Nutrient loss biomass burning

Figure 1. Generalized nutrient balance of ecosystems in the intervals between disturbance events. Natural disturbances such as wildfires, hurricanes, and floods as well as anthropogenic disturbances such as deforestation and biomass burning can dramatically influence nutrient inputs, internal cycles, and ecosystem outputs (losses).

Nutrient Losses Associated With Biomass Burning. Nutrient losses associated with slash fires occur through volatilization and convective losses of ash. Elements with low temperatures of volatilization (e.g. N, K, S, and some organic forms of P) will be lost in the highest quantities (Table III) (57). Conversely, Ca and Mg have volatilization temperatures higher than that recorded during most vegetation fires. Almost all fire-induced losses of these elements are due to particulate transfer by convective processes. 

Much of the surface soil erosion and hence nutrient loss occurs when deforestation and biomass burning removes and/or consumes the organic materials that protect the soil surface. Significant losses may occur by dry ravel or overland water erosion associated with precipitation events. Under a shifting cultivation system in a tropical deciduous forest ecosystem in Mexico, Maass et al. 61) reported first year losses of N, P, K, and Ca were 187, 27, 31, and 378 kg ha" respectively. In contrast, losses in adjacent undisturbed forests were less than 0.1 kg ha for all nutrients except Ca (losses were 0.1-0.5 kg ha for Ca). 

Figure 7. Another temperate coniferous forest site of the Pacific Northwest, USA following clearcutting and slash burning. Severe levels of deforestation result in large quantities of nutrient losses through wood export, biomass burning and accelerated erosion and leaching losses. (Photograph is by courtesy of Dian L. Cummings. ...

The burning of slash following deforestation, whether intentional or unintentional, results in far greater direct and indirect losses of nutrients than deforestation alone. This is particularly true in many tropical forests where only a small fraction (if any) of the aboveground biomass is removed prior to burning. Carbon losses from slash fires in the tropical dry forest were 4-5 fold greater than C losses from wood export (Table IV) (55). Slash fires in tropical dry forests resulted in N losses of 428-500 kg ha whereas fuel wood export of the relatively N-poor coarse woody debris amounted to approximately 41 kg N ha" Losses of P increase with increasing fire severity. P losses of 10-77 kg ha" as a result of severe fires is not uncommon (Table TV) (53, 58, 60). 

Losses from the system due to combustion of biomass can be quite high for carbon, nitrogen, and sulfur, and less for major nutrients like phosphorus, potassium, and calcium (Ewel et al. 1981, Frost and Robertson 1985, Kauffman et al. 1995, Sanchez 1976, Wright and Bailey 1982). However, immediate improvement in the soil chemical characteristics - higher pH, reduced aluminum toxicity, and higher nutrient content— is usually observed after forest cutting and burning in the Amazon basin (Buschbacher 1984, Dantas 1989, Dantas and Matos 1980, Diez et al. 1991, Falesi 1976, Seubert et al. 1977, Smyth and Bastos 1984). 

If not grazed, or if the pasture is not burned, most of the P absorbed in plant tops will be immobilized by plant tissue and, thus, will be unavailable in the soil. Transfer of P and N by nutrient resorption in forage plants and in the weedy vegetation prior to leaf fall induces low nutrient concentration in litter (Table 6.6) and may in part explain the decreasing amount of available soil P often associated with an increase in the weed biomass of a degrading pasture or with pasture age. Without this biological immobilization, however, these nutrients would be more vulnerable to loss through runoff and erosion. 

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