Increased Soil Fertility ... 107inner diameter tube auger. The soil samples were air-dried and sieved.
Aboveground plant parts in each micro-plot were sampled and oven-dried at
70°C to constant weight to determine aboveground total dry weight. Total N
contents and^15 N abundances in soil and plant samples were measured using a
VAP50 Kjeldahl meter (Gerhardt, Königswinter, Germany) and a Delta V
Advantage isotope mass spectrometer (Thermo Fisher, Waltham, MA, USA),
respectively. Total N uptake, N uptake from soil, N uptake from fertilizer,
retention of fertilizer-N in soil, and fertilizer-N loss were calculated according
to Huang et al. (2014).
Data were analyzed by analysis of variance (Statistix 8.0, Analytical
software, Tallahassee, FL, USA) and significant differences among means
were assessed by the least significant difference test (LSD) at the 0.05
probability level.
RESULTS AND DISCUSSION
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 (Figure 1a–c). These
demonstrate that long-term rice-oilseed rape rotation increased soil fertility in
this study. This might be attributed to that crop residues of oilseed rape were
retained in the field. More interestingly, our study showed that ratio of
available N to total N increased 25% from 2004 to 2015 (Figure 1d). This
indicates that soil N mineralization was improved by long-term rice-oilseed
rape rotation.
Our results are in agreement with those of Deng and Tabatabai (2000),
who reported that multicropping not only conserved organic matter and N, but
also enriched the active N pools in soils, thus resulting in increasing N
availablility.
There were no significant differences in yield attributes between N rates of
150 and 120 kg ha–^1 (Table 1). Panicles per panicle and total dry matter were
lower under N rate of 90 kg ha–^1 than under the other two N rates (150 and 120
kg ha–^1 ), but lower grain yield was not observed under the N rate of 90 kg ha–^1
because it was compensated for by higher spikelet filling percentage and
harvest index. The differences in total N uptake, N uptake from soil and N
uptake from fertilizer were insignificant among the three N rates (Table 1 and
Figure 2). These results are not entirely consistent with those of Huang et al.
(2008).