Measurement of microbial biomass N:C by chloroform fumigation-incubation

Abstract

. 1997. Measurement of microbial biomass N:C by chloroform fumigation-incubation. Can. J. Soil Sci. 77: 507-514. We present a calculation for soil microbial biomass N:C ratio determined from a 10-d incubation following chloroform fumigation. The calculation is based on a mathematical model of the N content of the pre-and post-fumigation soil microbial biomass and the growth yield of the biomass that develops after fumigation. Biomass N is calculated from the N:C ratio and biomass C. The mineralization of bacteria and fungi, with different N contents, added to fumigated soils was used to establish the model parameters. The model was tested against an independent set of measurements and considers two assumptions: 1) The ratio of N:C mineralized from killed biomass is equal to the ratio of N:C mineralized from soil nonbiomass constituents. 2) More realistically, the N and C mineralization in the fumigated soil, from sources other than killed biomass, is a residual fraction of the N and C mineralization in the unfumigated soil. Biomass C:N ratios calculated without a control correction (assumption 1) were, on average, 20% wider than corrected values (assumption 2). Biomass N calculated as the product of N:C and biomass C was compared with published values for several data sets. The new calculation method was robust even when net immobilization of N followed fumigation.Key words: Soil microbial biomass, nitrogen, chloroform fumigation, C:N ratio Harris, D., Voroney, R. P. and Paul, E. A. 1997. Mesure du rapport N:C de la biomasse microbienne par fumigation au chloroforme et incubation. Can. J. Soil Sci. 77: 507-514. Nous présentons une formule de calcul de rapport N:C de la biomasse microbienne (BM) du sol après fumigation au chloroforme suivie d'une période d'incubation de 10 jours. Les calculs sont basés sur un modèle mathématique de la teneur en N de la BM avant et après la fumigation et de l'accroissement de la BM observé après la fumigation. Le N de la BM est calculé à partir du rapport N:C et du C de la biomasse. La minéralisation des bactéries et des champignons microscopiques, de teneurs azotées différentes, incorporés aux sols fumigés est utilisée pour établir les paramètres du modèle. Le modèle, testé sur un ensemble indépendant de mesures, examine deux hypothèses : 1) le rapport N:C minéralisé à partir de la biomasse tuée est égale à celui obtenu pour les composantes du sol autre que la biomasse et 2) de façon plus réaliste, la minéralisation de N et de C dans le sol fumigé, à partir de matières autres que la biomasse est une fraction résiduelle de celle mesurfée dans le sol fumigé. Les rapports C:N de la biomasse microbienne calculés sans correction (hypothèse 1 temoins) étaient en moyenne 20 % plus larges que les valeurs corrigées (hypothèse 2). Le N de biomasse microbienne calculé comme étant le produit de N:C et du C de la biomasse est comparé aux valeurs publiées pour plusieurs ensemble de données. Il appert que la nouvelle méthode de calcul est valable même lorsque la fumigation est suivie d'une immo...

Effects of Goat Manure and Lime Addition on Phosphate Sorption by Two Soils from the Transkei Region, South Africa

“…Incorporation of P into the soil microbial biomass is reported to be another mechanism that could significantly increase the availability of P to plants (Harris, Voroney, and Paul 1997). The microbial biomass apparently increases P availability by immobilizing soil inorganic P and releasing it later through mineralization during biomass turnover (Iyamuremye, Dick, and Baham 1996).…”

Soil nitrogen dynamics and leaching under conservation tillage in the Atlantic Coastal Plain, Georgia, United States

“…The change in C:N ratios after fumigation is due to the release of microbial biomass following membrane rupture and from solubilization of nonpolar SOM components by the chloroform. The microbial C:N ratios were much lower, but still represented realistic values of soil microbial communities (Franzluebbers et al 1999;Harris et al 1997). This may result from the mineralization of variable proportions of the biomass N and may lead to net immobilization of N, especially if fungi are prevalent (Paul et al 1999).…”

“…) and that the total N removed in a cotton year (through the removal of cotton lint and seeds) is ~87 kg ; Strickland, unpublished data, 2013 and (Franzluebbers et al 1999;Harris et al 1997). Soil MBC and MBN are considered part of the active fraction of SOM, important for supplying plant nutrients, decomposing organic residues, and developing soil structure (Franzluebbers et al 2000;McCarty et al 1995).…”

Estimates of Gross Transformation Rates of Dairy Manure N Using15N Pool Dilution

“…The only direct measurement is to trace 15 N enrichment into the soil microbial biomass over time. This is complicated by uncertainty over the extraction efficiency and other factors in estimating soil microbial biomass N (Harris et al 1997;Horwath et al 1996;Voroney and Paul 1984). Alternatively, gross immobilization can be estimated as the difference between NH 4 þ consumption (C a ) and gross nitrification (n), where…”