TEMPERATURE EFFECT ON CARBON BIOMASS IN SOILS FROM TROPICAL AND TEMPERATE REGIONS

Four soils from various origins, (tropical and temperate regions) were amended with C labelled glucose (1mg C.g soil) and incubated at 15oC and 35oC to determine the temperature effect on the carbon turnover and on the microbial biomass. The temperature effect on the biomass increased with the glucose addition. The biomass mineralization rates were higher at 35oC than at 15oC and higher for Woburn and Pegwell soils (temperate region) than for Capinopolis and Janauba (tropical region). Specific respiration rate (SRR) of new biomass (from glucose) and old biomass showed different behaviors between soils. At 15oC, the turnover C was 207, 225, 115 and 141 days for Janauba, Capinopolis, Woburn and Pegwell soil, respectively. At 35oC, it was 92, 69, 69 and 33 days for the same soils. The residual C in the soil was higher at 35oC. The final total biomasses at 15oC and 35oC were correlated with the initial soil carbon content. There was an average of 31 and 8 mg of biomass C.g soil organic carbon, respectively at 15oC and 35oC. The initial carbon content was an important factor to explain the mineralization rate at 35oC.


INTRODUCTION
Soil microbial biomass is the living part of soil organic matter, other than living plant material and organisms greater than 500 mm 3 in volume.It is the agent of organic residue breakdown and can be considered as the soil)] For biomass 14 C determinations, 0.75 ml aliquots of the K 2 SO 4 soil extract solutions were placed into plastic scintillation vials with 20ml of Ultima Gold -TM (Packard Inst., Groningen, The Netherlands) as scintillation cocktail, sealed and mixed until clear.Samples were counted in a liquid scintillation counter (2500 TR Liquid Scintillation Analyser Packard) to a 2 Sigma value of 1.00 % or for 5 min.The counts detected (cpm) were converted to disintegration per minute (dpm) using a quench curve in the program for 14 C counting.The quench curve efficiency was 88.4%.Biomass 14 C was calculated as described above.The quantity of C-labelled biomass ( 14 C-Bc) synthesized from residual 14 C-labelled metabolities was calculated from equation 2, Wu (1990) (Bl) o = 14 C -Bc measured at the start (day 25, t=0); (Bl) t = 14 C -Bc measured at day t; (Bl) s = The amount of 14 C -Bc which was synthesised from the residual 14 C -labelled metabolites in the total 14 C -Bc measured at day t.(SA)m= the 14 C -specific activity of the metabolite fraction available for synthesis of new biomass.This was taken to be the same as the 14 C-specific activity of the K 2 SO 4 -extractable organic -C in the unfumigated soil.(SA)g = the 14 C-specific activity of the glucose originally added.
The Biomass specific respiration rates were calculated from total CO 2 -C evolved during the incubation within the different time intervals divided by the average of the amounts of biomass carbon measured at the first and last day of the interval and divided by the total of days in each interval period.
The mean separation was done by Duncan's Multiple test at 5% of probability.

