Nutrient Balances for Australian Natural Resource Management Zones

Earlier work in this project reported N, P and K balances for Australia, but the N balances did not include a value for fixed N, which in Australia with mixed cropping systems could contribute significantly to the national N budget. A review was undertaken by Dr SK Lam to develop a simple approach that growers could use to estimate the amount of contributed by grain legume N fixation based on pulse crop yield.
Biological nitrogen (N2) fixation is an important source of N in cropping systems. Globally, the amount of N fixed by crop legume-rhizobia symbioses is estimated to be 20–22 million tonnes per year (Herridge et al. 2008). The percentage of N derived from the atmosphere (Ndfa) can be determined by non-isotopic (N balance, N difference, ureide and acetylene reduction) and isotopic (15N natural abundance, 15N isotope dilution and 15N2 gas) methods (Unkovich et al. 2008). Nonetheless these methods are more or less impractical or cost-ineffective for growers to estimate how much N is fixed by their crops. Simple relationships between aggregated data on legume shoot dry matter production and N2 fixation provide a pragmatic approach to estimating N2 fixation (Unkovich et al. 2010a), but the assessment of net N contribution of N2 fixation to the system’s N budget would also require the amount of N fixed in roots and nodules as well as that removed in grains.
Here we first present three methods adopted in the literature for estimating N2 fixation from shoot dry matter of legumes. Using estimated data on shoot N fixation, root N fixation and grain N removal, we then assess the net contribution of legume N under different harvest indices for the major crop legumes (chickpea, faba bean, field pea, lentil, narrow-leaf lupin and vetch) grown in Australia on a basis of per tonne of grain yield.

Estimation of N2 fixation by legume shoot dry matter

1. Consolidation of existing data for all crops
Crop legumes generally fixed 15–25 kg shoot N for every tonne of shoot dry matter (Herridge et al. 2008), or an average of 21 kg (Unkovich et al. 2010a).

2. Regression analysis for individual crop
Unkovich et al. (2010a) assembled published and unpublished data on legume N accumulation and N2 fixation from Australian field studies into a database. Linear regressions have been fitted between the aggregated datasets of legume shoot dry matter production and shoot N fixed for each crop (Table 1).

Table 1 Linear regression analysis and adjusted r2 for crop shoot dry matter (x, t/ha) and shoot N fixed (y, kg/ha)

Legume	Regression equation	r2	kg N fixed/tonne of shoot dry matter (when x = 1)
Chickpea	y = –1.05 + 10.7 x	0.50	9.7
Faba bean	y = –1.5 + 23 x	0.79	21.5
Field pea	y = –1.73 + 20.6 x	0.53	18.9
Narrow leaf lupin	y = 4.03 + 14.2 x	0.76	18.2

Source: Unkovich et al. (2010a)


3. Determination of %Ndfa for individual crop
The relationships between shoot dry matter and N2 fixation can also be estimated using data on shoot %N and %Ndfa for individual crop (Unkovich et al. 2010a) by the equation:
Shoot N fixed (kg/ha) = shoot dry matter (kg/ha) × shoot %N × %Ndfa
The average values for the shoot %N and %Ndfa of the crops are presented in Table 2.


Table 2 Shoot %N and %Ndfa for the estimation of shoot N fixation

Legume	Shoot %N	%Ndfa	kg N fixed/tonne of shoot dry matter
Chickpea	2.40	41	9.84
Faba bean	2.83	65	18.40
Field pea	2.40	66	15.84
Lentil	2.57	60	15.42
Narrow leaf lupin	2.49	75	18.68
Vetch	3.24	80	25.92

Source: Unkovich et al. (2010a)

Net N contribution by legumes to N budget
Net contribution of legume N = whole plant legume N fixed – grain N removed
= shoot N fixed + root N fixed – grain N removed
The estimation of shoot N fixed is presented above. Root N fixed can be calculated using a ‘root N factor’ for individual crops as listed in Table 3. To account for the amount of N removed in grains, we use data on grain %N reported by Patterson and Mackintosh (1994).

Table 3 Shoot N: root N, root factor, grain %N and removal for different crop legumes

Legume	Shoot N: root N	Root factor a	Grain %N	kg N removed in grain/tonne of grain yield
Chickpea	1.25	1.80	3.54	35.4
Faba bean	2.13	1.47	3.84	38.4
Field pea	2.10	1.48	3.83	38.3
Lentil	1.80	1.56	4.88	48.8
Narrow leaf lupin	3.78	1.26	5.15	51.5
Vetch	2.10	1.48	5.00	50.0

a The ‘root N factor’ is 1 + 1/(shoot N: root N)
Source: Patterson and Mackintosh (1994); Unkovich et al. (2010a)
Variation in N2 fixation of crops is closely related to dry matter production. To provide a more practical assessment on the net N contribution by legumes, we calculated net N change based on a range of harvest indices as described in Herridge et al. (2008) (Table 4).

