Soil Characteristics Rather than Starter Phosphorus Control Active Carbon Pools and Enzyme Activities in High-Legacy-Phosphorus Soils

This study aimed to disentangle the relative influence of inherent soil properties and annual starter P fertilization on active carbon (C) pools and C-, nitrogen (N)-, and phosphorus (P)-cycling enzyme activities in silage corn production systems with high-legacy P. Six fields with Mehlich-3 P ranging from 53.5 to 332 mg kg−1 were investigated in 2020 and 2021 in the Fraser Valley, Canada. The experiments at each site consisted of five starter P rates (0, 5, 10, 15, and 20 kg P ha−1 as triple super phosphate) arranged in a randomized complete block design with four replicates. Soil samples were collected at the V3 and V6 stages of silage corn and analyzed for active C, soil enzyme activities, and chemical properties. N-acetyl-β-glucosaminidase varied significantly across the six sites, suggesting substantial differences in the rate of C and N cycling. For instance, in 2020, N-acetyl-β-glucosaminidase was similar at Sites 1 and 2 at V6 and was approximately three times (514.51 pmol MUF g−1 soil h−1) higher than at Site 3 (170.29 pmol MUF g−1 soil h−1). Similarly, in 2021, a 2.8-fold higher MBC observed at Site 4 at V6, compared with the averages of Sites 5 and 6, further confirms an active C pool. Meanwhile, sites with the lowest MBC concentrations were linked to acidic soils (pH 5.3), and a negative correlation between inherent site–year characteristics and enzyme activities confirm enzymes repression. Acid phosphatase at Site 4 was 3-fold higher than at Site 5 and Site 6, while alkaline phosphatase was detected only at Site 4. We conclude that long-term soil conditions are the main factors influencing biological functionality, thereby overshadowing transient fertilization. This indicates that Fraser Valley farmers can prioritize long-term soil health management and safely reduce starter P applications in these high-legacy systems.