Measuring gross nitrogen mineralization, and nitrification by ¹⁵ N isotopic pool dilution in intact soil cores

Abstract

The isotope dilution method for measuring gross rates of N mineralization, immobilization, and nitrification was applied to intact soil cores so that the effects of soil mixing were avoided. Soil cores were injected with solutions of either "NH: or "NO;; gross mineralization rates were calculated from the decline in "N enrichment of the NH: pool during a 24-h incubation; gross nitrification rates were calculated from the decline in "N enrichment of the NO; pool; gross rates of NH: and NO; consumption were calculated from disappearance of the I5N label. The assumptions required for application of this method to intact cores are evaluated. Sensitivity analysis revealed that homogeneous mixing of added "N with ambient pools was not a necessary assumption but that bias in distribution of added label, coincident with a non-random distribution of microbial processes, would cause significant errors in rate estimates. Rate estimates were also sensitive to errors in initial 15N and I4N pool size estimates, In a silt loam soil from a grassland site, abiotic processes consumed over 30% of the added "NH: within minutes of adding the label to sterilized soil. Extracting a subset of soil cores at the beginning of an incubation is recommended for obtaining initial pool size estimates. Gross immobilization is probably stimulated by addition of inorganic I5N substrate and, therefore, is overestimated by the isotope dilution method. As an alternative method, a non-linear equation is given for calculating the gross immobilization rate from the appearance of "N in chloroform-labile microbial biomass; but incomplete extraction of biomass N may result in low estimates. Details of the isotope dilution methodology (injection rates, concentrations, experimental artefacts, etc.) are described and discussed. When care is taken to understand the underlying assumptions and sources of error, the isotope dilution method provides a powerful tool for measuring gross rates of microbial transformations of soil nitrogen in intact soil cores. I N T R O D U C T I O NAn ideal method for measuring microbial transformations of nitrogen would cause minimal disturbance of the soil and would directly measure the process rate of interest. Tracer experiments with "N (e.g. measuring nitrification by adding lSNH: and analysing for ISNO;) can alter the process rate by adding substrate to a substrate-limited process. Net mineralization and net nitrification estimates obtained from buried bag incubations (Eno, 1960) confound two or more simultaneous processes. In contrast to these commonly used methods, isotope dilution permits independent estimation of gross rates of N mineralization and nitrification without adding substrate. Briefly,

Tansley Review No. 9515N natural abundance in soil‐plant systems

“…At most a few ^o of soil total N becomes available during a year. Recent '^N tracer work, based on calculations of pool dilutions and high temporal resolution (Davidson et al, 1991), suggests a turnover of inorganic N pools within one or a few days in both temperate grasslands and acid forest soils (Davidson, Stark & Firestone, 1990;Davidson, Hart & Firestone, 1992;Hart et al, 1994). Turnover times in agricultural soils should be comparable or even shorter.…”

Conversion of grassland to coniferous woodland has limited effects on soil nitrogen cycle processes

“…The nitrification pathway is important because of increased potential for gaseous N losses associated with nitrification and denitrification (Davidson et al, 1991), and leaching losses of NO 3 À in the soil solution. Gross nitrification and NO 3 À consumption estimates have illuminated the importance of nitrification in systems such as an old growth coniferous forest and other undisturbed coniferous forests (Davidson et al, 1992;Hart et al, 1994a;Stark and Hart, 1997) where nitrification previously was thought to be of little importance because of low net nitrification rates and small NO 3 À pools.…”

Nitrogen dynamics in Amazon forest and pasture soils measured by 15N pool dilution

“…Net N mineralization provides an index of plant available N in many systems (Nadelho er et al, 1983), but does not re¯ect the total amount of N cycling between organic matter and soil inorganic N. Gross rates of N mineralization and nitri®cation measured by 15 N dilution (Davidson et al, 1991) provide measured of the total amount of N mineralized and nitri®ed that can be contrasted with net N transformations. In this study, we compared rates of gross mineralization and nitri®cation with rates of net mineralization and nitri®-cation in soils of a reference forest and pastures of di erent ages in the Brazilian Amazon state of Rondoà nia where we have measured net rates over an annual cycle (Neill et al, 1995).…”