Dynamics of microbial biomass nitrogen in relation to plant nitrogen uptake during the crop growth period in a dry tropical cropland in Tanzania

Soil microbes are considered to be an important N pool in dry tropical croplands, which are nutrient poor. To evaluate the N contribution of soil microbes to plant growth in a dry tropical cropland, we conducted a maize cultivation experiment in Tanzania using different land management treatments (no input, plant residue application, fertilizer application, plant residue and fertilizer application, and non-cultivated plots). Over 104 experimental days, we periodically evaluated the microbial biomass N and C, plant N uptake, microbial respiration in situ and inorganic N in the soil. A significant amount of inorganic N was lost in all of the treatment plots as a result of leaching during the initial 60 days and inorganic N remained low thereafter (similar to 20-35 kg N ha-1 : 0-15 cm), whereas soil microbial respiration substantially decreased because of soil drying after 60 days (grain-forming stage). During the grain-forming stage (60-104 days), we found a distinct effect of plant N uptake on soil microbial dynamics, although we did not observe an obvious effect of plant residue and/or fertilizer application; microbial biomass N decreased drastically from 63-71 to 18-33 kg N ha-1 and the microbial biomass C : N ratio simultaneously increased (> 10-fold) in all maize-cultivated plots; these features were not observed in the non-cultivated plot. Plant N uptake over the same period was 26.6-55.2 kg N ha-1, which was roughly consistent with the decrease in microbial biomass N. These results indicate that strong competition for N occurred between soil microbes and plants over this period and N uptake by plants prevented microbial growth. Thus, we concluded that soil microbes contribute to plant growth by serving as a N source during the grain-forming stage in dry tropical cropland.