Resumen

BOTTNER, Pierre et al. Modelling the transformations of soil organic matter in fallow (dry Bolivian Altiplano). Ecología en Bolivia [online]. 2006, vol.41, n.3, pp.117-133. ISSN 1605-2528.

¹⁴C and ¹⁵N labelled wheat straw was incubated over 2 years in the 5-10 cm layer of a sandy soil, under a young 3 year old fallow plot ( =Jo) and an older 8 year old one (= Vi), in the central Bolivian Altiplano. Two kinds of straw were used: with N = 0.3% called N- and with N = 1.6% called N+. The analyses of total ¹⁴C, C, ¹⁵N and N at nine sampling dates were used in a five-compartment model of organic decomposition, MOMOS-6. The N content of each compartment was calculated using the C content divided by its C/N ratio. The model input (necromass = NC) is split into labile (VL) and stable (VS) plant material compartments, using the biochemical characteristics of NC. VL and VS are used by the microbial biomass (BM) which provides labile (HL) and stable humus (HS) compartments with C and N. The outputs are CO₂ released from BM and inorganic N. Each decomposition rate is multiplied by a temperature- and a soil moisture- response function. The soil moisture was predicted using SAHEL model. For both straw types, only 1/4 of the initial ¹⁴C remained in the soil after two years of incubation. With the N+ treatment, 60-70 % and with N- 55 % of the initial ¹⁴C were mineralised within the first 100 days. After 2 years of incubation, 45 and 60% of the initially added ¹⁵N remained in the soil for N+ and N-, respectively. Thus the N content of the straw lowered the mineralisation of ¹⁵N but also the initial mineralisation of ¹⁴C. The fallow age affected the decomposition rates much less than the straw quality. At the beginning of incubation, the predicted microbial biomass amounted to 30 and 13% of the remaining ¹⁴C and 75 and 50% of the remaining ¹⁵N for N+ and N- respectively. During the first 100 days, these values decreased to less than 8% for ¹⁴C and to 10-12% for ¹⁵N for both straw types. During this initial phase, 55-65% of the remaining ¹⁴C and 65-75% of the remaining ¹⁵N, accumulated progressively in the labile humus compartment (HL), which became the essential source of ¹⁴C and ¹⁵N for the micro-organisms and of inorganic N for plants. The stable humus compartment (HS) never exceeded 2% of the remaining ¹⁴C and 2.5 % of the remaining ¹⁵N. The microbial metabolic quotient (q ¹⁴CO₂) was higher with the N- treatment than with N+, due to a higher demand of maintenance energy for N-.

Palabras clave : Altiplano; Organic matter; Modelling; ¹⁴C; ¹⁵N.

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