Extraction and Activity Characterization of Urease From Various Crop Residues.

Nitrogen (N) fertilizer is increasingly surface applied to crops such as wheat and corn that are no-tilled planted into crop residues. The objectives of this study are to 1) extract urease from soybean and corn residues after grain harvest, and 2) characterize urease activity from different crop residues. When urea-based N sources are surface applied, urea is hydrolyzed to ammonia and N loss can occur through ammonia volatilization. The enzyme urease is responsible for urea hydrolysis in soils and plants. Previous research has shown that ammonia volatilization is greater when N is applied to crop residues compared to bare soil. However, little is known about the difference in urease activity among different residue types. If there are differences in urease activities in different residues than N management practices may need to be adjusted depending on the crop rotation.  Crop residues were taken from fields immediately after harvest, air dried and ground to pass a 1-mm sieve. A 25 mM sodium phosphate, 50 mM sodium chloride, 5 mM EDTA, and 0.1% 2-mercaptoethanol solution was used as the extracting solution. All extraction steps were carried out at 0-4 ^oC, unless stated otherwise. The crude urease extract was used in incubation studies to look at activity of the enzyme as well as subjected to analysis using fast flow liquid chromatography (FPLC) and SDS-PAGE to affirm urease was present.  Urease assays indicated that the enzyme is present in the extracting solution and that corn residue may have a lower activity than soybean residue. Within 4 hours of adding urea to soybean residue crude urease extract, 97.3 % of the urea-N had been converted to ammonia whereas only 4.5% of the urea-N was converted with corn residue crude urease extract.  FPLC elution peaks match the elution peaks from commercially available Jack bean urease (Canavalia ensiformis) (Sigma Aldrich, Type IX powder, U4002-100KU).  SDS-PAGE gels show that only the 0-40% ammonia sulfate fractionation was concentrated enough for the gel to detect, however this also matched jack bean urease for both corn and soybean residues. These results indicate that an active urease molecule can be extracted and identified using the above extraction solution. Corn residue had much lower activity in the incubation than the soybean and this indicates that soybean residue may need to be treated differently in a complete nutrient management plan.  Further research is needed to ascertain the differences among crop residues and even within individual species to better understand ammonia volatilization from no-till production systems.