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Soil microbes are an essential component of most terrestrial ecosystems; as decomposers they are responsible for regulating nutrient dynamics, and they also serve as a highly labile nutrient pool. Here, we evaluated seasonal variations in microbial biomass carbon (MBC) and nitrogen (MBN) as well as microbial activity (as qCO₂) for 16 months with respect to several factors relating to soil moisture and nutrients under different land management practices (plant residue application, fertilizer application) in both clayey (38% clay) and sandy (4% clay) croplands in Tanzania. We observed that MBC and MBN tended to decrease during the rainy season whereas they tended to increase and remain at high levels during the dry season in all treatment plots at both of our test sites, although soil moisture did not correlate with MBC or MBN. qCO₂ correlated with soil moisture in all treatment plots at both sites, and hence soil microbes act as decomposers mainly during the rainy season. Although the effect of seasonal variation of soil moisture on the dynamics of MBC, MBN, and qCO₂ was certainly greater than that attributable to plant residue application, fertilizer application, or soil texture, plant residue application early in the rainy season clearly increased MBC and MBN in both clayey and sandy soils. This suggests that plant residue application can help to not only counter the N loss caused by leaching but also synchronize crop N uptake and N release from soil microbes by utilizing these microbes as an ephemeral nutrient pool during the early crop growth period. We also found substantially large seasonal variations in MBC and MBN, continuously high qCO₂, and rapid turnover of soil microbes in sandy soil compared to clayey soil. Taken together, our results indicate that soil microbes, acting as both a nutrient pool and decomposers, have a more substantial impact on tropical sandy soil than on clayey soil.
