The determination of what has been termed an "active fraction" of soil organic matter (SOM) and the effect of management on the size and turnover of this fraction are the leading themes in the present thesis. The techniques employed to study the amount and turnover of this fraction include: long-term mineralization-incubations for determination of potentially mineralizable carbon (Co) and nitrogen (No), fumigation-incubation for determination of microbial biomass-C and N, and istotope techniques using 13C and 15N for determination of the dynamics of soil C and N. The field experiments include temperate agro-ecosystems, tropical forests and tropical agro-ecosystems cropped with pasture and sugar-cane.
The amount of microbial biomass-C in the temperate systems were in a range of 322-812 µg g-1 corresponding to 3-40% of total-C, while biomass-N ranged between 59-144 µg g-1 or 3.5-6.8% of total-N. In the tropical systems the amount of biomass-C ranged between 266-1463 µg g-1 corresponding to 1.3-4.8% of total-C, while biomass-N amounted to 35-170 µg g-1 or 2.5-4.2% of total-N. No significant differences between temperate and tropical systems with regard to microbial biomass and total SOM were found among comparable soil layers, despite very different litter inputs, soils and climatic conditions.
In the temperate agro-ecosystems, the turnover time of biomass-C was 2 yr, while 15N incorporated into microbial tissue seemed to remain in the microbes, indicating a very slow turnover of biomass-N. In the tropical systems, the turnover of microbial biomass-C was about 0.15 yr.
In the temperate systems, the amounts of N mineralized in long-term incubations increased with increasing additions of N-fertilizer and organic matter and amounted to 93-165 µg g-1 or 6.2-8.8% of total-N. In cases where the mineralization pattern allowed determination of No, the No ranged between 7-13% of total-N. The seasonal variation of No and the effects of management on No support the concept of an active fraction of SOM and that mineralization-incubations provide estimates of this fraction. In a top-soil layer in the tropical systems No amounted to 564-922 µg g-1 or 15-22% of total-N. The Co fraction corresponded to the active fraction as measured by means of the δ13C technique. The concomitant assessment of both C and N during long-term mineralization-incubations revealed two different SOM fractions having widely differing C/N ratios, namely about 2.5-3.0 and 34-37.
In one temperate system, the non-biomass active-N corresponded to 6% of total-N. The turnover of this fraction was estimated to 7-10 yr. In one tropical site, the active soil-C amounted to 21-25% of total-C and had a turnover time of about 2 yr as measured by means of the δ13C technique.
In a long-term tropical field experiment, the natural 13C abundance in particle size classes revealed the active SOM to be associated with sand- and silt-size particles (app. 25% of total-C), while the soil-C with intermediate turnover was associated with claysize particles (app. 25% of total-C), the turnover time being 4-6 and 59 yr, respectively. The remaining SOM was associated mainly with clay-size particles and was likely to constitute the stable passive fraction of SOM.
The dynamics of microbial biomass and indices of nitrification in the time series of introduced tropical pastures displayed a pattern, which suggested the pastures to degrade within a decade. However, the large amount of fairly stable SOM is likely to protect the soils from changes in management and given the nutrients lost form the system are replenished, the pastures may remain productive.
Linköping: Linköpings universitet , 1991. , 35 p.
1991-05-21, Sal Elysion, Hus-T, Universitetsområdet Valla, Linköping, 13:00 (Swedish)
Papers, included in the Ph.D. thesis, are not registered and included in the posts from 1999 and backwards.