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 (18:2ω6c and 16:1ω5c) 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.