, (ii) to compare this curve with published data, and (iii) to explore its potential for estimating spring rapeseed leaf N status.

A field experiment with seven levels of N fertilizer including 0, 50, 100, 150, 200, 250 and 300 kg.ha-1 was performed, and LDM and leaf N concentration (LNC) of two spring rapeseed cultivars were measured during the growing season. The procedure used for constructing the leaf Nc curve was first proposed by Justes et al. (1994). The N nutrition index (NNI) on each sampling date was calculated by dividing LNC by leaf Nc concentration. The accumulated N deficit (Nand) in leaves for each sampling date was determined by subtracting the N accumulation in leaves under the Nc condition from actual N accumulation in leaves under different N rates.

N application rate exhibited a significant effect on LDM during the vegetative growth of rapeseed. The LDM increased gradually with increasing N utilization. LDM ranged from 0.06-3.83 ton.ha-1 for Hyola 401 cv. and from 0.06-3.56 ton.ha-1 for Dalgan cv. LNC generally increased with increasing N application. LNC remained relatively stable during the early growth but decreased gradually as cultivars reached the flowering stage. LNC values ranged from 2.44-6.2% for Hyola 401 cv. and from 3.14-6.1% for Dalgan cv. The leaf Nc concentrations in Hyola 401 and Dalgan cultivars declined with increasing LDM and the curve for these cultivars was fitted according to the equation Nc=5.08LDM-0.06. The similar trends of decline in leaf N dilution have been previously reported in winter wheat, maize and rice, when N dilution was estimated on the whole plant or specific plant organs basis. NNI values began to decrease with the decrease in the quantity of applied N. NNI values varied from 0.72-1.14 in the Dalgan cv. and from 0.53-1.15 in the Hyola 401 cv. The NNI values were greater than one for non-N limiting treatments and less than one for N-limiting treatments. These results confirm that NNI can provide accurate and quantitative insight into the N nutrition status of rapeseed. The Nand values ranged between -11.61-107.09 kg N ha-1 in the Dalgan cv. and between -24.22-129.64 for the Hyola 401 cv. depending on the N dosage. Nand decreased with increased N rates, while intensified gradually over growth progress of rapeseed and reached its peak at flowering stage for N-limiting treatments. These results confirmed the usefulness of Nand for assessing N nutrition in spring rapeseed. There was a significant positive correlation between the changes in N application rates (ΔN) and the changes in NNI (ΔNNI), and between ΔN and the changes in Nand (ΔNand). Generally, the Nc dilution curve, NNI, and Nand derived from it well recognized nutrition status of two cultivars under N-limiting and non-N limiting conditions, and can be used to as a reliable indicator of the crop N status during the growing season.

The N concentration in the canopy leaves decreased with advancing maturity, while a higher N application rate generally exhibited a higher leaf N concentration. The present Nc dilution curve based on LDM provides an insight into N nutrition status in spring rapeseed plant and can serve as a novel tool to improve N fertilization management in rapeseed.