Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2005
Publication Date: 5/1/2006
Citation: Kumar, K., Rosen, C.J., Russelle, M.P. 2006. Enhanced protease inhibitor expression in plant residues slows nitrogen mineralization. Agronomy Journal. 98:514-521. Interpretive Summary: Nitrogen is the most limiting nutrient for most agricultural production in the world. Plant-available nitrogen comes from many sources, including soil organic matter, animal manures, composts, commercial fertilizer, and legume crops. Nitrogen availability is regulated in part by soil organisms that tie up and release nitrogen during the year. When nitrogen is converted to nitrate by these organisms, it is readily available to plants but also can be lost from the field and contaminate water or contribute to global warming. Because of these complexities, nitrogen management continues to be one of the critical functions farmers perform. Until now there have been few dependable ways to reduce the conversion to nitrate of organic forms of nitrogen in soil and plant residues. The first steps in this process are regulated by enzymes called proteases, which cut large proteins into smaller pieces that soil organisms can consume. In earlier research, we found that purified protease inhibitors were highly effective in slowing this process of nitrogen mineralization. In this report, we show that natural protease inhibitors in plants also can be effective. We found that we could predict nitrogen release from plant residues better when the amount of protease inhibitors was known than without this information. These results provide soil scientists, plant breeders, and agronomists with a new tool that could be developed to help prevent nitrate losses from farms.
Technical Abstract: Mineralization of organic N sources by extracellular proteases affects both the availability of inorganic N to plants and losses of N to the environment. In our earlier research, soil N mineralization was slowed by application of purified protease inhibitors. We hypothesized that elevated concentrations of protease inhibitors in plant residues also would reduce soil and plant residue N mineralization. Transgenic plants of Brassicacae (Brassica napus L.), Japonicum rice (Oryza sativa L.), and tobacco (Nicotiana attenuata Torr. Ex Watts) showing enhanced wound-inducible protease inhibitor production and their isogenic controls were grown in a greenhouse and leaves were mechanically wounded 3 d before shoot removal. One-half of the plants received 15N-labeled ammonium sulfate during growth. Transgenic and their isogenic controls did not differ in N concentration, C/N ratio, or lignin concentration in shoot residues, but protease inhibitor concentration was 1.5 to 2.3 times greater in the transgenic lines. In laboratory incubations of non-15N-labeled shoot residues with a loamy sand soil, inorganic N concentrations in leachate were significantly smaller from transgenic than isogenic controls for the first 30 d when the residues remained on the soil surface, then were larger at one or more dates thereafter. When residues were mixed with soil, differences were observed only for Brassica. The time course of N mineralization from static incubations of 15N-labeled residues mixed with soil were similar to those obtained from leaching experiments, and cumulative N mineralization followed the order Brassicacae > tobacco > rice residues. In general, transgenic residues mineralized between 22 to 27% less N than their isogenic control plant residues in the first 30 d. In contrast, however, no differences in soil N mineralization were detected between transgenic and isogenic residues. We conclude that protease inhibitors show potential for regulating short-term mineralization of N and recommend that protease inhibitor concentration in plant residues be included with measures of total N concentration, C/N ratio, and lignin concentration to improve prediction of short-term N mineralization from plant residues.