Enhanced Protease Inhibitor Expression in Plant Residues Slows Nitrogen Mineralization

Organic N mineralization by extracellular proteases affects inor-ganic N availability and loss. Soil Nmineralization is slowed by addition of purified protease inhibitors. We hypothesized that elevated concen-trations of protease inhibitors in plant residues would reduce soil and plant residue N mineralization. Isogenic controls and transgenic plants of Brassica (Brassica napus L.), Japonicum rice (Oryza sativa L.), and tobacco (Nicotiana tabaccum L.) showing enhanced wound-inducible protease inhibitor production were grown in a greenhouse, and leaves were mechanically wounded 3 d before shoot removal. Transgenic plants 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 in a loamy sand soil, inorganic N in leachate from transgenic plants was significantly lower than isogenic controls for the first 30 d when the residues remained on the soil surface and were higher at one or more dates thereafter.When residues weremixed with soil, differences were observed only for Brassica. Cumulative N mineralization in static incubations of residues mixedwith soil followed the order Brassica. tobacco. rice residues. In general, transgenic residues mineralized between 22 and 27 % less N than control plant residues in the first 30 d, but no differences in soil N mineralization were detected. Thus, protease inhibitor concentration of plant residues should be included with measures of total N concentration, C/N ratio, and lignin concentration to improve prediction and potentially management of short-term N mineralization from plant residues. MOST N PRESENT in soil is in organic forms, primarilyproteinaceous compounds. Soil acts as a sink for proteins from plants, animals, and microorganisms and also is an environment characterized by rapid protein hydrolysis under relatively wide ranges of soil temper-ature, water content, and pH (Hankin and Hill, 1978). Soil proteases originate from the same sources as pro-teins and are a mixture of heterogeneous, primarily ex-tracellular, enzymes with different molecular weights, structures, cofactor requirements, and substrate specific