Carbon Fluxes in Terrestrial Ecosystems

All three CO2-controlling processes (ocean soaking, photosynthesis, and carbonate formation) play an important role in maintaining equilibrium in the biosphere-atmosphere-hydrosphere system. The photosynthetic process is of great importance for living plants and microorganisms. The difference between total photosynthesis and respiration processes is defined as 'net primary production', NPP. The global NPP distribution in the Earth's major ecological zones is shown in Table 3.

Oceans, despite their much larger surface area, contribute much less than half of the global NPP. The reason is related to high nutrient deficiency in surface waters, which limits the photosynthesis process. Oceanic production is mainly concentrated in coastal zones, especially where upwelling of deep water brings the nutrients (P and N, of major interest) into the surface layer, 0-100 m. On land the photosynthetic process is also often limited by nutrient deficit; however, the influence of water storage and low temperature plays a more important role. That is why subtropical and tropical ecosystems contribute much more to global NPP than their proportional share.

The amount of annually decaying organic matter is the subject of speculation. However, some estimates might be done. For instance, in terrestrial ecosystems only, the humus accumulation of carbon in soils is about 70% of the total accumulation of CO2 in the atmosphere. We may presume therefore that the stable long-lived humic compounds acquire some 30% of carbon annually from the dead organs of plants, and the complete renewal of humus in soils extends over the period of (0.3-1.0) x 103 years. The variance depends on the moisture and temperature conditions in the region of question.

Terrestrial biomass is divided into a number of subreservoirs with different turnover times. Forest ecosystems contain 90% of all carbon in living matter on land but their NPP is only 60% of the total. About half of the primary production in forest ecosystems is in the form of twigs, leaves, shrubs, and herbs that only make up 10% of the biomass. Carbon in wood has a turnover time of the order of 50 years, whereas these times for carbon in leaves, flowers, fruits, and rootlets are less than a few years. When plant material becomes detached from the living plant, carbon is moved from phytomass reservoir to litter. 'Litter or litterfall' can refer to a layer of dead plant material on the soil surface. A litter layer can be a continuous zone without sharp boundaries between the obvious plant structures and a soil layer containing amorphous organic carbon. Decomposing roots are a kind of litter that seldom receives separate treatment due to difficulties in distinguishing between living and dead roots. Total litter is estimated as 60 x 10 C and total litterfall as 40 x 1091 C yr-1. The average turnover time for carbon in litter is thus about 1.5 years, although for tropical ecosystems with mean temperature above 30 °C, the litter decomposition rate is greater than the supply rate and so storage is impossible. For colder climates, NPP exceeds the rate of decomposition in the soil and organic matter in the form of peat is accumulated. The total global amount of peat might be estimated at 165 x 109t C. Average

Aboveground

Aboveground

0.16

Humins 3.8

0.16

Soil respiration

/

0.41

/ 7

■>■ 0.004

0.005

/

/

0.419

/

Undecomposed litter

/

/

Turnover in

0.5

/

10's of years

V

0.004

/

/ \

Fulvic acids

Turnover in

2.3

s

100's of years

Humins 3.8

Humic acids 3.8

Humic acids 3.8

Turnover in 1000's of years

Permanent accumulations in the lower profile

Figure 4 Detrital carbon dynamics for the 0-20cm layer of chernozem grassland soil. Carbon pool (kgCm~2) and annual transfers (kgCm~2yr~1) are shown. Total profile content down to 20 cm is 10.4 kgCm~2.

temperature at which there is a balance between production and decomposition is about 25 °C.

Humus is a type of organic matter in terrestrial ecosystems that is not readily decomposed and therefore makes up the carbon reservoir with a long turnover time (300-1000 years). An assessment of the various carbon pools for a temperate grassland soil is presented in Figure 4.

The undecomposed litter (4% of the soil carbon) has a turnover time measured in tens of years, and the 22% of the soil carbon in the form of fulvic acids is intermediate with turnover times of hundreds of years. The largest part (74%) of the soil organic carbon (humic acids and humins) also has the longest turnover time (thousands of years).

10 Ways To Fight Off Cancer

10 Ways To Fight Off Cancer

Learning About 10 Ways Fight Off Cancer Can Have Amazing Benefits For Your Life The Best Tips On How To Keep This Killer At Bay Discovering that you or a loved one has cancer can be utterly terrifying. All the same, once you comprehend the causes of cancer and learn how to reverse those causes, you or your loved one may have more than a fighting chance of beating out cancer.

Get My Free Ebook


Post a comment