Every year, the main research field at the Strawberry Center is divided in half: fumigated and non-fumigated. Within the first few months after transplanting, I always notice how vigorous the plants on the fumigated side are compared to the non-fumigated side. Could fumigation also be influencing nutrient availability, perhaps even creating a fertilizer effect?  In 2023, I set out to investigate that possibility.

To explore whether fumigation affects nitrogen availability in the soil, I designed a study examining how two fumigants—metam sodium and chloropicrin—interacted with the application of controlled-release fertilizer (CRF) during the month following fumigation. The study took place in a commercial strawberry field on the west side of Santa Maria in sandy loam soil. Ammonium and nitrate were measured in the top 6” of the bed before fumigation, and 14, 21, and 28 days after fumigation. The plots were bed fumigated and fertilized treatments were given 42.5 lb N/acre in the form of CRF.

Six treatments were evaluated:

1. No fumigation
2. No fumigation + CRF
3. Metam sodium
4. Metam sodium + CRF
5. Chloropicrin
6. Chloropicrin + CRF

My idea was that after fumigation, most microbes would be killed and the nutrients they are composed of would be released into the soil. But, no increase in total nitrogen was noticed at any of the time points after fumigation with either fumigant compared to the non-fumigated control. With both fumigants, ammonium concentrations increased after fumigation (Fig. 1), while nitrate decreased (Fig. 2).

There was no significant association between CRF and ammonium, nitrate, or total N in the month after fumigation. However, the chloropicrin + CRF treatment showed numerically lower nitrate and total nitrogen levels post-fumigation than the chloropicrin treatment without CRF.

So, fumigation didn’t change the total N but shifted the N pool to be balanced between ammonium and nitrate. Before fumigation, over 85% of the N pool was nitrate. But, at 14, 21, and 28 days after fumigation, it was closer to a 50:50 ratio of nitrate to ammonium with both fumigants. In non-fumigated plots, nitrate remained the dominant source of N, comprising over 90% of the N pool at all time points. These results align with studies such as Xiong et al. 2023 that showed a reduction in nitrate and an increase in ammonium post-fumigation due to inhibition of nitrifying bacteria populations. This reduction in nitrifying bacteria post-fumigation has been well researched (Castellano-Hinojosa et al. 2022; Fang et al. 2023; Xiong et al. 2023).

Surely this shift in the N pool also impacts beneficial microbial species that recolonize the soil after fumigation. Some studies have shown that beneficial bacteria like those in the genus Bacillus and fungi in the genus Mortiella increased after dazomet and chloropicrin fumigation (Li et al. 2022; Sun et al. 2023), while others show that having nitrate as the dominant N source promotes a beneficial microbiome (Zhu et al. 2023). The shift also plays a role in flowering. Shi et al. (2021) showed that having a 50:50 ratio of nitrate to ammonium caused the cultivar ‘Albion’ to produce fewer flowers compared to a ratio of 80:20.

It would be interesting to look at how the soilborne diseases of strawberries react to different ammonium-nitrate ratios. If soilborne pathogens favor nitrate as their N source, then populations of these pathogens would likely be tied to abundance of nitrifying bacteria. To find effective alternatives to fumigation, we need to understand why fumigation is so effective. More research focused on how soil nutrients are interconnected with the soil microbiome and soilborne pathogens could help us find more effective fumigant alternatives.