Nitrate content and total nitrogen content determination
Nitrate content was determined by the hydrazine reduction method
according to Cao et al. (2017). In brief, the samples were collected,
washed and ground into powder using mortar. The powder was transferred
to 2 mL centrifuge tube and 1 mL ddH2O was added,
following by boiling at 100℃ for 30 min. The cooled sample was
centrifuged at 12000 rpm for 10 min and 400 µL was collected for
analysis. The nitrate content was detected using AutoAnalyzer 3
continuous flow analytical system (AA3, SEAL Analytical, Germany). Total
nitrogen content was detected by Kjeldahl method based on a previously
published description (Magomya et al. 2014).