Soil Enzyme Activities Affect SOC and TN in Aggregate Fractions in Sodic-Alkali Soils, Northeast of China

Abstract

Soil enzymes strongly affect soil organic carbon (SOC) and nitrogen (TN) storage. However, few studies have focused on their relationships in aggregates, especially in sodic-alkali agricultural fields. In the current study, we hypothesized that the impact of soil enzymes on SOC and TN were different within aggregates for their heterogeneous distribution. Soils collected from the surface (0–20 cm) and subsurface (20–40 cm) layers of sodic-alkali agricultural fields in the northeast of China were separated via the dry sieve method into macro-aggregates (>2000 μm), meso-aggregates (250–2000 μm), and micro-aggregates (<250 μm). SOC, TN, microbial biomass carbon (MBC) and nitrogen (MBN), and C- and N-cycling enzymes, namely amylase (AMY), invertase (INV), β-glucosidase (GLU), catalase (CAT), β-N-acetylglucosaminidase (NAG), and urease (URE) in soil aggregates were tested and analyzed. High content of SOC and TN were observed in macro- and meso-aggregates in both layers, with the largest amount detected in meso-aggregates. The highest values of MBC and MBN were observed in meso-aggregates, followed by micro-aggregates for MBC and macro-aggregates for MBN. Soil enzymes were distributed heterogeneously in soil aggregates, where the activities of AMY, INV, and URE in both layers were in the order of meso-aggregates > macro-aggregates > micro-aggregates. The same trend was followed by NAG of surface soils, while in the subsurface soils, NAG activities increased with the increasing aggregate sizes. NAG activities in both layers decreased with decreasing aggregate sizes. The GLU activity rose with the decreasing aggregate sizes in both layers, contrary to CAT. Enzyme activities affect SOC and TN in soil aggregates, for NAG, INV, GLU, and URE are closely related to SOC and TN across aggregate sizes. The test indices mentioned above in the surface layer were higher than those in the subsurface layer. These results indicate that biophysical processes associated with C- and N-cycling enzymes may be vital to the SOC and TN sequestration within soil aggregates in sodic-alkali agricultural fields.