Table 4 Net N contribution (kg/ha) per tonne of grain yield under a range of harvest indices calculated by the three methods (Method 1: consolidation of existing data; Method 2: regression analysis; Method 3: %Ndfa)

Crop		Net N contribution (kg/ha) per tonne of grain yield
	Method	Harvest index
		0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9
Chickpea	1	342.7	153.7	90.7	59.2	40.3	27.7	18.7	11.9	6.7
	2	155.4	59.1	27.0	10.9	1.3	–5.1	–9.7	–13.2	–15.8
	3	141.8	53.2	23.7	8.9	0.1	–5.8	–10.0	–13.2	–15.7
Faba bean	1	270.3	116.0	64.5	38.8	23.3	13.1	5.7	0.2	–4.1
	2	297.5	128.4	72.1	43.9	27.0	15.7	7.7	1.7	–3.0
	3	232.0	96.8	51.7	29.2	15.7	6.7	0.2	–4.6	–8.4
Field pea	1	272.6	117.2	65.4	39.5	23.9	13.6	6.2	0.6	–3.7
	2	264.1	111.6	60.8	35.4	20.2	10.0	2.7	–2.7	–6.9
	3	196.2	79.0	39.9	20.4	8.6	0.8	–4.8	–8.9	–12.2
Lentil	1	278.8	115.0	60.4	33.1	16.7	5.8	–2.0	–7.8	–12.4
	2	NA	NA	NA	NA	NA	NA	NA	NA	NA
	3	191.8	71.5	31.4	11.4	–0.7	–8.7	–14.4	–18.7	–22.1
Narrow leaf lupin	1	213.1	80.8	36.7	14.7	1.5	–7.4	–13.7	–18.4	–22.1
	2	132.5	43.1	13.2	–1.7	–10.6	–16.6	–20.8	–24.0	–26.5
	3	183.8	66.2	27.0	7.4	–4.4	–12.3	–17.9	–22.1	–25.3
Vetch	1	260.8	105.4	53.6	27.7	12.2	1.8	–5.6	–11.2	–15.5
	2	NA	NA	NA	NA	NA	NA	NA	NA	NA
	3	333.6	141.8	77.9	45.9	26.7	13.9	4.8	–2.0	–7.4

NA: regression equation not available
The higher the harvest index, the lower the net contribution of crop legumes to N budget (Table 4, Fig. 1). Crop legumes may add up to 300 kg N/ha per tonne of grain yield (when the harvest index is 0.1) or remove around 30 kg N/ha per tonne of grain yield (when the harvest index is 0.9). This suggests that excessively vegetative crops with low seed yield may have compensatory benefits in terms of N input to the cropping system.
Key message: The average harvest index of major crop legumes grown in Australia is between 0.3 and 0.4 (Unkovich et al. 2010b), which represents an input of 7–65 kg N/ha for every tonne of grain produced.


Figure 1 Net N contribution by major crop legumes grown in Australia under various harvest indices (average across the three methods listed in Table 4)


References
Herridge, D.F., Peoples, M.B., Boddey, R.M., 2008. Global inputs of biological nitrogen fixation in agricultural systems. Plant and Soil 311, 1-18.
Petterson, D.S., Mackintosh, J.B., 1994. The chemical composition and nutritive value of Australian grain legumes. Grain Research and Development Corporation, Canberra.
Unkovich, M., Herridge, D., Peoples, M., Cadisch, G., Boddey, B., Giller, K., Alves, B., Chalk, P., 2008. Measuring plant-associated nitrogen fixation in agricultural systems. Australian Centre for International Agricultural Research, Canberra, 258 pp.
Unkovich, M.J., Baldock, J., Peoples, M.B., 2010a. Prospects and problems of simple linear models for estimating symbiotic N2 fixation by crop and pasture legumes. Plant and Soil 329, 75-89.
Unkovich, M., Baldock, J., Forbes, M., 2010b. Variability in harvest index of grain crops and potential significance for carbon accounting: examples from Australian agriculture. Advances in Agronomy 105, 173-219.