Short-Term Impacts of Fertilizer Addition on Soil Aggregate Composition and Microbial Populations in Conventional and Organic Farming Systems.

Understanding the response of soil microbes to nutrients and other external inputs is essential to understanding soil function in maintaining and restoring soil ecosystem services, such as creating soil structure. Microbial response to nutrient addition is rapid, with major changes in microbial community activity occurring in a matter of hours or days, whereas soil structure investigations concern timescales of months or years. We analyzed the effect of N fertilizer addition on soil aggregate distribution, microaggregate development and microbial population dynamics during 28 days following fertilizer application in soil microcosms. Organic and conventional farming system soils were collected from longterm plots at the Russell Ranch Sustainable Agricultural Facility in Davis, CA.   The addition of urea fertilizer resulted in an increase in water stable aggregates (microaggregates) as a portion of the total soil in the organic treated soils but no change in aggregate composition was observed in the conventional soils. Total bacteria, total archaea and bacterial ammonia oxidizer (AOB) numbers increased in organic soil microaggregates but remained constant in conventional soils through the four week experiment. Total bacteria and archaea, AOB, and denitrifier (nirK and nirS) numbers were higher in the organic treatment on day 28. Archaeal ammonium oxidizer numbers were not significantly different in the organic and conventional soils. The increase in microaggregates in organic soils in response to urea fertilizer addition suggests that using organic practices, such as adding high-carbon amendments, could also lead to soil structure improvements in conventional farming systems.