PRS™ soil supply rates of nitrate and ammonium in grasslands, temperate forests and boreal forests

Background/Question/Methods

Nitrogen (N) is often the most limiting nutrient to plants in grassland and forested ecosystems. PRS™-probes consist of an ion exchange resin membrane in a plastic support that is inserted into the soil and adsorbs soil ammonium (NH₄⁺-N) and nitrate (NO₃^--N) in a manner similar to plant roots. We provide a preliminary meta-analysis of published PRS™ supply rates of NH₄⁺-N and NO₃^--N of surface mineral soils in temperate grasslands and temperate and boreal forests. We used studies that had not received any recent (< 3 years) disturbance or anthropogenic treatment such as fertilizer or burning. PRS™-probe burial periods ranged from 16 to 79 days.

Results/Conclusions

We found no differences in PRS™ supply rates of NH₄⁺-N, NO₃^--N or total N among the three plant communities. Grassland sites had greater supply rates of NO₃^--N than NH₄⁺-N. Although most forest sites were dominated by NH₄⁺-N, supply rates of NH₄⁺-N were not different from NO₃^--N. Variation in supply rates was relatively high with coefficients of variation of 1.50, 2.14 and 1.42 for NH₄⁺-N, NO₃^--N and total N, respectively. Supply rates were log-normally distributed as supply rates were typically low, likely due to rapid utilization and immobilization, but were occasionally relatively high. Nitrogen supply rate and form in forest soils depend largely on litter quality and pH, while grassland N supply rate and form in grassland soils depend largely on moisture levels. This meta-analysis highlights the high variability in the N supply rates of soils, even within plant communities. While different sites may have varying potentials to supply soil N, weather during the burial period also has a large effect on soil N supply rates due to effects on soil N processes.