Increased Soil Fertility ... 105the N rate for rice is much less in the regions with rice-oilseed rape cropping
system. In our long-term rice-oilseed rape rotation experiment, an average rice
yield of nearly 10.0 t ha–^1 was achieved at an N rate of 150 kg ha–^1 (Huang et
al. 2011; Badshah et al. 2014; Huang et al. 2015). In this regard, as early as in
the mid-eighteen century, the great biologist Carl Linnaeus noted that oilseed
rape had a useful soil improving role that aided the performance of subsequent
crops. This is a role that is still vital today and oilseed rape is known as a break
crop–one that enhances soil quality and improves the yield of the next crop.
Here, we hypothesized that long-term rice-oilseed rape rotation can increase
soil fertility and consequently reduce the dependence on external N inputs and
the impact on environments in rice production. To test this hypothesis, we (1)
detemined the soil fertility change in a long-term rice-oilseed rape cropping
paddy field from 2004 to 2015 and (2) conducted a field and a^15 N micro-plot
experiment to compare yield attributes, N uptake and fertilizer-N fate among
three N rates on the long-term rice-oilseed rape cropping paddy field in 2015.
MATERIALS AND METHODS
A rice-oilseed rape rotation was carried out on a paddy field located at the
Research Farm of Hunan Agricultural Univeristy (28°11′N, 113°04′E, 32 m
a.s.l.), China during 2004–2015. The location has a moist sub-tropical
monsoon climate with a mean annual temperature of about 17.0 °C, mean
annual rainfall of about 1355 mm and mean annual sunshine hours of about
1677 h. The soil of the paddy field is a Fluvisol (FAO classification). In each
year, a hybrid rice cultivar Liangyoupeijiu was grown in the single rice-
growing season (from May to October). Rice plants received 150 kg N ha–^1 , 90
kg P 2 O 5 ha–^1 and 180 kg K 2 O ha–^1 in each growing season. Rice straw was
removed from the field after harvest. At 2 days after harvesting rice, a hybrid
oilseed rape cultivar Xiangzayou 6 was planted. Fertilizers were applied at
doses of 120 kg N ha–^1 , 36 kg P 2 O 5 ha–^1 and 42 kg K 2 O ha–^1 in each oilseed
rape-growing season. Oilseed rape straw was retained in the field after harvest.
In 2004 and 2015, soil samples were taken from the upper 20 cm of the soil
before planting rice to determine organic matter, total N and available N
contents. The organic matter content was determined by the potassium
dichromate method. The total N content was measured by the semi-micro
Kjeldahl method. The available N content was determined by the diffusion
method. Ratio of available N to total N was calculated by dividing the
available N content by the total N content.