+ 1000


C (g C m-2)








a Simulated values were generated by the Century model and are presented in parentheses. With permission from Paustian et al. (1992).

a Simulated values were generated by the Century model and are presented in parentheses. With permission from Paustian et al. (1992).

C and N levels result from increases in root C inputs. The high-N-content organic additions (farmyard manure and green manure) result in large increases in plant production and N uptake and substantial increases in soil C and N levels. Approximately 40% of the added N from the inorganic fertilizer and high-N organic additions goes into increased plant N uptake, while organic N additions result in greater stabilization of N in the soil (40% vs 20% for inorganic N fertilizer). Comparison of model results with observed data is quite favorable; however, more information will be needed to accurately predict the role of SOM in controlling global CO2 levels.

establishing pool sizes and kinetic constants

The various forms of organic matter in the soil-plant system and the complex changes that it undergoes can be rationalized using a pool structure. A pool can be defined as a compartment containing material of unique and uniform composition. An example of biological and chemical pool sizes can be found in the analysis of two Swedish agricultural systems (Table 16.6). The standing crop of barley fertilized with 120 kg N ha-1 is greater than that of alfalfa, but the primary production of alfalfa is greater when one considers both the two harvests per annum and the root turnover in the alfalfa crop. The shoot-to-root ratio of 6:1 for barley is typical of many agricultural crops. The perennial alfalfa with more roots than shoots is representative of perennial crops. The microbial biomass is equivalent to about 50% of the weight of the plant standing crop and greatly exceeds the meta-zoan biomass. The uncultivated alfalfa has a soil fauna biomass of 105 kg C ha-1 compared to the 55 kg C ha-1 in the barley field. The soil organisms account for slightly more than 3% of the total soil C in both systems, and direct microscopy of the bacteria and fungi shows the predominance of fungal biomass. The annual C balance of the two systems (Table 16.7) shows the root respiration as 11-12%

TABLE 16.6 Pool Sizes of Biotic Components in or above the Top 27 cm of Soil in Two Swedish Cropping Systems (kg C ha~')a

Cropping system




Plant shoots and surface litter

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