PARA UN PAÍS CON OPORTUNIDADES
7.2 Política social para la reducción de la pobreza
7.2.2 La reforma a la salud que Colombia necesita
CALC WHOLE-SOIL DECALC WHOLE-SOIL
Water-filled porosity during the incubation
Figure1b. Total volumetric porosity and percentage over the total of each pore-size class in macroaggregates (2-5 mm) samples in Calc and Decalc soils. Different letters indicate significant differences among soils in each pore size class analyzed (P < 0.05). Errors bars represent standard errors (n = 3).
No differences in bulk density (data not shown) and total volumetric porosity were observed between CALC and DECALC. However, a different pore-size distribution was observed in the two soils. Differences existed in three pore size classes: 6.8-10 µm, 2.3- 6.8 µm, and 0.2-2.3 µm (Figure 1a and 1b). In whole soil samples, CALC had a greater porosity in 6.8-10 µm and 0.2-2.3 µm pore size classes and smaller porosity than DECALC in 2.3-6.8 µm. The distribution of pores was different in macroagregates (2-5 mm), having CALC a greater proportion of their porosity than DECALC in 6.8-10 µm, 2.3-6.8 µm, and smaller in 0.2-2.3 µm. Assuming that pores smaller of 0.2 µm are too narrow for the accessibility of microorganism and pores above 10 µm were air filled at the matric potential (-33 kpa) of the incubation, the volume of pores accessible to microorganism was calculated as the sum of 6.8-10 µm, 2.3-6.8 µm, and 0.2-2.3 µm pore size classes. For whole soil samples there were no difference (P=0.454) in the calculated volume between CALC (30.0±0.3 %) and DECALC (27.1±3.6 %). For 2-5 mm macroaggregates CALC had a greater (P=0.005) volume percentage (31.6±1.1%) than DECALC (22.2±1.3 %).
Water-filled porosity during the incubation a a a a a b a a a a b b a a
0
10
20
30
40
50
60
Porosity (%) > 34 10-34 6.8-10 2.3-6.8 0.2-2.3 < 0.2%
Pore-size distribution (µµµµm)
Porosity (Aggregates 2-5 mm): Total and % over total
CALC AGGREGATES 2-5
DECALC AGGREGATES 2-5 Water-filled porosity
Soil incubations. At the end of the incubation, the total accumulated respired CO2-C was of 27.5 µg CO2-C mg-1 soil OC in CALC-intact
whole soil samples, significantly smaller (P=0.005) than 33.9 µg CO2-
C mg-1 soil OC observed DECALC-intact whole soil samples. For the disrupted samples, 30.8 µg CO2-C mg-1 soil OC were respired in
CALC-disrupted whole soil samples, for 46.7 µg CO2-C mg-1 soil OC
in DECLC-disrupted whole soil samples (P=0.061). In addition, significantly greater respiration occurred in disrupted than intact samples in DECALC (P=0.045), but not in CALC (P=0.481) (Figure 2).
Figure2. Cumulative C respired (µg CO2-C mg-1 soil OC) in 30 days incubation in Calc
and Decalc whole soil samples, intact and physically disrupted. Errors bars represent standard errors (n = 3).
For macroaggregates (2-5 mm) samples, the accumulated respiration at the end of the incubation was of 27.2 µg CO2-C mg-1 soil
OC in CALC intact samples, not significantly different (P=0.324) from the 31.8 µg CO2-C mg-1 soil OC in DECALC intact samples. Disrupted
samples did not show greater accumulated respiration than the non- disrupted ones in CALC (P=0.108) and only slight differences in DECALC (P=0.076). In CALC disrupted macroaggregates, total respiration was of 41.6 µg CO2-C mg-1 soil OC, not significantly
0 10 20 30 40 50 60 0 5 10 15 20 25 30 35 µ g C O 2- C /m g so il C Time (Day) Respired C Whole soil
CALC-DISRUPTED WHOLE SOIL CALC-INTACT WHOLE SOIL DECALC-DISRUPTED WHOLE SOIL DECALC-INTACT WHOLE SOIL
different (P =0.747) from DECALC disrupted macroaggregates (44.2 µg CO2-C mg-1).
