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

Abstract

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 (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.

Soil nitrogen dynamics under different quality and application methods of crop residues in maize croplands with contrasting soil textures in Tanzania

“…Meanwhile, it can be said that critical N deficiency did not occur after the application of maize residues either through incorporation or mulching. Sugihara et al (2010a) showed that microorganism behaved as a sink of N through the decomposition of incorporated crop residue without a severe N deficiency in Tanzanian maize cropland.…”

Effect of Nitrogen Sources on Microbial Biomass Nitrogen under Different Soil Types

“…Increasing microbial N uptake rates under sludge treatment leads to the possibility of improving nitrogen use efficiency in both soils. These results indicated that organic amendments with high C content as shown in sludge (Table 1) may increase the N immobilization by soil microbes, resulting in a lower rate and longer period of N uptake by plants [28]. Positive correlations were found between N uptake and both of the microbial N uptake and P fix in both clay soil (R = 0.96 and R = 0.98, resp.)…”

“…Nitrogen uptake in rice shoot was determined by multiplying the N concentration by shoot dry weight. The total amount of microbial N absorbed by rice plant was determined by N difference method at maximum tillering stage [8,[25][26][27][28]. Amount of microbial N was calculated from the difference between the extractable N from fresh soil and autoclaved soil samples under the same treatment as shown in the following equation.…”

In situshort-term carbon and nitrogen dynamics in relation to microbial dynamics after a simulated rainfall in croplands of different soil texture in Thailand

“…Therefore, it is necessary to reduce the N leaching resulting from heavy rainfall at the early crop growth stage (Shahandeh et al. 2004; Sugihara et al. 2010b).…”

“…In this season, the amount of N taken up by the crops is quite important for crop growth in later stages as well as yield. Therefore, it is necessary to reduce the N leaching resulting from heavy rainfall at the early crop growth stage (Shahandeh et al 2004;Sugihara et al 2010b). Judging from the THs-W plot where the level of inorganic N remained relatively steady for the first 100 h, increased In-situ-MB after the rainfall treatment was considered to immobilize the mineralized N after rewetting of the dry soil, and after 110 h, decreasing In-situ-MB in the THs-W plot appeared to supply 0.2 g N m )2 .…”