Comparison of methods to quantify soil microbial biomass carbon

ABSTRACT. Soil microorganism biomass is an important soil quality indicator. The microbial biomass of soil was determined by killing and lysing the soil microbes by fumigation with chloroform, irradiation with gamma rays, or irradiation with microwaves. Four soils with increasing carbon concentrations (5, 10, 15, and 30 g kg-1) were analyzed using four methods: the direct application of chloroform, chloroform fumigation, microwave irradiation, and gamma ray irradiation with radiation doses of 15, 25, 35, 45, and 60 KGy. The fungi and bacteria in the soil were quantified by plate counting. Microwave irradiation and gamma irradiation with doses equal to or above 25 KGy killed all the soil microorganisms, but the chloroform methods did not. The carbon liberation increased with higher gamma doses, while the microbe mortality rates were the same, indicating that carbon was liberated from organic matter sources other than microorganisms. The biomasses determined by the microwave method correlated with those determined by the fumigation and 25 KGy gamma irradiation methods, but their values differed among all methods for at least one soil type. Despite this discrepancy, all methods were consistent in ranking microbial biomasses in increasing order of soil carbon concentrations, which corresponds with decreasing land use intensities.

摘要：土壤微生物生物量（soil microorganism biomass）是重要的土壤质量指标。本研究采用氯仿熏蒸、γ射线辐照或微波辐照杀灭并裂解土壤微生物的方法，测定土壤微生物生物量。选取碳浓度依次递增的4种土壤（5、10、15和30 g·kg⁻¹），通过4种方法开展分析：直接氯仿施用、氯仿熏蒸、微波辐照，以及辐射剂量分别为15、25、35、45和60千戈瑞（KGy）的γ射线辐照法。采用平板计数法（plate counting）对土壤中的真菌与细菌进行定量检测。实验结果表明，当辐射剂量不低于25千戈瑞时，微波辐照与γ射线辐照均可完全杀灭土壤微生物，而氯仿处理法则无法做到这一点。碳释放量随γ射线辐照剂量的升高而增加，但微生物死亡率保持恒定，这说明碳源自微生物以外的有机质来源。微波法测得的生物量与氯仿熏蒸法及25千戈瑞γ射线辐照法的测定结果具有相关性，但针对至少一种土壤类型，不同方法的测定值存在差异。尽管存在上述差异，所有方法在按土壤碳浓度递增顺序对微生物生物量进行排序时结果一致，该排序结果与土地利用强度递减的趋势相契合。