Microbes regulate carbon cycling in soil systems, in part by excreting extracellular enzymes that decompose organic matter into usable energy and nutrient sources. Studies of potential extracellular enzyme activity in water-limited environments are required to improve our understanding of soil decomposition and microbial nutrient demands in drylands, which cover approximately 40% of Earth’s land surface. Our research examined soil microbial response to topography in a dryland watershed by quantifying exoenzyme activity (EEA) across pairs of divergent, water-shedding and convergent, water-gathering landscape positions. We established nine divergent and nine convergent plots in a Sonoran Desert scrub environment that represents the lower climate end member of the Catalina Critical Zone Observatory with annual precipitation and temperature averages of 45 cm and 18°C, respectively. Surface mineral soils were collected from each plot and analyzed for soil pH, organic matter content, and biogeochemistry. We quantified potential EEA for seven enzymes using fluorescently labeled substrates and summed the respective enzyme activities to assess total carbon (C), nitrogen (N), and phosphorus (P) mineralization between divergent and convergent landscapes.
Results/Conclusions
Soil pH is slightly acidic to neutral across the plots and did not significantly differ between landscape positions. Total C, P, and N positively correlated with pH, albeit insignificantly. We observed significant differences in potential EEA between divergent and convergent positions, likely a result of moisture availability in complex terrain. We found that total C and P activity were significantly higher in convergent, water-gathering landscape positions compared to adjacent water-shedding plots. Interestingly, total N activity showed no significant difference between divergent and convergent landscapes. We attribute the increase in total C and P activity to a greater accumulation of water and soluble geochemical constituents in the convergent sites. We hypothesize that total N activity may reflect seasonal patterns in rainfall, single storm events, or contrasting responses based on the form of available N (NH4+, NO3-). Our findings demonstrate that landscape position exhibits significant control on potential exoenzyme activity and requires additional consideration when quantifying microbial activity in dryland systems. Future work will integrate bulk soil geochemistry, particle size distribution, and seasonal variation in potential exoenzyme activity to further examine the importance of landscape position on decomposition rates in pulse-driven ecosystems.