Summary
The soil is important in sequestering atmospheric CO2 and in emitting trace gases (e.g. CO2, CH4 and N2O) that are radiatively active and enhance the ‘greenhouse’ effect. Land use changes and predicted global warming, through their effects on net primary productivity, the plant community and soil conditions, may have important effects on the size of the organic matter pool in the soil and directly affect the atmospheric concentration of these trace gases.

A discrepancy of approximately 350 × 1015 g (or Pg) of C in two recent estimates of soil carbon reserves worldwide is evaluated using the geo-referenced database developed for the World Inventory of Soil Emission Potentials (WISE) project. This database holds 4353 soil profiles distributed globally which are considered to represent the soil units shown on a 1/2° latitude by 1/2° longitude version of the corrected and digitized 1:5 M FAO–UNESCO Soil Map of the World.

Total soil carbon pools for the entire land area of the world, excluding carbon held in the litter layer and charcoal, amounts to 2157–2293 Pg of C in the upper 100 cm. Soil organic carbon is estimated to be 684–724 Pg of C in the upper 30 cm, 1462–1548 Pg of C in the upper 100 cm, and 2376–2456 Pg of C in the upper 200 cm. Although deforestation, changes in land use and predicted climate change can alter the amount of organic carbon held in the superficial soil layers rapidly, this is less so for the soil carbonate carbon. An estimated 695–748 Pg of carbonate-C is held in the upper 100 cm of the world's soils. Mean C: N ratios of soil organic matter range from 9.9 for arid Yermosols to 25.8 for Histosols. Global amounts of soil nitrogen are estimated to be 133–140 Pg of N for the upper 100 cm. Possible changes in soil organic carbon and nitrogen dynamics caused by increased concentrations of atmospheric CO2 and the predicted associated rise in temperature are discussed.

https://library.wur.nl/isric/fulltext/isricu_t47d6414d_001.pdf

Total carbon and nitrogen in the soils of the world

The use of fire as a tool to manipulate the environment has been instrumental in the human conquest of Earth, the first evidence of the use of fires by early hominids dating back to 1–1.5 million years ago [1]. Even today, most human-ignited vegetation fires take place on the African continent, and its widespread, frequently burned savannas bear ample witness to this. Although natural fires can occur even in tropical forest regions [2, 3], the extent of fires has greatly expanded on all continents with the arrival of Homo sapiens. Measurements of charcoal in dated sediment cores have shown clear correlations between the rate of burning and human settlement [4]. Pollen records show a shift with human settlement from pyrophobic vegetation to pyrotolerant and pyrophilic species, testimony to the large ecological impact of human-induced fires.

https://science.sciencemag.org/content/sci/250/4988/1669.full.pdf

Biomass Burning in the Tropics: Impact on Atmospheric Chemistry and Biogeochemical Cycles

Soil fertility and leaching losses of nutrients were compared between a Fimic Anthrosol and a Xanthic Ferralsol from Central Amazonia The Anthrosol was a relict soil from pre-Columbian settlements with high organic C containing large proportions of black carbon It was further tested whether charcoal additions among other organic and inorganic applications could produce similarly fertile soils as these archaeological Anthrosols In the first experiment, cowpea (Vigna unguiculata (L) Walp) was planted in pots, while in the second experiment lysimeters were used to quantify water and nutrient leaching from soil cropped to rice (Oryza sativa L) The Anthrosol showed significantly higher P, Ca, Mn, and Zn availability than the Ferralsol increasing biomass production of both cowpea and rice by 38–45% without fertilization (P<005) The soil N contents were also higher in the Anthrosol but the wide C-to-N ratios due to high soil C contents led to immobilization of N Despite the generally high nutrient availability, nutrient leaching was minimal in the Anthrosol, providing an explanation for their sustainable fertility However, when inorganic nutrients were applied to the Anthrosol, nutrient leaching exceeded the one found in the fertilized Ferralsol Charcoal additions significantly increased plant growth and nutrition While N availability in the Ferralsol decreased similar to the Anthrosol, uptake of P, K, Ca, Zn, and Cu by the plants increased with higher charcoal additions Leaching of applied fertilizer N was significantly reduced by charcoal, and Ca and Mg leaching was delayed In both the Ferralsol with added charcoal and the Anthrosol, nutrient availability was elevated with the exception of N while nutrient leaching was comparatively low

http://www.css.cornell.edu/faculty/lehmann/publ/PlantSoil%20249,%20343-357,%202003%20Lehmann.pdf

