2.1. Experiment site and design
The field experiment was conducted in a reclamation land in Zhangjiagang
city, Jiangsu province (31°57′N, 120°46′E, Fig. 1). The soil is composed
of alluvium from the Yangtze River and classified as Fluvo-aquic
according to the Chinese Soil
Taxonomy (Gong et al., 2001). The basic soil chemical properties are as
follows: pH (H2O) 8.00±0.30; SOC 3.86±1.35 g
kg−1; TN 0.41±0.16 g kg−1;
NH4+-N 1.36±0.34 mg
kg−1; NO3−-N
11.58±5.37 mg kg−1; available phosphorus (AP)
20.57±12.67 mg kg−1; and available potassium (AK)
7.34±1.95 mg kg−1.
A plot design with replicates was adopted with 60 m × 35 m (Fig. 1).
Rice-green manure (RG), rice-rape (RR), and rice-wheat (RW) rotations
were included. Five fertilization treatments were set as follow: without
N fertilizer (WN), farmer’s conventional urea fertilizing practice with
a thrice-split application (CU), single-dose application of BBU with the
conventional amount of N (CB1), twice-split application of BBU with the
conventional amount of N (CB2), single-dose application of BBU with 20%
reduction of N (RB1), and twice-split application of BBU with 20%
reduction of N (RB2). The BBU consisted of 75% CRU and 25%
conventional urea. The CRU coated with 10% waterborne polyacrylate was
purchased from Jiangsu Issas New Fertilizer Engineering & Technology
Co., Ltd., China and had an N content of 42% and an N release longevity
of 3 months. The farmer’s conventional N application rates are 300 and
270 kg N ha−1 for rice and wheat, respectively. There
was no fertilization during the rape oil and green manure seasons.
Superphosphate, potassium chloride, zinc sulfate, and silicon fertilizer
were incorporated into the 10−15 cm soil layer for all treatments at
rates of 75 kg P2O5ha−1, 120 kg K2O
ha−1, 3 kg Zn ha−1, and 0.8 kg
SiO2 ha−1 as basal fertilizers in rice
and wheat seasons.