The growth and nitrogen uptake of Rhodes grass grown on soils with varying histories of legume cropping

Rhodes grass (Chloris oayana) was grown under glasshouse conditions on soils that had previously grown from 1 to 6 soybean (Glycine max) or Siratro (Macroptillium atropurpureum) crops. Soil mineral N contents at sowing were higher in Siratro-cropped than soybean-cropped soils and increased with cropping history. Yields of Rhodes grass at the first harvest (14 weeks) were related to soil mineral N levels at sowing by the relationship Y = 138.7 (l~0.917e~~)Grass grown on all soybean soils was responsive to N at both harvests (14 and 28 weeks). Grass grown on soils that had grown three or more Siratro crops was non-responsive to N at the first harvest but responses to N were recorded on all Siratro soils at the second harvest. The amount of N removed by the grass crops was small in relation to the total amount present in the soil system. This resulted in no change in soil total N levels over the two crop periods. These results highlight the need to study N dynamics in crop systems rather than continue to measure N pool sizes when evaluating the contribution of biologically fixed N to the nutrition of subsequent non-legume crops

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