Nutrient availability and leaching in an archaeological Anthrosol and a Ferralsol of the Central Amazon basin: fertilizer, manure and charcoal amendments

widespread, usually transitional between semidesert or savanna and moist forest. About 40% of the earth's tropical and subtropical landmass is dominated by open or closed forest. Of this, 42% is dry forest, 33% is moist forest, and only 25% is wet and rain forest (sensu Holdridge, 42; 15). We will never know the true original or potential extent of dry forest because many savannas and scrub or thorn woodlands are thought to be derived from disturbed dry forest. Walter (110), for example, considers most or all of the grassland in India to have been derived from seasonal or dry forest. Some of the processes that cause this conversion are addressed later in this review. The largest proportion of dry forest ecosystems is in Africa and the world's tropical islands, where they account for 70-80% of the forested area. In South America they

Ecology of Tropical Dry Forest

Early studies of nutrient cycling in moist tropical forests described productive forests rich in nutrients (98, 114, 176) in which rates of primary production and the amounts of nutrients cycled clearly exceeded those in temperate zone forests. Reviews of global-scale patterns in biomass, production, and nutrient cycling reported these results as representative of tropical forests (130, 175). At the same time, tropical forest soils were described as acid, infertile clays that harden irreversibly to "laterite" when cleared (106), or as bleached quartz sands low in mineral nutrients (88). This apparent paradox was crystallized by Whittaker (174) in the statement "The tropical rain forest thus has a relatively rich nutrient economy perched on a nutrient-poor substrate" (p. 271). Reviews of more recent research on overall patterns of mineral cycling in the tropics (78, 124), and of important components such as biomass (20, 21), litterfall nutrients (123, 164), and decomposition (5), clearly show that patterns of nutrient cycling in tropical forests are diverse. It makes no more sense to describe a 'typical' tropical forest than a 'typical' temperate forest (33, 151). Variations in mineral cycling nonetheless follow coherent, explicable patterns in tropical forests. Our purposes in undertaking this review are: (a) to illustrate the patterns of nutrient cycling in moist tropical forests; (b) to identify the mechanisms which regulate those patterns; and (c) to show how those patterns affect the productivity, physiology, and population biology of tropical forests and their large-scale linkages with aquatic ecosystems and the atmosphere. We emphasize the cycling of nitrogen, phosphorus, potassium, calcium, and magnesium; these are the best-studied soil-derived nutrients, and they are the nutrients most likely to limit primary production and other ecosystem functions.

/pdf/nutrient-cycling-in-moist-tropical-forest-48sxol78p3.pdf

Nutrient cycling in moist tropical forest

In a 1 -ha plot of old-growth tierra firme forest near San Carlos de Rio Negro, Venezuela (northwest Amazon Basin), 88 trees - 10 cm dbh out of 786 died over a 10-yr period. Most deaths resulted in the formation of small (5-100 m2) canopy openings (gaps). Occasionally, large gaps are formed in this region when strong winds topple many trees together. In five small (single-treefall) gaps and one large (multiple-treefall) gap we studied changes in soil fertility and nutrient leaching, and also plant establishment, mortality, and growth during the first 4 yr following gap formation. Gaps were divided into four zones, or mi- crohabitats: a trunk zone, an open zone (between bole and forest edge), a crown zone, and a root-pit zone. Sampling was conducted in each microhabitat. Soil nutrient levels in single-treefall gaps did not differ in a predictable fashion in response to microhabitat within gap or gap age. Moreover, except for a small, short-term increase in NO3-N, leaching losses from single-treefall gaps did not differ from forest levels. Advance regeneration plays a dominant role in treefall gap succession at San Carlos. Under closed forest, advance regeneration has a mean annual survivorship of - 80%; height growth is only a few centimetres a year, and leaf retention times frequently exceed 4 yr. Four years after gap formation, advance regeneration accounted for 97% of all trees ?-1 m tall in the single-treefall gaps and 83% of all trees in the multiple-treefall gap. Almost all trees in both gap types were of primary forest species; pioneer trees comprised only a small fraction of the regrowth. In general, microhabitat within gaps did not influence plant density, or plant estab- lishment and mortality patterns. However, mortality was much higher for individuals that germinated after gap formation than for individuals present as advance regeneration. Plant growth within treefall gaps was influenced by gap size, gap microhabitat type, gap age, and plant size. Seedlings and saplings of forest trees in the multiple-treefall gap grew three times as fast as those in the single-treefall gaps. Within single-treefall gaps, height growth was greater in the trunk and open zones than in the crown zone, and trees generally grew more slowly as gaps aged. Furthermore, tree growth was positively correlated with tree size, causing the size differential between trees of different heights to expand as gaps aged. At San Carlos, tree seedlings do not grow to the canopy during a single gap event; rather, canopy closure occurs by growth of larger pole-sized trees that survived treefalls or by lateral expansion of bordering subcanopy trees. Our results indicate that neither gap size, microhabitat within gaps, nor gap age have measurable effects on nutrient loss, nor do they appear to affect plant density, plant estab- lishment, or plant mortality. Size, microhabitat, and temporal affects are minimized, in large part, because of the great importance of advance regeneration in gap succession.

