Fig. 4 shows the correlations between the contents of total
arsenic and TOC in the agricultural soils from the CAFO zone and
the Pearl River Delta. Animal manure is composed primarily of
organic matter, and contributes to soil fertility by increasing the
contents of organic matter and nutrients (e.g., nitrogen). Animal
wastes are disposed off on local farm lands as an organic fertilizer
in the CAFO zone, thus the surface soils receiving more animal
wastes should have higher levels of TOC and arsenic contributed
by the animal wastes. The content of arsenic in the surface soils
from the CAFO zone was weakly correlated to TOC
(r2 = 5.7 102, p = 0.047), while no significant linear correlation
(r2 = 2.7 105, p = 0.97) was observed between arsenic and TOC
contents in the agricultural soils from the Pearl River Delta. Chemical
fertilizers are overwhelmingly used on agricultural lands in the
Pearl River Delta. In contrast, animal wastes are the predominant
form of fertilizers used in the CAFO zone due to their ready availability
and the short transportation distance required. The correlation
between the contents of arsenic and organic carbon in the
surface soils from the CAFO zone suggests that the fertilization of
agricultural lands with animal wastes was a likely source of arsenic
in the surface soils. The r2 value of the linear model in the CAFO
zone is small because lithogenic materials remain the predominant
source of arsenic in the soils. As discussed earlier, the mean content
of soil arsenic (16.0 mg kg1) was not too much higher than
that (12.2 mg kg1) in the agricultural soils in the Pearl River Delta,
thus TOC in the surface soils could not be the only predictor for soil
arsenic content, even if the soil organic matter came completely
from the animal wastes.