A cool season wheat (Triticum aestivum)-spring mungbean (Vigna radiata)-monsoon rice (Oryza sativa) cropping pattern was implemented over 12 years, starting with wheat sown in November 25, 2004, at the Regional Wheat Research Centre, Shyampur, Rajshahi, Bangladesh (24o3'N, 88041E, 18 m above sea level). The trial involved a three-crop (wheat-mungbean-rice (W-M-R) annual rotation planted on raised beds and cultivated flats. Rice was transplanted (one 25-days-old seedling per hill) with hill-to-hill spacing of 15 cm and line-to-line spacing on the beds of 30 cm in late July and harvested in late November by hand. Wheat was planted at the nationally recommended seeding rate of 100 and 120 kg ha-1 for beds and conventional, respectively, in late November and harvested in late March. After harvest of wheat, mungbean was planted at the same time at the nationally recommended seeding rate of 35 kg ha-1 for mungbean in early April and harvested in mid July for both beds and conventional layout. The trial was originally established as a PRB experiment with two straw management practices (main plots-30% straw retention (SR) and 0% SR) and five N levels (subplots 0, 40, 80, 100 and 120% of recommended). The area of each subplot was 15 m2 (5×3 m). The experiment consisted of 20 subplots with four tillage/straw treatments (30% SR + PRB, 30% SR + CTP, 0% SR + PRB and 0% SR + CTP) and five N levels (0, 40, 80, 100 and 120% of recommended) with three replicates. After planting the wheat or rice, straw from the preceding cereal crop was returned as mulch into the plot from which it had been removed at harvest. After harvesting and threshing, the rice and wheat straw were returned without chopping as standing way. Total system productivity (TSP) was calculated using the method of Tanaka (1983). Harvested organs of crops vary widely in chemical composition. For example, the percentages of proteins, lipids and carbohydrates in rice grain are 8.8, 2.7 and 87%, respectively, while in wheat grain they are 12.1, 2.3 and 83.7% and in mungbean seeds, the composition is 24.1, 2.6 and 69%, respectively. The conversion efficiencies (the process of conversion from primary photosynthetic products, for example from glucose to biosynthesis of harvested organs or grains requires some energy for respiration) for starch, proteins and lipids are 0.84, 0.38 and 0.31, respectively. Based on the composition of harvested organs for mungbean and their conversion efficiencies, the equivalent yields for various crops can be calculated (Tanaka, 1983). For example, the equivalent yield of 1.35 in rice is computed as: 0.088/0.38+0.027/0.31+0.87/0.84.
Finally, total system productivity (TSP) for each treatment was calculated as the total annual productivity (or the annual total of economic yield of the individual crops) based on equivalent yields (1.35, 1.39 and 1.54, respectively, for rice, wheat and mungbean). TSP (rice-wheat-mungbean) = (rice grain yield*1.35) + (wheat grain yield*1.39) + (mungbean grain yield*1.54).
Estimation of nitrogen uptake
The result was expressed in percentage. N-uptake by grain and straw also were calculated by the following formulae:
N-uptake by grain (kg ha-1) = Total N (%) in grain × grain yield (kg ha-1)
100
N-uptake by straw (kg ha-1) = Total N (%) in straw × straw yield (kg ha-1)
100
Microsoft Excel and DMRT were used to determine the significant difference between treatments. Simple correlation was determined following the computer package SPSS. Treatment means were compared using Duncan’s Multiple Range Test (DMRT) at P ≤ 0.05 (Gomez and Gomez, 1984). The data were analyzed statistically following computer package MSTAT-C. All the data were statistically analyzed following the ANOVA technique and the significance of mean differences was adjusted by DMRT.