In seasonally dry ecosystems, moisture held in weathered rock (“rock moisture”) underlying soil can be a critical growth-sustaining resource to vegetation. In the upland forests of NW California, this is especially true during summer months when light and temperatures are relatively high but soil water becomes limiting to growth. The cycling of water in the unsaturated rock moisture zone not only impacts plant water status; it also controls the biogeochemical cycling, transport and availability of essential nutrients to plant growth. However, there is very little known about the cycling and bioavailability of essential nutrients, such as nitrogen, in the rock moisture zone. Therefore, we asked: (1) What is the content and chemical identity of N in the rock moisture zone year round?; and (2) Is there evidence of plant or microbial use of this N?.
Using the novel Vadose Zone Monitoring System at the Eel River Critical Zone Observatory in N California, we sampled rock moisture and gasses bi monthly at ~1.5 m intervals from the top of the unsaturated weathered rock zone (~1.5 m) to the transition into the saturated water table (~ 16m depth). Water samples were analyzed for TN, NH4+ and NO3-, as well as a suite of other solutes. Gasses were analyzed for CO2 and O2.
Results/Conclusions
The primary form of N in rock moisture is organic N. We have found that TN and NH4+ and NO3- in rock moisture range in concentration year-round (TN = 1-7 mg N/L; NH4 = <0.05-0.4 mg/L; NO3- = <0.05-0.6 mg/L), yet they reach concentrations on the same order of magnitude as those seen in some temperate ecosystem soils. All N pools changed in concentration seasonally and correspond to changes in CO2 and O2 depth profiles that we hypothesize reflect root and microbial respiration. Both TN and NH4+ show an accumulation with depth, while NO3- is overall low in concentration at all depths year round, with the exception of several high concentrations measured in summer months at 5 m depth. The dynamic seasonal changes of N pools in rock moisture suggest it is being cycled by plants and microbes at depth, and that future research into N cycling in forests should consider rock moisture as an important reservoir of N to ecosystems.