Managing Soil Organic Carbon Concentration by Cropping Systems and Fertilizers in the North China Plain - Principles of Sustainable Soil Management in Agroecosystems - page 197

land use change and adoption of recommended management practices (RMPs) lead
to improvement in SOC concentration, which follows a sigmoid curve and attains the
maximum value in 20 to 25 years after adoption of RMPs; moreover, the increase
may continue until the SOC attains equilibrium once again. The common RMPs
include mulch farming, conservation tillage, diverse cropping systems, cover crops,
agroforestry, and integrated nutrient management (Lal 2004). Thus, the incorpora-
tion of biomass-C increases the SOC concentration and improves soil quality under
intensive agricultural systems.

8.3.4 r eSultS of C hAngeS in the C ropping S yStem And
the i nCreASed u Se of f ertilizer over t ime
The regional cropping systems have been transformed from diverse crops in 1980
to monoculture in 1999, and back to diversity in 2006. In addition, there has been
an increase in use of chemical fertilizers over time. There are several factors that
influenced these changes.
The first factor is the transition from family farms to maximum profit farms,
which triggered the change from subsistence farming to focus on increased crop pro-
duction from 1980 to 1999 in response to economic development. Thus, these farms
increased the rate of fertilizer application. The application rate (kg ha -1 ) of N was 66,
without use of P and K fertilizer in 1980, 241 in 1999, and 277 in 2006 ( Table 8.4 ) for
the cropping system of winter wheat and maize, respectively.
When the family's food needs were met, these farmers were motivated to tran-
sition to the maximum profit farming. Cotton cultivation yielded more profit than
wheat-maize rotation during the second period, which led to the conversion of the
cropping system from wheat-maize to cotton. The surveys from farming households
show that the labor input for cotton was five times that of wheat and 2.6 times that
of maize, respectively. However, the application rate of N was 183 kg ha -1 during the
cotton cultivation in 2006 (Table 8.8). These data show that the farming households
preferred to dedicate much more labor and less materials to meet their goals of profit

TABLE 8.8
Different Application Rates of Fertilizers among Different Cropping Systems
in 2006

Human
Labor Fertilizer Rate (kg ha -1 )

Number of
Irrigation N P K MJ ha -1

Wheat 277 206 19 3.6 35

Maize 203 0 0 3 70

Cotton 183 168 100 2.5 182

Source: Data from the 2006 household survey.
Note: Approximate value of 0.8 MJ/working hour or 6-7 MJ/man day (Leach 1976).