Is microbial biomass measurement by the chloroform fumigation extraction method biased by experimental addition of N and P?

Abstract

The chloroform fumigation extraction (CFE) method determines microbial biomass carbon (MBC) or nitrogen (MBN) by calculating the increase in extractable carbon (C) or nitrogen (N) due to microbial lysis during chloroform fumigation. In China, many studies have focused on the impacts of N and phosphorus (P) addition on soil MBC and MBN in forest ecosystems, where substantial atmospheric N deposition has strongly acidified soils. The addition of nutrients may alter the extraction process applied in the CFE method, potentially influencing the MBC and MBN determined by the CFE method independently of the actual microbial biomass. In this study, we tested whether the MBC and MBN determined by the CFE method were biased by the experimental addition of N and P in strongly acidified Chinese forest soils by adding N and P to the soils immediately before chloroform fumigation, which should not affect the actual microbial biomass. P addition significantly elevated the dissolved organic carbon (DOC) content, especially after fumigation, while N addition significantly reduced the dissolved nitrogen (DN) content. The added N was subtracted using blank samples without soil. However, the altered DOC and DN contents did not affect the MBC and MBN contents determined by the CFE method. In conclusion, our study suggests that the CFE is a relatively robust method to test the impacts of nutrient addition on microbial biomass in the strongly acidified soils of Chinese forests. We also suggested that: (i) even if a fertilization experiment results in an elevated DOC content following P addition, it does not necessarily indicate a stimulation of DOC production by microbes; and (ii) the soil adsorption capacity or the strength of microbial N uptake during the extraction procedure applied in the CFE method may affect the determination of MBN by influencing the DN extraction efficiency.

Effects of 9 years of continuous field phosphorus fertilization on adsorption of dissolved organic matter in tropical forest soil

“…Our proposition suggests that the evidence traditionally used to support the concept of microbial P limitation is, in fact, a result of increased availability of carbon (C) due to abiotic processes triggered by the liberation of organic matter from soil mineral surfaces, which occurs as a consequence of P addition (Mori et al , 2023cMori 2023). This phenomenon can be attributed to the competition between P and organic C for binding sites on soil particles (Kaiser and Zech 1996;Guppy and Menzies 2005;Ruttenberg and Sulak 2011;Spohn and Schleuss 2019;Mori et al 2021b;Spohn et al 2022). It is noteworthy that the addition of P resulted in an increased detachment of organic compounds from mineral soils and an accelerated soil respiration, even in soils with high P availability (as indicated by bioavailable inorganic P concentrations of 326 and 58 mg kg −1 soil in the O and A horizons, respectively) (Spohn and Schleuss 2019), which supports the notion that the accelerated microbial respiration caused by P addition is not solely attributable to microbial P limitation.…”

The Relationship between Soil Particle Size Fractions, Associated Carbon Distribution and Physicochemical Properties of Historical Land-Use Types in Newly Formed Reservoir Buffer Strips

“…We extracted DOC with K 2 SO 4 and determined its level by dichromate digestion. MBC and MBN were measured with the fumigation extraction method [45,46], and we measured bulk density (BD) by using the oven-drying volumetric ring method after samples were oven-dried at 105 • C for 24 h to a constant mass. We then calculated BD as the ratio of the oven-dried undisturbed core weight to the cutting ring volume.…”

“…In the process of reservoir buffer strip evolution and restoration, soil texture is affected by internal factors of the soil parent material and external factors such as soil chemical composition, climate, water, and vegetation [45,80,81]. Dam-triggered flooding intensity also determines the trend of soil texture and nutrients [51,63].…”