Figure 3. Cumulative C respired (µg CO2-C mg-1 soil OC) in 30 days incubation in Calc
and Decalc macroaggregates (2-5 mm) soil samples, intact and physically disrupted. Errors bars represent standard errors (n = 3).
Both soils showed a phase of increasing respiration rates, and a later decrease after reaching a peak of maximum respiration rates (Figure 4). This peak of respiration in DECALC intact samples was greatergreater than for CALC intact (P=0.047) and among DECALC disrupted and CALC disrupted peaks no differences were found (P=0.125). It is also noticeable that in the two soils a lag in mineralization rates existed in the disrupted samples in comparison with intact samples. The time-lapse of this lag between intact and disrupted peaks was longer in DECALC than in CALC (10 days vs. 3 days). Disrupted samples also displayed greater respiration rates than intact samples in DECALC until the end of the incubation (P=0.040 at day 30). In CALC this was true only until day 11 (P=0.05 at day 11; P = 0.16 at day 13). It has to be reminded that both soils had been pre- incubated, and therefore these differences correspond to steady-state incubation dynamics. 0 10 20 30 40 50 60 0 5 10 15 20 25 30 35 µ g C O 2- C /m g s o il C Time (Day) Respired C macroaggregates (2-5 mm) CALC-DISRUPTED MACROAGGREGATES CALC-INTACT MACROAGGREGATES DECALC-DISRUPTED MACROAGGREGATES DECALC-INTACT MACROAGGREGATES
Figure 4. Evolution of C respiration rate (µg CO2-C mg-1 soil OC_h) in 30
days incubation in Calc and Decalc whole soil samples, intact and physically disrupted. Errors bars represent standard errors (n = 3).
Figure 5 shows the evolution of respiration rates in macroaggregates (2-5 mm). In both soils and for disrupted and intact samples, the trend was similar, with an initial increase up to a maximum rate, and a progressive decline over time. However, some differences were observed among soils and in relation to whole-soil samples. No differences were observed for DECALC intact vs. disrupted and CALC intact vs. disrupted macroaggregates at their peak, which occurred almost simultaneously at days 3 to 7. However, in both soils, respiration rates decreased faster and were smaller for intact than for disrupted macroaggregates up to day 21.
0,00 0,02 0,04 0,06 0,08 0,10 0,12 0 5 10 15 20 25 30 35 µ g C O 2- C m g C O _h Time (Day) Respired C Whole soil
CALC-DISRUPTED WHOLE SOIL CALC-INTACT WHOLE SOIL DECALC-DISRUPTED WHOLE SOIL DECALC-INTACT WHOLE SOIL
Figure 5. Evolution of C respiration rate (µg CO2-C mg-1 soil OC_h) in 30 days
incubation in Calc and Decalc macroaggregates (2-5mm) samples, intact and physically disrupted. Errors bars represent standard errors (n = 3).
DISCUSSION
Despite decades of equal C inputs and soil management, data in Table 1 confirmed previous observations (Fernández-Ugalde et al., 2014) that CALC samples stored more organic C per unit mass than DECALC (Table 1). This result supports the hypothesis of a lower availability for decomposition of the organic matter of the carbonated soil. Soil incubations carried out in this study partially confirmed this hypothesis, as they resulted in a greater total respiration per unit of soil C in DECALC intact than CALC intact soil after 30 days.
Considering the “regulatory gate” theory (Kemmit et al., 2008), abiotic processes such as water and/or gases availability would act as rate-limiting factors for organic matter mineralization rather than microbial processes or organic matter characteristics. Under the light of this theory, differences in porosity and water retention could be at the origin of the observed differences in total mineralization and hourly mineralization rates in our data. Previous work showed that DECALC and CALC soil had a different mineral matrix (Virto et al., 2013). The
0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 0 5 10 15 20 25 30 35 µ g C O 2- C m g C O _h Time (Day) Respired C macroaggregates (2-5 mm) CALC-DISRUPTED MACROAGGREGATES CALC-INTACT MACROAGGREGATES DECALC-DISRUPTED MACROAGGREGATES DECALC-INTACT MACROAGGREGATES
differences in fabric were confirmed by our observations of differences in porosity distribution observed among CALC and DECALC soils, both in whole soil samples < 5 mm and in 2-5 mm macroaggregates (Figures 1a and 1b).
