Monitoring belowground processes and responses of bioavailable soil nutrients in arid ecosystems

Background/Question/Methods

Understanding belowground responses, such as bioavailability of soil nutrients, is an important component of ecosystem process research. Measuring in situ bioavailable soil nutrients can be difficult without disturbing study sites, and arid environments pose additional difficulty in capturing relative soil nutrient bioavailability with the extremes in soil moisture and temperature. A tool commonly used for measuring in situ bioavailable soil nutrients is ion exchange membranes. We compared three separate studies that utilized ion exchange membranes (Plant Root Simulator (PRS™)-probes) in arid environments to measure relative differences in nutrient supply rates, as affected by various treatments and conditions. One study focused specifically on the usability of PRS™-probes at varying soil depths (15 and 40 cm), moistures (wet period: 1/23/04-4/23/04; dry period: 5/4/04-8/1/04), and burial times (30, 60, and 90 days) in the Mojave Desert. A second study examined the effects of nighttime warming (1-2˚C) and nitrogen deposition (2.0 g m^-2y^-1 NH₄NO₃) on community structure and ecosystem processes in a northern Chihuahuan Desert grassland, and used PRS™-probes to measure bioavailable soil nitrogen. The third study assessed ecosystem properties prior to and after downy brome (Bromus tectorum L.) control treatments (burning, herbicide, and mowing) in salt desert shrubland. The PRS™-probes were used to measure bioavailable soil nitrate, magnesium, potassium, and phosphorus.

Results/Conclusions

In the Mohave Desert, PRS™-probes were able to show differences in nutrient supply rate when compared across depths and burial periods. These differences were also observed temporally, suggesting that moisture did not impede PRS™-probe ability to detect differences in nutrient supply rate. The general trend showed an increase in nutrient supply rate with burial duration, but some desorbing and readsorbing of soil nutrients did occur. In a northern Chihuahuan Desert grassland bioavailable soil nitrate was significantly greater (28%; P = 0.01) in fertilized plots compared to controls. Bioavailable soil nitrogen was also significantly greater (P = 0.001) under grass canopy relative to interspaces. In salt desert shrubland downy brome abundance was negatively correlated with bioavailable soil nitrate (P <0.0001) and magnesium (P <0.01).  Burning treatments of downy brome did not result in an increase of bioavailable soil nutrients, but compared to the control treatments, it prevented a significant reduction in bioavailable soil nitrate. Herbicide treatments showed similar results to the burning treatments. These studies demonstrate that PRS™-probes are a useful tool for measuring relative differences in bioavailable soil nutrients in arid environments, delivering information integral to understanding ecosystem function and processes.