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).