| Tillage and residue management practices may lead to significant biological, chemical, and biochemical changes and a large input of cellulose into soils. Cellulose is degraded by cellulase activity, producing glucose (reducing sugar). Among the five colorimetric methods (anthrone-sulfuric acid, phenol-sulfuric acid, dinitrosalicylic acid, Somogyi-Nelson, and Prussian blue methods) evaluated, the Somogyi-Nelson method was the most suitable for determination of reducing sugars in soils. The method developed for assay of cellulase activity in soils involves determination of the reducing sugars produced when a soil sample is incubated with acetate buffer (50 mM, pH 5.5), carboxymethyl cellulose (CMC), and toluene at 30$spcirc$C for 24 h. The optimal pH of cellulase activity was 5.5. Reducing sugars produced increased with incubation time up to 7 days and with the amount of soil up to 7 g. The cellulase activity values obeyed the Michaelis-Menten kinetics, with K$sb{rm m}$ values ranging from 9.7 to 21.4 g CMC liter$sp{-1}$. The E$sb{rm a}$ values of the reaction ranged from 21.7 to 28.0 kJ mol$sp{-1}$. Denaturation of soil cellulase occurred at temperatures ranging from 60 to 70$spcirc$C; from 60 to 80% of the total cellulase enzyme remained active upon preheating at 100$spcirc$C for 2 h. Air-drying of 8 field-moist soils markedly decreased cellulase activity (avg = 45%). Depending on the source of the reference fungal cellulase protein used, the amounts of cellulase protein calculated for air-dried and field-moist soils ranged from $<$15 $mu$g to 38 mg kg$sp{-1}$ soil. Organic materials (crop residues, animal manures, and sewage sludges) studied contained significant amounts of reducing sugars and cellulase activity. Reducing sugar values of waterlogged soils increased upon toluene treatment of air-dried and field-moist soils. Among the 25 trace elements studied, Ag(I), Cu(I), Cd(II), Hg(II) and Al(III) were the most effective inhibitors of cellulase activity ($>$15% inhibition at 5 $mu$mol g$sp{-1}$ soil).;The activities of 14 enzymes involved in C, N, P, and S cycling in soils were greater in four replicated plots treated with no-till/double mulch than in those treated with other tillage systems and residue placement. The activities decreased significantly with increasing soil depth, and accompanied by a decrease in organic C content. The activities of the enzymes were significantly correlated with each other, with r values ranging from 0.33* to 0.96***, and significantly correlated with soil organic C, with r values ranging from 0.70*** to 0.93***, suggesting that a large proportion of soil enzymes are complexed with soil organic matter. |
