Models of biological nitrogen fixation of legumes. A review

Abstract

Leguminous crops have the ability to fix nitrogen (N) biologically from the atmosphere. This can benefit not only the legumes themselves but also any intercropped or subsequent crops, thus reducing or removing the need to apply N fertilizers. Improved quantification of legume biological nitrogen fixation (BNF) will provide better guidance for farmers on managing N to optimise productivity and reduce harmful losses to the environment. There are many techniques available for the direct quantitative measurement of legume BNF in the field and in controlled environments. However, these are time-consuming and therefore expensive, and generate data relevant only to the time and place of measurement. Alternatively, legume BNF can be estimated by either empirical models or dynamic mechanistic simulation models. Comparatively, simulation by a dynamic model is preferable for quantifying legume BNF, because of its capability to simulate the response of N fixation to a wide range of environmental variables and legume growth status. Currently there is no published review of the approaches used to simulate, rather than measure, legume BNF. This review of peer-reviewed literature shows that most simulation models estimate the N fixation rate from a pre-defined potential N fixation rate, adjusted by the response functions of soil temperature, soil/plant water status, soil/plant N concentration, plant carbon (C) supply and crop growth stage. Here, we highlight and compare the methods used to estimate the potential N fixation rate, and the response functions to simulate legume BNF, in nine widely-cited models over the last 30 years. We then assess their relative strengths in simulating legume BNF with varying biotic and abiotic factors, and identify the discrepancies between experimental findings and simulations. After this comparison, we identify the areas where there is the potential to improve legume BNF simulation in the future. These include; (1) consideration of photosynthetic C supply, (2) refining the various effects of soil mineral N concentration, (3) characterization and incorporation of excess soil water stress and other factors into models, and (4) incorporation of the effects of grazing, coexistence and competition with intercrops and weeds into models to improve their practical relevance to sustainable agricultural systems. This review clarifies, for the first time, the current progress in legume BNF quantification in simulation models, and provides guidance for their further development, combining fundamental experimental and modelling work. nitrogen fixation / soil mineral nitrogen / legume / simulation / review Contents

Climate-smart cropping systems for temperate and tropical agriculture: mitigation, adaptation and trade-offs

“…Symbiotic N 2 fixation which takes place in the top soil layers is extremely susceptible to drought (Liu et al, 2011). High temperatures may affect dramatically anthesis and seed set contributing to flower abortion (Daryanto et al, 2015).…”

Nitrogen fixation in Australian dairy systems: review and prospect

“…Whitehead (1995) suggests that N 2 fixation ceases below a soil temperature of 98C, but other evidence indicates that it occurs over a wider range of temperatures (~2À408C), and is relatively insensitive to temperature over quite a wide range (15À308C) (Liu et al 2010). Provided that there is adequate water available, white clover can maintain a constant N 2 fixation rate over the 20-338 temperature range (Ryle et al 1989) and thus the summer temperatures experienced in the Australian dairy regions should not be prohibitive to N 2 fixation.…”

“…Such so called mechanistic or dynamic simulation models are usually only semi-mechanistic as they typically include some empirical approaches. Liu et al (2010) reviewed a large number of approaches to modelling N 2 fixation and the reader is referred to this thorough exposé of N 2 fixation modelling, the detail of which is outside the scope of the present review.…”

Improving Nitrogen Use Efficiency in Crops for Sustainable Agriculture

“…The most important N-fixing agents are the symbiotic associations between crop and forage/fodder legumes and bacteria of the genus Rhizobia [31,32]. There are accurate estimations of annual inputs of symbiotically fixed N by legume crops.…”