Enhanced protease inhibitor expression in plant residues slows nitrogen mineralization

Organic N mineralization by extracellular proteases affects inorganic N availability and loss. Soil N mineralization is slowed by addition of purified protease inhibitors. We hypothesized that elevated concentrations 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 saliva 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 were mixed with soil, differences were observed only for Brassica. Cumulative N mineralization in static incubations of residues mixed with 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.