Managing Soil Organic Carbon Concentration by Cropping Systems and Fertilizers in the North China Plain - Principles of Sustainable Soil Management in Agroecosystems - page 194

SOC concentrations were 2.84 and 7.59 g kg -1 , respectively, in 1980. The data in
Table 8.2 illustrates that the SOC concentration gradually increased by approxi-
mately 38% from 1980 to 1999, which is in contrast to other studies (Zeidler et al.
2002), which argue that the SOC concentration is reduced to below the critical levels
because of high land use intensity. The coefficient of variation (CV) of the SOC con-
centration was 36 and 39 in 1980 and 1999, respectively (Tables 8.2 and 8.3). The CV
was similar between these two years, although SOC concentration increased from
5.01 to 6.93 g kg -1 .
In the 1980s, the application rate of N was 66 kg ha -1 , while P and K fertilizers
were not in use at the time. Farmers produced crops only for personal consumption
under low economic development, so crops were grown by extensive rather than
intensive cultivation. Thus, increase in production was achieved by expanding farm-
land areas and use of natural fertilizers because they had no money to buy chemical
fertilizers. Furthermore, the standard of living on a family farm in the 1980s was
low due to fuel shortages in rural areas. For this reason, crop residues were the main
source of fuel. And only some straw and roots were returned to the soil. Therefore,
the SOC concentration in the 1980s was low.
However, family farms improved production by increasing the rate of fertilizer
application during the periods of rapid economic development. The application rate
of N increased from 66 kg ha -1 in 1980 to 241 kg ha -1 in 1999, an increase of 260%
over <20 years (Table 8.4). At the same time, the crop yield increased from 1538 kg
ha -1 in 1980 to 5070 kg ha -1 in 1999, an increase of 229%. Consequently, the total
crop production increased from 87,517 to 324,943 Mg over <20 years. In contrast,
crop residues were no longer considered as an important fuel source because of eco-
nomic development and availability of electricity and natural gas. As a result, the
survey data showed that almost 85% of family farms returned crop residues to the
soil directly or indirectly.
The increase in crop production and the widespread practice of returning the
crop residues to the soil led to buildup of the SOC concentration, which attained
equilibrium at a higher level. Therefore, increase in the SOC concentration during
the first period can be explained by the input of biomass-C through retention of large
amounts of crop residues returned to the soil upon transition of farm household's

TABLE 8.4
Change in Fertilizer Use and Crop Yield in 1980, 1999, and 2006

Application Rate of Fertilizer
(kg ha -1 )

Year GDP per Capita (Yuan) Yield (kg ha -1 ) N P K

1980 167 1538 66 No data No data

1999 4425 5070 241 101 No data

2006 6842 5295 277 125 40