104 Min Huang, Xuefeng Zhou, Xiaobing Xie et al.
the long-term rice-oilseed rape cropping paddy field in 2015. Our results
showed that soil organic matter, total N and available N contents in the
upper 20 cm layer increased from 15.0 g kg–^1 , 1.40 g kg–^1 and 0. 14 g kg–^1
in 2004 to 34.4 g kg–^1 , 1.49 g kg–^1 and 0. 18 g kg–^1 in 2015, respectively.
Ratio of available N to total N increased 25% from 2004 to 2015. In the
field experiment, there were no significant differences in grain yield
(11.2–11.6 t ha–^1 ) and total N uptake (196–209 kg ha–^1 ) among the three
N rates. In the^15 N micro-plot experiment, the differences in total N
uptake, N uptake from soil and N uptake from fertilizer were insignificant
among the three N rates. The N uptake form soil and fertilizer occupied
81 – 83% and 17–19% of the total N uptake, respectively. Retention of
fertilizer-N in soil did not significantly affected by N rate, whereas
fertilizer-N loss significantly decreased with decreasing N rate. These
results suggest 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.Keywords: rice-oilseed rape rotation, soil fertirity, N input, fertilizer-N fate
INTRODUCTION
Soil quality is critical to crop productivity and sustainability in agro-
ecosystems (Shang et al. 2014). Improving nutrient cycling is an important
step towards stabilizing and optimizing soil quality (Fan et al. 2012). Well-
planned crop rotations, as compared to continuous monoculture systems, can
be expected to promote nutrient cycling efficiency and consequently enhance
crop productivity and reduce dependence on external fertilizer inputs (Bullock
1992; Zegada-Lizarazu and Monti 2011). In China, rice-wheat and rice-oilseed
rape are two long-established major rice-based rotation systems (Zou et al.
2008). However, long-term experiments indicate that yields of rice-wheat
cropping system are stagnant or even declining^ (Ladha et al. 2003). What is
worse, N fertilizer input has been excessively high for rice in the regions with
rice-wheat cropping system (Ju et al. 2009). In Jiangsu, a typical rice-wheat
cropping province in China, the average N rate for rice reaches 300 kg ha–^1 in
some counties^ (Peng et al. 2009). Because of the high rate of N application,
only 20–30% of N is taken up by the rice plant and a large proportion of N is
lost to the environment^ (Peng et al. 2006; Ju et al. 2009). The lost N has caused
substantial environmental problems such as increased greenhouse gas
emissions, enhanced N deposition and degradation of cropland and freshwater
(Ju et al. 2009; Fan et al. 2012; Guo et al. 2010; Liu et al. 2013). By contrast,