Gross rates and potentials of N cycling processes – Work package 4.4

Soil net N mobilization is the net result of rapid microbial gross mineralization and subsequent biological and non-biological N immobilization. Small changes in any of these rates can have pronounced effects on the net nutrient supply to the plant available pool and thereby affect primary productivity as well as rates of N-losses in N-limited systems. It is important therefore to examine how the treatments change these gross processes in order to understand the ecosystem responses and the longer-term mobilization of N in the system, described in WP4.3.

The turnover of N in the mineralization-immobilization step of the N cycle will be measured by an isotopic pool dilution approach. Replicate sets of soil samples labeled with ¹⁵NH₄⁺ and ¹⁵NO₃^- will be incubated in the field, and the rates of dilution of the label by ¹⁴NH₄⁺ and ¹⁴NO₃^- released from the decomposing substrate will be measured over short time intervals in order to determine rates of gross mineralization and gross nitrification, respectively. The immobilization is assessed from the ¹⁵N balance in the incubations after removal of the inorganic N fractions by KCl extractions. The measurements will be done at four events over one year beginning in the spring of the second treatment year (2006).

Potential nitrification (oxidation of NH₄⁺ to NO₃^-) and potential denitrification (reduction of NO₃^- to N₂O and N₂) will be measured in a laboratory study. The potential nitrification rates will be measured as accumulation of NO₂^- and NO₃^- during aerobic incubation of ammonium amended soil samples from the treated plots. The potential denitrification will be estimated by anaerobic incubation of soil samples under near-optimal conditions with added carbon sources and nitrate. Based on these results, we will evaluate if N₂ fluxes are likely to form a significant part of the ecosystem N-gas loss (see WP4.1).

Based on the vegetation and soil characteristics of the experimental site, we will also consider including measurements on biological N₂ fixation by the acetylene reduction technique in the laboratory or in the field.