Environmental implications of low nitrogen use efficiency in excessively fertilized hot pepper (Capsicum frutescens L.) cropping systems

A randomized-block greenhouse plot experiment was conducted on N utilization efficiency and N losses in intensive hot pepper (Capsicum frutescens L.) production systems typical of commercial practice in Shouguang, an important vegetable production area in Shandong province, northeast China. Crop yield and N utilization, soil mineral N dynamics and potential nitrate leaching losses were studied in control plots receiving no N fertilizer or organic manure and in experimental plots receiving 0, 600, 1200 or 1800 kg urea-N ha−1 plus a basal dressing of 15 t ha−1 air-dried poultry manure supplying 178 kg N ha−1. Ammonia volatilization from the soil surface was monitored. A microplot was established in each of the plots receiving 1200 kg urea-N ha−1 (the local average commercial fertilizer N application rate), the urea applied to the microplot was labelled with 10 atom % 15N tracer and residual soil 15N and 15N in harvested plant parts were determined. Previous intensive cropping led to a very high residual soil mineral N content (1117 kg N ha−1) before the experiment began and control plots gave a satisfactory mean fruit yield of 5.7 t DM ha−1 with no significant yield response to applied fertilizer and poultry manure. Only 10% of applied fertilizer N was recovered in the aboveground parts of the crop and about 52% was lost from the soil–plant system. Substantial nitrate leaching losses occurred using the two highest fertilizer N application rates but there was little NH3 volatilization from the soil surface. In a survey of 94 wells in Shouguang, the NO3−–N concentrations in water used for drinking and irrigation were determined. Almost half of 94 local wells surveyed had NO3−–N concentrations above the USEPA public drinking water maximum contaminant level of 10 mg L−1. The data indicate that N inputs for intensively managed vegetable crops in northeast China should be reduced urgently to maintain crop yields while avoiding serious long-term environmental damage.