Higher rates of nitrogen fertilization decrease soil enzyme activities, microbial functional diversity and nitrification capacity in a Chinese polytunnel greenhouse vegetable land

Little is known about effects of high nitrogen (N) fertilization on soil chemical properties, enzyme and microbial activities involved in N and C transformation. Soil EC, pH, mineral N, three enzyme activities, nitrification capacity, and microbial functional diversity were evaluated under conventional N rate (270 or 300 kg N ha−1 season−1, N4), 80% (N3), 60% (N2), 40% (N1) and 0% (N0) of N4 during a 2-year cucumber/tomato rotation in polytunnel greenhouse lands in Eastern China. Soil EC, NH 4 + -N and NO 3 − -N were significantly increased, whilst pH, activities of dehydrogenase, urease and neutral phosphatase, nitrification capacity, and microbial functional diversity decreased significantly with N application rate. Microbial diversity indices deducted from the C-source utilization pattern were significantly lower under the highest N4 rate in the same vegetable season, but varied with seasons among N rates. Both principal component analysis and C substrate utilization patterns displayed significant separation of soil microbial communities between the higher N4 or N3 and the lower N0, N1 and N2 rates. Our results indicated that a 40% reduction of the conventional N fertilization rate was practical to reduce excess N input while maintaining the sustainability of such greenhouse-based intensive vegetable systems in China’s Yangtze River Delta.