Control of soil phosphatase activities at millimeter scales in a mixed paper birch - Douglas-fir forest: The importance of carbon and nitrogen

Organic P can serve as an important source of P for plants and microbes when mineralized by extracellular phosphatases. Substrate induction, end-product repression and/or resource limitation regulate activities of phosphatase in bulk soils. Yet, factors controlling enzyme activities in fine-scale microsites may differ from those observed at larger scales. Understanding such differences is needed to improve estimates of global models of biogeochemical cycling. Imprinting of soil profiles using cellulose sheets infused with chromogenic substrates allows study of extracellular enzymes at mm scales under naturally occurring soil temperatures, with minimal disturbance to soil microbial communities. In this study, we used a soil imprinting approach to investigate soil chemical characteristics associated with mm-scale regions of high in situ phosphatase activities in a mixed paper birch - Douglas-fir forest in the southern interior of British Columbia. In addition, we tested whether the addition of simple (ammonium chloride plus sodium acetate) and complex (cellulose, collagen, chitin) forms of carbon (C) and/or nitrogen (N) to 1 cm2 microplots on soil profiles influenced in situ phosphatase activity. In unamended microplots, percent C was 30% higher on average (P = 0.05) and percent N was about 15% higher (P = 0.05) in high-phosphatase microsites. Extractable P did not differ between high and low-phosphatase microsites, regardless of the form of P measured. Within the first 24 h, no difference in imprintable phosphatase was observed between C and N addition treatments, but after 72 h, microplots receiving any substrate containing N had higher phosphatase activities than those receiving only water (P < 0.001). The results from both of our studies support a role for resource limitation in regulating phosphatase activities at this site because either (i) P became limiting in microsites with higher amounts of C and N, and/or (ii) microsites with higher C and N were the ones where these nutrients were in sufficient supply to allow microbes to excrete extracellular enzymes. We conclude that phosphatase excretion occurs in C + N-enriched soil microsites, but that any such phosphatase-active microsites located beyond the rhizosphere are unlikely to supply P to roots because of the low diffusion rates of orthophosphate.