I am interested in the range of mineralization rates of soil organic matter mineralization in tropical settings.
How fast can SOM be depleted?
Under temperate climates we are often talking about few percents of loss of SOM every year.
But for tropical conditions, is there data on this phenomenon?
What about a more specific case: when the vegetation is cleared, the soil is left bare. This implies that there will be no more flux of organic matter coming from plants on this soil.
At which rate do we observer a decline in soil organic matter or soil carbon in this particular setting?
I suppose it could be much faster than in temperate climate. Perhaps close to the 10% per year, or even more if you are in a region with important and well distributed rainfall.
I will share some of the findings from the literature in response.
Article Evaluation of the Rates of Soil Organic Matter Mineralizatio...
Dear Mr. Fungenzi,
this is a very interesting but complex question. You're right, higher temperatures of humid tropical regions stimulate microbial activity, thus decomposition of SOM in soil. This results in a rapid consumption of the available C and N in these climates. However, biochemical weathering rates are also high, leading to soils dominated by oxides, which can stabilize high amounts of SOM. Further, a distinct aggregation (especially micro-aggregation) and low pH values, both typical for tropical soils, will further decrease availability of SOM.
We just published a paper (open access), showing that despite comparable SOM amounts, the potentially mineralizable N is significantly smaller in warm humid tropical, compared to cool temperate regions.
https://www.nature.com/articles/s41598-019-42274-y
There is no general answer regarding your example, as it will strongly depend on the soils (mineral reactivity, aggregation, SUE of the microbial biomass and its composition). However, management actions (such as tillage or "removing all plants") might enlarge the bioavailable SOM pool, e.g. due to aggregate breakdown. Further, high amounts of DOM can get leached out of these soils upon high rainfall, thus leading to high losses of C.
Kind regards,
The high temperature at tropical areas speeds the rate of decaying of organic matters in soil. Soil that contains 1:1 clay does not have it organic matters decaying early compared to 2:1 clay soils. Because 2:1 clay soils have water in their interlayer, this makes it well living for microorganisms to live and serve as agent's of organic matters in soil. From study, most tropical areas are filled with 1:1 clay soils. Therefore, decomposition of organic matters in soil seems to have low rate.
I quite remember, when I buried three peels of banana in the soil at Ghana, it took closed to three weeks for it to decay.
Increased soil organic matter (SOM) improves the cation exchange capacity of tropical weathered soils, and liming is required to achieve high yields in these soils. Despite a decrease in SOM in the short term, liming may increase SOM with time by improving cation chemical bonds with soil colloids. Soil C may also be increased in high dry matter input cropping systems. We evaluated C changes in a Typic Rhodudalf as affected by four production systems with increasing residue inputs, with or without limestone or silicate. Soil use intensification by increasing the number of species in rotation as well as acidity remediation resulted in higher plant residue production. Introducing a green manure or a second crop in the system increased plant residue by 89% over fallow, but when a forage crop was used, plant residues more than doubled. Soil acidity amelioration increased plant residue deposition by 21% over the control. The introduction of a forage crop increased labile SOM and C contents in the particulate fraction, and lime or silicate application led to increases in the more stable SOM fraction. High amounts of plant residues (>70 Mg ha−1 in 5 yr) are effective in raising soil labile C, but the alleviation of soil acidity results in increased soil stable C irrespective of crop rotations in tropical weathered soils, and in this case plant residue deposition can be lower. Lime and silicate are equally effective in alleviating soil acidity and increasing soil C, probably due to the formation of cation bridges with soil colloids.
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