Soil Renewal and Sustainability - Principles of Sustainable Soil Management in Agroecosystems - page 494

seem to enhance the yields of eroded soil to some extent but fail to bring yields to
preerosion levels ( Fig u r e 17.4 ).
The Loess Plateau, located in Southwest China, is one of the most eroded regions
in the world. With sloping topography, approximately 70% of the area has suffered
soil erosion to different extents; 36% of these eroded areas lose more than 50 Mg/ha
annually (Zhu 1956). Soil erosion has significantly reduced crop production and
yield potential in the southern region of the Loess Plateau, which is also a critical
wheat production area of China.
The potential impact of the Loess Plateau soil loss on wheat production might
be inferred from erosion and wheat yield research. Jia et al. (2004) examined the
impact of soil erosion on wheat production and yield of slightly sloping loess farm-
land (Inceptisol) without fertilizer application. Wheat yield showed a positive incre-
mental exponential relationship with increases in topsoil depth. Yield decreased by
39.1% and 69.1% when simulated erosion removed 10 and 20 cm, respectively, of
topsoil compared to uneroded soil conditions, and increased by 53.1% when 10 cm
of topsoil was added to the field surface. For each 1 cm loss of topsoil, wheat yield
reduction ranged from 2.3% to 4.0%. In contrast, wheat yield increased by 5.3% for
each 1-cm increase in topsoil depth. A linear relationship between topsoil thickness
and grain weight was found. The study indicated that soil erosion not only reduced
wheat yield but also resulted in degradation of grain quality. Based on wheat sen-
sitivity to TSD from this study, erosion rates identified by Zhu (1956) in Southwest
China could have very severe impacts on wheat production in that region.
While maize, soybean, and wheat are among the most important commodities
globally and considering that soil erosion impacts to these crops is critical, other
crops play key roles in feeding the world population as well, and erosional influ-
ences are also critically important. Limited evidence shows a negative relationship
between thinning TSD and yields of potato, flack, and pea (Chen et al. 2003), with
pea being the most sensitive of these three crops. Spring wheat was also part of that
study, and similar to Jia et al.'s (2004) study, it was highly sensitive to TSD change.
When removing topsoil depths of 5, 10, and 20 cm, reductions in spring wheat yield
equaled 15.68%, 22.34%, and 37.92%, while reductions in pea were 9.79%, 26.20%,
and 41.38%, respectively. Adding 10 cm of topsoil to the field brought an increase of
16.86% and 12.21% to spring wheat and pea yield, respectively.
The Sichuan Province in Southwest China consists of purple soils (Entisols) and
is considered an agriculture center of China. In the region, 68% of the land is farmed,
of which 72% is defined as low to medium yielding, with average topsoil depths of
20 to 60 cm (He et al. 1990; Zhu et al. 2009). Thinner topsoil induced by soil ero-
sion has restricted crop growth and yield in the region. As displayed in Fig u r e 17. 5 ,
corn yield showed an exponential increase, while wheat displayed a positive loga-
rithmic relationship when increasing the topsoil thickness from 20 to 100 cm (Zhu
et al. 2009). For wheat, the largest impacts in yield were seen at TSD, ranging from
20 to 60 cm, while there was no significant difference in yield when the thickness
exceeded 80 cm.
Winter wheat and corn yields also increased with increasing topsoil thickness,
although the yield impact varied during different growth periods. The effects of top-
soil depth on crop yield could be partially explained by examining root growth under