Short-term partitioning of ammonium and nitrate between plants and microbes in an annual grassland

Abstract

We measured short-term (24 h) partitioning of 15NH 4 + and 15NO 3 − into plants and microbes in a California annual grassland. Experiments were done in early spring at the peak of plant growth (February), and in late spring (April) when plant senescence had begun. Soil moisture decreased from 31 to 9% during this period. We injected either 15NH 4 +, 15NO 3 −, 15NH 4 +NO 3 − or NH 4 15NO 3 − (≈ 2μg Ng −1 soil for each N-species) into the top 10cm of soil in cylinders which had been driven into the soil. After 24 h the soil and plants in the cylinders were harvested and we measured total N and 15N in inorganic forms, the microbial biomass (chloroform fumigation technique) and harvested plant material. Ammonium was the dominant source of N to both plants and microbes. In February, uptake rates of NH 4 + were 81 and 426 mg N m −2 day −1 for plants and microbes, respectively, while in April the rates were 110 and 639 mg N m −2 day −1. Rates of NO 3 − uptake were 41 and 81 mgNm −2day −1 in February and 83 and 146mgNm −2 day −1 in April for plants and microbes, respectively. Thus, microbes took up substantially more NH 4 + and NO 3 − than plants. On both sampling dates, NH 4 + concentrations in the top 10cm of soil ranges from 600 to 800mg N m −2, andNO 3 − concentrations were <50 mg N m −2 High rates of microbial NO 3 −3 uptake may have resulted from the occurrence of microsites that were depleted in NH 4 +. Even though plants competed better for NO 3 − than for NH 4 +, microbial uptake was a major factor controlling NO 3 − availability to plants. The high rates of NH 4 + and NO 3 − uptake by plants and microbes clearly demonstrate that the soil N pool bears little relationship to actual N availability.