Mineralogical impact on long-term patterns of soil nitrogen and phosphorus enzyme activities

Abstract

During long-term ecosystem development, both soil mineralogical composition and nutrient contents change, thus possibly altering microbial nutrient cycling by constraining substrate accessibility. In addressing the mineral impact on nitrogen (N) and phosphorus (P) cycling, we determined microbial abundances, activities of N-hydrolyzing (aminopeptidases, protease, urease) and P-hydrolyzing (phosphatase) enzymes and the potential substrate availability as well as their physicochemical and mineralogical controls in whole soil profiles along the 120 kyr-old Franz Josef chronosequence (New Zealand). Pedogenic soil iron (Fe) and aluminum (Al) resided initially (<1 kyrs) in metal-humus complexes, changed to poorly crystalline Fe and Al at intermediate-aged sites (1–12 kyrs) and into dominance of clay and crystalline Fe oxides at the oldest site. Despite this, organic C (OC) and organic N (ON) stocks increased only slightly with soil age, whereas organic P (OP) stocks decreased continuously. In organic layers, enzyme activities were mainly regulated by ON and OP concentrations, whereas in mineral soils, mineral–enzyme relations were more complex and included both, direct and indirect effects. Protease, urease, and phosphatase activities were inhibited by mineral interactions, especially with poorly crystalline Fe and Al oxides, whereas aminopeptidases were less affected by mineralogical properties. On a pedon basis, most N-hydrolyzing enzyme activities per ON stocks responded negatively to increasing stocks of poorly crystalline Fe and Al minerals, but were also affected by the C:N ratio of labile organic substrates. Profile-based phosphatase activities per OP stock were highest at the oldest sites having the largest stocks of clay and crystalline Fe oxides. Overall, our study indicates that long-term mineral changes create distinct patterns of nutrient accumulation and N- and P-enzyme activities at both horizon and pedon scale, with a variable extent of the mineralogical effect for the different N-hydrolyzing enzymes.