Background

Sampling to investigate rangeland soil responses to management at NDSU’s Hettinger Research Extension Center (HREC) has, to date, only been initiated once the grazing season has begun, which is often weeks if not months after prescribed fire. But it has been well-documented elsewhere that important plant-available species of nitrogen such as ammonium NH4 and nitrate NO3.

Thus, this study was designed to investigate NH4 and NO3 responses to fire immediately (1 day after fire), over the short term (7 days after fire), and into the next growing season (7 months after fire).

Study design

We used a Before-After-Control-Impact (BACI) design in which five patches intended to be burned were paired with adjacent patches to be left unburned. Soil samples were collected from three points within each patch no more than 24 hours prior to a prescribed fire. Within 24 hours of the burn, all points were resampled, and resampled again 7 days after the burns. All points were resampled in the spring, about 7 months after the burns.

Burns occurred over two days in the fall of 2020. On the second day, thermocouple dataloggers were installed at soil sampling points prior to the fire, recording data on temperatures at the soil surface and at 15 cm above the soil surface for 3 of the 5 prescribed burns.

Results

Observed temperatures

Mean temperature (n=3) for three burns on the second day of Rx fires at the Dayton Tract, fall 2020, with thermocouple sensors positioned at soil surface and 15 cm above soil surface.

Mean temperature (n=3) for three burns on the second day of Rx fires at the Dayton Tract, fall 2020, with thermocouple sensors positioned at soil surface and 15 cm above soil surface.

Nitrogen responses

Raw patterns

Pools of two plant-available N species at four times relative to burns at the Dayton tract, in paired burned and unburned patches.

Pools of two plant-available N species at four times relative to burns at the Dayton tract, in paired burned and unburned patches.

Change relative to control

Responses of two plant-available soil N pool at four times in burned patches relative to paired unburned control areas.

Responses of two plant-available soil N pool at four times in burned patches relative to paired unburned control areas.

Take-aways

Interpreting patterns

The immediate increase in ammonium NH4 following fire is exactly as expected, as is the return to pre-burn levels over the course of the week. This has not yet been described in these grasslands.

Lower nitrate NO3 one week after burning was not expected, but the long-term increase is consistent with the literature. The conventional pattern is for a substantial amount of the flush of ammonium to be converted to nitrate, which a longer-lasting form of mineralized N. Note that in the raw data, there was a slight increase in nitrate 7 days vs. 1 day after the fire, but there was also a substantial increase in the unburned patches, as well. This suggests some background pattern in N dynamics that the fire effect actually reduced in the burned patches.

Possible explanations for the 1-week dip in NO3:

  • Plants took up the ammonium immediately. This is unlikely (?) as these fall burns occurred near the end of the growing season, although the stand was dominated by cool-season grasses that could still have been active.

  • The fire set back microbes responsible for the NH4 \(\rightarrow\) NO3 conversion. This should be illumiated by PLFA microbial community analysis, for which samples are standing by in a USDA-ARS freezer. Not sure when/if these will actually get analyzed, though, unfortunately.

Regardless of what happened in that first week after the fire in the fall, plants in burned areas started the growing season with an abundance of NO3.

How long it lasts, and how it affects the plants, merits further investigation.