Greenhouse and laboratory investigations using four soils were conducted to determine the influence of seven herbicides on symbiotic nitrogen (N2) fixation by soybeans [Glycine max (L.) Merr. ‘Davis’]. Soils were treated with recommended rates of herbicides immediately after planting. After 8 to 10 weeks the nodulated root systems were assayed for nitrogenase activity using the acetylene (C2H2) to ethylene (C2H4) reduction technique, and total numbers and dry weights of nodules were determined. Metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] (0.3 kg/ha) decreased nodule dry weight and C2H4 production per plant per hour in a Convent sandy loam. Alachlor [2-chloro-2’,6’-diethyl-N-(methoxymethyl) acetanilide] (2.2 kg/ha) and prometryn [2,4-bis(isopropylamino)-6-(methylthio)-s-triazine] (0.6 kg/ha) also decreased C2H4 production per plant per hour in a Convent sandy loam, while metribuzin (0.6 kg/ha) increased C2H4 production per plant per hour and per milligram of nodule dry weight in a Mhoon silt loam. Prometryn (1.7 kg/ha) increased production of C2H4 per plant per hour in a Moreland silt loam. None of the herbicides affected C2H4 production, nodule number, or nodule dry weight in a Commerce silt loam.

EPTC (S-ethyl dipropylthiocarbamate), benefin (N-butyl-N-ethyl-α α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine], butralin[4-(1,1-dimethylethyl)-N-(1-methylpropyl)2,6-dinitrobenzenamine], diclofopmethyl {methyl 2-[4-(2,4-dichlorophenoxy)phenoxy]-propanoate} and 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid] were evaluated in the greenhouse for effects on growth, nodulation, and NH3 production of alfalfa (Medicago sativa L.) and medium red clover (Trifolium pratense L.). Some reduction in nodulation occurred from some rates of the herbicides, but reduction in nodulation was usually associated with reduced plant growth caused by herbicidal injury.

Field and greenhouse studies conducted in North Carolina in 1981 and 1982 evaluated the effect of the dimethylamine salt of 2,4-DB [4-(2,4-dichlorophenoxy) butyric acid] on soybeans. In the field, 2,4-DB at and above 0.07 kg ae/ha reduced soybean shoot mass, leaf nitrogen levels, and nodule activity. Soybean seed yields were reduced by 2,4-DB at and above 0.07 kg/ha. In greenhouse studies, total nodule activity was reduced by 0.07 and 0.14 kg/ha of 2,4-DB. Soybeans treated with 2,4-DB at initiation of flowering (R1 stage) or when pods were 1.9 cm long (R4 stage) yielded progeny with visible injury symptoms. Seedling emergence was also reduced in progeny of soybeans.

Nitrogenase activity in the surface soil and litter layers from a tropical lowland rain forest and an adjacent fernland in Sri Lanka was measured by the acetylene reduction method. The total amount of acetylene reduced, a measure of biological nitrogen fixation, was greater in the fernland (38.4 µmol m−2 h−1) than in the forest (15.3–29.2 µmol m−2 h−1). In both ecosystems the nitrogenase activity was greater (59–85%) in the soil than in the litter layer. The rates of acetylene reduction in soil and litter showed an inverse relationship with the decay constants of mixed litter and a positive relationship was found between the rate of acetylene reduction and the C:extractable nitrogen ratio of the soil. The relatively greater rate of nitro genase activity coupled with the slower rate of litter decomposition in the fernland indicates that accumulation of nitrogen may result in the soil and litter layers of the deforested areas during their aggrading phases.

The treatment of wheat roots with 2,4-dichlorophenoxyacetic acid (2,4-D) led to the formation of tumour-like structures, para-nodules, which were readily colonized by cyanobacteria. The amount of cyanobacteria found on 2,4-D treated roots was 3.6 times higher than that found on untreated roots. Cyanobacteria penetrated the para- nodules by migrating in between loosely arranged cells that covered their surfaces or by penetrating the space at the junction of root and para-nodule. In plants treated with 2,4-D and co-cultivated with cyanobacteria in medium without combined nitrogen, the rate of acetylene reduction was three times that seen in untreated but colonized roots. Addition of 2,4-D itself did not change the rate of acetylene reduction in free-living culture of Nostoc sp. strain 2S9B. In plants treated with 2,4-D and co-cultivated with Nostoc sp. strain 2S9B in medium without combined nitrogen the nitrogen content of roots but not shoots was significantly increased.

Symploca PCC 8002 Kützing is a filamentous cyanobacterium that lacks the specialized cells, known as heterocysts, that protect nitrogenase from O2 in most aerobic N2-fixing cyanobacteria. Nevertheless, Symploca is able to carry out N2 fixation in the light under aerobic conditions. When cultures were grown under light/dark cycles, nitrogenase activity commenced and increased in the light phase and declined towards zero in the dark. Immunolocalization of dinitrogenase reductase in sectioned Symploca trichomes showed that the enzyme was present only in 9% of the cells. These cells lacked any obvious mechanical protection against atmospheric O2 and their ultrastructural characteristics were similar to those of cells that did not contain any dinitrogenase reductase. The nitrogenase-containing cells possessed carboxysomes that were rich in ribulose-1,5-bisphosphate carboxylase/oxygenase and phycoerythrin, a light harvesting pigment of PS II. This indicates that these cells had a capacity for both N2 fixation and photosynthesis. The significance of the localization pattern for dinitrogenase reductase is discussed in the context of N2 fixation in Symploca PCC 8002.