Effect of cultivation on microbial carbon and nitrogen in dry tropical forest soil

Fifteen- and forty-year-old cropfields developed from a dry tropical forest were examined for soil organic C and total N and soil microbial C and N. The 15-year-old field had never been manured while the 40-year-old field had been fertilized with farmyard manure every year. The native forest soil was also examined. The results indicated that the native forest soil lost about 57% and 62% organic C and total N, respectively, in the 0- 10 cm layer after 15 years of cultivation. The microbial C and N contents of the forest soil were greater than those of the cultivated soils. Application of farmyard manure increased the biomass-C and -N levels in the cultivated soil but the values were still markedly lower than in the forest soil. There was an appreciable seasonal variation in biomass C and N, the values being highest in summer and lowest in the rainy season. During an annual cycle, biomass-C contents varied from 180 to 727 &#956;g g<SUP>&#8722;1</SUP> and N from 20 to 80 &#956;g g<SUP>&#8722;1</SUP>dry soil, and both were linearly related. Microbial biomass C represented 1.6%-3.6% of total soil organic C and microbial biomass N represented 1.7%-4.4% of soil organic N