Net nitrogen balances for cool-season grain legume crops and contributions to wheat nitrogen uptake: a review

Abstract

The removal of nitrogen (N) in grain cereal and canola crops in Australia exceeds 0.3 million t N/year and is increasing with improvements in average crop yields. Although N fertiliser applications to cereals are also rising, N2-fixing legumes still play a pivotal role through inputs of biologically fixed N in crop and pasture systems. This review collates Australian data on the effects of grain legume N2 fixation, the net N balance of legume cropping, summarises trends in the soil N balance in grain legume–cereal rotations, and evaluates the direct contribution of grain legume stubble and root N to wheat production in southern Australia. The net effect of grain legume N2 fixation on the soil N balance, i.e. the difference between fixed N and N harvested in legume grain (Nadd) ranges widely, viz. lupin –29–247 kg N/ha (mean 80), pea –46–181 kg N/ha (mean 40), chickpea –67–102 kg N/ha (mean 6), and faba bean 8–271 kg N/ha (mean 113). Nadd is found to be related to the amount (Nfix) and proportion (Pfix) of crop N derived from N2 fixation, but not to legume grain yield (GY). When Nfix exceeded 30 (lupin), 39 (pea) and 49 (chickpea) kg N/ha the N balance was frequently positive, averaging 0.60 kg N/kg of N fixed. Since Nfix increased with shoot dry matter (SDM) (21 kg N fixed/t SDM; pea and lupin) and Pfix (pea, lupin and chickpea), increases in SDM and Pfix usually increased the legume’s effect on soil N balance. Additive effects of SDM, Pfix and GY explained most (R2 = 0.87) of the variation in Nadd. Using crop-specific models based on these parameters the average effects of grain legumes on soil N balance across Australia were estimated to be 88 (lupin), 44 (pea) and 18 (chickpea) kg N/ha. Values of Nadd for the combined legumes were 47 kg N/ha in south-eastern Australia and 90 kg N/ha in south-western Australia. The average net N input from lupin crops was estimated to increase from 61 to 79 kg N/ha as annual rainfall rose from 445 to 627 mm across 3 shires in the south-east. The comparative average input from pea was 37 to 47 kg N/ha with least input in the higher rainfall shires. When the effects of legumes on soil N balance in south-eastern Australia were compared with average amounts of N removed in wheat grain, pea–wheat (1:1) sequences were considered less sustainable for N than lupin–wheat (1:1) sequences, while in south-western Australia the latter were considered sustainable. Nitrogen mineralised from lupin residues was estimated to contribute 40% of the N in the average grain yield of a following wheat crop, and that from pea residues, 15–30%; respectively, about 25 and 15 kg N/ha. Therefore, it was concluded that the majority of wheat N must be obtained from pre-existing soil sources. As the amounts above represented only 25–35% of the total N added to soil by grain legumes, the residual amount of N in legume residues is likely to be important in sustaining those pre-existing soil sources of N.

Growing vetches (Vicia villosa Roth) in irrigated cotton systems: inputs of fixed N, N fertiliser savings and cotton productivity

“…Lablab added more N than any legume, but this summer green manure crop required irrigation, and growers normally prefer to use their allocation of irrigation water for cotton; lablab is rarely grown commercially for that reason. While the amounts of N 2 fixed per tonne of shoot DM by vetch and faba beans determined in these experiments (27-30 kg shoot N fixed DM t )1 ) were similar to estimates for pasture and crop legumes grown elsewhere within this cropping zone of Australia (28 kg shoot N fixed DM t )1 , Hossain et al, 1995), they were higher than the average values generally reported for most annual forage legumes (20-25 kg shoot N fixed DM t )1 , Peoples and Baldock, 2001), or pulse crops grown in Australia (13-22 kg shoot N fixed t )1 DM, Rochester et al, 1998;Evans et al, 2001;, or in the farming systems of Europe and north America (e.g. 18-23 kg shoot N fixed DM t )1 , Jensen, 1987;Sparrow et al, 1995).…”

“…For example, while grain legume crops such as faba bean (Vicia faba L.) have been demonstrated to fix substantial quantities of N in Australia (e.g. Evans et al, 2001;Rochester et al, 1998;Unkovich et al, 1997) and elsewhere in the world (e.g. Jensen, 1987;Kilian et al, 2001;Sparrow et al, 1995), much of the fixed N is often removed in the harvested seed and the residual legume N may have little impact on following crops.…”

Nitrogen dynamics following grain legumes and subsequent catch crops and the effects on succeeding cereal crops

“…The grain legumes were expected to show positive partial N balances due to the fixation of N 2 (HauggaardNielsen et al 2003;Jensen 1996b). Positive differences between total N 2 fixed and grain N yield of 45-113, 65-97, and 27-47 kg N ha -1 have been reported for faba bean, lupin and pea, respectively (Evans et al 2001). Faba bean accumulated more N from the atmosphere and left more residual N in the soil than the other two grain legumes, possibly increasing the risk for N leaching (Thomsen et al 2001).…”

The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems

“…However, the net inputs of fixed N into soils depend upon the amounts of N z fixed relative to the amounts of N removed in the protein-rich legume products which can represent between 45-75% of the N in the above-ground biomass of crop legumes (van Kessel and Hartley, 2000;Walley et aI., 2007;Salvagiotti et aI., 2008). With the N content of legume grain representing between 30-40 kg N in every tonne (t) for most legume pulses and 60-65 kg N per t for soybean (Salvagiotti et al, 2008), the 60 million t of pulses and 250 million t of legume oilseeds (soybean and groundnut) harvested globally each year (Table 1; Herridge et aI., 2008), results in an annual removal of around 17 million t of N. One might expect some variation in the net returns of fixed N to soil as a result of local farming practices, the legume line used, or regional soil and climatic effects on the ability of different legume species to (i) grow and fix N z or (ii) partition N into grain (Evans et al, 2001;Maskey et aI., 2001;Peoples et al, 2001;Ojiem et al, 2007).…”