Symbiotically-fixed and soil-derived nitrogen in legumes grown in pots in soils with different amounts of available nitrate

Abstract

Symbiotically-fixed and soil-derived nitrogen have been measured in pot experiments for Medicago littoralis (medic), grown alone or with Lolium multiflorum (ryegrass) and for Pisum sativum (field pea). The four soils used contained organic matter labelled with 15N, and differed in their capacities to release available N. During a 4–12-week incubation each released inorganic N (NO − 3) of approximately constant 15N atom% enrichment. In one soil, the mineralized N was supplemented by 15NO − 3 of similar 15N atom% enrichment. Incubation of soils under intermittently moist and dry conditions increased N mineralization rates, but did not affect the 15N atom% enrichments of the released N. For all soils and treatments the amounts of soil-derived N taken up by plants equalled the amounts of available N in moist incubated, unplanted soils. The enrichment of ryegrass root N grown alone or with medic was slightly but consistently less than that of top N. Nitrogen of the legume nodules and pods (peas) was least enriched, followed by N of legume stems, leaves and roots; the 15N atom% enrichments of root N were 4–5 times those of nodule N. Peas generally outyielded and fixed more N than medic grown alone. Medic grown with ryegrass yielded least and fixed least N. For unamended soils, yields of legume dry matter and amounts of N fixed were greatest in Roseworthy or Avon sandy loam soils and least in Northfield clay loam. Addition of 15NO − 3 to Avon soil decreased N fixed by peas and by medic grown alone or with ryegrass. For this soil, soil-derived N of plant tops exceeded fixed N of roots, even for unamended soil where fixation by legumes was relatively high. Thus, complete removal of plant tops would have produced a net loss of N from the soil, the net loss increasing with increasing amounts of 15NO − 3 added.