However, not much evidence of the role of physical properties in promoting stabilization of organic matter in carbonated soils was found in our study. First, no differences in total porosity that can explain greater mineralization rates in DECALC in the field because of favoured aeration and/or infiltration rates existed. Second, CALC intact samples had the same water holding capacity at field capacity than DECALC intact samples. This means that lower humidity conditions for microorganisms cannot be argued as the reason of lower mineralization rates in CALC when samples are moist at this point.
In relation to physical protection within aggregates, macroaggregates intact samples respiration showed no differences among soils, which suggests that the observed differences in whole- soil samples were not due to organic matter stabilization within macroaggregates. However, the observed greater porosity accessible to microorganisms (0.2-10 µm) in CALC macroaggregates than DECALC macroaggregates indicates that physical conditions were more favourable for microbial respiration in CALC soil. Moreover, the fact that disruption induced greater decomposition in DECALC macroaggregates but not in CALC macroaggregates suggests that organic matter protection was less dependent on the physical protection in CALC macroaggregates, at least at the scale considered in this study.
All in all, the only remaining difference that could explain to some extent the observed differences in mineralization of whole-soil samples would be the greater proportion of pores in the 2.3-6.8 µm pore size class, and lower proportion of pores in the 6.8-10 µm class observed in DECALC in relation to CALC. However, as it has been shown in previous studies (Killham et al., 1993; Strong et al., 2004), organic matter mineralization is slower as the pore size observed decreases, which seems contradictory with the observed greater mineralization in DECALC.
These results suggest that, overall, the greater organic matter protection observed in CALC soil was not related to differences in physical characteristics. Therefore, a different nature of the organic materials stored in each soil or different processes of chemical stabilization in CALC and DECALC soils may be responsible of greater organic matter protection. Respiration rates (Figure 4) suggested indeed that organic matter stored in DECALC whole-soil samples were more labile than those in CALC. This was so for intact and disturbed samples, despite the longer time-lag in respiration peaks in the disturbed samples, probably due to a negative effect of disruption over microorganisms’ activity.
As a result, it cannot be excluded that possible differences in the biological lability of organic matter between the two soils could explain the differences in mineralization observed. Biochemical enhanced stability of the organic matter in the presence of Ca2+ cations has
been already demonstrated (Baldock and Skjemstad, 2000). Moreover, the quality of inherited pedogenic soil organic carbon stored in a soil may determine the resulting amount and quality of the soil organic C remaining after long-term cultivation (Plante et al., 2010). Analyses that confirm differences in the characteristics of soil organic matter when carbonates are present are needed to elucidate to what extent the reaction with the carbonated matrix results in a less biologically labile organic matter.
CONCLUSIONS
Short-term laboratory incubations of intact and physically disrupted whole soil and macroaggregates (2-5 mm) of two neighboring Mediterranean semiarid soils differing in the presence (CALC) or absence (DECALC) of carbonates confirmed previous observations of a greater organic matter stabilization in CALC soil. We attribute this to a reduced bioavailability of the stored organic matter. However, differences in the physical structure of the two soils were insufficient to explain the observed differences in organic matter bioavailability. Therefore, other organic matter stabilization mechanisms are likely responsible of the greater protection of organic matter in the carbonated soil. A greater chemical recalcitrance in
CALC does not seem a likely explanation because, on one hand, the nature and amount of organic inputs was similar for decades in both soils, and on the other hand, recalcitrance per se has been shown to be rather rare in soil organic materials (Lutzow et al., 2006). As a result, the reduced biological lability of organic matter observed in CALC could be attributed to a different nature of organic matter in this soil resulting of the interaction between organic materials and the carbonated matrix. This hypothesis needs to be explored, as it has practical consequences both in terms of the amount of organic C potentially stored in these soils, and in the relationship between this C and the soil potential fertility, among other consequences.
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IV
CHAPTER IV