Water use efficiency in perennial forage species: Interactions between nitrogen nutrition and water deficit
Introduction
Water and N supply are two major limitations in crop production (Sinclair and Rufty, 2012). Recent reviews on the interactions between water and nitrogen (N) highlight the reduction in water use efficiency caused by N deficit and the reciprocal impact of water deficit on the N economy of crops and pastures (Sadras et al., 2016; Sinclair and Rufty, 2012; Gonzales-Dugo et al., 2010). Crop responses to the combination of water and N supply is thus important for evaluating the interactions among crop management, species, cultivar and environment.
Many studies analysed water use efficiency (WUE) as the relationship between biomass or grain yield and evapotranspiration (ET) (Dagdelen et al., 2006; Fischer, 1979; Garafolo and Rinaldi, 2013; Kresovic et al., 2016; Suyker and Verna, 2009; Tolk et al., 1998; Yimam et al., 2015). Separating the components of ET, soil evaporation (E) and crop transpiration (T) (Cooper et al., 1987; de Wit, 1958):highlights that biomass per unit ET (WUE) increases with transpiration efficiency (W/T) and smaller E/T fraction. Further, TE is related to CO2 assimilation, transpiration, stomatal control of gas exchange, and plant nutritional status (Jones, 2004; Monneveux et al., 2006; Schulze and Hall, 1982), thus providing a solid physiological basis to investigate effects of N on WUE. Methods to separate E and T include statistics (Hanks et al., 1969), modelling (Fandino et al., 2015; Sanchez et al., 2015; Sutanto et al., 2012), microlysimeters (Villalobos and Fereres, 1990), and isotope discrimination (Wang et al., 2015; Yidana et al., 2016).
Nitrogen deficit reduces both (i) the crop leaf area index (LAI, Bélanger et al., 1992a), hence increasing E/T (Ritchie, 1972); and (ii) leaf photosynthesis and radiation use efficiency (RUE, biomass per unit of intercepted radiation) (Bélanger et al., 1992a; Gastal and Bélanger, 1993; Sadras et al., 2016; Sinclair and Muchow, 1999). On the other hand, water deficit can decrease N availability in legumes by impairing N2 fixation (Durand et al., 1987), and in both legumes and grasses by decreasing availability of mineral N in soil, as mineralisation and N transport from bulk soil to rhizosphere are constrained in dry soil (Hungria and Vargas, 2000; Marino et al., 2007; Serraj et al., 1999). The ratio between N uptake and ET or T has been used to capture N-water interactions, and their effect on water use efficiency (French and Schultz, 1984a; Sadras, 2004; Sadras and Rodriguez, 2010; Sadras et al., 2016).
This paper focuses on perennial forage species, and the objectives are to examine the following hypothesis:
- 1.
Does TE vary between seasonal growth cycles, in relation to seasonal variation in growth potential (water and N non-limiting) and climatic conditions?
- 2.
How do crop N status and water availability affect TE? As a consequence, how to use crop N status as a benchmark for comparisons of TE among different crops or growing conditions?
- 3.
Is the ratio between crop N uptake and transpiration useful for analysing interactions between water and N affecting crop growth and transpiration efficiency?
- 4.
Are the water-N relations different according to the main source of N, namely N2-fixation in legumes or soil mineral N in grasses?
- 5.
To test these hypotheses, we compared tall fescue (Festuca arundinacea Schreb.) and alfalfa (Medicago sativa L.) for which production is directly linked to shoot biomass.
Section snippets
Conceptual framework
We used dynamic accumulation variables recorded during the regrowth of alfalfa and tall fescue: (i) shoot biomass Wt (t ha−1), (ii) N uptake Nt (kg ha−1) and (iii) evapotranspiration ETt (mm), where the subscript “t” is time (day) since beginning of regrowth (date of cutting). Data were sourced from previous publications (Section 3).
Data set
We compiled a database of shoot biomass, N uptake, and evapotranspiration for tall fescue and alfalfa recorded at approximately weekly intervals during several regrowth periods following cutting of stands (Table 1). Crops were grown at INRA Lusignan (46°24′55 N, 0°07′09 E) between 1979 and 1985. The main features as well as the data sources are described in Table 1. For details of the method used for each species, year and treatment see Durand et al. (1989) and Lemaire et al. (1989) for
Dynamics of biomass accumulation and water use
Fig. 3 shows the dynamics of shoot biomass accumulation against cumulative evapotranspiration during each regrowth period for alfalfa and tall fescue, and Table 2 shows the parameters of fitted regressions. For all treatments the quadratic coefficient of the quadratic regression Wt vs ETt was not different from 0 at P > 0.05, supporting the assumption of a constant TE during each regrowth period. For three treatments (dry alfalfa, 2nd cut in 1982; for tall fescue in 1979, irrigated +N, and dry
Accuracy and limits of the method for separating soil evaporation and crop transpiration
The use of a linear regression between accumulation of biomass and evapotranspiration allows for an approximated separation of soil evaporation and transpiration in stands of perennial forages. Two conditions are necessary for this approximation: (i) the Wt-ETt regression must be restricted to Wt > 1 t ha−1 that corresponds to LAI > 2 to meet the assumption of minor soil evaporation; and (ii) TE during the period is constant. The first assumption is robust, but constancy of TE during a period
Conclusion
The simplified method of separating soil evaporation and transpiration by plotting the dynamic of crop biomass accumulation Wt vs the dynamic of water consumption ETt can be useful for analyzing regrowth of forages after cutting. The assumption of constant transpiration efficiency is the main required condition for the accuracy of this approach.
Water shortage led to N deficiency and a decline in transpiration efficiency strictly proportional to the N deficiency induced by drought. As a result,
Acknowledgments
The authors are grateful to Brazilian National Council for Scientific and Technological Development (CNPq) for providing financial assistance (PDE – Project 233600/2014-0). The Grains Research and Development Corporation (grant DAS-00166) supports VOS.
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