Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2005
Publication Date: 2/2/2006
Citation: Martens, D.A., Jaynes, D.B., Colvin, T.S., Kaspar, T.C., Karlen, D.L. 2006. Soil amino acid enrichment following soybean in an Iowa corn-soybean rotation. Soil Science Society of America Journal. 70:382-392. Interpretive Summary: Soil nitrogen (N) is composed of an organic fraction and an inorganic or mineral N fraction. The mineral fraction can account for up to 5% of the total soil N with the remaining 95%, an organic fraction composed of amino acids, amino sugars and unidentified N compounds. Understanding the amount of organic N that cycles to mineral N would provide scientists a way to predict the amount of N the soil could provide, potentially limiting the over application of N fertilizers. Ten Iowa soils from a privately managed 16 ha field were sampled each spring and fall from 1997 – 1999 and the surface 15 cm soil was prepared and analyzed for organic N composition. The amount of organic N following corn production decreased 255 kg N ha-1 while the soil organic N content increased following soybean production by 247 kg N ha-1. The spring total amino acid N content and certain microbial amino acids were significantly correlated with fall corn yields suggesting that understanding the soil organic N content and production rotation may provide an important tool the producer may use to reduce N fertilizer additions. The benefits may include reduced N costs to producers and reduced N nutrient pollution in the environment while not impacting economic yields.
Technical Abstract: Understanding soil amino acid enrichment may help explain why rates of N fertilization required to attain maximum corn (Zea mays L.) yields are usually lower for corn following soybean (Glycine max L.) than for corn following corn. Our objectives were to quantify the amino acid (AA) pool within a 16 ha field and to correlate those results with corn yield. Spring and fall measurements of organic N content (0 – 15 cm soil) as AAs and amino sugars (ASs) were made using samples from 10 soil map units identified in a typical central Iowa field and collected between 1996 and 1999. The chemical extraction method determined an average 87% of the total N content (n = 10 soils) as AA or AS, but gave reduced AA-N recovery from depression soils that experienced periods of water ponding. The total AA concentrations measured in May were positively correlated (r2 = 0.84, ' < 0.01) with corn yield during a dry year (1997), seven out of 10 soils provided near maximum yields. A wetter 1999 boosted overall corn yields 6.6%, six out of eight soils provided near maximum yields, but resulted in a poorer relationship between May AA concentrations and corn yield. Microbial compounds measured for the May 1997 sampling date as glucosamine, galactosamine and ornithine were also positively correlated with corn yield (r2 = 0.84, ' < 0.01; r2 = 0.94, ' < 0.001; r2 = 0.93, ' < 0.001, respectively). The AA concentration decreased during corn production from May to September 1997 an average of 255 kg N ha-1 (n = 9 soils), but increased following soybean growth by 247 kg N ha-1 (n = 9 soils) in 1998. The chemical extraction methodology identified soils that may not require the amount of N fertilizer currently being applied, thus decreasing the potential for N loss to surface and ground water resources without decreasing opportunities to achieve optimum yield.