Organic agriculture is a nitrogen limited farming system that relies on biologically fixed nitrogen to support production. The source of this biological nitrogen fixation (BNF) comes primarily from legumes and the symbiotic relationship formed with rhizobium in the root nodules. Maximising this process within organic cropping systems is required to sustain crop yields and can bring productivity gains but nitrogen fixing potential and management options to help reach this are not as well understood as they might. Providing  better estimates of the BNF from different legumes can help design agroecological cropping systems that better rely on the service of BNF and can help implement strategies that enhance nitrogen availability.

Here is a summary of one review paper published this year from French researchers conducting met-analysis on 39 studies across 76 (mostly European) locations. The objective of the paper was to provide an estimate of BNF for organically managed leguminous crops according to different cropping practices. Results show that significant differences exist between legume type, with forage legumes capable of fixing more nitrogen that pulses,  and also in crop duration with increased BNF from crops species with longer growing periods. There was a strong link between above-ground biomass and % nitrogen derived from the atmosphere (%Ndfa) as well as a large variability in performance  within crop species and cropping practice within different geographical locations.

The study found found an overall 87 kgN. per ha per  Ndfa value when pooling all legume crop species and legume cropping practices.  Forage crops species like clover and lucerne exhibited a higher %Ndfa compared to pulses (131 kgN/ha/year compared to 76 kgN/ha/ year). Clover fixed around 96 kgN/ha/year. Lucerne, clover and faba bean were the best performing species.

Pulses and forage legumes in mixtures with non-leguminous crops exhibited lower levels of N fixation possibly due to the competition for other resources leading to lower biomass of the legume. In terms of cropping practice, cover crops had lower N fixation (54kgN) than temporary leys (90 kgN) and perennial legumes (174 kgN). Intercrops (adjusted to monocrop plant density equivalent) did not show increased N fixation compared to monocrops.

Site effect accounted for half the variability in N fixation so an appreciation of farm pedoclimatic conditions must be taken into account for the N fixation estimates.

Around 69% of crop above ground biomass N was found to be derived from the atmosphere with forage legumes having 78% to pulses 67%. Pulses in mixtures and a higher proportion of N derived from the atmosphere possibly due to increased competition for resources from the non-legume. Perennial crops have higher Ndfa but lower %Ndfa because they have higher levels of soil N availability during the year, as opposed to cover crops that have lower Ndfa but higher %Ndfa as there is less available soil N. There was found to be a high variation in %Ndfa between species with faba bean having the highest value of 81%. Fixation rates tend to  be higher for shorter crop duration and faster growth rate than slower growing crops that result in lower fixation rates.

Net soil N budgets for pulses were found to be negative in a third of cases meaning more N is exported in the harvested pulse than left in the soil, with grain harvest offsetting N input from fixation leading to soil N depletion. In nearly half of cases, net N budgets were found to be in excess of 10kgN/ha. There was a negative correlation between N content of the grain and net soil N budget. It should be noted that negative N budgets were generally linked to soils higher in mineral N.

A comparison of Organic and non-organic revealed a surprising result. Ndfa in organic farming was often lower than conventional systems, possibly due to lower biomass production and increased plant stress affecting nodulation. Even more surprising despite this lower biomass, %Ndfa was generally equivalent to non-organic,  showing that efficiency of N fixation in organic systems may be lower than non-organic systems, despite lower levels of available soil mineral N. This may be the most relevant point of all, with the implication that organic farming needs to improve forage legume management in order to increase Ndfa, with crop nutrition likely to play important role in this,  given P, K and S all contribute to N-fixation levels in legumes. Plant growth is the main driver of Ndfa and anything that can help increase the biomass of legumes can improve N fixation, particularly appropriate crop nutrition and where possible reductions in other biotic and abiotic stresses that can be expected to negatively affect plant growth.

Read the full review paper here:  BNF Review 2023.pdf