How does nitrogen fertilizer forms, dose and soil Pb pollution affect spinach qualities?

This study focuses on the effect of different nitrogen fertilizer forms (NFF), fertilizer dose (NFD), soil Pb content and the interactions between nitrogen and Pb on spinach qualities. It was evaluated by measuring nitrate-nitrogen content in spinach leaves and Pb accumulation in the leaves and roots. Not only the interactions between Pb, NFF and NFD but also the leaves greenness value (SPAD) during the growth period of spinach was statistically investigated. The result shows that spinach biomass was not affected by NFF but affected by NFD, such as raising NH(4)-N could increase spinach biomass while raising NO(3)-N decreased the biomass. Comparison between different NFF such as urea, nitrate and ammonia fertilizer shows that ammonia-nitrogen fertilizer had always the lowest nitrate accumulation in spinach leaf while nitrate-nitrogen fertilizer produces exclusively higher NO(3)-N. It was observed that ammonia nitrogen is beneficial to spinach leaves N accumulation at a low nitrogen fertilizer level but it will lead to a reverse result at a higher N level in the presence of Pb. Urea-N facilitates Pb accumulation in spinach root and NH(4)-N does in leaves. Soil Pb pollution decreases the spinach quality while NO(3)-N fertilizer application could bring lower leaf and root Pb than urea and ammonia fertilizer. The good correlation between SPAD values and leaf nitrate concentration showed that SPAD could be adopted to effectively pre-evaluate nitrate in spinach which will be potentially used in determining spinach harvest time.