https://www.jstor.org/stable/pdfplus/1941024.pdf

Vegetation Dynamics in Amazonian Treefall Gaps

(1) Changes in vegetation structure, species composition, and species dominance were studied over the first twenty-two months of succession after the cutting and burning of several mature tierra firme (i.e. non-flooded) forest sites in south Venezuela in the upper Rio Negro region of the Amazon Basin. To explain the observed succession, additional field investigations were undertaken. (2) Four months after burning the density of plants was less than 1 m-2. The density of successional woody plants, forbs, and grasses increased rapidly from the fourth to the tenth month of succession. There was little change in plant density between the tenth and twenty-second month, but a large change in vegetation height occurred during this time. By the twenty-second month a loose canopy of Cecropia spp. was present at 5 m height. (3) Many of the forest tree species sprouted from the stump when cut, but burning killed sprouts and significantly reduced the size of the seed bank. Nevertheless, the first successional woody colonizers established from seed which survived the burn. The first forb and grass colonizers established from seeds dispersed onto the site after the burn. (4) The common successional woody species had bator bird-dispersed seeds. Many forbs and grasses had wind-dispersed seeds. (5) Population densities were low during the first months of succession because of the low density of propagules and because the low rainfall during this time was not favourable for seedling germination or establishment. The rapid increase in plant density after about the fourth month occurred because the early colonizers had then begun to produce, and disperse locally, large numbers of seeds. (6) There were six distinct microhabitat types present on the sites after burning. Most species tested showed distinct microhabitat preferences.

http://www.jstor.org/stable/pdfplus/2259689.pdf

Early plant succession after cutting and burning in the upper rio negro region of the amazon basin

Charcoal is common in the soils of mature rain forests within 75 kilometers of San Carlos de Rio Negro in the north central Amazon Basin. Carbon-14 dates of soil charcoal from this region indicate that numerous fires have occurred since the mid-Holocene epoch. Charcoal is most common in tierra firme forest Oxisols and Ultisols and less common in caatinga and igapo forest soils. Climatic changes or human activities, or both, have caused rain-forest fires.

https://science.sciencemag.org/content/sci/227/4682/53.full.pdf

Amazon Rain-Forest Fires

A study of ecosystem recovery following forest cutting, forest cutting and burning, and forest clearing by bulldozing was conducted in Amazon caatinga forest in the upper Rio Negro Region of southern Venezuela. Ecosystem recovery was evaluated by measuring vegetation composition, biomass, nutrient accumulation, soil characteristics, and nutrient leaching. As disturbance increased in intensity, the early successional vegetation changed from primary forest trees (cut treatment) to successional woody species (cut and burned treatment) to forbs and grasses (bulldozed treatment). Soil nutrient levels were greater in both the cut and the cut and burned treatment plots than in the control forest three years after the disturbances presumably because of steady transfers of nutrients from the forest slash to the soil. Soil nutrient levels in the bulldozed plot were much lower than the control forest because of top soil removal. Aboveground biomass at three years was 1291 g m2 in the cut site, 870 g m2 in the cut and burned site, and 77 g m2 in the bulldozed site. Given these rates of biomass accumulation, approximately 100 yr will be required for both the cut and the cut and burned sites to reach biomass levels characteristic of mature caatinga forest, while more than 1000 yr will be necessary in the case of the bulldozed site.

https://www.jstor.org/stable/pdfplus/3544671.pdf

Ecosystem Recovery in Amazon Caatinga Forest after Cutting, Cutting and Burning, and Bulldozer Clearing Treatments

In a study of plant succession during and following slash-and-burn agriculture near the town of San Carlos de Rio Negro in southern Venezuela we found that: 1. Weeds established just as readily in plots with Manihot esculenta (the principal crop plant of the region) present as in plots with M. esculen:a removed. 2. Repeated farm-plot weedings caused woody plants to decline in numbers and biomass and herbaceous plants to increase. 3. Forbs and grasses dominated immediately following farm site abandonment, but by one year these had begun to senesce and fast-growing successional woody species (particularly Vismia spp.) were common. Standing crop biomass at one year was 773 grams dry weight m-2. 4. Several microhabitat types were present on abandoned farm sites. Grasses and forbs showed no microhabitat preference, whereas successional woody individuals had their best establishment near slash and under fruit trees.

Kathleen Clark

Papers

Vegetation Dynamics in Amazonian Treefall Gaps

Early plant succession after cutting and burning in the upper rio negro region of the amazon basin

Amazon Rain-Forest Fires

Ecosystem Recovery in Amazon Caatinga Forest after Cutting, Cutting and Burning, and Bulldozer Clearing Treatments

Successional patterns associated with slash-and-burn agriculture in the upper Rio Negro region of the Amazon Basin.