RESULTS AND DISCUSSION
The biomass development: The biomass ( 12 C, total 12+14 C and 14 C) in the end of incubation time at 15ºC and 35ºC and their variation during the incubation period are shown in TABLE 2 and Figure 1.The biomasses decreased as the temperature increased.With glucose, the total 12+14 C biomass decreased by 79%, 64%, 79% and 73%, for Janauba, Capinopolis, Woburn and Pegwell soils respectively, following incubation at 35ºC .Without glucose, the 12 C biomass decreased only 43%, 52%, 67% and 31%.
The new biomass formed with the glucose addition, represented by 14 C biomass decreased 72%, 57%, 72% and 67% when the temperature increased for Janauba, Capinopolis, Woburn and Pegwell soils.Thus, the biomass which developed following glucose addition seemed more sensitive to higher temperatures than the original biomass.Joergensen et al. (1990), also, showed declining of biomasses in temperate soil, when the incubation was done at 35ºC.
The changes in the pattern of microbial biomass could affect the N mineralization and immobilization processes of each soil during the plant development.
At 15ºC, with glucose, the Janauba and Pegwell soils showed the same biomass pattern from native organic C; Capinopolis and Woburn soils decreased the biomass native organic 12 C, (TABLE 2).The same behaviour was reported by Anderson & Domsch (1985) and Brookes et al. (1987).The corresponding decline at 25ºC was 16-45%.Broadbent (1953) reported an increase in the rate of decomposition of native organic matter when addition of plant material was made, i.e., there was a positive priming effect.Thus, the soil priming effect following glucose addition seemed to have different behaviour at 15ºC.After an initial fluctuation, soil microbial biomass apparently drifted slowly upward at 15ºC only for Woburn and Pegwell soil in unamended and amended soil.It could be said that no significative variation was observed after 20 days of incubation.After this period, Janauba and Capinopolis soils, from tropical regions, apparently increased in a linear pattern (Figure1) probably in consequence of the cryptic growth of the microbial population.
For a given cohort of biomass, the 14 C decay can be expressed by the first-order equation: BCt = BCo* e -k't , ( Equation 3) Where BCo = the initial amount of biomass 14 C at time = 0; BCt = the amount remaining after time t; k' = the decay constant rate per day of initial biomass; For this system, T ( turnover) = 1/k' ( Equation 4) These equations, as mentioned by Wu (1990) are true only for a biomass cohort which do no replenished as the organism die and it is necessary to correct the decay constant rate (k value).
At 15ºC there was a decline in 14 C biomass as described by the equations: Scientia Agricola -Temperature effect on CARBON biomass in soils ... http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-90161... From these equations it is possible to make some observations as the temperature effect on biomass.The k'values were higher at 35ºC than at 15ºC.For the 14 C biomass, the k values indicate higher mineralization rates for temperate Woburn and Pegwell soils, and smaller ones for tropical Janauba and Capinopolis.However, the Co values (initial 14 C biomass) were higher for Woburn and Pegwell.It means that the reserve of microbial carbon and available energy was greater in English soils than in the Brazilian ones, emphasising the observation of the sensibility of biomass from temperate soils to high temperatures (Jenkinson et al. 1991, Grisi 1997).
The half-life was calculated from t(1/2) = ln2/k' When an energy source is added to the soils, normally the micro-organisms multiply rapidly until the substrate is nearly exhausted and then decrease again.The CO 2 evolution must have the same pattern.However, the pattern seen in these experiments was different as shown by the specific respiration rate -(mg CO 2 evolved.* g -1 * biomass * day -1 ), TABLES 3 and 4. The data for CO 2 evolved are in Vasconcellos (1994).
The Janauba and Capinopolis biomass showed an increasing pattern; Woburn and Pegwell a constant biomass pattern, (Figure 1).
The SRR was constant, during the incubation period, for amended soils at 15ºC.The SRR for Janauba were 39 and 16 mg CO 2 /g of total and 14 C biomass per day, respectively; for Capinopolis, 47 and 22 mg of CO 2 /g per day, for Woburn, 48 and 13 mg of CO 2 /g per day and for Pegwell 39 and 8 mg of CO 2 /g of biomass per day.In a sharp contrast, biomass 14 C felt during the incubation period.The biomass at 35ºC did not show a specific SRR pattern in unamended soils.For amended soil without the first point as initial temperature adaptation, the SRR showed constant values from 15 to 70 days.For Capinopolis it was 57 and 41 mg of CO 2 g -1 total biomass and 14 C biomass per day.
As pointed out by Santruckova & Straskraba (1991) the increase of specific respiration could be due to effect of stress.But, the water content of the soils at 15ºC varied from 50 to 46% of WHC; at 35ºC the range was 50% to 31% of WHC.However, during the last week of incubation period the water content was corrected by weight.By another way it is possible that energy for biomass maintenance and CO 2 evolved is coming from various sources (carbon inputs, organic matter, dead microbial cells, endocellular reserve, etc) as showed by Grisi (1997).
As point out by Jenkinson et al. (1991) changes in the climate pattern can alter the total carbon storage in soil.In this way, there is a characterisation of vicious cycle, global warming will accelerate the decomposition of soil organic matter, the CO 2 thus formed further increasing global warming and decreasing the soil biomass microbiology.
These results also emphasise the importance of management practice on soil organic carbon and with the possibility to quantify the organic matter quality, mainly knowing that continuous crop (and plowing processes) cause losses in the natural reserves of organic matter (Zadorin &Yumagulova, 1983).
The biomass turnover: As point out by Jenkinson & Parry (1989) the internal recycling of biomass metabolites can affect the estimate of turnover time.In consequence the uncorrect rate (k') or the degra-dation of 14 C-Biomass C was first obtained by fitting a first-order regression equation to the residual 14 C-Biomass C as shown in the equations above.
The true rate constant of 14 C-Biomass C degradation (k) in the soils can be obtained from measured 14 C-Biomass C subtracted from the 14 C-Biomass C synthesised from metabolities as shown in TABLE 5.The 14 C-biomass synthesised was calculated by the equation 2 with the specific activities data in TABLE 6.
The declines in total biomass C during the incubation period was faster in amended soils than in unamended control soils.As Wu (1990) has pointed out, it means that with new energy source the turnover of biomass C would be faster than in unamended soil.The faster decline of total biomass C with glucose is due to the enhanced turnover of biomass due to the residual effects of glucose incorporation.The rate of biomass C turnover in unamended soils can be calculated from the following formula: D P -is the proportion of the total biomass C that declines between the incubation period in the glucoseamended treatment minus the proportion of the total biomass C that declines in the unamended treatment.k and ku are the rate constants of biomass turnover in amended and unamended soils, respectively at a specific time (t) in days.The ku value is determined by solving the equation above.
Unfortunately, at 15ºC, the results observed for the biomass C in unamended treatments were not consistent.
During the incubation period the microbial population could use the soil organic matter as substrate for its development (Grisi, 1997).
In the amended treatments, after a decline from time 0 to time 20, the total biomass showed an increasing linear pattern for all soils, except for Capinopolis, which showed no specific biomass variation.
At 35ºC, the corrected constant rate of Biomass C turnover for unamended soil are given in TABLE 7 and  Figure2.The biomass in the Janauba and Capinopolis soils had turnover times (1/ku) of 92 and 69 days; Woburn and Pegwell, 69 and 33 days.Thus, biomass C in the Pegwell soil was most sensitive to temperature.It can be calculated from ku values in  45% in Pegwell soil. Van Veen et al. (1981) showed that clay soils have a greater capacity to preserve microbial biomass than sandy soils.In our experiment, the clay content did not vary widely (16.7% to 25.4%).
At 15ºC, for amended soils, the turnover time were 207 days for Janauba, 225 day for Capinopolis, 115 days for Woburn and 141 days for Pegwell .According to Jenkinson et al. (1987), the air temperature conversion factor is 3.81 at 25ºC.If that factor is used to convert the turnover times of biomass C obtained in the laboratory to those expected under field conditions, the following values are obtained at 15ºC: Janauba, 2.2 years; Capinopolis 2.3 years; Woburn, 1.2 years and Pegwell 1.5 years.Wu (1990) showed biomass C turnover times of 1.3 years in the Woburn soil, and 1.8-2.2years in a range of soils from Rothamsted.
TABLE 8 gives the 14 C balance.Probably because biomass activity was lower at 35ºC the 14 C remaining was higher at 35ºC.It is necessary to note that the total 14 C evolved at 15ºC and at 35ºC was very close.

