Distribution of extracellular enzyme activity is related to microbial biomass and organic carbon throughout the top meter of soil
For all assayed EEs, EE activity per mass of soil declined logarithmically with depth (p < 0.001, Fig. 1 and S2), but about 50% of the total-profile EE activity kg-1 soil in the top meter occurred below 20 cm (Fig. 2A). However, the proportion of the EE activity below 20 cm differed by the soil order for many of the assayed EEs (Table S1). Mollisols had about a 1.5 times greater percentage of the sum of C- and N-acquiring EE activity kg-1 soil below 20 cm than Inceptisols or Ultisols (p < 0.050 for all comparisons, Fig. 2B). For AP, the 39% higher proportion in the subsoil for Mollisols compared to Inceptisols was only marginally significant (p = 0.063).
There were also differences in the percentage of MB and SOC in the subsoil among soil orders (MB: p < 0.001, SOC: p = 0.013), with Mollisols having an almost two times greater proportion of MB and SOC below 20 cm than Inceptisols (MB: p = 0.006, SOC: p = 0.013; Fig. 2B). While the proportion of MB below 20 cm was significantly higher in Mollisols compared to Ultisols (about 1.5 times greater, p = 0.001), the difference in the proportion of SOC below 20 cm between Mollisols and Ultisols was only marginally significant (p = 0.057).
Microbial biomass-normalized EE activity increased with depth for all enzymes (Fig. S3; all: p < 0.05). The strongest increases were for LAP and AP, which increased six- and seven-fold, respectively, while NAG and BG increased by 85% and 103%, respectively. Throughout the top meter, over 80% of MB-normalized EE activity occurred below 20 cm (Fig. S4A). However, because the proportion of MB below 20 cm also varied among soil orders, the proportion of MB-normalized EE activity below 20 cm was consistent among soil orders for most assayed EEs (AG: p = 0.333, BG: p = 0.175, CB: p = 0.278, BX: p = 0.211, NAG: p = 0.027, LAP: p = 0.537, AP: p = 0.048; Fig. S4A). Nevertheless, the proportion of MB-normalized NAG activity below 20 cm was 15% greater in Ultisols compared to Inceptisols (p = 0.025), and the proportion of MB-normalized AP activity below 20 cm was 17% greater in Ultisols compared to Mollisols (p = 0.042).
There were inconsistent patterns of EE activity normalized by SOC with depth. N-acetylglucosaminidase normalized by SOC decreased with depth (p = 0.004); AG, LAP, and AP increased with depth (AG: p = 0.016, LAP: p = 0.002, AP: p < 0.001), and BG, CB, and BX did not change with depth (BG: p = 0.322, CB: p = 0.344, BX: p = 0.198; Fig. S5). Similar to the proportion of MB-normalized EE activity below 20 cm, the proportion of EE normalized by SOC below 20 cm averaged about 80% and did not differ among soil orders (all: p > 0.1; Fig. S6).