Ecología en Bolivia
versión impresa ISSN 1605-2528
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 llave : Altiplano; Organic matter; Modelling; 14C; 15N.