Nitrogen fixation in peas (Pisum sativum)

Abstract

Field experiments were conducted at Lincoln College, Canterbury, from 1979-81 to assess the effect of peas (pisum sativum) on subsequent crops and evaluate factors influencing their N-fixing activity.

In two trials, spring-sown peas harvested green or dry were grown to contrast with barley. On a low fertility soil, wheat yielded 73 and 61 percent more grain respectively following the two pea harvests when compared with barley. Wheat after lupins yielded 85% more than after barley. Seed size in wheat was 31.8 mg after barley but 39.7 mg after legumes.

In the second trial, which was on a more fertile soil, winter-grown ryegrass produced 34 and 23 per cent more biomass after vining and seed peas respectively. Pea residues were removed and contained 6.8 and 2.7 g N m⁻² at green and dry pea stages respectively. Nitrogen harvest index (NHI) was 0.47 and 0.85 at the respective stages. Peas harvested dry relied on soil N for 50 per cent of their N requirements. Also on a high fertility soil, winter ryegrass after spring-sown peas and lupins yielded 25 and 11% respectively more than after wheat.

Incorporation of green pea residues to the soil would further increase subsequent crop yield, but most N is removed in seed at final harvest. These trials clearly showed the benefits from growing peas compared with cereals in crop rotations. The NHI and soil N uptake, however, limits soil nitrogen increases after peas.

The following four factors influencing pea N fixation were evaluated with peas cv. Puke sown in late spring (13/11/80). Treatments were 1) 8 t ha⁻¹ straw incorporated at sowing; 2) irrigation during flowering and pod filling; 3) 4.5 g N m⁻² at nodule formation; and 4) 4.5 g N m⁻² at flowering: Overall treatment responses were small because of adequate soil nitrogen and 114 mm rainfall during pod-filling. Nitrate (0 - 20 cm) was 34 ppm N at sowing and straw halved this to 20 ppm N, 36 days after sowing. Straw increased, and nitrogen reduced, N-fixation but dry seed yield was unaffected by these treatments. Irrigated green pea yield (644 g m⁻²) was increased by 8% when nitrogen was applied at flowering. Irrigation increased dry seed yield from 305 to 343 g m⁻²

Trial 4 -examined 1) the effect of autumn (7/5/1980) and spring (12/9/80) sowing, and 2) moisture stress, natural rainfall and irrigation applied from flowering on indeterminate (cv. Partridge) and determinate (cv. Whero) peas. Available soil nitrate of more than 6.5 ppm N (0 - 20 cm) reduced reliance on N fixation. Autumn sowing increased N fixation by 27 percent, primarily through an extended period of N fixation. Irrigation of Partridge stimulated vegetative growth at the expense of seed development. At final harvest Partridge and Whero residues contained 16.8 and 7.1 g N m⁻² respectively. Soil moisture and sowing date did not significantly influence seed yield. Lupins cv. Unicrop sown in winter (24/6/80) and spring (12/9/80) yielded 453 and 251 g m⁻² respectively.

In Trial 5, eight pea cultivars were grown without irrigation and seven acetylene reduction assays used to assess N-fixation during growth. Cultivars and their seed yields (g m⁻²) were Whero (262), Partridge (96), Huka (360), Rovar (284), Puke (269), Pania (306), Tere (229), Small Sieve Freezer (270). The highest NHI was recorded in the earliest maturing cultivar, Tere (0.85) and the lowest in Partridge (0.38) because of late flowering during drought stress. Whero and Partridge reached peak N fixation activity (5 µmoles C₂H₄ plant⁻¹h⁻¹) 18 and 39 days before flowering respectively. Other cultivars reached peak activity soon after the "start of flowering. Field pea N fixation was twice that of garden peas. In all cultivars, reliance on N fixation for nitrogen requirements during reproductive growth diminished as soil nitrogen uptake increased.

Diurnal variation in N fixation activity in cv. Whero was measured at three hourly intervals over three 24 h periods, during bud formation, flowering and pod-filling. The optimum time for a one hour assay was between 1100 and1400 h NZST but this was not constant. Extrapolation from a single assay to a daily N fixation total could lead to 40% error. There was no distinct diurnal cycle at bud formation, but at flowering (during drought stress), N fixation increased during the night. In contrast, during pod-filling, N fixation decreased during the night. This was attributed to insufficient carbohydrate for nodule function. Mean N fixation increased from the first to last cycle.

Peas enhanced soil fertility when compared with cereals in all trials. Although water stress frequently limits pea yield in Canterbury, when water is adequate, available soil nitrogen will reduce the reliance of peas on N fixation.