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Title: Soil N dynamics related to soil C and microbial changes during long-term incubation

item PAUL, ELDOR - Colorado State University
item Follett, Ronald
item HADDIX, MICHCELLE - Colorado State University
item Pruessner, Elizabeth

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/2011
Publication Date: 10/1/2011
Citation: Paul, E., Follett, R.F., Haddix, M., Pruessner, E.G. 2011. Soil N dynamics related to soil C and microbial changes during long-term incubation. Soil Science. 176: 527-536.

Interpretive Summary: 1) Knowledge of the different kinetics of the C and N constituents can be used in the management of both soil -C and -N because the two are not necessarily maintained by identical controls. We also argue that N mineralization should not be modeled exclusively on C kinetics and a knowledge of C:N ratios. 2) The size of the microbial biomass pool although very small after extended incubation did not affect C and N mineralization kinetics and in one soil was not large enough to carry out nitrification of the mineralized ammonium. 3) Soil N is mineralized to a much smaller extent than C. This could be due to a greater extent of microbial re-utilization of N. It also must be attributable to greater protection of what appear to be proteinaceous-like compounds by the soil matrix. 4) A single stage acid hydrolysis was shown to be as effective as the more complicated two stage procedure. Acid hydrolysis provided large differences in MRT of the soil C and some recognition of the proportion of resistant C and N but it is not known whether this is attributable to chemical structure or interactions with the soil matrix or possibly the interaction of both. 5) Pyrolysis resulted in more meaningful fractionation based on the amount of N not pyrolyzed and the lack of discrimination of the 13C constituents. We recommend this procedure be further investigated as a possible pretreatment in identifying the resistant fraction for modeling soil organic matter dynamics. More attention to possible discrimination effects with stable isotopes is recommended.

Technical Abstract: Knowledge of the pools and fluxes of C and N soil components is required to interpret ecosystem functioning and improve biogeochemical models. Two former grassland soils, where wheat or corn are currently growing, were studied by kinetic analysis of microbial biomass C and N changes, C and N mineralization rates, acid hydrolysis and pyrolysis. Nearly twice as much C as N was mineralized during incubation. Modeling of changes during incubation demonstrated that two-pool first-order kinetics effectively described losses of microbial biomass C and N and concurrent N mineralization. Loss of microbial biomass N during incubation accounted for a significant portion of the N mineralized. Microbial biomass N content and soil N mineralization rates were strongly affected by soil type and soil management. Nitrification but not N mineralization was inhibited during the latter stages of incubation in one of the soils. We believe nitrifier populations had dropped below effective levels. Non-acid hydrolyzable C was increased in both amount and mean residence time by cultivation and incubation. Hydrolysis removed a large amount of N as compared to pyrolysis. The pyrolysis technique should be investigated to identify the recalcitrant forms of C and N in soils. The dynamics of soil C and soil N, although related, are not identical, thus management can be targeted to soil C or N cycling in ecosystem functioning or to soil organic matter dynamics in global change.