Activities of Nitrogen-Mineralization Enzymes Associated with Soil Aggregate Size Fractions of Three Tillage Systems

Nitrogen mineralization occurring near the soil surface of agroecosystems determines the quantity of plant-available N, and soil enzymes produced by microorganisms play significant roles in the N mineralization process. Tillage systems may influence soil microbial communities and N mineralization enzymes through alterations in total soil C and N. Soil aggregates of different sizes provide diverse microhabitats for microorganisms and therefore influence soil enzyme activities. Our objective was to test the hypothesis that activities of N mineralization enzymes increase with aggregate size and in no-till compared with tilled systems. Potential activities of N-acetyl glucosaminidase (NAG), arylamidase, L-glutaminase, and L-asparaginase were measured in five aggregate size fractions (<0.25, 0.25-0.5, 0.5-1, 1-2, and 2-4 mm) obtained from soils of three long-term (22-yr) tillage systems (no-till, chisel plow, and moldboard plow). All enzyme activities were significantly (P < 0.05) greater in no-till than in tilled systems and positively correlated (P < 0.005) with potential N mineralization. Potential activities of NAG, L-glutaminase, and arylamidase were significantly greater (P < 0.05) in the intermediate (0.5-1-mm) aggregate size than in other size fractions. All enzyme activities were positively correlated with total soil C (P < 0.0001), N (P < 0.05), and microbial biomass C (P < 0.05). Aggregate size had significant effects on NAG, arylamidase, and L-glutaminase activities but the magnitudes were small. Fungal biomarkers (18:2w6c and 16:1w5c) determined by the phospholipid fatty acid (PLFA) method were significantly greater in the no-till than in tilled systems and positively correlated with all enzyme activities. This suggests that no-till management enhances activities of N mineralization enzymes by enhancing the proportion of fungal organisms in the soil microbial community.