Microbial decomposition of leached or extracted dissolved organic carbon and nitrogen from pasture soils

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Microbial decomposition of extracted and leached dissolved organic carbon (DOC) and nitrogen (DON) was demonstrated from three pasture soils in laboratory incubation studies. DOC concentration in water extracts ranged between 29 and 148 mg C L−1 and DON concentration ranged between 2 and 63 mg N L−1. Between 17 and 61 % of the DOC in the water extracts were respired as CO2 by microbes by day 36. DON concentrations in the extracts declined more rapidly than DOC. Within the first 21 days of incubation, the concentration of DON was near zero without any significant change in the concentration of NO3 − or NH4 +, indicating that microbes had utilized the organic pool of N preferentially. Decomposition of leached DOC (ranged between 7 and 66 mg C L−1) and DON (ranged between 6 and 11 mg N L−1) collected from large lysimeters (with perennial pasture; 50 cm diameter × 80 cm deep) followed a similar pattern to that observed with soil extracts. Approximately 28 to 61 % of the DOC in leachates were respired as CO2 by day 49. The concentration of DON in the leachates declined to below 1 mg N L−1 within 7–14 days of the incubation, consistent with the observations made with extractable DON. Our results clearly show that DOC and DON components of the dissolved organic matter in pasture soils, whether extracted or leached, are highly decomposable and bioavailable and will influence local ecosystem functions and nutrient balances in grazed pasture systems and receiving water bodies.