Grain legumes in crop rotations under low and variable rainfall: are observed short-term N benefits sustainable?
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REVIEW ARTICLE
Grain legumes in crop rotations under low and variable rainfall: are observed short-term N benefits sustainable? Christopher J. Smith & Phillip M. Chalk
Received: 30 December 2019 / Accepted: 22 May 2020 # Springer Nature Switzerland AG 2020
Abstract Increased grain yields are frequently obtained when a cereal follows a grain legume in sequence compared with a cereal-cereal rotation. The biotic (disease break) and abiotic (N supply) components of the observed benefits are identified and methods for differentiating the components are discussed. Annual measurements of the N balance of grain legume-cereal rotations are extremely variable when measured over short time periods and are therefore not useful as indicators of cropping system N sustainability. While measurement of long-term changes in total soil N is a valid index of N sustainability, this approach is impractical. We suggest an alternative avenue for the assessment of N sustainability by using simulation modelling, after validation, which takes climatic and biophysical parameters into account.
Keywords APSIM . Crop rotations . Grain legumes . N benefit . 15N . Sustainability
Responsible Editor: Euan K. James. C. J. Smith (*) : P. M. Chalk CSIRO Agriculture, GPO Box 1700, 2601 Canberra, ACT, Australia e-mail: [email protected]
Introduction The annual benefit of inserting a legume crop into a cerealbased crop sequence has been well documented (Chalk 1998; Evans et al. 2001). There are several ways in which a benefit can be expressed. The most common agronomic parameter is the cereal grain yield differential between the legume-cereal and cereal-cereal crop sequences at zero rate of N fertilizer. Several questions arise, however, as to the source of the benefit and whether grain legume-cereal rotations are sustainable in the long-term (Schwenke et al. 1998). One or several annual measurements of a yield or N benefit may not necessarily translate into longterm N sustainability of the rotation due to the site, season and legume-cereal specificity of the yearly measurements. The objectives of this mini-review are (i) to provide a brief overview of the various approaches for identifying and quantifying the sources of the yield benefit in legume-cereal rotations in Eastern Australia (ii) to identify potential indicators of N sustainability and (iii) to illustrate an alternative approach to evaluating N sustainability. We refer initially to cereals in the rotation, but other non-legume rotation crops could include oilseeds (e.g. canola) or fibre crops (e.g. cotton).
Annual yield benefits P. M. Chalk e-mail: [email protected]
Components
P. M. Chalk Faculty of Veterinary and Agricultural Sciences, University of Melbourne, 3010 Parkville, VIC, Australia
The most obvious yet complex benefit derives from the N added to the cropping system by the legume through
Plant Soil
biological N2 fixation (BNF) (Chalk 1998; Fillery 2001). However, there is considerable uncertainty about the net N benefit of the legume due to the large amount of N
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