Carbon inputs to soils

Compared to baseline levels, soil organic C and N increased by 22% in the organic treatment and <1 % in the conventional treatment (Clarke et al. 1998). Carbon inputs to the organic treatment (poultry and green manures) were 6.2 times greater than to the conventional system (Cunapala and Scow 1998). 

The quantity of litter input provides the second critical link between NPP and decomposition because NPP governs the quantity of organic matter inputs to decomposers. When biomes are compared at steady state, heterotrophic respiration (i.e., the carbon released by processing of dead plant material by decomposer organisms and animals) is approximately equal to NPP. In other words, net ecosystem production (NEP), the rate of net carbon sequestration, is approximately zero at steady state, regardless of climate or ecosystem type. This indicates that the quantity and quality of organic matter inputs to soils, as determined by... 

H. Keith, J. M. Oades, and J. K. Martin. Input of carbon to soil from wheat plants. Soil Biology ami Biochemistry 18 455 (1986). 

Input rates of organic C into the soil system are hard to quantify, particularly for natural ecosystems and to a lesser extent for agricultural ecosystems. Whereas quantity and quality of carbon inputs via litter fall and plant residues after harvest might be directly measurable, inputs via roots and rhizodeposition are more difficult to assess. 

Important, poorly-constrained variables in this carbon balance include temporal and spatial variations in precipitation and RO DIC, carbonate dust, and pedogenic carbonate and soil C02. In addition, ocean and lake response to river carbon input is not well known. Although marine and freshwater aquatic organisms can be fertilized by increases in N, P, Si, Fe, Zn and C02 (Cassar et al. 2004 Zondervan 2007), it is not clear how much of the... 

Jenkinson, D. S., and Coleman, K. (1994). Calculating the annual input of organic matter to soil from measurements of total organic carbon and radiocarbon. Eur. J. Soil Sci. 45, 167-174. 

The turnover time (x) of a reservoir is its mixing or refresh rate, and is the time it would take for the reservoir to completely empty if there were no further inputs. For soils, it is a measure of the first-order kinetics for decay (x = Ilk). At steady state, it is calculated as the inventory divided by the total inputs (or total outputs) to the reservoir. To calculate the turnover time for a soil C reservoir at steady state, we would divide the mass of SOM (C) by the total carbon fluxes (.S ) from the reservoir or x = C/S. Fluxes would include decomposition to C02 and leaching of dissolved organic. 

Figure 6.7. Simplifed soil carbon cycling scheme. Major inputs (plant litter) to and outputs (respiration and erosion) from the soil carbon reservoir. The observed flux of C out of the soil can be modeled by assuming three pools of carbon an active pool with a turnover time on the order of years, an intermediate pool with a turnover time on the order of decades to centuries, and a passive pool with a turnover time on the order of millennia. The decomposition constant is k = 1/t. Subscripts a, i, and p refer to the active, intermediate, and passive C pools, respectively. Adapted with permission from Amundson, R. (2001). The carbon budget in soils. Annu. Rev. Earth Planet. Sci. 29, 535-562.

Carbohydrate partitioning from shoot to roots can comprise a substantial proportion of plant-photosynthetic C02 fixation (20-70%), and 4-70% of this fraction can be released into the soil as organic rhizodeposition (Lynch and Whipps, 1990 Grayston et al., 1996). This is not only a significant loss of reduced carbon but can also contribute by 30-40% to the total input of soil-organic matter with considerable impact as a source of carbon and nitrogen for soil microorganisms. 

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