Soil heating and available nitrogen responses

Analysis of fall 2020 burn data from Dayton Tract

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 have immediate and short-term responses to fire.

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).

Methods

Sampling

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.

Modeling

We used a soil heat transfer model developed by Campbell et al. (1994) and applied to grassland fire science by Choczynska & Johnson (2009) to estimate heat penetration below each surface thermocouple.

Dynamic variables included initial soil temperature, maximum soil surface temperature, and time to maximum temp, all derived from heating curves from the thermocouple placed on the soil surface. Soil bulk density and particle fractions for each sample point were queried from the SSURGO database via soilDB::SDA_spatialQuery.

Soil moisture content was fetched from NOAA’s Climate Prediction Center’s daily soil moisture model for the day of the burn and was constant through each simulation. All other model terms were held constant from the defaults in the C++ program provided by Choczynska & Johnson. Each simulation lasted 10 minutes and the maximum value for each cm depth in that period is reported.

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.

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.

Change relative to control

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

N with respect to soil heating

Soil heating model results

Take-aways

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 illuminated by PLFA microbial community analysis, for which samples are standing by in a USDA-ARS freezer. Doesn’t look like 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.

Heating really seemed limited to the top 3-4 cm. Despite considerable variability in surface heating, 1 cm temperatures generally converged. Soil heating at depths greater than 5 cm was pretty negligible 10 minutes after heating began. It is likely that 10 cm samples are too general to parse effects on soil microbe communities.