UC Berkeley

Under a collaborative research and development (CRADA) agreement, LBNL scientists investigated the effect of the semi-humic substance, Proximus, on nitrogen dynamics in two contrasting soils. We specifically tested four hypotheses that Proximus (1) binds ammonium and reduces nitrification; (2) does not bind ammonium but chemically inhibits nitrification; (3) stimulates microbial immobilization of N in biomass; (4) stimulates denitrification. We performed two primary experiments, one in the laboratory using a pure culture of a nitrifying bacterium, and one in a greenhouse setting testing the fate of nitrogen and response of microbial communities in soils with and without Proximus. Our findings can be summarized as follows: • Proximus does not chemically inhibit nitrification – this conclusion was reached by analysis of the response of Nitrosomonas europaea (ATCC 19718) to Proximus addition at field relevant concentrations, relative to the known nitrification inhibitor Nitropyrin. No inhibition of growth or ammonia oxidation to nitrite was observed at field relevant concentrations. • In soils with higher clay and silt content with relatively low N fertilizer application rates (100lbs/ac), the effect of Proximus on nitrogen concentrations and microbial communities was minimal. • In sandier soils with low organic matter and clay content and with relatively low N fertilizer application rates (100lbs/ac), Proximus aided delayed availability of inorganic nitrogen in the system. • Using a stable isotope tracer (15N) we observed differential effects of Proximus on N uptake by soil microbial biomass. In sandy soils, although Proximus slowed the release of NH4+ from fertilizer, Proximus did not have any significant effect on nitrogen assimilation by soil microbes. However, in more clay rich Iowa soils, while Proximus did not affect the magnitude or timing of total NH4+ or NO3- availability, it did reduce the availability of fertilizer N to soil microbes, delaying uptake into microbial biomass. • Proximus addition did not significantly affect overall microbial (bacterial, archaeal, fungal) community composition, although specific genera of bacteria and fungi were found to be enriched or repressed due to Proximus addition. • Over the 5 week observation period, Proximus did not stimulate any significant increase in denitrification relative to fertilizer alone. In summary, we see no evidence that Proximus inhibits nitrifiers or nitrification directly, nor does it stimulate more nitrogen loss through denitrification than fertilizer application alone. We do not find strong evidence for the direct role of microbial biomass stimulation as a Proximus mode of action. A consideration for future analyses and experimental design is the fertilization rate simulated in these experiments which may be considered low relative to more intensive agricultural practices. A study of dose dependence of Proximus application rate across soil types would also illuminate the importance of potential interactions with soil mineralogy. A further consideration is the interactions between plants and soil microorganisms not considered here. Root deposition of carbon enhances microbial growth and nutrient demand and we expect that Proximus impacts on the soil microbiome and nutrient retention would differ in the presence of growing plants.