Biomass and Correlations:
Final total biomass at 15ºC and 35ºC were very close correlated with initial soil carbon content, (r values were 0.982 and 0.992, respectively), but not with the clay content.There was an average of 31 and 8 mg of biomass C.g -1 of C, respectively at 15 and 35ºC .The residual 14 C was closely correlated with clay content at 15ºC.No correlation were found at 35ºC.There was correlation between the decomposition rate at 35ºC and the initial soil organic carbon.

CONCLUSIONS
-The biomass mineralization rates were higher at 35ºC than at 15ºC and higher for Woburn and Pegwell (soils from temperate region) than for Janauba and Capinopois (soils from tropical region).
-The specific respiration rate of new biomass from glucose and old biomass showed different behaviour between soils, but not between temperate and tropical region; -At 15ºC, the turnover C was 207, 225, 115 and 141 days for Janauba, Capinopolis (soils from tropical regions), Woburn and Pegwell (soils from temperate regions) respectively.At 35ºC, it was 92, 69, 69 and 33 days for the same soils.These results emphasised the effect of global warming on soil biomass and on the stability of the soil quality.
-The residual 14 C remaining in the soil was higher at 35ºC.
-The final total biomasses at 15ºC and 35ºC were correlated with the initial soil carbon content.There was an average of 31 and 8 mg of biomass C.g -1 soil organic carbon, respectively at 15ºC and 35ºC.
-The initial carbon content was an important factor to explain the mineralization rate at 35ºC being important for soil management practice to improve the soil organic matter and the sustentability of food production incubatiom time (days) (from day 25 onwards); (Bc) t = Total biomass C measured at day t;

Figure 1 -
Figure 1 -Biomass C variation at 15ºC as incubation temperature.Symbols are for experimental points and complete lines to fitted points.Biomass value for each individual soil followed by different letters are significantly different, (Duncan p < 0.05).

Figure 2 -
Figure 2 -Biomass C variation at 35ºC as incubation temperature.Symbols are for experimental points and complete lines to fitted points.Biomass value for each individual soil followed by different letters are significantly different, (Duncan p < 0.05).