Search for: All records

Associative N2 fixation (ANF) is widespread but poorly characterized, limiting our ability to estimate global inputs from N2 fixation. In some places, ANF rates are at or below detection most of the time, but occasionally and unpredictably spiking to very high rates. Here we test the hypothesis that plant phenology and rainfall events stimulate ANF episodes. We measured ANF in intact soil cores in switchgrass (Panicum virgatum L.) in Michigan, USA. We used rain exclusion shelters to impose three rainfall treatments with each receiving 60 mm of water over a 20-day period but at different frequencies. We concurrently established a treatment that received ambient rainfall, and all four treatments were replicated four times. To assess the effects of plant phenology, we measured ANF at key phenological stages in the ambient treatment. To assess the effects of rainfall, we measured ANF immediately before and immediately after each wetting event in each treatment involving rainfall manipulation. We found that the previous day’s rainfall could explain 29% of the variation in ANF rates within the ambient treatment alone, and that bulk soil C:N ratio was also positively correlated with ANF, explaining 18% of the variation alone. Wetting events increased ANF and the magnitude of response to wetting increased with the amount of water added and decreased with the amount of inorganic N added in water. ANF episodes thus appear to be driven primarily by wetting events. Wetting events likely increase C availability, promote microbial growth, and make rhizosphere conditions conducive to ANF. 

Abstract. The active fraction of soil organic carbon is an important component of soil health and often isquickly assessed as the pulse of CO2 released by re-wetting dried soils in short-term(24–72 h) assays. However, soils can lose carbon (C) as they dry and, if soil samples vary in moisture content at sampling, differential C loss during the pre-assay dry-down period maycomplicate the assay's interpretations. We examined the impact of pre-assay CO2 loss ina long-cultivated agricultural soil at initial moisture contents of 30 %, 50 %, and 70 %water-filled pore space (WFPS). We found that 50 % and 70 % WFPS treatments lost more C duringdrying than did those in the 30 % WFPS treatment and that dry-down losses led to a 26 %–32 % underestimate of their CO2 pulses. We developed a soil-specificcorrection factor to account for these initial soil moisture effects. Future C mineralizationstudies may benefit from similar corrections.