Abstract
The decomposition of alfalfa (Medicago sativa L.) residues can provide significant amounts of N to subsequent crops, but most of the data on this subject has been obtained from 1-2 year old alfalfa stands. The objective of this study was to determine the biomass of alfalfa taproots and their N content in irrigated alfalfa stands that are more than 2 years old. Twenty-two commercial irrigated alfalfa fields were evaluated in the Ebro Valley (Northeast Spain) from 2006 to 2010. The taproot biomass in the arable layer (0 to 30 cm depth) ranged from 1.8 to 10.1 Mg ha-1 and averaged 4.8 Mg ha-1. In contrast, the N concentration in alfalfa taproots was constant among fields and averaged 24.6 g N kg-1. The total amount of N contained in alfalfa taproots (0-30 cm depth) ranged from 47 to 96 kg N ha-1 in 55% of the fields, ranged from 97 to 200 kg N ha-1 in 22% of the fields, and exceeded 200 kg N ha-1 in 23% of the fields. The N content of the irrigated alfalfa taproots studied here is in the upper range previously reported in other areas, mainly with younger alfalfa stands. Based on the current finding, a classification of the quality of irrigated alfalfa stands is proposed to improve the estimates of the residual-N effects of alfalfa on subsequent crops.
Highlights
The decomposition of alfalfa (Medicago sativa L.) residues can provide significant amounts of N to subsequent crops, but most of the data on this subject has been obtained from 1-2 year old alfalfa stands
Alfalfa crops are usually grown for periods of at least 3 years, receive 5 to 7 cuts per year, and yield from 11 to 23 Mg ha–1 of dry matter per year, depending on such factors as cultivar, stand age and the method of irrigation, etc. (Delgado et al, 2005; Lloveras et al, 2008)
Most of the existing information was obtained from one- or two-year old alfalfa crops, but the N contributions of older alfalfa stands could be higher than those previously reported in the literature (Kelner et al, 1997)
Summary
The decomposition of alfalfa (Medicago sativa L.) residues can provide significant amounts of N to subsequent crops, but most of the data on this subject has been obtained from 1-2 year old alfalfa stands. Previous works conducted in different areas of the world reported that the N content in alfalfa roots ranged from 21 to 259 kg N ha–1 (Heichel et al, 1984; Pettersson et al, 1986; Bruuslema & Christie, 1987; Kelner et al, 1997; Justes et al, 2001; Hakl et al, 2007).
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